WO2022104483A1 - 2d/3d alloyed halide perovskites: method for their preparation and use in solar cells - Google Patents
2d/3d alloyed halide perovskites: method for their preparation and use in solar cells Download PDFInfo
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- WO2022104483A1 WO2022104483A1 PCT/CA2021/051657 CA2021051657W WO2022104483A1 WO 2022104483 A1 WO2022104483 A1 WO 2022104483A1 CA 2021051657 W CA2021051657 W CA 2021051657W WO 2022104483 A1 WO2022104483 A1 WO 2022104483A1
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- perovskite
- perovskites
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- 150000004820 halides Chemical class 0.000 title claims abstract description 95
- 238000000034 method Methods 0.000 title claims description 27
- 238000002360 preparation method Methods 0.000 title description 20
- 229910020816 Sn Pb Inorganic materials 0.000 claims abstract description 38
- 229910020922 Sn-Pb Inorganic materials 0.000 claims abstract description 38
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- 229910001502 inorganic halide Inorganic materials 0.000 description 2
- RQQRAHKHDFPBMC-UHFFFAOYSA-L lead(ii) iodide Chemical compound I[Pb]I RQQRAHKHDFPBMC-UHFFFAOYSA-L 0.000 description 2
- LLWRXQXPJMPHLR-UHFFFAOYSA-N methylazanium;iodide Chemical compound [I-].[NH3+]C LLWRXQXPJMPHLR-UHFFFAOYSA-N 0.000 description 2
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- YUOWTJMRMWQJDA-UHFFFAOYSA-J tin(iv) fluoride Chemical compound [F-].[F-].[F-].[F-].[Sn+4] YUOWTJMRMWQJDA-UHFFFAOYSA-J 0.000 description 2
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- 125000006017 1-propenyl group Chemical group 0.000 description 1
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- 125000003903 2-propenyl group Chemical group [H]C([*])([H])C([H])=C([H])[H] 0.000 description 1
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- APFVFJFRJDLVQX-UHFFFAOYSA-N indium atom Chemical compound [In] APFVFJFRJDLVQX-UHFFFAOYSA-N 0.000 description 1
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- 238000011835 investigation Methods 0.000 description 1
- OCVXZQOKBHXGRU-UHFFFAOYSA-N iodine(1+) Chemical compound [I+] OCVXZQOKBHXGRU-UHFFFAOYSA-N 0.000 description 1
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- 125000001971 neopentyl group Chemical group [H]C([*])([H])C(C([H])([H])[H])(C([H])([H])[H])C([H])([H])[H] 0.000 description 1
- 231100000252 nontoxic Toxicity 0.000 description 1
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- 229910052697 platinum Inorganic materials 0.000 description 1
- 229920000123 polythiophene Polymers 0.000 description 1
- 125000004805 propylene group Chemical group [H]C([H])([H])C([H])([*:1])C([H])([H])[*:2] 0.000 description 1
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- 125000002914 sec-butyl group Chemical group [H]C([H])([H])C([H])([H])C([H])(*)C([H])([H])[H] 0.000 description 1
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- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 description 1
- 229910001887 tin oxide Inorganic materials 0.000 description 1
- QPBYLOWPSRZOFX-UHFFFAOYSA-J tin(iv) iodide Chemical compound I[Sn](I)(I)I QPBYLOWPSRZOFX-UHFFFAOYSA-J 0.000 description 1
- 238000002371 ultraviolet--visible spectrum Methods 0.000 description 1
- 125000000391 vinyl group Chemical group [H]C([*])=C([H])[H] 0.000 description 1
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Classifications
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
- H10K85/50—Organic perovskites; Hybrid organic-inorganic perovskites [HOIP], e.g. CH3NH3PbI3
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
- H10K30/50—Photovoltaic [PV] devices
Definitions
- the present invention relates generally to halide perovskites. More specifically, the present invention relates to halide perovskites comprising substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof.
- the halide perovskites according to the invention have a 2D/3D structure and comprise alloys that may or may not involve lead, for example tin-lead (Sn-Pb) alloys.
- organic-inorganic halide perovskites have stolen the show among other photovoltaic materials mainly because of a relatively simple fabrication process which involves deposition, high power conversion efficiencies (PCEs), and tunable bandgap from 1.15 to 3.06 eV obtained among others, by swapping different cations,
- PSCs The state-of-the-art perovskite solar cells
- Pb water-soluble lead
- Pb halide perovskite in contact with water or humid air form water-soluble by-products of Pb, which can accumulate within the food chain and reach the human body [7]
- Sn-based perovskites are toxicologically safe, highest-performing lead-free solar cells [10,11]. Unfortunately, Sn is more stable in the 4 + oxidation state, causing critical stability issues as well as lower photovoltaic performance for Sn-based PSCs, as a result of the formation of lattice vacancies [12], The Sn lattice vacancies enhance the background hole carrier concentration (p-doping) in Sn-based perovskites [12,13], Sn-based perovskite devices demonstrate subpar open-circuit voltage (V oc ) mainly due to severe charge carrier recombination in the solar cell triggered by p-doping [14], Different approaches including morphological control of the Sn perovskites, minimization of oxygen exposure during the device preparation, and the addition of SnF2, have been explored and found to be useful in improving PCE of Sn-based perovskite devices.
- Zhao et al. reported one of the highest-performing Sn PSCs with an efficiency of 8.12% falling far short of Pb halide PSCs [10], On the other hand, adding Pb content into Sn perovskites can stabilize 2+ oxidation state of Sn.
- Alloyed Pb-Sn perovskites are thus alternate strategies towards stable, less toxic solar cells without compromising the photovoltaic performance.
- alloyed Sn-Pb perovskites exhibit broader absorption of photons extending to near-infrared spectrum (up to 1050 nm with optical bandgap of 1.18 eV).
- MAPbl 3 , MAPbl 3 and FASnl 3 lack absorption in near-infrared spectrum; MAPbl 3 , FASnl 3 and FASnl 3 have limited optical bandgaps of 1.55 eV (up to 800 nm), 1.48 eV (up to 838 nm) and 1.30 eV (up to 950 nm), respectively [15],
- halide perovskites which comprise substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof.
- the halide perovskites according to the invention have a 2D/3D structure and comprise tin-lead (Sn-Pb) alloys.
- the inventors have designed and prepared halide perovskites, wherein at least part of the small organic cations formamidinium (FA) and methylammonium (MA) is replaced by an organic cation having a size which is larger than the size of FA and/or MA.
- such larger size organic cation is an ammonium or an amidinium.
- A is an organic cation selected from ammonium cation and amidinium cation
- FA is formamidinium
- B is an alloy involving at least two of Sn, Pb and Ge;
- X is a halogen atom.
- A is an ammonium cation of general formula I below wherein Ri to R4 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl.
- R1 to R3 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl;
- L is present or absent and is a group comprising one or more of (CH 2 ) and (CH);
- Q is present or absent and is a 5 to 12-member ring or bicycle ring, with the proviso that at least one of Q and L is present.
- Q is present or absent and is a 5 to 12-member ring or bicycle ring; and n is an integer from 0-6.
- Ri are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl; n is an integer from 1-6; and m is an integer from 0-4.
- A is an ammonium cation of general formula V below wherein n is an integer from 1-6.
- A is an amidinium cation of general formula I' below wherein R 5 to R 9 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl.
- Rs to Rs are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl;
- L' is present or absent and is a group comprising one or more of (CH 2 ) and (CH); and Q' is a 5 to 12-member ring or bicycle ring.
- A is an amidinium cation of general formula III' below wherein:
- Q' is a 5 to 12-member ring or bicycle ring; and n' is an integer from 0-6.
- R'i are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl; n' is an integer from 1-6; and m' is an integer from 0-4.
- A is an organic cation selected from an ammonium cation of general formula I, II, III, IV orV and an amidinium cation of general formula I', II', III', IV' or V';
- FA formamidinium
- MA methylammonium
- y 0.01-0.99
- x 0.01-0.99
- X is a halogen atom.
- PEA y (FA x MA 1-x ) 1-y Sn x Pb 1-x l 3 (P2) wherein:
- PEA is phenylethylammonium cation
- FA is formamidinium
- a method of preparing a 2D/3D alloyed halide perovskite comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with an organic cation having a size which is larger than the size of FA and/or MA.
- a method of preparing a 2D/3D alloyed halide perovskite comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with a larger organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'.
- FA formamidinium
- MA methylammonium
- a method of manufacturing a solar cell device comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with an organic cation having a size which is larger than the size of FA and/or MA.
- a method of manufacturing a solar cell device comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with a larger organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'.
- a method of manufacturing a solar cell device comprising replacing at least part of the cations formamidinium (FA) and methylammonium (MA) with phenylethylammonium (PEA) cation.
- a method of manufacturing a solar cell device comprising preparing the 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above.
- a method of manufacturing a solar cell device comprising using the 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above.
- a solar cell device which comprises the 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above.
- Figure 1 a) Absorption spectra of 3D perovskite films and comparison of absorption spectra of 3D perovskite vs. 2D/3D perovskite films; b) (PEA) y (FA 0.3 MA 0.7 ) 1-y Sn 0.3 Pb 0.7 l 3 ; c) (PEA)y (FA 0.5 MA 0.5 ) 1-y Sn 0.5 Pb 0.5 l 3 ; d) (PEA) y (FA 0.7 MA 0.3 ) 1-y Sn 0.7 Pb 0.3 l 3 .
- Figure 3 Morphology of a) (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 ; b) (PEA) 0.05 (FA 0.3 MA 0.7 ) 0.95 Sn 0.3 Pb 0.7 l 3 ; c) (PEA) 0.1 (FA 0.3 MA 0.7 ) 0.90 Sn 0.3 Pb 0.7 l 3 ; d) (FASnl 3 ) 0.5 (MAPbl 3 ) 0.5 ; e) (PEA) 0.05 (FA 0.5 MA 0.5 ) 0.95 Sn 0.5 Pb 0.5 l 3 ; f) (PEA)o.1 (FA 0.7 MA 0.3 ) 0.90 Sn 0.5 Pb 0.5 l 3 ; g) (FASnl 3 ) 0.7 (MAPbl 3 ) 0.3 ; I 3 )
- PEA 0.05 (FA 0.7 MA 0.3 ) 0.95 Sn 0.7 Pb 0.3 l 3 ; i) (PEA) 0.1 (FA 0.7 MA 0.3 ) 0.90 Sn 0.7 Pb 0.3 l 3 .
- Figure 4 AFM topographical images with roughness of a) (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 ; b) (PEA) 0.05 (FA 0.3 MA 0.7 ) 0.95 Pb 0.7 Sn 0.3 l 3 ; c) (FASnl 3 ) 0.5 (MAPbl 3 ) 0.5 ; d) (PEA) 0.05 (FA 0.5 MA 0.5 ) 0.95 Pb 0.5 Sn 0.5 l 3 ; e) (FASnl 3 ) 0.7 (MAPbl 3 ) 0.3 ; f) (PEA) 0.05 (FA 0.7 MA 0.3 ) 0.95 Pb 0.3 Sn 0.7 l 3 .
- Figure 5 SEM images and elemental mapping of Pb, Sn and I of a) (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 ; b) (FASnl 3 ) 0.5 (MAPbl 3 ) 0.5 ; c) (FASnl 3 ) 0.7 (MAPbl 3 ) 0.3 ; d) (PEA) 0.05 (FA0.3 MA 0.7 ) 0.95 Pb 0.7 Sn 0.3 l 3 ; e) (PEA) 0.05 (FA 0.5 MA 0.5 ) 0.95 Pb 0.5 Sn 0.5 l 3 ; f) (PEA) 0.05 (FA 0.7 MA 0.3 ) 0.95 Pb 0.3 Sn 0.7 I 3 .
- Figure 6 XRD spectra of FASnl 3 and MAPbl 3 perovskite films.
- Figure 7 Comparison of XRD spectra of different compositions of 3D perovskites and 2D/3D perovskites: a) (PEA) y (FA0.3 MA 0.7 ) 1-y Sn 0.3 Pb 0.7 l 3 , b) (PEA) y (FA 0.5 MA 0.5 ) 1-y Sn 0.5 Pb 0.5 I 3 and c) (PEA) y (FA 0.7 MA 0.3 ) 1-y Sn 0.7 Pb 0.3 I 3 ; and variation of FWHM of the (110) diffraction peak of: d) (PEA) y (FA0.3 MA 0.7 ) 1-y Sn 0.3 Pb 0.7 I 3 , e) (PEA) y (FA 0.5 MA 0.5 ) 1-y Sn 0.5 Pb 0.5 I 3 and f) (PEA) y (FA 0.7 MA 0.3 ) 1-y Sn 0.7 Pb 0.3 l 3 .
- Figure 8 High resolution XPS spectra of Sn 3d in a) (FASnl 3 ) 0.3 (MAPbl3) 0.3 ; b) (PEA) 0.05 (FA 0.3 MA 0.7 ) 0.95 Sn 0.3 Pb 0.7 I 3 .
- the red and blue curve corresponds to Sn 2+ and Sn 4+ oxidation states, respectively.
- Figure 9 XPS survey spectra of (FASnI 3 ) 0.3 (MAPbI 3 ) 0.7 and (PEA)o.os (FA0.3 MA 0.7 ) 0.95 Pb 0.7 Sn 0.3 I 3 .
- Figure 10 Statistical distribution of efficiency of a) (FASnl 3 ) 0.3 (MAPbl3) 0.7 ; b) (PEA) 0.05 (FA0.3 MA 0.7 ) 0.95 Sn 0.3 Pb 0.7 l 3 ; c) (PEA) 0.1 (FA0.3 MA 0.7 ) 0.95 Sn 0.3 Pb 0.7 I 3 devices.
- Figure 11 Statistical distribution of efficiency of a) (FASnl 3 ) 0.5 (MAPbI 3 ) 0.5 ; b) (PEA) 0.05 (FA 0.5 MA 0.5 ) 0.95 Sn 0.5 Pb 0.5 I 3 ; c) (PEA) 0.1 (FA 0.5 MA 0.5 ) 0.95 Sn 0.5 Pb 0.5 I 3 devices.
- Figure 12 Statistical distribution of efficiency of a) (FASnl 3 ) 0.7 (MAPbl3) 0.3 ; b) (PEA) 0.05 (FA 0.7 MA 0.3 ) 0.95 Sn 0.7 Pb 0.3 I 3 ; c) (PEA) 0.1 (FA 0.7 MA 0.3 ) 0.95 Sn 0.7 Pb 0.3 I 3 devices.
- Figure 13 j. V curv es of a) (PEA) y (FA 0 .3MA 0.7 ) 1-y Sn 0.3 Pb 0.7 l3; b) (PEA) y (FA 0.5 MA 0.5 )i.
- Figure 14 Dependence of a) J sc ; b) Voc; c) FF on the incident light intensity of the 3D PSCs and 2D/3D PSCs.
- Figure 15 Topography and surface potential profile of a) (FASnl 3 ) 0.3 (MAPbl3) 0.7 , b) (PEA) 0.05 (FA 0.3 MA 0.7 ) 0.95 (Sn 0.3 Pb 0.7 ) I 3 , C) (FASnl 3 ) 0.5 (MAPbl 3 ) 0.5 , d) (PEA) 0.05 (FAO.5 MAO.5 ) 0.95 (Sn 0.5 Pb 0.5 ) I 3 , e) (FASnl 3 ) 0.7 (MAPbl3) 0.3 and f) (PEA) 0.05 (FA 0.7 MA0.3 ) 0.95 (Sn 0.7 Pb 0.3 ) I 3 .
- Figure 16 J-V hysteresis of a) (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 ; b) (PEA) 0.05 (FA0.3 MA o.7 ) 0.95 Sn 0.3 Pb 0.7 I 3 devices.
- Figure 17 Degradation of 3D perovskite devices vs. 2D/3D perovskite devices a) under 28 ⁇ 2% RH humidity in the dark; b) under N2 at 1.5G AM Sun illumination; c) under 28 ⁇ 2% RH humidity at 1.5G AM Sun illumination.
- ammonium refers to cations having a general formula I as depicted herein.
- aminodinium refers to cations having a general formula I' as depicted herein.
- the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “include” and “includes”) or “containing” (and any form of containing, such as “contain” and “contains”), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.
- alkyl represents a monovalent group derived from a straight or branched chain saturated hydrocarbon comprising, unless otherwise specified, from 1 to 15 carbon atoms and is exemplified by methyl, ethyl, n- and /so-propyl, n-, sec-, iso- and tert-butyl, neopentyl and the like and may be optionally substituted with one, two, three or, in the case of alkyl groups comprising two carbons or more, four substituents.
- alkylene represents a saturated divalent hydrocarbon group derived from a straight or branched chain saturated hydrocarbon by the removal of two hydrogen atoms, and is exemplified by methylene, ethylene, isopropylene and the like.
- alkenyl represents monovalent straight or branched chain groups of, unless otherwise specified, from 2 to 15 carbons, such as, for example, 2 to 6 carbon atoms or 2 to 4 carbon atoms, containing one or more carbon-carbon double bonds and is exemplified by ethenyl, 1-propenyl, 2-propenyl, 2-methyl-1-propenyl, 1-butenyl, 2- butenyl and the like and may be optionally substituted with one, two, three or four substituents.
- alkynyl represents monovalent straight or branched chain groups of from two to six carbon atoms comprising a carbon-carbon triple bond and is exemplified by ethynyl, 1-propynyl, and the like and may be optionally substituted with one, two, three or four substituents.
- cycloalkyl represents a monovalent saturated or unsaturated non-aromatic cyclic hydrocarbon group of three to eight carbon atoms, unless otherwise specified, and is exemplified by cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, bicyclo[2.2.1]heptyl and the like.
- halogen or “halo” as used interchangeably herein, represents F, Cl, Br and I.
- the inventors have designed and prepared halide perovskites which comprise substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof.
- the halide perovskites according to the invention have a 2D/3D structure and comprise tin-lead (Sn-Pb) alloys.
- the inventors have designed and prepared halide perovskites, wherein at least part of the small organic cations formamidinium (FA) and methylammonium (MA) is replaced by an organic cation having a size which is larger than the size of FA and/or MA.
- such larger size organic cation is an ammonium or an amidinium.
- the inventors explored different compositions of Pb- Sn alloyed perovskite by stoichiometrically mixing FASnl 3 and MAPbl 3 to utilize near-infrared photons of the solar spectrum [16]. It is known in the art that the trap states at grain boundaries (GBs) and primarily at the perovskite surface, instigate lower photovoltaic performance, current-voltage hysteresis and to some degree stability in 3D alloyed power conversion efficiencies (PSCs).
- GBs grain boundaries
- PSCs 3D alloyed power conversion efficiencies
- a drawback associated to Sn-based alloyed perovskites is inferior stability under ambient environment since Sn +2 can readily oxidize to stable Sn 4+ when Sn perovskites exposed to ambient environment.
- ion migration in the halide perovskites plays a critical role in the stability of the PSCs.
- the passivation of iodide-rich perovskite surface reduces the ion migration in the material, improving the stability of the devices.
- reduced ion migration in 2D/3D alloyed perovskites decreases the current-voltage hysteresis in the derived devices.
