Perovskite Solar Cells
Perovskite Solar Cells
Perovskite Solar Cells
9
Perovskite solar cells
Amol Nande1, Swati Raut2, S.J. Dhoble2
1
Guru Nanak College of Science, Ballarpur, Maharashtra, India; 2Department of Physics, RTM
Nagpur University, Nagpur, Maharashtra, India
9.1 Introduction
In the 19th and 20th centuries, the human race increased fossil fuel energy consumption to
satisfy its demand for electrical energy. Unfortunately, fossil fuels are limited but global de-
mand for energy for day-to-day use continues to grow. Moreover, carbon emissions have also
grown on average by 2% [1]. Furthermore, fossil fuels have many disadvantages regarding
extraction, transportation, and consumption. There are high risks involved in the transporta-
tion and accidental spills of fossil fuels, which can cause very serious problems. Therefore hu-
mankind has to find alternative sources of easily available and clean energy. The available
options for clean and safe energy are solar energy, wind energy, biomass, and geothermal
energy. Of these energy sources, solar energy is easily available and can be directly converted
to electrical energy [2].
Solar energy offers an easy solution to produce electricity in a clean and safe way. The ma-
terials or devices that convert solar energy to electrical energy are termed photovoltaic or so-
lar cells. Daryl Chapin, Gerald Pearson, and Calvin Fuller from Bell's Laboratory invented a
silicon-based solar cell that ultimately revolutionized the photovoltaics industry [3,4]. Due to
advancements in fundamental physics and device fabrication along with silicon wafer-based
solar technology, other thin film technologies such as GaAs, Cu(In,Ga)Se2, CdS, CuZnSnSe2,
CdTe, organic molecule and polymer solar cells, quantum dot solids, dye-sensitized solar
cells, and perovskite solar cells (PVSCs) have been established and researchers are intensively
investigating materials and systems for efficient solar cells [5e8]. The major photovoltaics in-
dustries still dominate silicon photovoltaics, but it has high manufacturing and installation
costs [9e11], although it is expected that the cost of silicon photovoltaics will decrease. But
for now, PVSCs are a potentially proven candidate for photovoltaic technology. The materials
used are cheap and easily processable organiceinorganic hybrid perovskite semiconductors
used as absorber materials [11e14]. This is the third generation of solar cells and the first
work on this subject was published in 2009 by Kojima et al. [15]. Researchers have seen var-
iations in power conversion efficiencies from 3.78% to 28% and they are expecting to see
Energy Materials
https://doi.org/10.1016/B978-0-12-823710-6.00002-9 249 Copyright © 2021 Elsevier Ltd. All rights reserved.
250 9. Perovskite solar cells
further improvements in efficiencies in the coming years [13,16,17]. This chapter discusses the
photovoltaic and solar cell properties of perovskite materials. First, the chapter explains
perovskite materials, where we focus on their common structure and the literature devoted
to inorganic and organic perovskite materials. This will lead to synthesis techniques, espe-
cially for perovskite solar cells, which consist of topics such as device anatomy, requirements
of each layer, working principles, and characterization techniques. Later, the chapter dis-
cusses the fabrication approach and device evaluation of PVSCs. The chapter concludes
with key challenges and the future outlook of PVSCs.
Perovskite was first used for calcium titanium oxide (CaTO3) and is named after Lev Per-
ovski. Later, perovskite was used to explain any material having the same structure as
CaTO3. To express perovskite material, the more generalized formula ABX3 is deployed,
where A (such as metal ions and organic cations) and B (such as metal ions) are cations
and X (such as oxides and halides) is the anion. Also, their valences are in a ratio of 1:2:1
with a cubic structure of conventional perovskite. Cations A are larger than cations B, as
shown in Fig. 9.1 [18]. The figure shows that perovskite materials have a cubic crystal struc-
ture where the smallest cation B is surrounded by an octahedral anion. Twelvefold octahedral
coordination with 12 X anion neighbors is covered by cation A. It is well known that when
discussing the stability of perovskite structures, including oxidesand
pffiffiffi halides,Goldschmidt's
tolerance factor is widely accepted. This is given as t ¼ ðrA þrX Þ 2ðrB þrX Þ , where, rA, rB,
FIGURE 9.1 Ideal perovskite crystal structure in which B cations are linked at the corners of an octahedron to
form a cubic lattice [18]. Used with permission granted from Copyright © 2001, American Chemical Society.