- optical bandgaps of (FASnl 3 ) x (MAPbl3) 1-x and (PEA) y (FA X MA 1-x ) 1-y Sn x Pb 1-x I 3 were determined from diffuse reflectance measurements.
- Figure 2 shows steady-state photoluminescence (PL) peaks of the (PEA) y (FA X MAi- x) 1-y Sn x Pb 1-x I 3 films.
- the optical band gaps obtained from PL match with absorption onset observed in the absorption spectra indicative of direct bandgap nature in (PEA) y (FAx MA 1-x ) 1- y Sn x Pb 1-x I 3 materials ( Figure 2a).
- Figure 2a we compare the PL intensity of 3D alloyed perovskites with 2D/3D alloyed perovskites in Figures 2b-d. A noticeable enhancement in PL intensity has observed after stoichiometrically replacing FA/MA with PEA in alloyed perovskites.
- the FA/MA substitution with 10% PEA cations shows highest PL intensity in all alloyed perovskite compositions.
- the increase in PL intensity in a semiconductor correlates with reduction of nonradiative recombination in a material.
- the XRD pattern also suggests that (FASnl 3 )x (MAPbl 3 ) 1-x adopts a crystal structure type consisting of Sn and Pb atoms randomly occupying corner-sharing ([Sn 1-x Pb x l 6 ] -4 ) octahedra [21],
- the crystal structure is unchanged after replacing a small amount of FA/MA by PEA in alloyed perovskites. But, we noticed better crystallinity for all 2D/3D compositions in comparison to 3D counterparts.
- the intensity of the XRD peaks notably increased for alloyed perovskites with PEA, which could be from higher crystallinity of the perovskite as well as good film coverage on the substrate.
- X-ray photoelectron spectroscopy (XPS) measurements were carried out.
- Figure 8 shows the two XPS Sn 3d peaks (corresponding to 3d 5/2 and 3d 3/2 peaks) of (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 and (PEA) 0.05 (FA0.3 MA 0.7 ) 0.95 Sn 0.3 Pb 0.7 I 3 films.
- Figure 9 shows the survey spectrum of alloyed perovskites.
- 3D alloyed perovskite was highly susceptible to Sn 2+ oxidation evident from presence of peak corresponding to Sn 4+ .
- Sn 3d 5/2 peak located at 486 eV
- Sn 3d 5/2 peak could deconvoluted into two peaks located at 485.97 eV and 486.7 eV corresponding to Sn 2+ and Sn 4+ oxidation states.
- Sn 3d 3/2 peak (located at 495.5 eV) comprised of two peaks located at 494.35 eV and 495.34 eV associated with Sn 2+ and Sn 4+ oxidation states [13], Also, additional peaks at 483.65 eV and 492.55 eV emerges in the XPS spectrum of 3D alloyed perovskite films which correspond to zero-valent Sn (Sn°).
- the Sn 2+ oxidation to Sn 4+ is a strong indication of Sn vacancy formation in Sn-based perovskites [23]
- the intrinsic defects such as Sn vacancies (through the Sn 2+ oxidation) in the 3D alloyed perovskite generate p-type conductivity in the semiconductor [24]
- the defect formation energy of Sn vacancy is lower amongst other point defects due to the strong Sn 5s— I 5p antibonding coupling, which implies that Sn vacancy is the dominant intrinsic defect producing high hole (p-type) carrier density in Sn-based perovskites [25]
- the high background hole carrier density within 3D alloyed perovskites stimulates predominantly monomolecular (or trap-assisted) recombination processes.
- 2D/3D alloyed perovskite does not show any signs of Sn 2+ oxidation or metallic Sn formation.
- the XPS studies reveal that PEA cations play a key role in inhibiting Sn 2+ oxidation and Sn formation in alloyed perovskite films.
- First principle calculations have shown that the ionic size of organic cations plays a vital role in governing formation energy of Sn vacancies in Sn-based perovskites [14], The larger ionic size of organic cation reduces the Sn 5s— I 5p antibonding coupling, leading to lower the Sn vacancy formation energies in the Sn-based perovskites.
- the V oc and FF strongly depend on charge carrier recombination in the solar cell and Sn-rich perovskites are likely to heavily p-doped limiting the V oc and FF of the devices.
- the J-V curves and corresponding external quantum efficiency (EQE) graphs of solar cells are shown in Figure 13.
- the EQE shows photoresponse of the solar cell in the NIR region.
- the calculated J sc from EQE closely matches with J sc obtained from J-V curve except in (FASnl 3 ) 0.7 (MAPbl 3 ) 03 . This mismatch only is seen in Sn-rich 3D perovskite probably because of quicker degradation of the samples.
- ⁇ 95
- 2D/3D perovskite devices which implies photogenerated charge carriers in 3D alloyed perovskite device are not efficiently transported to the electrodes [26]
- Voc versus logarithmically scaled light intensity.
- recombination rate is proportional to the product of charge carrier densities [27].
- the recombination rate reduces producing better FF.
- the FF increases with decreasing light intensity in 2D/3D alloyed PSCs, which suggest bimolecular recombination dominate in the device.
- FF decreases with decreasing light intensity in 3D alloyed perovskite device which indicates trap-assisted recombination dominates in these devices [27]
- the rate of charge carriers recombining with trapped charges increases with decreasing light intensity resulting in lower FF in 3D alloyed perovskite devices. It should be noted that the number of traps does not change with lowering the light intensity.
- the charge carrier traps such as Sn vacancies, mobile iodide ions are mainly located at GBs and on the perovskite film surface [27],
- single crystal CsSnI 3 shows very low hole concentration ( ⁇ 10 17 cm -3 ) compared to polycrystalline perovskite films ( ⁇ 10 19 cm -3 ) due to the absence of grain boundaries [13,25],
- the topographical images suggest that a plate-like crystal film growth for 2D/3D alloyed perovskite films which reduces the number of grain boundaries compared to their 3D analogs.
- the mobile ions are known to be responsible for the hysteresis phenomenon in PSCs [34,35], Under an applied electric field, iodide anions can move generating ionic current in ionic crystals like halide perovskites [36,37], The inhibition of Sn vacancy formation may also lead to reduction of mobile iodide ions in 2D/3D perovskites.
- By controlling mobile iodide ions we have also effectively tamed the J-V hysteresis in alloyed PSCs.
- Figure 17b displays the degradation of devices under AM 1.5G illumination in an inert atmosphere.
- Figure 17c shows that the degradation of alloyed perovskite devices is rapid in the co-presence of light and modest moisture.
- 2D/3D perovskite devices are more resilient to the degradation, retaining 20% of initial performance after 24 hours whereas 3D perovskite devices wholly degraded.
- Tin iodide (SnI 2 ) , tin fluoride (SnF2), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Sigma Aldrich.
- Lead iodide (PbI 3 ) was purchased from Acros Organics.
- Phenyl-Cei-butyric acid methyl ester (PC 61 BM) and poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS; CleviosTM P VP Al 4083) were obtained from 1 -Materials and Heraeus, respectively. All chemicals were used as received.
- Example 1 - Preparation of FASnl 3 precursor solution (1.1 M): The 1.1M of FASnh precursor solution was prepared by adding equimolar FAI (189.2 mg) and SnI 2 (409.77 mg) powders and 20 mol% of SnF2 (17.3 mg) into a mixture of 1 mL of mixture of DMF: DMSO (4:1 v/v). The solution was stirred for half an hour at room temperature inside the glove box.
- Example 3 Preparation of 3D (FASnI 3 ) 0.3 (MAPbI 3 ) 0.7 composition: The composition was obtained by stoichiometrically mixing 300 microliter of the FASnl 3 precusor solution prepared at Example 1 and 700 microliter of the MAPbl 3 precursor solution prepared at Example 2.
- Example 4 Preparation of 3D (FASnI 3 ) 0.5 (MAPbI 3 ) 0.5 composition: The composition was obtained by stoichiometrically mixing 500 microliter of the FASnl 3 precusor solution prepared at Example 1 and 500 microliter of the MAPbl 3 precursor solution prepared at Example 2.
- Example 5 Preparation of 3D (FASnI 3 ) 0.7 (MAPbI 3 ) 0.3 composition: The composition was obtained by stoichiometrically mixing 700 microliter of the FASnl 3 precusor solution prepared at Example 1 and 300 microliter of the MAPbl 3 precursor solution prepared at Example 2.
- the 2D/3D alloyed perovskite precursor solutions were prepared by stoichiometrically substituting FAI/MAI with 5% and 10% PEAI in (FASnI 3 )x (MAPbl3) 1-x precursor solution.
- Example 6 Preparation of 2D/3D (PEA) 0.05 (FA X MAI. X ) 0.95 Sn x Pb 1-x I 3 perovskite precursor solution: We first prepared PEA 0.05 FA 0.95 SnI 3 and PEA 0.05 MA 0.95 PbI 3 precursors as described above at Examples 1-2 wih suitable modifications. We replaced 5% (in weight) of MAI and FAI replaced with PEAI compared to 3D FASnl 3 and MAPbl 3 perovskite precursor solutions. The solutions were stirred for half an hour at room temperature inside the glove box.
- Example 7 Preparation of 2D/3D (PEA) 0.1 (FA X MAI. X ) 0.90 Sn x Pb 1-x I 3 perovskite precursor solution:
- PEA 2D/3D
- PEA 0.1
- FA X MAI. X 0.90
- Sn x Pb 1-x I 3 perovskite precursor solution We first prepared PEA 0.1 FAo.9oSnh and PEA 0.1 MAogoPbh precursors as described above at Examples 1-2 wih suitable modifications.
- the solutions were stirred for half an hour at room temperature inside the glove box.
- Example 8 Preparation of 2D/3D (PEA) 0.05 (FA X MAI. X ) 0.95 Sn x Pb 1-x h perovskite solution: The solutions were mixed as described above for the 3D perovskite precursor solutions (Examples 3-5).
- Example 9 Preparation of 2D/3D (PEA) 0.1 (FA X MAI. X ) 0.90 Sn x Pb 1-x I 3 perovskite solution: The solutions were mixed as described above for the 3D perovskite precursor solutions (Examples 3-5).
- Example 10 - Solar celle fabrication Patterned ITO-coated glasses were cleaned by sonication in detergent followed by sequential washing with deionized water, acetone, and isopropanol. After drying under air flow, the substrate surface was cleaned by oxygen plasma for 10 minutes under rough vacuum.
- the PEDOT: PSS solution was spin-coated on top of an ITO-coated glass substrate at 45000 rpm for 45 seconds; PEDOT: PSS performs as the hole transporting layer.
- the PEDOT: PSS film was then dried in air on a hot plate (set at 170°C) for 10 minutes. After drying, the substrate is transferred to a nitrogen-filled glovebox for further use.
- the 3D and 2D/3D alloyed perovskite absorber layer was spin-coated on the PEDOT : PSS film at 5,000 rpm for 60 seconds. Diethyl ether was dropped onto the spinning substrate. Then the PCeiBM solution (20 mg/mL in chlorobenzene) was spin-coated on top of the perovskite film at 1000 rpm for 45 seconds to form a 20 nm thick electron transporting layer. Finally, the film was transferred to a thermal evaporation chamber inside the nitrogen filled glove box. The chamber was pumped down to 1 x 10" 6 Torr for silver deposition. The 100 nm thick silver top electrode was deposited through a shadow mask that defines the active device area as 0.06 cm 2 for the solar cells.
- Example 11 - Perovskite Film Characterization Topography of perovskite film surface was obtained by using Bruker MultiMode8 AFM. The absorption spectra were collected by using a Lambda 750, UV-Visible-NIR spectrometer (Perkin Elmer). Steady-state PL spectra were obtained from a Fluorolog®-3 system (Horiba Jobin Yvon). XRD measurements were carried out using a Panalytical X-Pert PRO MRD X-Ray diffractometer. The oxidation of Tin element probed by XPS (ESCALAB 220I-XL spectrometer) equipped with an Al Ka (1486.6 eV) monochromatic source.
- XPS XPS
- Al Ka 1486.6 eV
- KPFM measurements were done using Lift mode in Bruker MultiMode8 AFM under ambient conditions. KPFM measurements were performed using a Pt/lrtip (Bruker, SCM-PIT) with a lift height of 20 nm by applying AC voltage of 1V.
- Example 12 - Solar cell characterization Solar cell performance was measured using a class ABA LED solar simulator which was calibrated to deliver simulated AM 1.5 sunlight at an irradiance of 100 mW/cm 2 (The irradiance was calibrated using an NREL- calibrated KG5 filtered silicon reference cell). Current-voltage curves were recorded using a source meter (Keithley 2400, USA). External quantum efficiency (EQE) measurement was conducted by using an IQE200B system (Newport Corporation).
- EQE External quantum efficiency
- Example 13 - Device stability tests The stability of devices were tested in the same device configuration without any encapsulation.
- the 2D and 3D perovskite devices were placed inside a desiccator.
- the relative humidity (28 ⁇ 2% RH humidity) was measured with a digital humidity sensor.
- the perovskite devices were placed under constant AM1.5G illumination inside the N2 filled glovebox. Devices are also tested under constant AM1.5G illumination at 28 ⁇ 2% RH humidity.
- Suitable large organic cations equivalent to the methylammonium (MA) cation and different from phenylethylammonium (PEA), ammonium cations may also be used to replace methylammonium (MA) and formamidinium (FA) in the halide perovskite according to the invention.
- Such organic cations include for example those depicted in general formulae I, II, III, IV and V as depicted herein.
- iodine may be replaced by chorine (Cl) or bromine (Br).
- the alloy in the halide perovskite of the invention may include any suitable metals.
- Such metals may be for example tin (Sn), lead (Pb), germanium (Ge).
- alloys in the perovskite according to the invention may be for example Sn-Pb, Sn-Ge or Ge- Pb. In embodiments of the invention, the alloy is Sn-Pb.
- Keywords lead-free perovskite solar cells, alloyed perovskites, Sn oxidation, trap-assisted recombination, stability
- PES bulkier phenylethylammonium
- halide perovskites mainly the hybrid organic- inorganic perovskites 2 but also the all-inorganic perovskites, 3,4 have attracted the major research interest in the photovoltaic community mainly because of their simple solution deposition fabrication process, rapidly increasing and high power conversion efficiencies (PCEs) and spectacular optical and electronic properties, including tunable bandgap from 1.15 to 3.06 eV by swapping different cations and halides in the perovskite structure. 5 7
- Pb perovskite solar cells involve water-soluble Pb species, which hamper the commercialization of this technology. 8 Once in contact with water or moisture, Pb perovskites quickly form water-soluble by-products of Pb, which can accumulate within the food chain and so human body. 9 It is thus highly necessary to restrict the use of Pb by replacing it partially or entirely with less toxic or, even better, non-toxic metals. Most likely, appropriate candidates for Pb replacement without compromising too much photovoltaic properties are elements in the same group (group 4) as Pb. The immediate candidates then appear as Sn and Ge, both of which can be more easily oxidized from the 2 + to 4 + oxidation states than Pb.
- Sn-based PSCs thus demonstrate subpar open-circuit voltage (V oc ) mainly due to severe trap- assisted recombination triggered by Sn vacancy-mediated unintentional doping/defects.
- V oc subpar open-circuit voltage
- Different approaches including morphological control of the Sn perovskites, minimization of oxygen exposure during the device preparation, and the addition of Sn salts, have been explored and found to be useful in improving the performance of Sn-based PSCs.
- Wang et al. reported one of the Sn PSCs with an efficiency of 9.41% by employing quasi-two-dimensional (2D) Sn perovskites, which is still falling far short of Pb PSCs.
- alloying Pb with Sn perovskites seems to be a promising, balanced strategy towards achieving stable, less toxic solar cells without much compromising the photovoltaic performance.
- alloyed Sn-Pb perovskites bring an added advantage, extended absorption of photons to the near-infrared (NIR) spectral range (up to 1050 nm with an optical bandgap of 1.18 eV).
- typical hybrid organic-inorganic halide perovskites such as MAPbI 3 , FASnl 3 .
- the trap-assisted recombination coefficient which is defined as a specific rate at which oppositely charged ions combine at traps, is ⁇ 70 times higher than those in Pb-based ones. 23 It suggests a prospect of improving the performance of Sn-Pb alloyed PSCs by reducing the defects associated with oxidation of Sn 2+ to Sn 4+ and/or Sn vacancy formation.
- the defects formed in the course of perovskite film preparation can be detrimental to the stability of the PSCs. So, minimizing the defects in Sn-Pb perovskites improves stability. The defects also play a detrimental role in current density-voltage (J-V) curve hysteresis in PSCs through defect migration at grain boundaries (GBs) and perovskite film interfaces.
- J-V current density-voltage
- the peak close to the optical absorption edge is ascribed to the bound excitonic state due to the Columbic attraction of electrons and holes (the hydrogenic absorption peak).
- the peak is located (at 1.85 eV) significantly distant from the absorption onsets (at 1.18 eV, 1.22 eV, 1.26 eV), which basically excludes the hydrogenic absorption origin of the peak.
- GIWAXS Grazing-incidence wide-angle X-ray scattering
- the Sn 3d 5/2 peak (located at 495.41 eV) comprises two peaks located at 494.27 eV and 494.71 eV, associated with Sn 2+ and Sn 4+ oxidation states. 17
- the (PEA) 0.05 (FA0.3 MA 0.7 ) 0.95 Sn 0.3 Pb 0.7 I 3 only shows two sharp, non- deconvolutable peaks, with no Sn 4+ peaks.
- the XPS spectra of (PEA) 0.05 (FA 0.7 MA 0.3 ) 0.95 Sn 0.7 Pb 0.3 I 3 perovskites also showed similar behavior compared to their control samples ( Figure S7).
- the resultant defects can stimulate non-radiative (or trap-assisted) recombination processes, which is detrimental to photovoltaic performance.
- the PEA-FA/MA alloyed perovskites do not show any detectable signals of Sn 4+ , enhanced photovoltaic performance and stability are thus expected.
- PEA bulkier organic cations
- V oc and fill factor (FF) reduced as the Sn content increased (Table 1), with Sn-rich ((FASnl 3 ) 0.7 (MAPb 13)0.3) showing the lowest V oc and FF amongst all the compositions.
- Sn-rich perovskites are highly likely to contain a higher number of defects limiting the V oc and FF of their devices. Specifically, the defects in the bandgap would attract electrons/holes and act as non-radiative recombination centers, which primarily impacts the V oc of corresponding solar cells.
- V oc was significantly improved.
- the V oc of (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 PSCs was boosted from 0.72 V to 0.80 V.
- the highest V oc enhancement (of 0.23V) was observed for (PEA) 0.1 (F Ao. ?M A 0.3 )0.90 Sn 0.7 Pb 0.3 I 3 , followed by (PEA) 0.1 (FA 0.5 MA 0.5 )o.9o Sn 0.5 Pb 0.5 I 3 (with an improvement of 0.15 V).
- J sc there is no clear trend of variation with incorporating PEA among various alloyed perovskite compositions. But in a particular alloyed perovskite compositional group, the J sc in the devices follows a similar trend as observed in XRD data, especially in Sn-rich perovskites, where reduction of crystallinity with higher amount of PEA lowered the J sc .