FIGURE 9.2 Tolerance versus octahedral map for perovskite compounds [19]. Used with permission of the Inter-
national Union of Crystallography.
[11,12,18]. Perovskite materials have either oxygen anions (oxides) or halogens. Perovskite
materials with halides are of great interest due to their exceptional optoelectronic properties.
Therefore it is observed that the materials have halides (Cl, Br, I, and F) at the X position in
ABX3 and their optoelectronic properties make them better candidates for solar cell applica-
tion compared to oxides. Also, the published work shows that highly efficient perovskite ma-
terials for solar cells contain lead (Pb) or tin (Sn) at position B. These compounds have low
bandgaps that enhance photon absorption in visible and near-infrared regions. One can
tune the perovskite bandgap by varying the composition of perovskite elements. Also, by
changing any elements in the ABX3 perovskite stoichiometry, the bandgap can be tuned
throughout the visible spectrum. The energy bandgap depends on the size of cation A,
and it is observed that by increasing the size of A, the bandgap size decreases and vice versa
[23e25]. Also, one can tune the bandgap by keeping the same composition of materials and
by changing the quantum confinement in perovskite nanoparticles. There are organometallic
perovskite materials for which the experimental bandgap value of ~1.6 eV is an ideal
bandgap value for single junction solar cells.
Another interesting parameter used for photovoltaics is absorption coefficient, which de-
termines the intensity of light as it passes through a given material. It is observed that perov-
skite materials, especially organometallic perovskite materials, have higher absorption
coefficient compared to other photovoltaic materials [26e28]. Along with absorbance coeffi-
cient, carrier diffusion length also plays an important role in the performance of the solar cell
device; the higher the carrier diffusion length, the better the performance of the device. Liter-
ature dictates that perovskite materials exhibit longer carrier diffusion lengths compared to
traditional semiconductors or solar materials; for example, the single crystal of MAPbI3
showed ~175 mm [29]. Thus this large diffusion length allows a planar heterojunction with
a selective charge interface for perovskite materials. This gives perovskite materials both p-
type and n-type interfaces, which form p-i-n or n-i-p structures. The performance of these
structures is similar to mesoporous structures. Thus perovskite materials are both excellent
light absorbers and electron and hole transporters, which make them outstanding candidates
for solar cells.
The structure and morphology of perovskite materials or layers play a vital role in their
inherent properties and hence their solar cell performance. The homogeneity and uniformity
of the photovoltaic grains are also responsible for the performance of solar cells. The crystal-
line nature, grain size, and quality of perovskite thin film depend highly on depositing
parameter and substrate. Here, we explain the most commonly used techniques to deposit
dense and uniform perovskite layers. The perovskite absorber can be deposited by
solution-based methods (such as dip coating, spin coating, inkjet printing, doctor blading,
etc.), vapor-based methods (such as thermal evaporation, chemical vapor deposition, sputter-
ing, pulsed laser deposition, sequential evaporation, etc.), and hybrid vaporesolution-based
deposition methods. Some of these are described next.
[45]. This method is used to deposit perovskite thin film over a large surface. In this process,
one could replace the spin-coating step with the inkjet printing technique or the dip-coating
technique. This method allows comparatively fast formation of perovskite even at room tem-
perature, and controls deposition rate and substrate temperature to deposit BX2. By varying
these two conditions, the formation of size, shape, and arrangement of grains can be
controlled. This will provide a controlled porous nanostructured BX2 thin film, which defines
the formation and structure of the perovskite layer when AX is deposited onto it.
Abbreviations: CsPbI3, Cesium lead iodide; Spiro-OmeTAD, 2,20 ,7,70 -Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 0 -spirobifluorene;
Spiro-MeOTAD, 2,20 ,7,70 -Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 0 -spirobifluorene; PEDOT:PSS, poly(3,4-ethyl-
enedioxythiophene) polystyrene sulfonate; P3HT, Poly(3-hexylthiophene-2,5-diyl); PTTA, purified terephthalic acid.
bromine were used as a halide in the perovskite combination. Sn and Ge have good optoelec-
tronic properties similar to Pb. However, the perovskite structure of these used materials is
less stable and hence efficiency is less as compared to previously studied perovskite
materials. Further studies are required to understand the optical and electrical properties
of Sn or Ge used for perovskite materials [73e76]. A mixture of Sn and Ge is used instead
of monometallic counterparts due to its stable narrow bandgap absorbers [77]. Instead of
FIGURE 9.4 (A) Schematic for a mesoporous perovskite solar cell (PVSC) indicating different layers of the device.
(B) Schematic for the n-i-p approach to the synthesis of the PVSC indicating different layers. (C) Schematic for the p-i-n
approach to the synthesis of the PVSC indicating different layers.
that if the thickness of nanoporous materials is increased up to certain values (~300 nm), the
number of pores filled with perovskite materials will also increase, which causes an increase
in pore-filling fraction. Therefore an increase in charge carriers is observed and hence the
transportation of charge carriers increases. This leads to increases in efficiencies at the elec-
tron transport layer interface and hence there is a rise in the PCE of PVSC devices [32,92].
The highest PCE recorded for MA chloride PVSCs, which are prepared using the mesoporous
approach, is 24.02% [93].
The working PVSCs is explained as e when photons are incident on substrate it passes
from the transparent electrode to absorbing layer like perovskite layer. Incident photons
are absorbed by the absorbing layer; excitons are generated and propagate to the perovskitee
electron transport layer interface. At the interface, excitons are separated into electrons and
holes. The electrons are collected by the electronehole layer and holes are collected by
hole transport layers. The PCE of PVSCs depends on transparency of the electrode or photo-
anode, mobility, and collection of photogenerated charges. Also, a metal electrode in the
topmost layer in PVSCs, which is directly in contact with the environment, affects the stability
of devices. Therefore it is necessary to understand the requirements and properties of each
layer.
potential barrier (between electron transport layer and perovskite layer) for electrons and
block the holes [105,106]. The common electron transport materials are TiO2, SnO2, ZnO,
fullerene butyric acid methyl ester (PCBM), tin-doped zinc oxide, SrTiO3, B2S3, BaSnO3,
and IndeneeC60 bisadduct as well as doped metal oxides [107e110]. The basic requirements
to qualify for the layer are high electron mobility, easy dissolution in organic solvent, inability
to absorb ultraviolet light, and good air stability. Also, there must be good alignment be-
tween the perovskite layer and the electron transport layer [111].
The electron transport layer can be deposited using atomic layer deposition, electrochem-
ical deposition, the solution-processed method, the hydrothermal method, radiofrequency
sputtering, and electrodeposition [112e115]. These methods, like atomic layer depositions,
facilitate large surface deposition of the electron transport layer; the large surface area of
the electron transport layer (ZnO, TiO2, and SnO2) allows better contact with the perovskite
layer. Better contact provides efficient extraction of electrons and hence increases the PCE
value of PVSCs. To increase the extraction ability and improve electron-transporting proper-
ties, metal oxide electrodes are doped with n-type impurity [115]. Because of their high elec-
tron mobility, transparency to ultraviolet-visible light, and high electron affinity, carbon
materials like fullerene and its derivatives are used as electron transport layers [116,117].