- the PSCs employing Sn-Pb alloyed perovskites with 5% bulkier PEA cations produced the highest PCE. We hypothesis this is due to a trade-off between local lattice strain or/and crystallinity and the number of defects in the alloyed perovskites.
- Figures S8-S10 shows the statistical distribution of efficiency of 20-30 devices in each compositional group.
- J-V curves and corresponding external quantum efficiency (EQE) spectra of solar cells are shown in Figures 6 and SI 1. It can be seen from the EQE measurements that the photoresponse of all the solar cells reaches up to the near-infrared (NIR) region.
- NIR near-infrared
- the calculated J sc values from EQE basically matched with those obtained from the J-V curves, except for (FASnl 3 ) 0.7
- Figure 7a shows the dependence of J sc on incident light intensity by fitting to a power law (I).
- t 1/2 as a time for the PCE of the device to drop to 50% of its initial value for standardizing the comparison between different devices.
- Figure S13a shows the degradation of devices under a modest moisture environment (28 ⁇ 2% RH humidity) in the dark. The degradation was significantly slowed down by substituting FA/MA with 5% of PEA in (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 .
- Figure SI 3b displays the degradation of devices under AM1.5G illumination in an inert atmosphere.
- Figure S13c shows that the degradation of alloyed perovskite devices became faster in the co-presence of light and modest moisture.
- PEA-FA/MA perovskite devices were still more resistant to the degradation, retaining 20% of initial performance after 24h, whereas FA/MA perovskite devices were wholly degraded during the same period.
- the defects in Sn-Pb perovskites induced by Sn vacancies not only play a key role in determining photovoltaic performance (PCE and J-V hysteresis) but also the stability of the devices.
- the defects in perovskites are vulnerable to oxygen or moisture filtration and accelerate the degradation of devices under ambient conditions.
- 49 Reduced defects (see DFT calculations and PL discussion) and enhanced crystallinity (see XRD and GIWAXS discussion) with the incorporation of 5% PEA in Sn-Pb perovskites contribute to enhanced stability of the PEA-FA/MA Sn-Pb PSCs.
- Figure 8 a) Height topography and b-c) surface potential profile of (FASnl 3 ) 0.3 (MAPbl 3 ) 0.7 , d) topography and e-f) surface potential profile of (PEA) 0.05 (FA0.3 MA 0.7 ) 0.95 (Sn 0.3 Pb 0.7 ) I 3 .
- the average CPD calculated using Gwyddion software is also provided at the bottom of each Figure.
- CPD contact potential difference
- KPFM Kelvin probe force microscopy
- the charge-separation process in the complete device is mimicked using the AFM probe in the KPFM measurement, and SPV represents the V oc or internal electric field in the solar cell.
- the SPV increased from 0.059 V to 0.077 V with 5% PEA in alloyed perovskite film, indicating an increase in the internal electric field in PEA-FA/MA perovskite devices; therefore, we can reasonably conclude that photogenerated carriers are more efficiently separated in the PEA-FA/MA perovskite film than FA/MA perovskite film.
- the KPFM measurements essentially suggest reduced recombination events for the devices employing PEA-FA/MA perovskite film compared to their control samples.
- Methylammonium iodide (CH 3 NH 3 I), Formamidinium iodide (CH(NH2)2l) and phenylethylammonium iodide ( C 6 H 5 (CH 2 ) 2 NH 3 I) were purchased from Dyesol.
- Tin iodide (SnE), tin fluoride (SnF2), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Sigma Aldrich.
- Lead iodide (Pb I 2 ) was bought from Acros Organics.
- Phenyl- C61-butyric acid methyl ester (PC 61 BM) and poly (3, 4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS; CleviosTM P VP Al 4083) were obtained from 1-Materials and Heraeus, respectively. All chemicals were used as received. Alloyed perovskite precursor solution: FASnI 3 precursor solution (1.1 M) was prepared by adding equimolar FAI and SnI 2 powders and 20 mol% of SnF 2 into a mixture of DMF: DMSO (4: 1 v/v).
- MAPbI 3 precursor solution (1.1 M) was prepared by adding MAI and PbI 2 in a mixture of DMF: DMSO (4: 1 v/v).
- (FASnl 3 ) x (MAPbl 3 ) 1-x (x 0.3, 0.5, 0.7) solutions were prepared by stoichiometrically mixing FASnI 3 and MAPbE solutions.
- the PEA-FA/MA alloyed perovskite precursor solutions were prepared by stoichiometrically substituting FAI/MAI with 5% and 10% PEAI in (FASnI 3 ) x (MAPbl 3 ) 1-x precursor solution.
- the FA/MA and PEA-FA/MA alloyed perovskite absorber layer were spin-coated on the PEDOT : PSS film at 5000 rpm for 60 s. Diethyl ether was dropped onto the spinning substrate. The spin-coated films were annealed at 60°C for 5 minutes. Then the PC 61 BM solution (20 mg/mL in chlorobenzene) was spin-coated on top of the perovskite film at 2000 rpm for 60 s to form a 20 nm thick electron transporting layer. Finally, the film was transferred to a thermal evaporation chamber inside the nitrogen-filled glove box. The chamber was pumped down to 1 x 10 -6 Torr for silver deposition.
- Perovskite Film Characterization Topography of perovskite film surface was obtained by using a Bruker MultiMode8 AFM. Absorption spectra were collected by using a UV-Visible- NIR spectrometer Lambda 750 (Perkin Elmer). Steady-state PL spectra were obtained from a Fluorolog®-3 system (Horiba Jobin Yvon) using a 444 nm laser. XRD measurements were carried out using a Panalytical X-Pert PRO MRD X-Ray diffractometer.
- the oxidation of the tin element was probed by an XPS spectrometer (ESCALAB 220I-X L) equipped with an Al K ⁇ (1486.6 eV) monochromatic source.
- the grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were done at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF).
- the GIWAXS patterns were collected by a MarCCD detector, which is mounted vertically at around 194 mm from the sample. The exposure time was less than 50 s, and a grazing incidence angle with respect to the surface plane is 1°.
- KPFM measurements were done using the tapping mode in a Cypher AFM (Model: Cypher S) under ambient conditions. KPFM measurements were performed using a Platinum-coated Silicon tip. A white LED lamp was used to carry out KPFM measurements under illumination.
- Solar cell characterization Solar cell performance was measured using a class ABA LED solar simulator, which was calibrated to deliver simulated AM 1.5 sunlight irradiance of 100 mW/cm 2 . The irradiance was calibrated using an NREL-calibrated KG5 filtered silicon reference cell. Current density -voltage (J-V) curves were recorded using a source meter (Keithley 2400, USA). External quantum efficiency (EQE) measurements were conducted by using an IQE200B system (Newport Corporation).
- Device stability tests The stability of devices was tested without any device encapsulation.
- the PEA-FA/MA and FA/MA perovskite devices were placed inside a desiccator under an ambient environment.
- the relative humidity 28 ⁇ 2% RH humidity
- the relative humidity 28 ⁇ 2% RH humidity
- the perovskite devices were placed under constant AM1.5G illumination inside an N2 filled glovebox. Devices were also tested under constant AM1.5G illumination at 28 ⁇ 2% RH humidity.
- a 1 x4x4 F-centered Appoint sampling of the Brillouin Zone (BZ) was used for the structural optimizations of both the 8x 1 x 1 bare FASnI 3 and mixed (PEA 4 FA 10 )Sn 12 I 38 supercells.
- E form E tot ( Vsn q ) - E tot + ⁇ Sn + q( ⁇ VBM +E F ), (2)
- the chemical potentials of FASnI 3 constituents were estimated by imposing the thermodynamic equilibrium of FASnI 3 with the SnE phase. We thus apply the following two constraints:
- BSSE Basis Set Superposition Error
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Abstract
Halide perovskites comprising substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof. The halide perovskites according to the invention have a 2D/3D structure and comprise alloys that may or may not involve lead, for example tin-lead (Sn-Pb) alloys.
Description
TITLE OF THE INVENTION
2D/3D ALLOYED HALIDE PEROVSKITES: METHOD FOR THEIR PREPARATION AND USE IN SOLAR CELLS
FIELD OF THE INVENTION
[0001] The present invention relates generally to halide perovskites. More specifically, the present invention relates to halide perovskites comprising substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof. The halide perovskites according to the invention have a 2D/3D structure and comprise alloys that may or may not involve lead, for example tin-lead (Sn-Pb) alloys.
BACKGROUND OF THE INVENTION
[0002] Halide perovskites have general ABX3 crystal structure, where A = methylammonium (MA), formamidinium (FA), caesium (Cs) and rubidium (Rb); B = lead (Pb) and and/or tin (Sn); X = chlorine (Cl), bromine (Br) and/or iodine (I) [1], In recent years, organic-inorganic halide perovskites have stolen the show among other photovoltaic materials mainly because of a relatively simple fabrication process which involves deposition, high power conversion efficiencies (PCEs), and tunable bandgap from 1.15 to 3.06 eV obtained among others, by swapping different cations, metals, and halides in the perovskite structure [2-5],
[0003] The state-of-the-art perovskite solar cells (PSCs) involve water-soluble lead (Pb) precursors which hamper commercialization of the associated technology [6], Pb halide perovskite in contact with water or humid air form water-soluble by-products of Pb, which can accumulate within the food chain and reach the human body [7], It is highly desirable to find a replacement to Pb, and the easiest way to do so without compromising photovoltaic properties is replacing Pb with elements in the same group (group 4) as Pb. The immediate candidates are tin (Sn) and germanium (Ge) which have multiple oxidation states (2+ and 4+). However, computational studies show that the electronic configuration of Pb2+ in the ABX3 perovskite structure is crucial for the photovoltaic behavior of the cell [8,9],
[0004] To date, Sn-based perovskites are toxicologically safe, highest-performing lead-free solar cells [10,11], Unfortunately, Sn is more stable in the 4+ oxidation state, causing critical stability issues as well as lower photovoltaic performance for Sn-based PSCs, as a result of the formation of lattice vacancies [12], The Sn lattice vacancies enhance the background hole
carrier concentration (p-doping) in Sn-based perovskites [12,13], Sn-based perovskite devices demonstrate subpar open-circuit voltage (Voc) mainly due to severe charge carrier recombination in the solar cell triggered by p-doping [14], Different approaches including morphological control of the Sn perovskites, minimization of oxygen exposure during the device preparation, and the addition of SnF2, have been explored and found to be useful in improving PCE of Sn-based perovskite devices. Recently, Zhao et al. reported one of the highest-performing Sn PSCs with an efficiency of 8.12% falling far short of Pb halide PSCs [10], On the other hand, adding Pb content into Sn perovskites can stabilize 2+ oxidation state of Sn.
[0005] Alloyed Pb-Sn perovskites are thus alternate strategies towards stable, less toxic solar cells without compromising the photovoltaic performance. Advantageously, alloyed Sn-Pb perovskites exhibit broader absorption of photons extending to near-infrared spectrum (up to 1050 nm with optical bandgap of 1.18 eV). For instance, typical halide perovskites such as MAPbl3, MAPbl3 and FASnl3 lack absorption in near-infrared spectrum; MAPbl3, FASnl3 and FASnl3 have limited optical bandgaps of 1.55 eV (up to 800 nm), 1.48 eV (up to 838 nm) and 1.30 eV (up to 950 nm), respectively [15],
[0006] Countries around the world adhere to lead-free directives which set a maximum amount of lead in electronic devices. For example, the European Union adheres to the “Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment,” RoHS1 in 2003 and the subsequent RoHS2 in 2011 . These Directives restrict to 0.1% in weight the maximum concentration of lead in each homogeneous material contained in any electronic devices, i.e. , the perovskite material within a perovskite solar cell. North America also makes similar restriction to the content of lead in the electronic devices. Meanwhile, it should be noted that highly efficient halide perovskites materials contain more than 10% lead in weight.
[0007] There is a need for efficient halide perovskites that are environmentally friendly and safe for humans. In particular, there is a need for halide perovskites that do not contain Pb in free form or any form that may lead to water-soluble precursors thereof.
SUMMARY OF THE INVENTION
[0008] The inventors have designed and prepared halide perovskites which comprise substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof. The halide perovskites according to the invention have a 2D/3D structure and comprise tin-lead (Sn-Pb) alloys.
[0009] In particular, the inventors have designed and prepared halide perovskites, wherein at least part of the small organic cations formamidinium (FA) and methylammonium (MA) is replaced by an organic cation having a size which is larger than the size of FA and/or MA. In embodiments of the invention, such larger size organic cation is an ammonium or an amidinium.
[0010] The invention thus provides the following in accordance with aspects thereof:
(1) A 2D/3D alloyed halide perovskite of general formula P below
Ay(FAxMA1-x)1-yBX3 (P) wherein:
A is an organic cation selected from ammonium cation and amidinium cation;
FA is formamidinium;
MA is methylammonium; y=0.01-0.99; x=0.01-0.99;
B is an alloy involving at least two of Sn, Pb and Ge; and
X is a halogen atom.
(2) The 2D/3D alloyed halide perovskite according to (1) above, wherein B is an alloy selected form Sn-Pb, Sn-Ge and Ge-Pb; preferably B is the alloy Sn-Pb or Ge-Pb; more preferably B is the alloy Sn-Pb.
(3) The 2D/3D alloyed halide perovskite according to (1) or (2) above, wherein X is selected from I, F, Cl and Br; preferably X is selected from I, Cl and Br; more preferably X is I.
(4) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein
A is an ammonium cation of general formula I below
wherein Ri to R4 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl.
(5) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is an ammonium cation of general formula II below
wherein:
R1 to R3 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl;
L is present or absent and is a group comprising one or more of (CH2) and (CH); and
Q is present or absent and is a 5 to 12-member ring or bicycle ring, with the proviso that at least one of Q and L is present.
(6) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is an ammonium cation of general formula III below
wherein:
Q is present or absent and is a 5 to 12-member ring or bicycle ring; and n is an integer from 0-6.
(7) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is an ammonium cation of general formula IV below
wherein:
Ri are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl; n is an integer from 1-6; and m is an integer from 0-4.
(8) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein
A is an ammonium cation of general formula V below
wherein n is an integer from 1-6.
(9) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is phenylethylammonium (PEA) cation
(10) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein
A is an amidinium cation of general formula I' below
wherein R5 to R9 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl.
(11) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is an amidinium cation of general formula II' below
wherein:
Rs to Rs are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl;
L' is present or absent and is a group comprising one or more of (CH2) and (CH); and
Q' is a 5 to 12-member ring or bicycle ring.
(12) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein
A is an amidinium cation of general formula III' below
wherein:
Q' is a 5 to 12-member ring or bicycle ring; and n' is an integer from 0-6.
(13) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is an amidinium cation of general formula IV' below
wherein:
R'i are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl; n' is an integer from 1-6; and
m' is an integer from 0-4.
(14) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is an amidinium cation of general formula V' below
wherein n' is an integer from 1-6.
(15) The 2D/3D alloyed halide perovskite according to any one of (1)-(3) above, wherein A is phenylethylammonium (PEA) cation; B is the alloy Sn-Pb; and X is I.
(16) The 2D/3D alloyed halide perovskite according to any one of (1)-(15) above, wherein y=0.05 or y=0.1; and x=0.3 or x=0.5 or x=0.7.
(17) The 2D/3D alloyed halide perovskite according to any one of (1)-(15) above, wherein y=0.05; and x=0.3 or x=0.5 or x=0.7.
(18) The 2D/3D alloyed halide perovskite according to any one of (1)-(15) above, wherein y=0.1 ; and x=0.3 or x=0.5 or x=0.7.
(19) A 2D/3D alloyed halide perovskite of general formula P1 below
Ay(FAxMA1-x)1-ySnxPb1-xX3 (P1) wherein:
A is an organic cation selected from an ammonium cation of general formula I, II, III, IV orV and an amidinium cation of general formula I', II', III', IV' or V';
FA is formamidinium;
MA is methylammonium; y=0.01-0.99; x=0.01-0.99; and
X is a halogen atom.
(20) The 2D/3D alloyed halide perovskite according to (19) above, wherein A is phenylethylammonium (PEA) cation.
(21) The 2D/3D alloyed halide perovskite according to (19) above, wherein y=0.05 or y=0.1 ; and x=0.3 or x=0.5 or x=0.7.
(22) The 2D/3D alloyed halide perovskite according to (19) above, wherein y=0.05; and x=0.3 or x=0.5 or x=0.7.
(23) The 2D/3D alloyed halide perovskite according to (19) above, wherein y=0.1; and x=0.3 or x=0.5 or x=0.7.
(24) A 2D/3D alloyed halide perovskite of general formula P2 below
PEAy(FAxMA1-x)1-ySnxPb1-xl3 (P2) wherein:
PEA is phenylethylammonium cation;
FA is formamidinium;
MA is methylammonium; y=0.05 or y=0.1 ; and x=0.3 or x=0.5 orx=0.7.
(25) A 2D/3D alloyed halide perovskite, which is:
(PEA)0.05 (FA0.3 MAQ.7 )0.95 (Sn0.3 Pb0.7) I3
(PEA)o.1 (FA0.3 MAQ.7 )0.90 (Sn0.3 Pb0.7) I3
(PEA)0.05 (FA0.5 MAO.5 )0.95 (Sn0.5 Pb0.5) I3
(PEA)o.1 (FAO.5 MAO.5 )0.90 (Sn0.5 Pb0.5) I3
(PEA)0.05 (FA0.7 MA0.3 )0.95 (Sn0.7 Pbo.3) I3 or
(PEA)o.1 (FA0.7 MA0.3 )0.90 (Sn0.5 Pbo.3) I3.
(26) A method of preparing a 2D/3D alloyed halide perovskite, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with an organic cation having a size which is larger than the size of FA and/or MA.
(27) A method of preparing a 2D/3D alloyed halide perovskite, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with a larger organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'.
(28) A method of preparing a 2D/3D alloyed halide perovskite, comprising replacing at least part of the cations formamidinium (FA) and methylammonium (MA) with phenylethylammonium (PEA) cation.
(29) The 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above obtained by the method as defined in any one of (26)-(28) above.
(30) A method of manufacturing a solar cell device, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with an organic cation having a size which is larger than the size of FA and/or MA.
(31) A method of manufacturing a solar cell device, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with a larger organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'.
(32) A method of manufacturing a solar cell device, comprising replacing at least part of the cations formamidinium (FA) and methylammonium (MA) with phenylethylammonium (PEA) cation.
(33) A method of manufacturing a solar cell device, comprising preparing the 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above.
(34) A method of manufacturing a solar cell device, comprising using the 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above.
(35) A solar cell device which comprises the 2D/3D alloyed halide perovskite as defined in any one of (1)-(25) above.
[0011] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
[0013] In the appended drawings:
[0014] Figure 1: a) Absorption spectra of 3D perovskite films and comparison of absorption spectra of 3D perovskite vs. 2D/3D perovskite films; b) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 l3; c) (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 l3; d) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 l3.