It is observed that this layer directly affects the stability and performance of the PVSC de-
vice. For better performance, the layer should be resistant to air and humidity, transparent to
ultraviolet-visible light, and have high ability to extract electrons from the perovskite layer,
high electron mobility, high tolerance to holes to avoid recombination of electronehole pairs,
and zero degradation over time. It can be concluded that the electron transport layer should
have negligible degradation because degradation in the layer causes a decrease in carrier
collection and damages ohmic contact with the electrode. Therefore efficiency of PVSCs
decreases.
absorber layer was deposited in the PVSC device; therefore the electrode could protect or
decrease the transfer of moisture from air to the perovskite layer. Metals, like silver,
aluminum, and gold, are the most commonly used electrodes in PVSCs. Metal electrodes
have stability issues as well as affect the PCE values. Silver and aluminum electrodes undergo
corrosion from ion migration from perovskite layers. These generated ions from metal halides
have stability issues with perovskite cells [128e130].
Other elements like chromium and carbon are used for metal electrodes. A chromium elec-
trode has been used in MAPbBr3 PVSC devices. According to studies [131,132], chromium
increases the interfacial resistance and reduces the amount of interfacial recombination.
Sometimes a chromium layer is deposited between the metal electrode and perovskite layer,
which prevents diffusion of the metal electrode into the perovskite layer, and leads to stable
PCE. As the layer is chemically inert toward iodine, it does react with PVSCs [133,134]. Re-
searchers have also started working on hole transport layer-free PVSC devices by using car-
bon electrodes [60,65]. A thick carbon electrode protects the perovskite layer from humidity
and increases the stability of the device. PVSCs with porous carbon back electrodes usually
exhibit exceptional long-term stability under light and ambient conditions [135].
Thus to fabricate stable PVSCs, diffusion barrier layers should be inserted to separate the
metal electrodes and perovskite layer. Although electrodes do not directly affect the efficiency
of PCE values of PVSCs, they affect their stability.
In PVSCs, the perovskite layer acts as an absorber layer, which absorbs light and gives rise
to free electrons and holes. These photogenerated free charge carriers diffuse and drift under
the influence of an electric field. The holes move toward the hole transport layer while elec-
trons move toward the electron transport layer. The efficiency of extraction depends on the
transporting ability of holes and electrons toward their respective layers. When holes and
electrons are collected by respective electrodes, the electrons dissipate energy and produce
power before returning to the device at the opposite electrode and recombine with the
hole. If electrons are not extracted by the electron transport layer from the perovskite layer,
then it will eventually recombine with the hole. This is nothing but radiative transition, which
emits a photon of energy equal to the bandgap of the perovskite material.
Fig. 9.5 depicts the general working principle of PVSC devices. When light falls on PVSCs,
perovskite acts as an absorber layer and absorbs light leading to the creation of free carriers
(electronseholes). Charge separation occurs through the injection of photogenerated elec-
trons like TiO2 into the electron transport layer, e.g., spiro-MeOTAD. When a large number
of photons are incident on PVSCs, a number of electrons are generated in the perovskite
absorber. Therefore electrons are captured by the electron transport layer from which elec-
trons move externally to the metal electrodes. However, photogenerated holes are extracted
from the perovskite layer and these electrons flow to the metal electrode (such as gold, silver,
or aluminum). The holeeelectron recombines externally to produce electrical potential due to
the absorption of light [25,28]. At zero cell voltage, the cell current reaches its maximum
limiting value and is called a short-circuit current. This depends on the production of charge
FIGURE 9.5 Schematic diagram showing the working principle of perovskite solar cells.
carrier in the perovskite layers and the extraction of charge carriers by the respective charge
carrier layers. This directly affects the efficiency and stability of PVSCs. Thus it is concluded
that the device configuration of PVSCs is a perovskite layer sandwiched between the electron
and hole blocking layer. This configuration leads to stable and efficient photovoltaic devices.
This section deals with specific characterizations and supporting examples from the liter-
ature used for PVSC devices. Also, a brief introduction to general characterization is dis-
cussed, which is used to analyze PVSC devices; detailed information on these general
devices is provided in Chapter 3.
cell. Open-circuit current density is the voltage at which no net current flows through the de-
vice and it depends on the bandgap of the absorber, in this case the bandgap of the perovskite
layer. The power of the solar cell is estimated by combining applied voltage and current pro-
duced in the PVSCs. Maximum power of the devices is the product of the current maximum
power point and voltage maximum power point. One can also estimate the fill factor of the
solar cells; it is the ratio of maximum power of the devices to the product of short-circuit cur-
rent density and open-circuit voltage.
The JeV characteristics of PVSC change with the illumination time of solar cells. The per-
formance generally increases with the increase in illumination time as there is an increase in
fill factor and open-circuit voltage. This effect is termed the light-soaking effect. This effect is
due to the presence of mobile ions (ABþ and Ie) inside the perovskite layer [136,137]. When
the JeV scan is performed from negative to positive voltage and vice versa, PVSCs show a
difference in response time. This phenomenon is termed hysteresis [65,92]. Apart from
scan speed, temperature, and prescanning conditions, hysteresis depends on the movement
of ions in the perovskite layer and the PVSC architecture [92,138e140]. If hysteresis is only
because of movement of ions in the perovskite layer, then hysteresis can be tuned by chang-
ing either the electronehole transport layer or the device architecture [136,141,142].
The inherent hysteresis and soaking effect change the JeV characteristics and therefore
change the PCE values of PVSCs. Therefore it is obviously an essential tool for PVSCs [142].
FIGURE 9.6 (A) An illustration of surface morphology using scanning electron microscopy for perovskite solar
cell (PVSC) devices. (B) Illustration of cross-sectional analysis of a PVSC device. ITO, Indium tin oxide; PCBM, butyric
acid methyl ester; RGO, reduced graphine oxide. Copied with permission. Copyright © 2014 Elsevier.
that in the interface between layers, the crystallinity of absorbing layers and uniformity of
layers directly affect the stability and performance of PVSC devices [3,6,21,55,92].
It is important to study the interface between the layers as well as to discover the unifor-
mity and exact thickness of each layer in the devices. The interface between layers and an es-
timate of the exact thickness of each layer cross-sectionally can be studied using cross-section
SEM images [152,153]. Fig. 9.6B shows the cross-section of a PVSC device. This confirms the
fabrication quality of the device and the uniformity and thickness of each layer [153].
FIGURE 9.7 Standard examples showing particle size analysis and cross-sectional morphology using trans-
mission electron microscopy [156]. ITO, Indium tin oxide.
FIGURE 9.8 Contact mode atomic force microscopy study of the hysteresis of ion relationships for control films
(AeD) and hybrid films (EeH) with IeV characteristics [160]. Copyright © 2015, Springer Nature.
PCE values of PVSCs [15,30,31]. Instead of using a single halide in perovskite materials, a
mixed halide CH3NH3PbI3exClx composition was used. It was observed that PCE values
and stability increased [31,161]. Furthermore, modification of the nanoporous layer, which
replaced conducting material (TiO2) with nonconducting material (Al2O3), improved the
open-circuit voltage of PVSCs, which increased the PCE to 10.90% [161]. Later, it was shown
that the performance of PVSC devices increased for CH3NH3PbI3exBrx mixed halide perov-
skites. The result was that efficiency increased for lower concentrations of Br, and for higher
concentrations perovskite films provided better stability against humidity [162]. Subsequently,
researchers tried to change deposition methods: instead of single-step deposition they used
two-step deposition for perovskite films, which improved their morphology [163]. The effi-
ciency of PVSCs further increased when solvent deposition was replaced by thermal evapora-
tion and CH3NH3PbI3exClx mixed perovskite. The observed efficiency for this was 15.40%
[164]. Further increase in efficiency of PVSC devices was reported for poly(triarylamine) as
a hole transport material and CH3NH3PbI3exBrx mixed halide perovskites [25,163]. There fol-
lowed a further modification in deposition technique, and cations of larger radii were used,
which had a symmetrical cubic phase and increased the t factor. This modification improved
the devices. Researchers also tried different proportions of inorganic cations and halide anions
in mixed perovskite, which allowed their properties to be tuned [66,165,166].