[0015] Figure 2: a) Steady-state photoluminescence spectra of 3D perovskite films and comparison of steady-state photoluminescence spectra of 3D perovskites vs. 2D/3D perovskites; b) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 l3; c) (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 l3; d) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 l3. y=0 , 5% and 10%.
[0016] Figure 3: Morphology of a) (FASnl3)0.3 (MAPbl3)0.7; b) (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 l3; c) (PEA)0.1 (FA0.3 MA0.7)0.90 Sn0.3 Pb0.7 l3; d) (FASnl3)0.5 (MAPbl3)0.5; e) (PEA)0.05 (FA0.5
MA0.5)0.95 Sn0.5 Pb0.5 l3; f) (PEA)o.1 (FA0.7 MA0.3)0.90 Sn0.5 Pb0.5 l3; g) (FASnl3)0.7 (MAPbl3)0.3; I3)
(PEA)0.05 (FA0.7 MA0.3)0.95 Sn0.7 Pb0.3 l3; i) (PEA)0.1 (FA0.7 MA0.3)0.90 Sn0.7 Pb0.3 l3.
[0017] Figure 4: AFM topographical images with roughness of a) (FASnl3)0.3 (MAPbl3)0.7; b) (PEA)0.05 (FA0.3 MA0.7)0.95 Pb0.7 Sn0.3 l3; c) (FASnl3)0.5 (MAPbl3)0.5; d) (PEA)0.05 (FA0.5 MA0.5)0.95 Pb0.5 Sn0.5 l3; e) (FASnl3)0.7 (MAPbl3)0.3; f) (PEA)0.05 (FA0.7 MA0.3)0.95 Pb0.3 Sn0.7 l3.
[0018] Figure 5: SEM images and elemental mapping of Pb, Sn and I of a) (FASnl3)0.3 (MAPbl3)0.7; b) (FASnl3)0.5 (MAPbl3)0.5; c) (FASnl3)0.7 (MAPbl3)0.3; d) (PEA)0.05 (FA0.3 MA0.7)0.95 Pb0.7 Sn0.3 l3; e) (PEA)0.05 (FA0.5 MA0.5)0.95 Pb0.5 Sn0.5 l3; f) (PEA)0.05 (FA0.7 MA0.3)0.95 Pb0.3 Sn0.7 I3.
[0019] Figure 6: XRD spectra of FASnl3 and MAPbl3 perovskite films.
[0020] Figure 7: Comparison of XRD spectra of different compositions of 3D perovskites and 2D/3D perovskites: a) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 l3, b) (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 I3 and c) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 I3; and variation of FWHM of the (110) diffraction peak of: d) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 I3, e) (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 I3 and f) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 l3.
[0021] Figure 8: High resolution XPS spectra of Sn 3d in a) (FASnl3)0.3 (MAPbl3)0.3; b) (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3. The red and blue curve corresponds to Sn2+ and Sn4+ oxidation states, respectively.
[0022] Figure 9: XPS survey spectra of (FASnI3)0.3 (MAPbI3)0.7 and (PEA)o.os (FA0.3 MA0.7 )0.95 Pb0.7 Sn0.3 I3.
[0023] Figure 10: Statistical distribution of efficiency of a) (FASnl3)0.3 (MAPbl3)0.7; b) (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 l3; c) (PEA)0.1 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 devices.
[0024] Figure 11 : Statistical distribution of efficiency of a) (FASnl3)0.5 (MAPbI3)0.5; b) (PEA)0.05 (FA0.5 MA0.5)0.95 Sn0.5 Pb0.5 I3; c) (PEA)0.1 (FA0.5 MA0.5)0.95 Sn0.5 Pb0.5 I3 devices.
[0025] Figure 12: Statistical distribution of efficiency of a) (FASnl3)0.7 (MAPbl3)0.3; b) (PEA)0.05 (FA0.7 MA0.3)0.95 Sn0.7 Pb0.3 I3; c) (PEA)0.1 (FA0.7 MA0.3)0.95 Sn0.7 Pb0.3 I3 devices.
[0026] Figure 13: j.V curves of a) (PEA)y (FA0.3MA0.7)1-y Sn0.3 Pb0.7 l3; b) (PEA)y (FA0.5MA0.5)i. y Sn0.5 Pb0.5 l3; c) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 I3 devices; d-f) corresponding EQE spectra of the devices, (y = 0, 0.05 and 0.1 represented in grey, red and blue colors, respectively.)
[0027] Figure 14: Dependence of a) Jsc; b) Voc; c) FF on the incident light intensity of the 3D PSCs and 2D/3D PSCs.
[0028] Figure 15: Topography and surface potential profile of a) (FASnl3)0.3 (MAPbl3)0.7 , b) (PEA)0.05 (FA0.3 MA0.7 )0.95 (Sn0.3 Pb0.7) I3, C) (FASnl3)0.5 (MAPbl3)0.5, d) (PEA)0.05 (FAO.5 MAO.5 )0.95 (Sn0.5 Pb0.5) I3, e) (FASnl3)0.7 (MAPbl3)0.3 and f) (PEA)0.05 (FA0.7 MA0.3 )0.95 (Sn0.7 Pb0.3) I3.
[0029] Figure 16: J-V hysteresis of a) (FASnl3)0.3 (MAPbl3)0.7; b) (PEA)0.05 (FA0.3 MAo.7)0.95 Sn0.3 Pb0.7 I3 devices.
[0030] Figure 17: Degradation of 3D perovskite devices vs. 2D/3D perovskite devices a) under 28±2% RH humidity in the dark; b) under N2 at 1.5G AM Sun illumination; c) under 28±2% RH humidity at 1.5G AM Sun illumination.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0031] Before the present invention is further described, it is to be understood that the invention is not limited to the particular embodiments described below, as variations of these embodiments may be made and still fall within the scope of the appended claims. It is also to be understood that the terminology employed is for the purpose of describing particular embodiments, and is not intended to be limiting. Instead, the scope of the present invention will be established by the appended claims.
[0032] In order to provide a clear and consistent understanding of the terms used in the present specification, a number of definitions are provided below. Moreover, unless defined otherwise, all technical and scientific terms as used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure pertains.
[0033] As used herein, the term “ammonium” refers to cations having a general formula I as depicted herein.
[0034] As used herein, the term “amidinium” refers to cations having a general formula I' as depicted herein.
[0035] The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one”, but it is also consistent with the meaning of “one or more”, “at least one”, and “one or more than one”. Similarly, the word “another” may mean at least a second or more.
[0036] As used in this specification and claim(s), the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “include” and “includes”) or “containing” (and any form of containing, such as “contain” and “contains”), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.
[0037] The term “alkyl” or “alk” as used herein, represents a monovalent group derived from a straight or branched chain saturated hydrocarbon comprising, unless otherwise specified, from 1 to 15 carbon atoms and is exemplified by methyl, ethyl, n- and /so-propyl, n-, sec-, iso- and tert-butyl, neopentyl and the like and may be optionally substituted with one, two, three or, in the case of alkyl groups comprising two carbons or more, four substituents.
[0038] The term “alkylene” as used herein, represents a saturated divalent hydrocarbon group derived from a straight or branched chain saturated hydrocarbon by the removal of two hydrogen atoms, and is exemplified by methylene, ethylene, isopropylene and the like.
[0039] The term “alkenyl” as used herein, represents monovalent straight or branched chain groups of, unless otherwise specified, from 2 to 15 carbons, such as, for example, 2 to 6 carbon atoms or 2 to 4 carbon atoms, containing one or more carbon-carbon double bonds and is exemplified by ethenyl, 1-propenyl, 2-propenyl, 2-methyl-1-propenyl, 1-butenyl, 2- butenyl and the like and may be optionally substituted with one, two, three or four substituents.
[0040] The term “alkynyl” as used herein, represents monovalent straight or branched chain groups of from two to six carbon atoms comprising a carbon-carbon triple bond and is exemplified by ethynyl, 1-propynyl, and the like and may be optionally substituted with one, two, three or four substituents.
[0041] The term “cycloalkyl” as used herein, represents a monovalent saturated or unsaturated non-aromatic cyclic hydrocarbon group of three to eight carbon atoms, unless otherwise specified, and is exemplified by cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, bicyclo[2.2.1]heptyl and the like.
[0042] The term “halogen” or “halo” as used interchangeably herein, represents F, Cl, Br and I.
[0043] The inventors have designed and prepared halide perovskites which comprise substantially no lead (Pb) in free form or any form that may lead to water-soluble precursors thereof. The halide perovskites according to the invention have a 2D/3D structure and comprise tin-lead (Sn-Pb) alloys.
[0044] In particular, the inventors have designed and prepared halide perovskites, wherein at least part of the small organic cations formamidinium (FA) and methylammonium (MA) is replaced by an organic cation having a size which is larger than the size of FA and/or MA. In embodiments of the invention, such larger size organic cation is an ammonium or an amidinium.
[0045] In embodiments of the invention, the inventors explored different compositions of Pb- Sn alloyed perovskite by stoichiometrically mixing FASnl3 and MAPbl3 to utilize near-infrared photons of the solar spectrum [16], It is known in the art that the trap states at grain boundaries (GBs) and primarily at the perovskite surface, instigate lower photovoltaic performance, current-voltage hysteresis and to some degree stability in 3D alloyed power conversion efficiencies (PSCs).
[0046] By employing 2D/3D alloyed perovskites as solar absorber, we show that the traps at GBs and perovskite film surface are passivated, which led to an improvement in photovoltaic performance. The 2D/3D alloyed perovskite is realized by stoichiometrically replacing in alloyed perovskite materials, formamidinium/methylammonium (FA/MA) cations with a larger cation, phenylethylammonium (PEA) cation. We tuned the PEA-to-FA/MA mixing ratio by substituting different amounts of FA/MA with PEA cations to achieve maximum PCE in all different compositions of alloyed perovskites.
[0047] We tested a series (PEA)y (FAX MA1-x)1-y Snx Pb1-x I3 (y= 0, 0.05 and 0.1 , x=0.3, 0.5, and 0.7) compositions and obtained a PCE of 13.60±0.12% for PEA0.05 FA0.3 MA0.7 Sn0.3 Pb0.7 I3 devices.
[0048] A drawback associated to Sn-based alloyed perovskites is inferior stability under ambient environment since Sn+2 can readily oxidize to stable Sn4+ when Sn perovskites exposed to ambient environment.
[0049] In this work, the 2D/3D alloyed PSCs shows better stability (half-life, t1/2 = 200 hours) under 28±2% relative humidity compared to that of 3D alloyed PSCs (t1/2 = 80 hours). It is known in the art that ion migration in the halide perovskites plays a critical role in the stability of the PSCs. The passivation of iodide-rich perovskite surface reduces the ion migration in the material, improving the stability of the devices. Also, reduced ion migration in 2D/3D alloyed perovskites decreases the current-voltage hysteresis in the derived devices.
[0050] The inventors are aware of other documents known in the art as follows: for 2D perovskites documents [39] and [40]; for 2D/3D perovskites document [41],
[0051] The optical bandgaps of (FASnl3)x (MAPbl3)1-x and (PEA)y (FAX MA1-x)1-y Snx Pb1-x I3 were determined from diffuse reflectance measurements. Figure 1a shows optical absorption by (FASnl3)x (MAPbl3)1-x (x= 0, 0.3, 0.5, 0.7 and 1) compounds. The optical absorption spectrum of mixed Pb-Sn perovskites shows absorption onset has shifted to N-IR region compared to pure Pb (x=0) or Sn (x=1) perovskites. The lowest optical bandgap found to be 1.18 eV for (FASnl3)0.7 (MAPbl3)03, following 1.22 eV for (FASnl3)0.5 (MAPbl3)0.5 and 1.26 eV for (FASnl3)0.3 (MAPbl3)0.7. Substituting FA/MA with small quantities of PEA does not change optical absorption onset considerably (Figures 1b-d). But we have noticed a change in the shape of the optical absorption spectra as well as optical absorption intensity. Interestingly, we have observed disappearance of hydrogenic absorption peaks near optical band gap (located at -1.85 eV, marked on the absorption spectra) for 2D/3D alloyed perovskites in comparison with absorption spectra of 3D alloyed perovskites [17], It is noted that as the Sn content was increased in 3D alloyed perovskites the intensity of hydrogenic peak steadily decreased. The hydrogenic absorption peaks ascribed to bound excitonic states due to the columbic attraction of electrons and holes. The disappearance of hydrogenic absorption
peaks indicates a reduction in the coulombic attraction of charge carriers in the 2D/3D alloyed perovskites [18],
[0052] Figure 2 shows steady-state photoluminescence (PL) peaks of the (PEA)y (FAX MAi- x)1-y Snx Pb1-x I3 films. The optical band gaps obtained from PL match with absorption onset observed in the absorption spectra indicative of direct bandgap nature in (PEA)y (FAx MA1-x)1- y Snx Pb1-x I3 materials (Figure 2a). Further, we compare the PL intensity of 3D alloyed perovskites with 2D/3D alloyed perovskites in Figures 2b-d. A noticeable enhancement in PL intensity has observed after stoichiometrically replacing FA/MA with PEA in alloyed perovskites. A trend was seen in the PL intensity enhancement, displaying an increase in the PL intensity with higher amounts of FA/MA substitution with PEA cations. The FA/MA substitution with 10% PEA cations shows highest PL intensity in all alloyed perovskite compositions. The increase in PL intensity in a semiconductor correlates with reduction of nonradiative recombination in a material.
[0053] The topography of alloyed perovskites films obtained by Atomic Force Microscopy (AFM) are shown in Figure 3. The topographical images show pin-hole free, dense alloyed perovskite films. We noticed a gradual increase in crystal grain size as the Sn content in alloyed perovskite increased. The 2D/3D alloyed perovskite films showed a plate-like topography, and it is more evident in 2D/3D perovskites with a higher percentage of organic cation introduced (Figures 3c, f and i). As a consequence, a noticeable reduction in root mean square roughness for 2D/3D perovskite films occurred to that of their 3D analog (Figure 4). Previous reports shown that 2D perovskites crystallize as plates because the crystallization along a-b crystallographic axes occurs more rapidly than the c-axis [19], Substituting small organic cations in 3D perovskite even with a low percentage of larger PEA cations influences the perovskite film growth due to the formation of additional 2D perovskite phases. The plate-like film formation reduces the overall grain boundary area which is advantageous for reducing the charge carrier recombination in the perovskite film. The SEM energy-dispersive X-ray spectroscopy (EDS) showed that Sn and Pb are homogenously distributed throughout the alloyed perovskite film (Figure 5). As can be seen on elemental maps of Sn, Pb and I in alloyed perovskites, there is no phase separation in the film
[0054] We characterized the structure of alloyed perovskites by X-ray diffraction (XRD). The FASnl3 has only one peak within the 26 range of 22°-25° indexed to (113) plane in the Amm2
space group whereas MAPbl3 exhibits two peaks within the 29 range of 22°-25° indexed to (211) and (202) planes in the tetragonal (14 cm) space group (Figure 6 shows the XRD spectrum of FASnl3 and MAPbI3) [20], The XRD spectra of alloyed perovskites (Figure 7) display only one peak within the 29 range of 22°-25°, which indicate that alloyed perovskites adopt orthorhombic crystal structure. The XRD pattern also suggests that (FASnl3)x (MAPbl3)1-x adopts a crystal structure type consisting of Sn and Pb atoms randomly occupying corner-sharing ([Sn1-x Pbxl6]-4) octahedra [21], The crystal structure is unchanged after replacing a small amount of FA/MA by PEA in alloyed perovskites. But, we noticed better crystallinity for all 2D/3D compositions in comparison to 3D counterparts. The intensity of the XRD peaks notably increased for alloyed perovskites with PEA, which could be from higher crystallinity of the perovskite as well as good film coverage on the substrate.
[0055] We characterized the change in the crystallinity by tracking variation in full width half maximum (FWHM) of major XRD peak located at 14° corresponding to (110) crystalline plane. The FWHM dropped from 0.23916° to 0.21455° and 0.24673° to 0.23099° when we substituted FA/MA with 5% of PEA in (FASnl3)0.3 (MAPbI3)0.7 and (FASnl3)0.5 (MAPbl3)0.5 respectively. The lowering of FWHM implies improvement of crystallinity of the materials. When we substituted FA/MA with 10% of PEA in alloyed perovskites, FWHM has slightly increased. On the contrary, Sn-rich composition showed a different trend. The FWHM increases with the amount of PEA in (FASnl3)0.7 (MAPbl3)0.5 although we observed a nominal increase in XRD peak intensity. In summary, compositions with 5% PEA showed the least FWHM indicating higher crystallinity for (PEA)0.05 (FAX MAI.X)0.95 Snx Pb1-x I3 (x= 0.3 and 0.5) perovskites.
[0056] To investigate the role of PEA cations in preventing the oxidation of Sn2+ in 2D/3D alloyed perovskites, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Figure 8 shows the two XPS Sn 3d peaks (corresponding to 3d5/2 and 3d3/2 peaks) of (FASnl3)0.3 (MAPbl3)0.7 and (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 films. Figure 9 shows the survey spectrum of alloyed perovskites. In contrast to 2D/3D alloyed perovskite, 3D alloyed perovskite was highly susceptible to Sn2+ oxidation evident from presence of peak corresponding to Sn4+. For the latter, Sn 3d5/2 peak (located at 486 eV) could deconvoluted into two peaks located at 485.97 eV and 486.7 eV corresponding to Sn2+ and Sn4+ oxidation states. Similarly, Sn 3d3/2 peak (located at 495.5 eV) comprised of two peaks located at 494.35 eV and 495.34 eV associated with Sn2+ and Sn4+ oxidation states [13], Also, additional
peaks at 483.65 eV and 492.55 eV emerges in the XPS spectrum of 3D alloyed perovskite films which correspond to zero-valent Sn (Sn°). The presence of peak corresponds to Sn° attributed to the formation of metallic Sn in the 3D alloyed perovskite film [22], Previous studies have shown that X-ray beam irradiation on Sn-based perovskite from XPS spectrometer induces the metallic Sn formation (reduction of Sn), which can be used to evaluate the stability of the material [22], The formation of metallic Sn shows that 3D alloyed perovskites are highly vulnerable to beam damage. The Sn2+ oxidation to Sn4+ is a strong indication of Sn vacancy formation in Sn-based perovskites [23], The intrinsic defects such as Sn vacancies (through the Sn2+ oxidation) in the 3D alloyed perovskite generate p-type conductivity in the semiconductor [24], The defect formation energy of Sn vacancy is lower amongst other point defects due to the strong Sn 5s— I 5p antibonding coupling, which implies that Sn vacancy is the dominant intrinsic defect producing high hole (p-type) carrier density in Sn-based perovskites [25], Furthermore, the high background hole carrier density within 3D alloyed perovskites stimulates predominantly monomolecular (or trap-assisted) recombination processes. Conversely, 2D/3D alloyed perovskite does not show any signs of Sn2+ oxidation or metallic Sn formation. The XPS studies reveal that PEA cations play a key role in inhibiting Sn2+ oxidation and Sn formation in alloyed perovskite films. First principle calculations have shown that the ionic size of organic cations plays a vital role in governing formation energy of Sn vacancies in Sn-based perovskites [14], The larger ionic size of organic cation reduces the Sn 5s— I 5p antibonding coupling, leading to lower the Sn vacancy formation energies in the Sn-based perovskites. We hypothesize that introduction of larger organic cations (PEA) increases the formation energy of Sn vacancies in 2D/3D alloyed perovskites. This leads to reduced Sn2+ oxidation or formation of Sn vacancies in the 2D/3D alloyed perovskites. Additionally, the absence of Sn reduction (shoulder peak corresponding to Sn° in XPS spectra) reveal the stability of the 2D/3D alloyed perovskites under X-ray irradiation. Moreover, no signs of damage by X-ray beam demonstrates the superior chemical stability of the 2D/3D alloyed perovskites.