Around 2014, experiments began with electron transport layers. The reported electron
transport material was TiO2, which was replaced by fluorine-doped TiO2 and a TiO2 combi-
nation. Later, this combination was replaced by ITO as transparent material combined with a
thin layer of zinc oxide [115]. Other metal oxides (like ZrO2, NiO2, SiO2, and SnO2) and nano-
core shell nanoparticles (such as Al2O3/ZnO, TiO2/MgO, and WO3/TiO2) were used as elec-
tron transport material and for charge retardation or to avoid recombination of
photogenerated holes and electrons, which enhanced PCE values [127,167e169]. As dis-
cussed in an earlier chapter, materials with a lower highest occupied molecular orbital
band but a higher lowest unoccupied molecular orbital band than the active perovskite layer
can be used as electron extraction or electron transport layer [170]. Researchers also tried to
make metal oxide-free PVSC devices. Organic materials like P3HT, PCBM composites to
achieve better efficiency in p-i-n architecture of the PVSCs device [127,171e173]. It is
observed that when organic material was used as an electron transport layer and a perovskite
layer was deposited onto it, perovskite materials crystallized well to form compact, uniform,
smooth, and pinhole-free film [174]. So, an organic electron transport layer provides flexible
PVSC devices and uniform perovskite films, and increases stability and PCE values of PVSC
devices. The flexible PVSC device even fabricates under ambient air and humidity providing
a PCE value of ~21% and a high open-circuit voltage of 1.15 V [46].
In 2006, dye-sensitized solar cells based on an organicemetal hybrid semiconductor within
a nanoporous TiO2 layer achieved a PCE of 2.2% [25]. Miyasaka suggested the use of
organicemetal perovskite (CH3NH3PbI3) for solar cells [15]. As discussed earlier in this chap-
ter, the crystal structure of perovskite is defined as ABX3; Miyasaka’s group chose A to be the
organic molecule, B as Pbemetal, and X as Brehalide. The observed efficiency value was
~3.8% as shown in Fig. 9.9. In the last decade, researchers have extensively worked on
PVSC; the progress in PCE values is shown in Fig. 9.9, which was plotted by taking values
from published research papers and published solar efficiency tables every year. So far, the
maximum recordable power efficiency achieved for PVSCs is 25.2% [175]. The overall perfor-
mance of PVSCs is shown in Fig. 9.9. It is clear that perovskites are the fastest developing
FIGURE 9.9 Year versus power conversion efficiency for perovskite solar cells. PCE, Power conversion efficiency.
Data taken from research papers and from a solar efficiency table published every year.
solar cell technology. However, more work has to be performed to increase their stability and
bring them to the commercial world.
The PCE values of PVSCs depend on many parameters: quality of perovskite film, electron
transport layer, hole transport layer, metal electrode, and interface between layers. A high-
quality polycrystalline perovskite layer and continuous defect-free junctions with electron
transport layer and hole transport layer are essential to obtain stable and high-
performance solar cells [176]. It is obvious that uniform grains with a minimal area of grain
boundaries provide faster transportation of charge carriers which increases the performance
of PVSCs [177]. The uniform grains can be achieved by using double cation perovskites like
formamidinium/MA, which provide better thermal stability but have high hysteresis in JeV
characterizations. Table 9.2 summarizes notable PVSC devices with their PCE values, cutoff
voltage, and current density.