[0057] To test photovoltaic properties of alloyed perovskites, we employed ITO/PEDOT: PSS/Perovskite/PCBM/Ag solar cell architecture. The results are outlined in Table 1 below. Between the 3D alloyed perovskites, (FASnl3)03 (MAPbl3)0 5 showed highest PCE followed by (FASnl3)0.5 (MAPbl3)0.5 and (FASnl3)0.7 (MAPbl3)03. The open-circuit voltage (Voc) and fill factor (FF) reduced as the Sn content in the composition increased. The Sn-rich ((FASnl3)0.7
(MAPbl3)0.3) showed lowest Voc and FF amongst all compositions. The Voc and FF strongly depend on charge carrier recombination in the solar cell and Sn-rich perovskites are likely to heavily p-doped limiting the Vocand FF of the devices.
Table 1. Photovoltaic properties of 3D and 2D/3D alloyed PSCs
[0058] gy substituting FA/MA with PEA in alloyed perovskite compositions, the Voc has significantly improved. The Voc and amount of FA/MA substituted with PEA showed a linear relationship, improving the Voc with an increase in the amount of PEA in 2D/3D perovskite compositions. By replacing FA/MA with 10% PEA in (FASnl3)0.3 (MAPbl3)0.7, the Voc boosted to 0.80±0.002 V from 0.72±0.002 V. The highest Voc enhancement (of 0.23V) is observed for (PEA)0.1 (FA0.7 MA0.3)0.90 Sn0.7 Pb0.3 I3, followed by (PEA)0.1 (FA0.5 MA0.5)0.90 Sn0.5 Pb0.5 I3 (with an improvement of 0.15 V). It can be understood by a skilled person in a such a way that “FASnI3-rich” alloyed perovskites contain more intrinsic defects, so the defect passivation is more evident in (PEA)0.1 (FA0.7 MA0.3)o.9o Sn0.7 Pb0.3 I3, hence the largest Voc enhancement. On the other hand, fill factor (FF) reached it is maximum for the samples with 5% PEA. With 10%
PEA substitution, FF slightly declined or almost remained constant. The increase in PEA content in the perovskites may change the orientation of semiconducting perovskite layers on the substrate which could negatively affect charge carrier diffusion length and reduce the FF. Elimination of Sn2+ oxidation in the (PEA)0.05 (FAX MAI.X)0.95 Snx Pb1-x l3 reduces the p-doping and hence the trap-assisted recombination, which resulted in improved Voc and FF in the 2D/3D alloyed perovskite devices.
[0059] The jsc has slightly increased in (PEA)0.05 (FAX MAI.X)0.95 Snx Pb1-x l3 (x= 0.3 and 0.5) devices. The largest Jsc enhancement noticed in (PEA)0.05 (FA03 MA0.7)0.95 Sn0.3 Pb0.7 I3 followed by (PEA)0.05 (FA0.5 MA0.5)0.95 Sn0.5 Pb0.5 I3 devices compared to their 3D alloyed perovskites counterparts. Jsc has slightly decreased or remained unchanged in (PEA)0.1 (FAX MAI-X)0.90 Snx Pb1-x l3 (x= 0.3 and 0.5). However, we have noticed an inconsistent trend in (PEA)y (FAX MA1-x)1-y Sn0.7 Pb0.3 I3 devices compared to other compositional groups, where Jsc decreases as the amount PEA is increased. The trend in photocurrent correlates with the crystallinity of the perovskites. The (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 perovskite shows higher crystallinity in (PEA)y (FA0.3 MA0.7)y Sn0.3 Pb0.7 I3 compositional group which is also reflected in photocurrent of the solar cell showing highest average Jsc of 25.79 mA cm2. It is also valid for (PEA)y (FA0.5 MA0.5)y Sn0.5 Pb0.5 I3 group, displaying highest crystinality and Jsc for (PEA)o 05 (FA0.5 MA0.5)y Sn0.5 Pb0.5 l3. The crystallinity measurements of (PEA)y (FAX MA1- x)1-y Sn0.7 Pb0.3 I3 (FASnh-rich) perovskites explain the anomalous trend observed in Jsc of the solar cells. As we have seen, the crystallinity of the FASnl3-rich perovskites reduces when the amount of substituent PEA cations is increased. The reduction in the crystallinity causes the photocurrent drop in (PEA)y (FAX MAi.x)i.y Sn0.7 Pb0.3 I3 solar cells. Figures 10-12 show statistical distribution of efficiency of 20-30 devices in each compositional group.
[0060] The J-V curves and corresponding external quantum efficiency (EQE) graphs of solar cells are shown in Figure 13. The EQE shows photoresponse of the solar cell in the NIR region. The calculated Jsc from EQE closely matches with Jsc obtained from J-V curve except in (FASnl3)0.7 (MAPbl3)03. This mismatch only is seen in Sn-rich 3D perovskite probably because of quicker degradation of the samples.
[0061] Figure 14a shows the dependence of Jsc on incident light intensity fit to a power law (I The Jsc deviates from linear dependence on incident light intensity (α = 95) in 3D
alloyed perovskite devices compared to that of 2D/3D perovskite devices which implies photogenerated charge carriers in 3D alloyed perovskite device are not efficiently transported to the electrodes [26], To investigate recombination of photogenerated carriers during solar cell operation near Voc, we plotted Voc versus logarithmically scaled light intensity. By linearly fitting the plot using:
where q is the elementary charge, n is the ideality factor, kb is the Boltzmann's constant and T is the absolute temperature.
[0062] we obtained a slope of 1.1 kbT/q (n is close to unity) for 2D/3D perovskite devices, which suggests bimolecular recombination process dominates during solar cell operation. The deviation of the slope from kbT/q in 3D perovskites devices (1.2 kbT/q) demonstrates the dominance of trap-assisted recombination close to the Voc of the solar cells. FF is a more significant parameter as the solar cell is operated near a maximum power point. The dependence of FF on incident light intensity unravels the nature recombination process in the solar cell during the device operation. In pure bimolecular recombination process, recombination rate is proportional to the product of charge carrier densities [27], At low light intensity (i.e., low charge carrier densities), the recombination rate reduces producing better FF. The FF increases with decreasing light intensity in 2D/3D alloyed PSCs, which suggest bimolecular recombination dominate in the device. On the other hand, FF decreases with decreasing light intensity in 3D alloyed perovskite device which indicates trap-assisted recombination dominates in these devices [27], The rate of charge carriers recombining with trapped charges increases with decreasing light intensity resulting in lower FF in 3D alloyed perovskite devices. It should be noted that the number of traps does not change with lowering the light intensity.
[0063] The PL spectra and XPS analysis of alloyed perovskites and light intensity dependent analysis of the solar cell (by plotting dependence of Jsc, Voc, and FF on light intensity) confirm that trap-assisted recombination limits the performance of 3D alloyed perovskite solar cells. By replacing FA/MA cations with a nominal quantity of PEA cations, we have seen a significant reduction in the trap-assisted recombination in the alloyed perovskites. The charge carrier traps such as Sn vacancies, mobile iodide ions are mainly located at GBs and on the perovskite film surface [27], For example, single crystal CsSnI3 shows very low hole concentration (< 1017 cm-3) compared to polycrystalline perovskite films (~1019 cm-3) due to
the absence of grain boundaries [13,25], The topographical images suggest that a plate-like crystal film growth for 2D/3D alloyed perovskite films which reduces the number of grain boundaries compared to their 3D analogs. To understand the trap-assisted recombination at perovskite surface and GBs of alloyed perovskite films, we measured electrical properties of perovskite film surface using Kelvin probe force microscopy (KPFM). The surface potential (SP) maps of the samples measured in the dark are shown in Figure 15. The GBs show lower SP than the grain interiors (Gl) as observed in previous literature reports [28,29], By substituting FA/MA with 5% of PEA in alloyed perovskites, the SP has reduced at GBs as well as in the Gl (Figures 15b, d and f). Also, SP difference between GBs and GIs is reduced in 2D/3D alloyed perovskites vs. 3D perovskites. The reduction of SPGI - SPGB in 2D/3D alloyed perovskites implies that electron transport across the GBs would be smoother [28,30], The reduction of SPGI - SPGB is accompanied by passivation of GBs in perovskite. Also, the average SP of 2D/3D perovskite films show an increase compared to that of 3D perovskite films.
[0064] The largest average SP increase (31 .66 mV) observed in (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.7 Pb0.3 I3 film (-19.11 mV) compared to that of 3D perovskite film (-50.77 mV). Then followed by (PEA)0.05 (FA0.5 MA0.5)0.95 Sn0.5 Pb0.5 I3 film (from -59.81 mV to -33.87 mV, an increase of 25.94 mV) and (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 film (from 224.8 mV to 231.2 mV, an increase of 6.4 mV). The increase in average SP in 2D/3D perovskite films suggests passivation of surface trap states in the material [31], The additional traps such as mobile iodide ions can be generated in the Sn-based perovskite films due to Sn vacancies since the mobile iodide ions in the halide perovskites are related to vacancy density of adjacent atoms [23], Also, density functional theory calculations have shown the presence of iodide-rich trap sites on the grain surfaces of halide perovskite film [32], The trap states at GBs and on the GIs of perovskite film may cause severe interfacial recombination in PSCs [33], We identify trap states at GBs and perovskite surfaces as sources of nonradiative recombination channels in 3D alloyed perovskites. We passivated the nonradiative recombination channels at the surface of alloyed perovskite films by substituting FA/MA by PEA cations potentially reducing the interfacial recombination in corresponding PSCs. We conclude that the reduction of nonradiative recombination in alloyed perovskites as well as on the perovskite film surface led to predominant Voc improvement in 2D/3D alloyed PSCs.
[0065] We also observed a large decrease in J-V hysteresis in 2D/3D alloyed perovskite devices compared to that of 3D alloyed perovskite devices (Figure 16). The mobile ions are known to be responsible for the hysteresis phenomenon in PSCs [34,35], Under an applied electric field, iodide anions can move generating ionic current in ionic crystals like halide perovskites [36,37], The inhibition of Sn vacancy formation may also lead to reduction of mobile iodide ions in 2D/3D perovskites. By controlling mobile iodide ions, we have also effectively tamed the J-V hysteresis in alloyed PSCs.
[0066] The photovoltaic performance degradation of unencapsulated 3D and 2D/3D alloyed perovskite devices was monitored under different environmental conditions to evaluate the stability of the devices. Here, we define t1/2 as a time for the PCE of the device to drop to 50% of its initial value for standardizing the comparison between different devices. Figure 17a shows degradation of devices under modest moisture environment (28±2% RH humidity) in the dark. The degradation significantly stalled by substituting FA/MA with 5% of PEA in (FASnl3)0.3 (MAPbl3)0.7. The t1/2 for (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 was found to be 200h appreciably better than that (t1/2 = 87 hours) of corresponding 3D perovskite devices. It has been reported that water molecules react with the small organic cations (such as MA) leading to the disintegration of the 3D perovskite structure. The large organic molecules like PEA act as nanoencapsulation layer protecting perovskites from the attack of water molecules. The nanoencapsulation by PEA is more central in Sn-based alloyed perovskites since oxidation state of Sn is extremely sensitive to the ambient environment.
[0067] Figure 17b displays the degradation of devices under AM 1.5G illumination in an inert atmosphere. The (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 devices exhibited better photostability compared to 3D perovskite devices (t1/2 = 30 hours vs. t1/2 = 15 hours). Light seems to be more harmful to alloyed perovskite devices than moisture. Figure 17c shows that the degradation of alloyed perovskite devices is rapid in the co-presence of light and modest moisture. However, 2D/3D perovskite devices are more resilient to the degradation, retaining 20% of initial performance after 24 hours whereas 3D perovskite devices wholly degraded. We have seen that 2D/3D alloyed perovskites demonstrating a better chemical stability under X-ray irradiation during the XPS analysis, which displays the robustness of the 2D/3D alloyed perovskites. Recently, it has been recognized that ionic migration towards the perovskite/metal cathode interface causing a large dipole at the interface, which is now identified as one of the major mechanism associated with device performance degradation
[38], Under illumination, mobile iodide ions in the perovskite devices react with diffused Ag metal atoms through a thin layer of PCBM. We hypothesize that the reduction of ionic migration by Sn vacancy modulation in 2D/3D alloyed perovskite devices improves the stability of these devices.
[0068] As will be understood by a skilled person, in this work, we investigated a variety of Pb- Sn alloyed perovskite compositions by stoichiometrically mixing FASnl3 and MAPbl3 to exploit near-infrared photons of the solar spectrum. The alloyed Sn-Pb PSCs exhibit broader photoresponse extending up to 1050 nm. Despite the NIR photoresponse of alloyed PSCs, the efficiency was found to be inferior compared to the state-of-the-art Pb PSCs mainly due to trap-assisted recombination in the material. We reduced the trap-assisted recombination by designing a new kind of materials called 2D/3D alloyed perovskites through replacing FA/MA with PEA cations. We analyzed the nature of traps states by combining a series of characterization results such as topography and surface potential profile of alloyed perovskite films, J-V curve behavior and dependence of photovoltaic parameters on illumination intensities of the devices, and so forth. The results show that the trap states at GBs and perovskite film surface play a critical role in the photovoltaic performance of alloyed perovskites. We hypothesize that the occurrence of Pb-I or Sn-I antisite defects is higher in 3D alloyed perovskites prepared by mixing two ionic crystals with different crystal structures. The improved crystallinity combined with reduced trap states in (PEA)0.05 (FAX MAI.X)0.95 Snx Pb1-x I3 (x = 0.3 and 0.5) contributes to the higher photovoltaic performance of the solar cells. Instead, the improvement of efficiency in Sn-rich 2D/3D PSCs is more associated with passivation of trap states than the crystallinity of these materials. The passivation of iodide- rich alloyed perovskite surface helps in reducing J-V hysteresis and enhancing the stability. In summary, the present invention provides for an approach to improve photovoltaic performance and stability with reduced J-V hysteresis simultaneously in Sn-Pb alloyed perovskite devices.
[0069] The present invention is illustrated in further details by the following non-limiting examples.
[0070] Materials: Methylammonium iodide (CH3NH3I), Formamidinium iodide (CH(NH2)2l or HC(=NH2)NH2l) and phenylethylammonium iodide (C6H5(CH2)2NH3I) were purchased from Dyesol. Tin iodide (SnI2) , tin fluoride (SnF2), dimethylformamide (DMF) and dimethyl sulfoxide
(DMSO) were purchased from Sigma Aldrich. Lead iodide (PbI3) was purchased from Acros Organics. Phenyl-Cei-butyric acid methyl ester (PC61BM) and poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS; Clevios™ P VP Al 4083) were obtained from 1 -Materials and Heraeus, respectively. All chemicals were used as received.
Preparation of perovskite precursor solutions
[0071] Example 1 - Preparation of FASnl3 precursor solution (1.1 M): The 1.1M of FASnh precursor solution was prepared by adding equimolar FAI (189.2 mg) and SnI2 (409.77 mg) powders and 20 mol% of SnF2 (17.3 mg) into a mixture of 1 mL of mixture of DMF: DMSO (4:1 v/v). The solution was stirred for half an hour at room temperature inside the glove box.
[0072] Example 2 - Preparation of MAPbl3 precursor solution (1.1 M): The 1.1M of MAPbh precursor solution was prepared by adding MAI (174.86 mg) and PbI2 (507.1 mg) in a mixture of 1 mL of DMF: DMSO (4:1 v/v). The solution was stirred for half an hour at room temperature inside the glove box.
Preparation of 3D perovskite precursor solutions
[0073] The 3D (FASnI3)x (MAPbl3)1-x (x = 0.3, 0.5, 0.7) solutions were prepared by stoichiometrically mixing FASnl3 and MAPbl3 solutions. The following three compositions were prepared.
[0074] Example 3 - Preparation of 3D (FASnI3)0.3 (MAPbI3)0.7 composition: The composition was obtained by stoichiometrically mixing 300 microliter of the FASnl3 precusor solution prepared at Example 1 and 700 microliter of the MAPbl3 precursor solution prepared at Example 2.
[0075] Example 4 - Preparation of 3D (FASnI3)0.5 (MAPbI3)0.5 composition: The composition was obtained by stoichiometrically mixing 500 microliter of the FASnl3 precusor solution prepared at Example 1 and 500 microliter of the MAPbl3 precursor solution prepared at Example 2.
[0076] Example 5 - Preparation of 3D (FASnI3)0.7 (MAPbI3)0.3 composition: The composition was obtained by stoichiometrically mixing 700 microliter of the FASnl3 precusor solution
prepared at Example 1 and 300 microliter of the MAPbl3 precursor solution prepared at Example 2.
Preparation of 2D/3D alloyed perovskite precursor solutions -
(PEA)y (FAx MA1-x)1-y Snx Pb1-x I3
[0077] The 2D/3D alloyed perovskite precursor solutions were prepared by stoichiometrically substituting FAI/MAI with 5% and 10% PEAI in (FASnI3)x (MAPbl3)1-x precursor solution.
[0078] Example 6 - Preparation of 2D/3D (PEA)0.05 (FAX MAI.X)0.95 Snx Pb1-x I3 perovskite precursor solution: We first prepared PEA0.05FA0.95SnI3 and PEA0.05MA0.95PbI3 precursors as described above at Examples 1-2 wih suitable modifications. We replaced 5% (in weight) of MAI and FAI replaced with PEAI compared to 3D FASnl3 and MAPbl3 perovskite precursor solutions. The solutions were stirred for half an hour at room temperature inside the glove box.
[0079] Example 7 - Preparation of 2D/3D (PEA)0.1 (FAX MAI.X)0.90 Snx Pb1-x I3 perovskite precursor solution: We first prepared PEA0.1FAo.9oSnh and PEA0.1MAogoPbh precursors as described above at Examples 1-2 wih suitable modifications. We replaced 10% (in weight) of MAI and FAI replaced with PEAI compared to 3D FASnl3 and MAPbl3 perovskite precursor solutions. The solutions were stirred for half an hour at room temperature inside the glove box.
Preparation of 2D/3D alloyed perovskite solutions - (PEA)y (FAX MAi.x)i-y Snx Pbi.x I3
[0080] Example 8 - Preparation of 2D/3D (PEA)0.05 (FAX MAI.X)0.95 Snx Pb1-x h perovskite solution: The solutions were mixed as described above for the 3D perovskite precursor solutions (Examples 3-5).
[0081] Example 9 - Preparation of 2D/3D (PEA)0.1 (FAX MAI.X)0.90 Snx Pb1-x I3 perovskite solution: The solutions were mixed as described above for the 3D perovskite precursor solutions (Examples 3-5).
[0082] Example 10 - Solar celle fabrication: Patterned ITO-coated glasses were cleaned by sonication in detergent followed by sequential washing with deionized water, acetone, and
isopropanol. After drying under air flow, the substrate surface was cleaned by oxygen plasma for 10 minutes under rough vacuum. The PEDOT: PSS solution was spin-coated on top of an ITO-coated glass substrate at 45000 rpm for 45 seconds; PEDOT: PSS performs as the hole transporting layer. The PEDOT: PSS film was then dried in air on a hot plate (set at 170°C) for 10 minutes. After drying, the substrate is transferred to a nitrogen-filled glovebox for further use. The 3D and 2D/3D alloyed perovskite absorber layer was spin-coated on the PEDOT : PSS film at 5,000 rpm for 60 seconds. Diethyl ether was dropped onto the spinning substrate. Then the PCeiBM solution (20 mg/mL in chlorobenzene) was spin-coated on top of the perovskite film at 1000 rpm for 45 seconds to form a 20 nm thick electron transporting layer. Finally, the film was transferred to a thermal evaporation chamber inside the nitrogen filled glove box. The chamber was pumped down to 1 x 10"6 Torr for silver deposition. The 100 nm thick silver top electrode was deposited through a shadow mask that defines the active device area as 0.06 cm2 for the solar cells.
[0083] Example 11 - Perovskite Film Characterization: Topography of perovskite film surface was obtained by using Bruker MultiMode8 AFM. The absorption spectra were collected by using a Lambda 750, UV-Visible-NIR spectrometer (Perkin Elmer). Steady-state PL spectra were obtained from a Fluorolog®-3 system (Horiba Jobin Yvon). XRD measurements were carried out using a Panalytical X-Pert PRO MRD X-Ray diffractometer. The oxidation of Tin element probed by XPS (ESCALAB 220I-XL spectrometer) equipped with an Al Ka (1486.6 eV) monochromatic source. KPFM measurements were done using Lift mode in Bruker MultiMode8 AFM under ambient conditions. KPFM measurements were performed using a Pt/lrtip (Bruker, SCM-PIT) with a lift height of 20 nm by applying AC voltage of 1V.
[0084] Example 12 - Solar cell characterization: Solar cell performance was measured using a class ABA LED solar simulator which was calibrated to deliver simulated AM 1.5 sunlight at an irradiance of 100 mW/cm2 (The irradiance was calibrated using an NREL- calibrated KG5 filtered silicon reference cell). Current-voltage curves were recorded using a source meter (Keithley 2400, USA). External quantum efficiency (EQE) measurement was conducted by using an IQE200B system (Newport Corporation).
[0085] Example 13 - Device stability tests: The stability of devices were tested in the same device configuration without any encapsulation. For moisture stability tests, the 2D and 3D
perovskite devices were placed inside a desiccator. The relative humidity (28±2% RH humidity) was measured with a digital humidity sensor. For photostability tests, the perovskite devices were placed under constant AM1.5G illumination inside the N2 filled glovebox. Devices are also tested under constant AM1.5G illumination at 28±2% RH humidity.
[0086] As will be understood by a skilled person, other suitable large organic cations equivalent to the methylammonium (MA) cation and different from phenylethylammonium (PEA), ammonium cations, may also be used to replace methylammonium (MA) and formamidinium (FA) in the halide perovskite according to the invention. Such organic cations include for example those depicted in general formulae I, II, III, IV and V as depicted herein.
[0087] Also, as will be understood by a skilled person, other suitable large organic cations equivalent to the formamidinium (FA) cation, amidinium cations, may also be used similarly to phenylethylammonium (PEA) to replace methylammonium (MA) and formamidinium (FA) in the halide perovskite according to the invention. Such organic cations include for example those depicted in general formulae I', II', III', IV' and V' as depicted herein.
[0088] Also, as will be understood by a skilled person, iodine may be replaced by chorine (Cl) or bromine (Br).
[0089] Moreover, as will be understood by a skilled person the alloy in the halide perovskite of the invention may include any suitable metals. Such metals may be for example tin (Sn), lead (Pb), germanium (Ge). Accordingly, alloys in the perovskite according to the invention may be for example Sn-Pb, Sn-Ge or Ge- Pb. In embodiments of the invention, the alloy is Sn-Pb.
[0090] The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.
[0091] The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.
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Other aspects and embodiments of the invention are described below.
Reducing non-radiative recombination in Pb-less halide perovskite films by incorporating bulky phenylethylammonium cations
Keywords: lead-free perovskite solar cells, alloyed perovskites, Sn oxidation, trap-assisted recombination, stability
Abstract: Alloyed tin-lead (Sn-Pb) perovskite solar cells (PSCs) exhibit broader photoresponse up to 1050 nm; however, their efficiency was inferior to that of Pb analogs. Trap-assisted recombination associated with Sn vacancies formed through Sn2+ oxidation is a major detrimental factor, limiting their efficiency. Here, by incorporating bulkier phenylethylammonium (PEA) cations into Sn-Pb alloyed perovskites, we reduced the non- radiative recombination by lowering the number of defects associated with Sn vacancies. This was supported by a series of characterizations, such as photoluminescence, X-ray photoelectron spectroscopy, and surface potential measurements. Theoretical calculations suggest higher formation energy for Sn vacancies in long-chain organic cation (PEA) containing Sn-perovskites compared to the short-chain organic cation (formamidinium (FA)) based Sn-perovskites. Furthermore, we calculated formation energy for Sn vacancies in Sn- perovskite structures containing mixed organic cations ((PEAyFA1-yjSnE) for the first time. Fascinatingly, (PEAyFAi.y)Snl3 exhibits even reduced propensity of Sn vacancy formation, which accounts for the reduced number of defects in the co-presence of FA and PEA in Sn-Pb perovskite structure. As a result, a champion device efficiency of 15.1% was achieved with PEA. Broadly, our calculations present a robust theoretical framework for recent reports using bulky cations to improve the performance of PSCs.
1. Introduction
Halide perovskites adopt the ABX3 general stoichiometric relation, where A = methylammonium (MA=CH3NH3+), formamidinium (FA=+HC(NH2)2), caesium (Cs) and/or rubidium (Rb); B= lead (Pb) and/or tin (Sn); X= chlorine (Cl), bromine (Br) and/or iodine (I).1 In recent years, halide perovskites (hereafter simply perovskites), mainly the hybrid organic- inorganic perovskites 2 but also the all-inorganic perovskites,3,4 have attracted the major research interest in the photovoltaic community mainly because of their simple solution deposition fabrication process, rapidly increasing and high power conversion efficiencies (PCEs) and fascinating optical and electronic properties, including tunable bandgap from 1.15 to 3.06 eV by swapping different cations and halides in the perovskite structure.5 7
The state-of-the-art perovskite solar cells (PSCs) involve water-soluble Pb species, which hamper the commercialization of this technology.8 Once in contact with water or moisture, Pb perovskites quickly form water-soluble by-products of Pb, which can accumulate within the food chain and so human body.9 It is thus highly necessary to restrict the use of Pb by replacing it partially or entirely with less toxic or, even better, non-toxic metals. Most likely, appropriate candidates for Pb replacement without compromising too much photovoltaic properties are elements in the same group (group 4) as Pb. The immediate candidates then appear as Sn and Ge, both of which can be more easily oxidized from the 2+ to 4+ oxidation states than Pb. Computational studies have shown that the electronic configuration of Pb2+ in the ABX3 perovskite structure is crucial for its remarkable photovoltaic behavior.10 13 Therefore,
although Sn-based perovskites are highest-performing lead-free solar cells in terms of PCE,14’15, the fact that Sn is more stable in the 4+ oxidation state causes critical stability issues.16 Meanwhile, they still show much lower photovoltaic performances than Pb perovskites because of the formation of Sn vacancies associated with the oxidation of Sn2+ to Sn4+. These vacancies act as non-radiative recombination centers in Sn-based perovskites.16,17 Sn-based PSCs thus demonstrate subpar open-circuit voltage (Voc) mainly due to severe trap- assisted recombination triggered by Sn vacancy-mediated unintentional doping/defects.18,19 Different approaches, including morphological control of the Sn perovskites, minimization of oxygen exposure during the device preparation, and the addition of Sn salts, have been explored and found to be useful in improving the performance of Sn-based PSCs. Recently, Wang et al. reported one of the Sn PSCs with an efficiency of 9.41% by employing quasi-two-dimensional (2D) Sn perovskites, which is still falling far short of Pb PSCs.20 On the other hand, alloying Pb with Sn perovskites seems to be a promising, balanced strategy towards achieving stable, less toxic solar cells without much compromising the photovoltaic performance.21 Moreover, alloyed Sn-Pb perovskites bring an added advantage, extended absorption of photons to the near-infrared (NIR) spectral range (up to 1050 nm with an optical bandgap of 1.18 eV). In contrast, typical hybrid organic-inorganic halide perovskites such as MAPbI3, FASnl3. and FASnR lack absorption in the NIR range because of their relatively higher optical bandgaps of 1.55 eV (up to 800 nm), 1.48 eV (up to 838 nm), and 1.30 eV (up to 950 nm), respectively.22 Unfortunately, the performance of Sn-Pb perovskites is limited by the mentioned Sn2+ — > Sn4+ oxidation. The most successful strategy for inhibiting such a detrimental process has been incorporating SnF2 into Sn-based perovskites. However, even with an optimum amount of
SnF2, the trap-assisted recombination coefficient, which is defined as a specific rate at which
oppositely charged ions combine at traps, is ~ 70 times higher than those in Pb-based ones.23 It suggests a prospect of improving the performance of Sn-Pb alloyed PSCs by reducing the defects associated with oxidation of Sn2+ to Sn4+ and/or Sn vacancy formation.
More recently, several groups, including ours have shown that 2D perovskites employing bulky cations are resistant against moisture and oxygen, but at the expense of PCE of the solar cell.24 27 Herein, we explore the suppression of defects formation through substituting small organic cations in alloyed perovskites with bulky organic cations for the first time with the purpose of increasing both PCE and stability simultaneously. To be more specific, different compositions of Sn-Pb alloyed perovskites are realized by stoichiometrically mixing FASnI3 and MAPbE (labeled as FA/MA alloyed perovskites).21 By substituting FA/MA with bulkier PEA cations in alloyed perovskites (labeled as PEA-FA/MA alloyed perovskites), we suppressed the oxidation of Sn2+ during the thin film preparation. By employing them as a solar absorber, a significant reduction of trap-assisted recombination was achieved in the device, therefore, a dramatic improvement of 40% in photovoltaic performance for Sn-rich PSCs. Notably, theoretical calculations suggest that the formation energy of Sn vacancies (under Sn- rich conditions, same as our experimental conditions) in Sn-based PEA containing perovskites (PEA2Snl4) is quantitatively larger than that of Sn vacancies in FA containing Sn-based perovskites (FASnI3). More intriguing is the fact that hypothetical mixed (PEAyFA1-y)SnE perovskites we assembled and optimized have shown an even lower tendency of Sn vacancy formation. Thus, assuming a reduced impact of the metal ion nature on the B-site vacancy formation energy for PEA- and FA-based perovskites and PEA/FA mixed ones, our results support the fact that less defects are likely to form in those Sn-Pb perovskite films characterized by the co-presence of both short (FA) hydrophilic and long (PEA) hydrophobic organic
cations. The reduction in the number of defects and also increase of the crystallinity in PEA- FA/MA perovskites were supported by X-ray diffraction (XRD) and synchrotron radiation techniques, photoluminescence measurements and elemental composition analysis by X-ray photoelectron spectroscopy (XPS) of the perovskite materials. It is worth noting that, we did not detect any 2D perovskite formation in the material after incorporating bulky PEA in the alloyed perovskites, as evidenced by XRD and synchrotron-based structural characterization, either due to the very low concentration of PEA we added herein or that PEA might just act as an additive in the perovskite at lower concentration without forming any 2D perovskite phases in the material. We hypothesize, the amount of PEA in perovskites and nature of perovskites influence the formation of 2D phases and orientation of perovskites. During the preparation of this manuscript, we found few reports using different bulkier organic cations to improve performance and stability of alloyed perovskites. But these reports rely on altering the crystal orientation of perovskite layers through in-situ growth of 2D perovskite phases to improve the solar cell performance.20,28 On the other hand, our work provides new physical insight about the reduction of Sn defect formation upon the incorporation of PEA molecules in Sn-Pb perovskites, which is strongly supported by experimental results and theoretical calculations. We tested photovoltaic properties of a series of (PEA)y (FAX MA1-x)1-y Snx Pb1-x I3 (y= 0, 0.05 and 0.1, x=0.3, 0.5, and 0.7) compositions and found that the highest average PCE of 13.60±0.69% was obtained for PEA0.05FA0.3MA0.7Sn0.3Pb0.7I3 devices. Our strategy simultaneously addresses another major drawback of Sn-based perovskites, i.e., the inferior stability under ambient environment. The PEA-FA/MA alloyed PSCs showed better stability (half-life, t1/2 = 200 I3) under 28±2% RH humidity than that of control FA/MA alloyed PSCs
(t1/2 = 80I3). The defects formed in the course of perovskite film preparation can be detrimental
to the stability of the PSCs. So, minimizing the defects in Sn-Pb perovskites improves stability. The defects also play a detrimental role in current density-voltage (J-V) curve hysteresis in PSCs through defect migration at grain boundaries (GBs) and perovskite film interfaces. By reducing the number of defects in PEA-FA/MA perovskite films verified by surface potential measurements, we eliminated the J-V hysteresis in the derived PSCs. In contrast, FA/MAPSCs exhibited significant J-V hysteresis.
2. Results and discussion
The optical absorption onsets of (FASnl3)x (MAPbl3)1-x and (PEA)y (FAX MA1-x)1-y Snx Pbi- x I3 (x= 0, 0.3, 0.5, 0.7 and 1) were determined from ultraviolet-visible (UV-Vis) absorption
Figure 1. a) UV-vis absorption spectra of FA/MA perovskite films and comparison of b) absorption spectra and c) steady-state photoluminescence (PL) spectra of FA/MA perovskite vs. PEA-FA/MA perovskite films, (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 I3. Dotted lines in a) denote absorption onsets of perovskites. measurements (Figure la and SI). Mixed Sn-Pb perovskites show the shift of the absorption onset to the NIR region compared to pure Pb (x=0) or Sn (x=l) perovskites. The lowest was 1.18 eV for (F ASnI3)0.7 (MAPbI3)0.3, followed by 1.22 eV for (FASnI3)0.5 (MAPbI3)0.5 and 1.26 eV for (FASnl3)0.3 (MAPbI3)0.7, as marked by dotted lines in Figure la. Substituting FA/MA
with a small amount of PEA does not change optical absorption onset considerably (Figures lb and SI). But we noticed a change in the shape of the optical absorption spectra and optical absorption intensity. Interestingly, the peak located at ~ 1.85 eV (marked with an arrow in the absorption spectrum) of FA/MA alloyed perovskites disappeared for PEA-FA/MA alloyed perovskites.29 In general, the peak close to the optical absorption edge is ascribed to the bound excitonic state due to the Columbic attraction of electrons and holes (the hydrogenic absorption peak). In our case, the peak is located (at 1.85 eV) significantly distant from the absorption onsets (at 1.18 eV, 1.22 eV, 1.26 eV), which basically excludes the hydrogenic absorption origin of the peak. The change in the shape of absorption spectra could be associated with excitonic properties of PEA-FA/MA alloyed perovskites, and more detailed studies focusing on photogenerated species in the low bandgap, alloyed perovskites are needed to understand this peculiar feature.30
The optical bandgaps obtained from PL measurements match well with the absorption onsets observed in the absorption spectra, indicative of the direct bandgap nature in (PEA)y (FAX MA1-x)1-y Snx Pb1-x I3 materials (Figure S2a). Further, we compared the PL intensity of FA/MA alloyed perovskites with PEA-FA/MA alloyed perovskites in Figure 1c and Figure S2 (b-c). A noticeable enhancement in PL intensity was observed after stoichiometrically replacing FA/MA partially with PEA in alloyed perovskites. Essentially, the more PEA is introduced, the higher the PL intensity. Such an observation indicates the decrease of defects in the PEA- FA/MA alloyed perovskites, which otherwise serve as charge recombination sites. This finding is also consistent with previous theoretical results that clearly show the integration of bulky cations in perovskite structure caused the increase of PL of perovskites.31
The topography of alloyed perovskite films obtained by atomic force microscopy (AFM) is shown in Figure 2. Nearly pin-hole free, dense alloyed perovskite films were formed in all the samples. However, a gradual increase in crystal grain size was noticed as the Sn content in alloyed perovskites increased. With the incorporation of PEA, the alloyed perovskite film surface showed a plate-like feature, being more evident at a higher PEA concentration (10%).
Consistently, a noticeable reduction in the root mean square (RMS) roughness was observed with respect to control samples. Our observation is in line with previous reports, which showed
that incorporation of PEA would encourage perovskites to crystallize as plates because the crystallization along a-b crystallographic axes occurs more rapidly than that along the c-axis.24
Figure 2. AFM topographic images of (a-c) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 I3 (y=0% (a), 5% (b) and 10% (c)), (d-f) (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 I3 (y=0% (d), 5% (e) and 10% (f)), (g-i) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 I3 (y=0% (g), 5% (I3) and 10% (i)).
Energy-dispersive X-ray spectroscopy (EDS) studies reveal that Sn and Pb are homogeneously distributed throughout the alloyed perovskite films, and there is no observable phase separation in these films (Figures S3 and S4).
Figure 3. 6-26 XRD spectra of different compositions of FA/MA and PEA-FA/MA perovskites: a) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 I3, b) (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 I3 and c) (PEA)y (FA0.7 MA0.3)1-y Sn0.7 Pb0.3 I3, and GIWAXS patterns of d) (FASnl3)0.3 (MAPb 13)0.7 and e) (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 films.
We characterized the crystalline structure of alloyed perovskites by 6-26 XRD. As shown in Figure S5, FASnI3 has only one peak, within the 26 range of 22°-25°, indexed to the (113) plane in the orthorhombic Amm2 space group, whereas MAPbI3 exhibits two peaks in the same range indexed to the (211) and (202) planes in the tetragonal (14 cm) space group.32 The 6-26
XRD spectra of alloyed perovskites (Figure 3(a-c)) display only one peak within the 26 range of 22°-25°, which strongly suggests that alloyed perovskites adopt the orthorhombic crystal structure.33 Moreover, it seems that Sn and Pb atoms randomly occupy the metal sites of comer-sharing ([Sn1-x PbxI6] -4) octahedra.34 The crystal structure is unchanged after replacing a small quantity of FA/MA cations by PEA cations in PEA-FA/MA alloyed perovskites. The bulky organic cations like PEA are typically used to synthesis 2D perovskites, but we did not observe any low angle peaks (26 <10°) corresponding to low dimensional perovskites as well as absorption onsets, so we can reasonably conclude that there is no experimentally observable 2D perovskite phase formation when a low amount of PEA incorporated in the material.35 The intensity of the XRD peaks, in particular the (110) peak at 14.1°, notably increased for all the alloyed perovskites with the introduction of PEA, which could be due to higher crystallinity of the alloyed perovskite and/or better film coverage on the substrate.