It is also observed that although perovskite materials have excellent defect-tolerant proper-
ties, the ionic migration in perovskite makes their characterization more difficult. Ionic migra-
tion in perovskite layers also affects the interface between perovskite and the charge transport
layers [176]. This is also one of the reasons for the presence of hysteresis in JeV characteristics.
Later, studies showed that hysteresis decreases if lithium, potassium, or sodium and their bis(-
trifluoromethane) sulfonimide salt are doped to the TiO2 electron transport layer in triple
cation perovskite solar cells; also, PCE values increased [188,189]. Surface analysis showed
that sulfur chemically bridges TiO2 and perovskite at their interface by forming TieSePb
bonding, thereby forming a structural continuity. This decreases grain boundary effects;
hence, the hysteresis effect decreases and the PCE value increases to 21.10% in this case
[190]. Another factor to improve PVSCs is the presence of hysteresis in JeV characteristics.
Based on the discussions, it can be concluded that for superior performance and better sta-
bility of PVSCs (1) high optical absorption coefficient of perovskite thin film, (2) long carrier
diffusion length and suppressed recombination, (3) defect tolerance, and (4) well-balanced
charge transfer are needed.
TABLE 9.2 Notable perovskite solar cell devices with power conversion efficiency, cutoff voltages, and
short-current density.
Power
consumption JSC
Fabricated devices efficiency (%) VOC (V) (mA/cm2) References
Abbreviations: CsPbI3, Cesium lead iodide; Spiro-OmeTAD, 2,20 ,7,70 -Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 0 -spirobifluorene;
Spiro-MeOTAD, 2,20 ,7,70 -Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,90 -spirobifluorene; PEDOT:PSS, poly(3,4-ethyl-
enedioxythiophene) polystyrene sulfonate; P3HT, Poly(3-hexylthiophene-2,5-diyl); PTTA, purified terephthalic acid; Peht, Phospha-
tidylethanol; FTO, Fluorine doped tin oxide; ITO, Indium tin oxide; Perovskite/HIT tandem cell, Perovskite/ Heterojunction
amorphous/Crystalline Silicon tandem cell.
Solar cells are considered to be the best alternative for fossil fuels and other energy sources.
However, traditional solar cells have good PCE but installation and production charges are
quite high. So, PVSCs are ideal alternatives. Although PVSCs are certified with high PCEs
such as 25.20%, they still face problems such as toxicity, long-term stability, and cost effec-
tiveness. As highly efficient PVSCs mostly contain Pb atoms, which are toxic and can cause
serious problems like cancer and mental retardation, it has been a big challenge to make Pb-
free PVSCs with high PCE values and better stability. At the same time, devices should have
long-term stability in terms of temperature and efficiency because Si cell stability is more than
20 years. Moreover, perovskite absorbers and charge transporting layers are sensitive to
moisture and temperature, which may affect the long-term stability of PVSCs. Furthermore,
for commercialization, these solar cells should be cost effective for everyone.
be durable at least for 20 years. However, these perovskites are a better option for relatively
short-life applications, e.g., in the car and electronic industries. For these applications, the re-
quirements are low cost of modules, Pb-free PVSCs, light weight, and stability up to 10 years.
Also, to replace traditional solar cells, PVSCs should be Pb free (to avoid pollution), highly
efficient, thermally stable, and have long-term stability. These can be achieved by modifying
perovskite films so that absorption increases, the quality of the carrier transporting layer is
maintained to increase the extraction of charge carriers and avoid degradation of the perov-
skite layer, and the quality of electrodes is preserved so that external recombination can easily
happen and be protected from the ambient environment.