We further probed the change in the crystal size by tracking the variation in the full width at full width at half maximum (FWHM) of the 6-26 XRD peak located at 14°corresponding to the (110) crystalline plane (Figure S6). The FWHM dropped from 0.239° to 0.214° and from 0.246° to 0.230° in (FASnI3)0 3 (MAPbI3)0.7 and (FASnI3)0 5 (MAPbI3)0.5 when FA/MA was substituted with 5% of PEA, respectively. The change in FWHM correlates to local strain and crystallinity. The decrease of FWHM implies an increase in the crystallinity as well as reduced lattice strain. Especially, the reduction of FWHM of XRD peak located at (110) signifies lowering lattice strain along the inorganic bonding direction of I-Pb-I, I-Sn-I. But further integration of PEA to 10% caused an increase of the FWHM, which was still below that of the PEA-free sample. It is clear that 5% PEA integration can produce highly crystalline alloyed perovskites among (PEA)y (FA0.3 MA0.7)1-y Sno.3 Pb0.7 I3, (PEA)y (FA0.5 MA0.5)1-y Sn0.5 Pb0.5 I3
(y=0%, 5%, and 10%) compositions. On the contrary, the Sn-rich composition showed a different trend. The FWHM gradually increased with the amount of PEA (5 and 10%) in (FASnl3)0.7 (MAPbl3)0.3, suggesting the PEA introduction led to a decrease in the crystallinity.
Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were also conducted. GIWAXS further confirms that the 5% PEA incorporation improves the crystallinity. The GIWAXS pattern of (FASnl3)0.3 (MAPb 13)0.7 and (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 films are illustrated in Figure 3d and e, respectively. The latter exhibits more intense Bragg reflection at q = 10.0 nm -1 and 20 nm-1 with respect to the former, implying enhanced crystallinity for PEA-FA/MA alloyed perovskites and consistent with XRD results (Figure 3a-c). An additional Bragg reflection at 21.5 nm-1, observed only for PEA-FA/MA alloyed perovskites, corresponds to the crystalline phase of SnF2.36 We assume that overall enhancement of the crystallinity in PEA-FA/MA alloyed perovskites also accentuated the Bragg reflection related to the SnF2 crystalline phase.
To investigate the role of PEA cations in preventing the oxidation of Sn2+ in alloyed perovskites, XPS measurements were carried out (Figure S7 and Figure 4a and b). For the (FASnl3)0.3 (MAPbl3)0.7, the Sn 3d5/2 peak (located at 486.08 eV) could be well deconvoluted into two peaks located at 485.75 eV and 486.3 eV, corresponding to Sn2+ and Sn4+ oxidation states (Figure 4a). Likewise, the Sn 3d5/2 peak (located at 495.41 eV) comprises two peaks located at 494.27 eV and 494.71 eV, associated with Sn2+ and Sn4+ oxidation states.17 In contrast, the (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 only shows two sharp, non- deconvolutable peaks, with no Sn4+ peaks. The XPS spectra of (PEA)0.05 (FA0.7 MA0.3)0.95 Sn0.7 Pb0.3 I3 perovskites also showed similar behavior compared to their control samples (Figure S7). The oxidation of Sn2+ to Sn4+ is a strong indication of the formation of Sn vacancies in
Sn-based perovskites, which are the dominant defects in Sn-based perovskites due to their low defect formation energy among all the point defects.37 39
Figure 4. High resolution XPS spectra of Sn 3d in a) (FASnl3)0.3 (MAPbl3)0.7 and b) (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3, (yellow line: measured XPS spectrum; red and black lines: deconvoluted peaks; violet line: background).
The resultant defects can stimulate non-radiative (or trap-assisted) recombination processes, which is detrimental to photovoltaic performance. The PEA-FA/MA alloyed perovskites do not show any detectable signals of Sn4+, enhanced photovoltaic performance and stability are thus expected.
First principle calculations have shown that the size of organic cations could play a vital role in governing the formation energy of Sn vacancies in Sn-based perovskites.18,40 The larger ionic size could indeed reduce the Sn 5s— I 5p antibonding coupling, leading to higher Sn vacancy formation energies.
We are aware of the complexity of our systems, and their atomistic modeling is a very cumbersome task. Microscale effects may go beyond those described by ab-initio approaches and play a significant role in such mixed cation systems. Acknowledging such
potential issues, we tried to rationalize the experimental observations with our theoretical modeling. More specifically, we attempt to establish the effect of organic cations on the formation energy of Sn vacancies, employing the electron exchange-correlation functional of
Perdew-Burke-Ernzerhof (PBE) (see the "Computational Details" section).41
Here, we calculated the formation energies of Sn vacancies in systems that can both mimic the 3D FASnI3 system and, at the same time, host PEA cations still keeping the 3D dimensionality (as experimentally found, i.e., no 2D domains are detected after adding PEA cations).
We have accordingly constructed a first model consisting of an 8x1x1 supercell of the orthorhombic FASnI3 system and a second one obtained, starting from the initial one, after removing a SnE layer and 2 FA cations in order to host four bulky PEA cations, nominally (PEA4FA10)Sn12l38. In this way, we would like to mimic the PEA positioning at the interface/grain boundaries, which remains an incredibly challenging task in chemical environments like the present one. For both systems, we have fixed the lattice parameters as obtained from the 3D FASnI3 optimization, i.e., keeping the 3D dimensionality even after introducing PEA cations in the system. The optimized structures are reported in Figure 5a. We have then created Sn vacancies in both systems and re-optimized ionic positions (defective
structures of neutral Sn vacancies are reported for both systems in Figures. 5b-5d). Results of the formation of energy for Sn vacancies are reported in Figures 5e-5f.
Figure 5. Optimized structure of a) the pristine FASnI3 8x 1 x 1 supercell and b) the neutral Sn vacant system. In c) and d) the same for the mixed (PEA4FA10) Sn12l38 perovskite model. The red arrow in b) and d) show the Vsn positions. [Large mauve: Sn; purple: I; cyan: N; brown: C; white: H atoms]. The corresponding formation energy plots in the Sn-rich conditions for the two cases are reported in (e-f) where the electron chemical potential is referred to as the top of the valence band.
The bandgap employed here was obtained using PBE calculations (further details are provided in the "Computational Details" section). We are aware of the intrinsic underestimation of the bandgap of semiconductors at the PBE level due to the presence of the so-called self-interaction error; simultaneously, the large number of atoms in our systems (~200) prevents us from using more accurate post-DFT approaches. Nevertheless, we are confident of the reliability of our results for what regards the trend in the vacancy formation energy at the band edges (see Figure 5d) mainly because we are consistently treating similar systems at the same level of theory. As previously reported in the literature, such a crystal has an orthorhombic Amm2 space polar group,32,42 an additional feature to be considered to determine the vacancy formation energy. Our calculated bandgap energy for the polar FASnI3 8x 1 x 1 supercell is 0.63 eV, and that for (PEA4FA10)Sn12I38 one is 0.960 eV. Still considering only the equilibrium with SnE (the co-presence of PEA in the final compound is expected to reduce the presence of oxidized Sn+4 species)43 we obtain that the VSn formation energy in FASnI3 is a process always thermodynamically more favored (than in our (PEA4FA10)Sn12I38 mixed model) in each range of the electron chemical potential. In particular, it is more likely to be formed at n-type conditions (bottom of the Conduction Band, CBM) in both cases, as observed by comparing Figures 5d-5e, respectively. However, still, the energy required for the Sn vacancy to be formed is quantitatively higher (~4 eV) in the case of (PEA4FA10)Sn12I38 than in the FASnI3 supercell, as observed by comparing Figures 5e-5f, respectively.
Once more, it is worth stressing that (PEA4FA10)Sn12I38 structure is only one of the several hypothetical, stable and metastable, structures that aim at reproducing the randomness of experimental conditions (such model has an intermediate stoichiometry between 3D and 2D
but in a bare 3D crystal) and that modeling closer to the complicated experimental situation should involve microscale effects whose investigation goes beyond the target of DFT analysis. The model we suggested and calculated aims to mimic the PEA positioning at the interface/grain boundaries even if we know that maybe a lot larger models would be required, models whose size would not be accessible to ab-initio calculations. It is worth noting that the results reported here for the bare FASnI3 supercell show an enhanced tendency for the charged Sn vacancies to be formed (more exothermic) if compared with previously reported data.18 Such difference can be reasonably ascribed to several factors (atomic orbitals vs plane waves; PBE vs PBE-D3; nature of the pseudopotentials). We anyway believe that the polar nature of the FASnI3 space group and even more specifically the periodicity of the employed supercell (8x1x1) is the origin of the discrepancies. Indeed, even if on one side such supercell allows for the accommodation of the PEA bulky cations without leaving any sub stoichiometry and/or Sn-I dangling bonds, on the other hand a 1x1 periodicity in the be plane seems to reduce the rotation of FA cations and the I-Sn-I tilting angle of the inorganic network. Such structural features anyway apply to both FASnI3 and (PEA4FA10)Sn12I38 supercells thus not affecting (if not in terms of numerical absolute values) the final reported trends.
Supported by our modeling, we reasonably state that the introduction of bulkier organic cations (PEA) increases the formation energy of Sn vacancies in PEA-containing alloyed perovskites under our Sn-rich conditions. Fewer Sn-vacancies in PEA-FA/MA Sn-Pb perovskites are highly encouraging for the photovoltaic applications. We anticipate that the optimum PEA-containing alloyed perovskites would exhibit superior PCE and stability compared to control samples.
To test the photovoltaic properties of alloyed FA/MA and PEA-FA/MA perovskites, we employed a solar cell architecture of indium doped tin oxide (ITO)/ poly (3, 4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS)/perovskite/phenyl-C6i-butyric acid
Figure 6. J-V curves of a) (PEA)y (FA0.3 MA0.7)1-y Sn0.3 Pb0.7 13, and b) corresponding EQE spectra of the devices, calculated Jsc shown in graph (y = 0 (gray), 5% (red) and 10% (blue)). methyl ester (PCBM)/silver (Ag). Among the FA/MA alloyed perovskites, (FASnl3)0.3 (MAPb 13)0.7 showed the highest PCE followed by (FASnl3)0.5 (MAPb 13)0.5 and then (FASnl3)0.7 (MAPbI3)0.3.
Table 1. Photovoltaic parameters of the PSCs based on FA/MA and PEA-FA/MA alloyed
perovskites.
Both Voc and fill factor (FF) reduced as the Sn content increased (Table 1), with Sn-rich ((FASnl3)0.7 (MAPb 13)0.3) showing the lowest Voc and FF amongst all the compositions. As Voc and FF are strongly dependent on charge carrier recombination in the solar cell, Sn-rich perovskites are highly likely to contain a higher number of defects limiting the Voc and FF of their devices. Specifically, the defects in the bandgap would attract electrons/holes and act as non-radiative recombination centers, which primarily impacts the Voc of corresponding solar cells.44
By partially substituting FA/MA with PEA in alloyed perovskites, Voc was significantly improved. In particular, with 10% PEA, the Voc of (FASnl3)0.3 (MAPbl3)0.7 PSCs was boosted from 0.72 V to 0.80 V. The highest Voc enhancement (of 0.23V) was observed for (PEA)0.1 (F Ao. ?M A0.3)0.90 Sn0.7 Pb0.3 I3, followed by (PEA)0.1 (FA0.5 MA0.5)o.9o Sn0.5 Pb0.5 I3 (with an improvement of 0.15 V). It can be easily understood in such a way that "Sn-rich" alloyed perovskites contain more defects, so the effect of defect reduction with PEA on Voc is more evident. On the other hand, FF reached its maximum for the samples with 5% PEA. With the further increase to 10%, FF remained almost unchanged or slightly declined. As we have seen in the XRD discussion, the increase in PEA content in the perovskites changes the orientation of semiconducting perovskite layers on the substrate or lattice strain along inorganic bonding direction I-Pb-I, I-Sn-I, which could negatively affect charge carrier diffusion length and reduce the FF.48 Overall, elimination of Sn2+ oxidation (see XPS analysis discussion) thereby reduction of defects (see PL and DFT calculation discussion) in the (PEA)0.05 (FAX MAI-X)0.95 SnxPb1-x l3 reduced the trap-assisted recombination, which resulted in improved Voc and FF in the PEA-FA/MA alloyed perovskite devices.
As for Jsc, there is no clear trend of variation with incorporating PEA among various alloyed perovskite compositions. But in a particular alloyed perovskite compositional group, the Jsc in the devices follows a similar trend as observed in XRD data, especially in Sn-rich perovskites, where reduction of crystallinity with higher amount of PEA lowered the Jsc. By and large, the PSCs employing Sn-Pb alloyed perovskites with 5% bulkier PEA cations produced the highest PCE. We hypothesis this is due to a trade-off between local lattice strain or/and crystallinity and the number of defects in the alloyed perovskites. Figures S8-S10 shows the statistical distribution of efficiency of 20-30 devices in each compositional group.
J-V curves and corresponding external quantum efficiency (EQE) spectra of solar cells are shown in Figures 6 and SI 1. It can be seen from the EQE measurements that the photoresponse of all the solar cells reaches up to the near-infrared (NIR) region. The calculated Jsc values from EQE basically matched with those obtained from the J-V curves, except for (FASnl3)0.7
(MAPbI3)0.3. Since EQE was measured after J-V curve measurements and the more considerable mismatch was observed only in the Sn-rich FA/MA perovskites, we speculate it is due to the severe degradation of Sn-rich samples, which led to lower calculated photocurrents.
Figure 7. Dependence of a) Jsc, b) Voc, and c) FF against the incident light intensity of the
FA/MA PSCs and PEA-FA/MA PSCs.
Figure 7a shows the dependence of Jsc on incident light intensity by fitting to a power law (I The fitting parameter (a = 0.95) deviated from the linear dependence in FA/MA
alloyed perovskite devices compared to that (a = 1) of PEA-FA/MA perovskite devices, which implies that photogenerated charge carriers in FA/MA alloyed perovskite devices are not as efficiently transported to the electrodes as in PEA-FA/MA perovskite devices.46 To investigate the recombination of photogenerated carriers during solar cell operation near Voc; we plotted Voc versus logarithmically scaled light intensity (Figure 7b).
By linearly fitting the plot using
Where q is the elementary charge, n is the ideality factor; kb is the Boltzmann's constant, and T is the absolute temperature, we obtained a slope of 1.1 kbT/q (n is close to unity) for PEA- FA/MA perovskite devices, which strongly suggests that bimolecular recombination process dominates close to the Voc of the solar cells. The larger deviation of the slope (1.2 kbT/q) of FA/MA perovskite devices from kbT/q means trap-assisted recombination plays a larger role in the FA/MA system with respect to the PEA-FA/MA system.47 The FF is a more significant parameter as the solar cell is operated near a maximum powerpoint. The dependence of FF on incident light intensity unravels the nature of the recombination process in the solar cell during the device operation. In the pure bimolecular recombination process, the recombination rate is proportional to the product of charge carrier densities.48 Therefore, at a low light intensity (i.e., low charge carrier densities), the recombination rate reduces, and a better FF is obtained. This is the trend that was observed for the PEA-FA/MA alloyed PSCs (Figure 7c), which is in line with the dominance of the bimolecular recombination in these devices. On the other hand, FF decreased with decreasing light intensity in FA/MA alloyed perovskite devices, which unambiguously supports the dominance of the trap-assisted recombination in these devices.48 Since the number of traps does not change with lowering light intensity, it can be easily understood that the rate of recombination of photogenerated charge carriers with trapped charges increases with decreasing light intensity, and therefore results in a lower FF. The study clearly shows that the introduction of PEA effectively changed the dominant charge carrier recombination process in our PSCs.
The photovoltaic performance degradation of unencapsulated, best-performing (in terms of PCE) 5% PEA-containing alloyed perovskite devices was monitored under different environmental conditions to evaluate their stability compare with that of their control samples. Here, we define t1/2 as a time for the PCE of the device to drop to 50% of its initial value for standardizing the comparison between different devices. Figure S13a shows the degradation of devices under a modest moisture environment (28±2% RH humidity) in the dark. The degradation was significantly slowed down by substituting FA/MA with 5% of PEA in (FASnl3)0.3 (MAPbl3)0.7. The t1/2 for (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 I3 was found to be 200h appreciably better than that (t1/2 = 87 I3) of corresponding FA/MA perovskite devices.
Figure SI 3b displays the degradation of devices under AM1.5G illumination in an inert atmosphere. The (PEA)0.05 (FA0.3 MA0.7)0.95 Sn0.3 Pb0.7 13 devices exhibited better photostability compared to FA/MA perovskite devices (t1/2 = 30h vs. t1/2 = 15I3). Light seems to be more harmful to alloyed perovskite devices than moisture. Figure S13c shows that the degradation of alloyed perovskite devices became faster in the co-presence of light and modest moisture. However, PEA-FA/MA perovskite devices were still more resistant to the degradation, retaining 20% of initial performance after 24h, whereas FA/MA perovskite devices were wholly degraded during the same period. The defects in Sn-Pb perovskites induced by Sn vacancies not only play a key role in determining photovoltaic performance (PCE and J-V hysteresis) but also the stability of the devices. The defects in perovskites are vulnerable to oxygen or moisture filtration and accelerate the degradation of devices under ambient conditions.49 Reduced defects (see DFT calculations and PL discussion) and enhanced crystallinity (see XRD and GIWAXS discussion) with the incorporation of 5% PEA in Sn-Pb perovskites contribute to enhanced stability of the PEA-FA/MA Sn-Pb PSCs.
Based on all the above-described characterizations on materials (PL, AFM, SEM, XPS, XRD, GIWAXS) and devices (J-V and EQE measurements, light intensity-dependent analyses of the solar cell performance parameters), a reasonable conclusion can be made: i) more defects are present in the FA/MA alloyed PSCs, and the trap-assisted recombination indeed dominates their performance and ii) by replacing FA/MA cations with a small quantity of PEA cations, the number of defects can be considerably reduced and thereby there is a significant drop in the trap-assisted recombination in the alloyed perovskites. This conclusion is further supported by a large decrease in J-V hysteresis observed in PEA-FA/MA alloyed perovskite devices compared to that of FA/MA alloyed perovskite devices (Figure SI 3). Among the several suggested possible reasons for the detrimental phenomenon of the hysteresis of the J-V curve, the defect migration in PSCs is considered the main responsible.50,51 It has been previously shown that charged defects such as MA/FA and Pb/Sn vacancies close to the charge selective layers obstruct the charge extraction at selective contacts, causing the solar cell hysteresis.52 By reducing the number of charged defects, we believe the defect migration has been substantially reduced in PEA-containing Sn-Pb perovskites and therefore lowering the J-V hysteresis in the devices. As noted, defects close to the interfacial region between perovskite and charge selective layers predominately influence the J-V hysteresis.53,54
Figure 8. a) Height topography and b-c) surface potential profile of (FASnl3)0.3 (MAPbl3)0.7, d) topography and e-f) surface potential profile of (PEA)0.05 (FA0.3 MA0.7 )0.95 (Sn0.3 Pb0.7) I3. The average CPD calculated using Gwyddion software is also provided at the bottom of each Figure.
To understand defects at perovskite/selective layer interface, contact potential difference (CPD) of the samples was measured in the dark and under illumination by Kelvin probe force microscopy (KPFM), as shown in Figure 8. By substituting FA/MA with 5% of PEA in alloyed perovskites, the CPD was reduced at GBs and in the grain interior (GI) (Figure 8b, c, e, f). The surface photovoltage (SPV) estimated by subtracting the average CPD in the dark from that measured under illumination, provides the information about photocharge carrier separation at the perovskite/PEDOT: PSS interface.55,56 Under illumination, the charge-separation process in the complete device is mimicked using the AFM probe in the KPFM measurement, and SPV
represents the Voc or internal electric field in the solar cell.57The SPV increased from 0.059 V to 0.077 V with 5% PEA in alloyed perovskite film, indicating an increase in the internal electric field in PEA-FA/MA perovskite devices; therefore, we can reasonably conclude that photogenerated carriers are more efficiently separated in the PEA-FA/MA perovskite film than FA/MA perovskite film.58,59 The KPFM measurements essentially suggest reduced recombination events for the devices employing PEA-FA/MA perovskite film compared to their control samples. This clearly demonstrates that the number of defects is lowered in Sn- Pb perovskites with the inclusion of 5% PEA, which also reduces the defect migration, leading to the eradication of J-V hysteresis in resultant PSCs.60,61 Not secondarily, partly replacing MA/FA with more bulky PEA cations has the advantage of massively reducing the polarizability of the final device, 12,62-64 the another factor which is strongly believed to affect the J-V hysteresis in the FA/MA PSCs, further stressing the significant advantages of using PEA-containing alloyed perovskites in PSCs.
3. Conclusions
In this work, we have used bulky aromatic cations to improve both the efficiency and stability of Sn-Pb perovskite solar cells. By incorporating PEA cations to Sn-Pb perovskites, we observed key advancements in perovskite semiconductor material quality. The overall crystallinity of the alloyed perovskites was significantly improved, as evidenced by XRD and GIWAXS, suggestive of a reduced number of defects that are further confirmed by PL spectroscopy. Consistently, DFT calculations suggested that the formation energy for Sn vacancies in perovskite models with mixed FA/PEA organic moieties is sensitively higher,
thus less likely to occur than in the case of pristine short-chain FA containing Sn-based perovskites (FASnI3).
The advantages of reduction of defects associated with Sn vacancies in Sn-Pb perovskites are threefold: 1) the PCE enhanced by up to ~40% in Sn-rich 5% PEA-containing alloyed PSCs and 15% in Pb-rich alloyed PSCs compared to their control samples due to reduced non- radiative recombination in the alloyed perovskites; 2) eliminated J-V hysteresis in PEA- FA/MA Sn-Pb PSCs attributable to lowered Sn vacancy defect migration and 3) improved device stability due to the reduced number of active defect sites in alloyed perovskites where accelerated material degradation can take place. In short, by modifying defect chemistry in Sn- Pb perovskites realized through incorporating the optimum amount of PEA, we improved the photovoltaic performance and stability of alloyed PSCs. Our work on passivating defects with bulky organic cations in Sn-Pb perovskites opens new opportunities in designing highly efficient, stable Pb-less alloyed perovskite solar cells.
4. Materials and Methods
Materials: Methylammonium iodide (CH3NH3I), Formamidinium iodide (CH(NH2)2l) and phenylethylammonium iodide ( C6H5 (CH2)2NH3I) were purchased from Dyesol. Tin iodide (SnE), tin fluoride (SnF2), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Sigma Aldrich. Lead iodide (Pb I2) was bought from Acros Organics. Phenyl- C61-butyric acid methyl ester (PC61BM) and poly (3, 4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS; Clevios™ P VP Al 4083) were obtained from 1-Materials and Heraeus, respectively. All chemicals were used as received.
Alloyed perovskite precursor solution: FASnI3 precursor solution (1.1 M) was prepared by adding equimolar FAI and SnI2 powders and 20 mol% of SnF2 into a mixture of DMF: DMSO (4: 1 v/v). MAPbI3 precursor solution (1.1 M) was prepared by adding MAI and PbI2 in a mixture of DMF: DMSO (4: 1 v/v). (FASnl3)x (MAPbl3)1-x (x = 0.3, 0.5, 0.7) solutions were prepared by stoichiometrically mixing FASnI3 and MAPbE solutions. The PEA-FA/MA alloyed perovskite precursor solutions were prepared by stoichiometrically substituting FAI/MAI with 5% and 10% PEAI in (FASnI3)x (MAPbl3)1-x precursor solution.
Solar cell fabrication: Patterned ITO-coated glasses were cleaned by sonication in detergent followed by sequential washing with deionized water, acetone, and isopropanol. After drying under airflow, the substrate surface was cleaned by oxygen plasma for 10 min under a rough vacuum. PEDOT: PSS solution was spin-coated on top of the ITO-coated glass substrate at 4500 rpm for 45 s; PEDOT: PSS performs as the hole transporting layer. The PEDOT: PSS film was then dried in the air on a hot plate (set at 170 °C) for 10 min. After drying, the substrate was transferred to a nitrogen-filled glovebox for further use. The FA/MA and PEA-FA/MA alloyed perovskite absorber layer were spin-coated on the PEDOT : PSS film at 5000 rpm for 60 s. Diethyl ether was dropped onto the spinning substrate. The spin-coated films were annealed at 60°C for 5 minutes. Then the PC61BM solution (20 mg/mL in chlorobenzene) was spin-coated on top of the perovskite film at 2000 rpm for 60 s to form a 20 nm thick electron transporting layer. Finally, the film was transferred to a thermal evaporation chamber inside the nitrogen-filled glove box. The chamber was pumped down to 1 x 10 -6 Torr for silver deposition. The 100 nm thick silver top electrode was deposited through a shadow mask that defines the active device area as 0.06 cm2 for the solar cells.
Perovskite Film Characterization: Topography of perovskite film surface was obtained by using a Bruker MultiMode8 AFM. Absorption spectra were collected by using a UV-Visible- NIR spectrometer Lambda 750 (Perkin Elmer). Steady-state PL spectra were obtained from a Fluorolog®-3 system (Horiba Jobin Yvon) using a 444 nm laser. XRD measurements were carried out using a Panalytical X-Pert PRO MRD X-Ray diffractometer. The oxidation of the tin element was probed by an XPS spectrometer (ESCALAB 220I-X L) equipped with an Al Kα (1486.6 eV) monochromatic source. The grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were done at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF). The GIWAXS patterns were collected by a MarCCD detector, which is mounted vertically at around 194 mm from the sample. The exposure time was less than 50 s, and a grazing incidence angle with respect to the surface plane is 1°. The GIWAXS patterns were examined afterward using the Fit 2D software and displayed in scattering vector q coordinates with q=4πsinoθ/λ, where θ is half of the diffraction angle, and 1 is the incident X-ray wavelength (0.124 nm). KPFM measurements were done using the tapping mode in a Cypher AFM (Model: Cypher S) under ambient conditions. KPFM measurements were performed using a Platinum-coated Silicon tip. A white LED lamp was used to carry out KPFM measurements under illumination.
Solar cell characterization: Solar cell performance was measured using a class ABA LED solar simulator, which was calibrated to deliver simulated AM 1.5 sunlight irradiance of 100 mW/cm2. The irradiance was calibrated using an NREL-calibrated KG5 filtered silicon reference cell. Current density -voltage (J-V) curves were recorded using a source meter (Keithley 2400, USA). External quantum efficiency (EQE) measurements were conducted by using an IQE200B system (Newport Corporation).
Device stability tests: The stability of devices was tested without any device encapsulation.
For moisture stability tests, the PEA-FA/MA and FA/MA perovskite devices were placed inside a desiccator under an ambient environment. The relative humidity (28±2% RH humidity) was continuously monitored with a digital humidity sensor. For photostability tests, the perovskite devices were placed under constant AM1.5G illumination inside an N2 filled glovebox. Devices were also tested under constant AM1.5G illumination at 28±2% RH humidity.
Computational details: We performed DFT calculations by means of the electron exchange-correlation functional of Perdew-Burke-Emzerhof (PBE),41 as implemented in the SIESTA code. The mesh cutoff was set to 300 Ry, while norm-conserving pseudopotentials of the Troullier-Martins type65 for the description of the core electrons were used along with standard triple-ς plus polarization (TZP) for Sn, C, N, H, and double-ς plus polarization (DZP) for iodine atoms. Calculations were converged when residual forces were lower than 0.04 eV/A. A 1 x4x4 F-centered Appoint sampling of the Brillouin Zone (BZ) was used for the structural optimizations of both the 8x 1 x 1 bare FASnI3 and mixed (PEA4FA10)Sn12I38 supercells.
We initially have calibrated our setup by optimizing the (pseudo)cubic FASnI3 system.26 Results show a calculated lattice parameter of 6.42 A, in excellent agreement with both theoretically66 and experimentally reported.32 Similarly, interesting is the fact that the band structure of the cubic primitive system (see Figure S14) shows a perfect agreement with other theoretical data, further validating our choice.67,68
The optimized lattice parameters for the investigated supercells are a= 50.63 A, b= 8.73 A, and c= 8.94 A. The formation energy of the Sn vacancy (Vsn) was calculated via the following equation:
Eform= Etot ( Vsnq) - Etot + μSn + q(εVBM+EF), (2)
Where Etot (Vsn q) is the energy of the defective system in its charged state (q=0 , -1, -2), Etot is the energy of the neutral non-defective structure, μSn is the chemical potential of Sn, £VBM represents the Kohn-Sham eigenvalue (top of the valence band, TVB), EF is the electron chemical potential (referenced to the TVB of the pristine, non-defective, crystal). Following the experimental conditions (vide infra), the chemical potentials of FASnI3 constituents were estimated by imposing the thermodynamic equilibrium of FASnI3 with the SnE phase. We thus apply the following two constraints:
Where μi and μFA are chemical potentials of iodine and FA, respectively, while AH(FASnI3) and AH(SnE) are formation enthalpies of FASnI3 and SnE, respectively. Here the chemical potentials of Sn and I have been referenced to the value of the E molecule and Sn bulk metal ( β-tin, I4(l)/amd, Z=4), respectively.69 We applied an additional constraint of Sn-rich conditions (i.e., μSn = 0) for a realistic representation of our alloyed perovskite systems, which
contain a surplus of Sn through the addition of 10 mol% of SnF2. Thus, the formation energy of Vsn in the Sn-rich condition was calculated (Figure 5e).
BSSE (Basis Set Superposition Error),70 whose calculation is of paramount significance in molecular complexes, was calculated and found to be almost negligible in neutral Sn vacancy formation in FASnI3. Regardless, we decided not to include BSSE in calculations because of the complexity of the systems here investigated. On the other hand, to test the validity of our setup, we benchmarked our results comparing the formation energy of the neutral Sn vacancy in the case of FASnI3, calculated with our chosen basis and with a full DZP basis, finding 2.32 eV in the latter and 2.20 eV in the former case, further assessing the reliability of our setup.
ASSOCIATED CONTENT.
Supporting Information
The following files are available free of charge available on ACS Publications website.
Additional absorption spectra, PL spectra, SEM-EDS images, XRD, XPS, J-V graphs, statistical distribution of photovoltaic parameters, device stability. (PDF)
AUTHOR INFORMATION
Corresponding Authors
* Dongling Ma, E-mail: ma@emt.inrs.ca
* Giacomo Giorgi, E-mail: giacomo.giorgi@unipg.it
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENT
Financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada, in the context of a NSREC Discovery Grant (Grant No.: RGPIN-2015-06756) and a NSERC Strategic Grant (Industry partner: Canadian Solar Inc.; Grant No.: STPGP493998) is greatly appreciated. D.M is also grateful for the financial support from Quebec Center for Functional Materials (CQMF), Canada. D.T.G acknowledges scholarship support from the Fonds de recherche du Quebec-Nature et technologies (FRQNT) under the Programme de Bourses d'Excellence (Merit Scholarship Program for Foreign Students). M.P. thanks INFN for financial support through the National project Nemesys and for allocated computational resources at CINECA. G. G. acknowledges PRACE (Grant No. Pral7_4466 "DECONVOLVES") and ISCRA ("2D-OIHPs" HP10BGUJ6X) for awarding access to resource Marconi based in Italy at CINECA. The authors thank beamline BL14B 1 at Shanghai Synchrotron Radiation Facility (SSRF) for providing the beam time.
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Table of Contents (TOC) graphic
Claims
1. A 2D/3D alloyed halide perovskite of general formula P below
Ay(FAxMA1-x)1-yBX3 (P) wherein:
A is an organic cation selected from ammonium cation and amidinium cation; FA is formamidinium;
MA is methylammonium; y=0.01-0.99; x=0.01-0.99;
B is an alloy involving at least two of Sn, Pb and Ge; and X is a halogen atom.
2. The 2D/3D alloyed halide perovskite according to claim 1 , wherein B is an alloy selected form Sn-Pb, Sn-Ge and Ge-Pb; preferably B is the alloy Sn-Pb or Ge-Pb; more preferably B is the alloy Sn-Pb.
3. The 2D/3D alloyed halide perovskite according to claim 1 or 2, wherein X is selected from I, F, Cl and Br; preferably X is selected from I, Cl and Br; more preferably X is I.
4. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an ammonium cation of general formula I below
wherein Ri to R4 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl.
5. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an ammonium cation of general formula II below
wherein:
Ri to R3 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl;
L is present or absent and is a group comprising one or more of (CH2) and (CH); and Q is present or absent and is a 5 to 12-member ring or bicycle ring, with the proviso that at least one of Q and L is present.
6. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an ammonium cation of general formula III below
wherein:
Q is present or absent and is a 5 to 12-member ring or bicycle ring; and n is an integer from 0-6.
7. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an ammonium cation of general formula IV below
wherein:
Ri are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl; n is an integer from 1-6; and m is an integer from 0-4.
8. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an ammonium cation of general formula V below
wherein n is an integer from 1-6.
9. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is phenylethylammonium (PEA) cation
10. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an amidinium cation of general formula I' below
wherein Rs to Rg are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl.
11. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an amidinium cation of general formula II' below
wherein:
R5 to R8 are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl;
L' is present or absent and is a group comprising one or more of (CH2) and (CH); and Q' is a 5 to 12-member ring or bicycle ring.
12. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an amidinium cation of general formula III' below
wherein:
Q' is a 5 to 12-member ring or bicycle ring; and n' is an integer from 0-6.
13. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an amidinium cation of general formula IV' below
wherein:
R'i are each independently selected from H, alkyl, cycloalkyl, alkene, alkyne, aryl and alkylaryl; n' is an integer from 1-6; and m' is an integer from 0-4.
14. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is an amidinium cation of general formula V' below
wherein n' is an integer from 1-6.
15. The 2D/3D alloyed halide perovskite according to any one of claims 1-3, wherein A is phenylethylammonium (PEA) cation; B is the alloy Sn-Pb; and X is I.
16. The 2D/3D alloyed halide perovskite according to any one of claims 1-15, wherein y=0.05 or y=0.1 ; and x=0.3 or x=0.5 or x=0.7.
17. The 2D/3D alloyed halide perovskite according to any one of claims 1-15, wherein y=0.05; and x=0.3 or x=0.5 or x=0.7.
18. The 2D/3D alloyed halide perovskite according to any one of claims 1-15, wherein y=0.1 ; and x=0.3 or x=0.5 or x=0.7.
19. A 2D/3D alloyed halide perovskite of general formula P1 below
Ay(FAxMA1-x)1-ySnxPb1.xX3 (P1) wherein: A is an organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'; FA is formamidinium; MA is methylammonium; y=0.01-0.99; x=0.01-0.99; and
X is a halogen atom.
20. The 2D/3D alloyed halide perovskite according to claim 19, wherein A is phenylethylammonium (PEA) cation.
21. The 2D/3D alloyed halide perovskite according to claim19, wherein y=0.05 or y=0.1; and x=0.3 or x=0.5 or x=0.7.
22. The 2D/3D alloyed halide perovskite according to claim 19, wherein y=0.05; and x=0.3 or x=0.5 or x=0.7.
23. The 2D/3D alloyed halide perovskite according to claim 19, wherein y=0.1; and x=0.3 or x=0.5 or x=0.7.
24. A 2D/3D alloyed halide perovskite of general formula P2 below
PEAy(FAxMA1-x)1-ySnxPb1.xl3 (P2) wherein:
PEA is phenylethylammonium cation;
FA is formamidinium;
MA is methylammonium; y=0.05 or y=0.1; and x=0.3 or x=0.5 or x=0.7.
25. A 2D/3D alloyed halide perovskite, which is:
(PEA)0.0 5(FA0.3 MA 0.7 )0 .95 (Sn0.3 Pb 0.7 ) I3
(PEA)0 .1 (FA0.3 MA 0.7 )0 .90 (Sn0.3 Pb 0.7 ) I3
(PEA)0.0 5(FA0.5 MA 0.5 )0 .95 (Sn0.5 Pb 0.5 ) I3
(PEA)0 .1 (FA0.5 MA 0.5 )0 .90 (Sn0.5 Pb 0.5 ) I3
(PEA)0.0 5(FA0.7 MA 0.3 )0 .95 (Sn 0.7 Pb 0.3 ) I 3 or
(PEA)0 .1 (FA0.7 MA 0.3 )0 .90 (Sn0.7 Pb 0.3 ) I3
26. A method of preparing a 2D/3D alloyed halide perovskite, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with an organic cation having a size which is larger than the size of FA and/or MA.
27. A method of preparing a 2D/3D alloyed halide perovskite, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with a larger organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'.
28. A method of preparing a 2D/3D alloyed halide perovskite, comprising replacing at least part of the cations formamidinium (FA) and methylammonium (MA) with phenylethylammonium (PEA) cation.
29. The 2D/3D alloyed halide perovskite as defined in any one of claims 1-25 obtained by the method as defined in any one of claims 26-28.
30. A method of manufacturing a solar cell device, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with an organic cation having a size which is larger than the size of FA and/or MA.
31. A method of manufacturing a solar cell device, comprising replacing at least part of the small organic cations formamidinium (FA) and methylammonium (MA) with a larger organic cation selected from an ammonium cation of general formula I, II, III, IV or V and an amidinium cation of general formula I', II', III', IV' or V'.
32. A method of manufacturing a solar cell device, comprising replacing at least part of the cations formamidinium (FA) and methylammonium (MA) with phenylethylammonium (PEA) cation.
33. A method of manufacturing a solar cell device, comprising preparing the 2D/3D alloyed halide perovskite as defined in any one of claims 1-25.
34. A method of manufacturing a solar cell device, comprising using the 2D/3D alloyed halide perovskite as defined in any one of claims 1-25.
35. A solar cell device which comprises the 2D/3D alloyed halide perovskite as defined in any one of claims 1-25.
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