9.12 Conclusion
It is interesting to note that perovskite materials were invented over a century ago. These
materials and their thin films have witnessed technological applications in the fields of tran-
sistors, light-emitting diodes, spintronics, and superconductivity. Due to their outstanding
optical properties and absorbance in the visible near-infrared region, it has been noticed
that they could be used in solar cell devices. This possibility is enhanced due to high effi-
ciency, low-cost starting materials and an easy fabrication approach. Both inorganic and
organometallic perovskites have proved their acceptance as absorbance layers in solar cells
and could replace traditional solar cell materials [14,127].
The wide range bandgap and high absorption coefficient of perovskite materials makes
them an ideal absorber material for solar cells. Also, the literature shows that perovskites
have high carrier mobilities and long carrier lifetime resulting in long carrier diffusion length.
High diffusion length decreases the possibility of recombination of photogenerated carriers.
These devices can be synthesized effectively by two approaches: n-i-p and p-i-n junction
structures. Another approach (which is not discussed here) is the perovskite tandem
approach, which is proven to have good efficiency and stability. However, some important
issues like hysteresis in JeV characteristics, long-term stability, and toxicity need to be
addressed.
These PVSCs are easy to fabricate and have strong solar absorption and low nonradiative
recombination rate and efficiency, making them a replacement for traditional solar cells.
PVSCs also have high carrier mobility to increase the overall PCE values. However, better
performing PVSCs contain Pb, which is highly toxic. Hence, this raises the toxicity issue dur-
ing the fabrication, development, and disposal of devices. Also, most of the PVSCs undergo
degradation when they are exposed to moisture. Although the efficiency of organometallic
PVSCs is good, the materials are highly unstable in humid environments. It is possible to
replace organometallic perovskites with inorganic perovskites. Still, a lot of research is
required to obtain commercial PVSC devices. However, it is still considered a better candi-
date for traditional solar cells than other semiconductor solar cells.
References
[1] V. Smil, Energy Transitions: Global and National Perspectives, ABC-CLIO, 2016.
[2] S. Sareen, H. Haarstad, Legitimacy and Accountability in the Governance of Sustainable Energy Transitions,
Elsevier, 2020.
[31] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-
superstructured organometal halide perovskites, Science 338 (6107) (2012) 643e647.
[32] G.E. Eperon, V.M. Burlakov, P. Docampo, A. Goriely, H.J. Snaith, Morphological control for high performance,
solution-processed planar heterojunction perovskite solar cells, Adv. Funct. Mater. 24 (1) (2014) 151e157.
[33] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.-S. Lim, J.A. Chang, Y.H. Lee, H.-j. Kim, A. Sarkar,
M.K. Nazeeruddin, Efficient inorganiceorganic hybrid heterojunction solar cells containing perovskite com-
pound and polymeric hole conductors, Nat. Photon. 7 (6) (2013) 486.
[34] M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu, J. Etheridge, A. Gray-Weale, U. Bach, Y.B. Cheng, L. Spiccia,
A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells,
Angew. Chem. Int. Ed. 53 (37) (2014) 9898e9903.
[35] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Compositional engineering of perovskite ma-
terials for high-performance solar cells, Nature 517 (7535) (2015) 476e480.
[36] D. Bi, C. Yi, J. Luo, J.-D. Décoppet, F. Zhang, S.M. Zakeeruddin, X. Li, A. Hagfeldt, M. Grätzel, Polymer-
templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%,
Nat. Energy 1 (10) (2016) 1e5.
[37] W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A.J. Neukirch, G. Gupta, J.J. Crochet, M. Chhowalla, S. Tretiak,
M.A. Alam, High-efficiency solution-processed perovskite solar cells with millimeter-scale grains, Science
347 (6221) (2015) 522e525.
[38] H. Tsai, W. Nie, J.-C. Blancon, C.C. Stoumpos, R. Asadpour, B. Harutyunyan, A.J. Neukirch, R. Verduzco,
J.J. Crochet, S. Tretiak, High-efficiency two-dimensional RuddlesdenePopper perovskite solar cells, Nature
536 (7616) (2016) 312e316.
[39] Z. Wang, Q. Lin, F.P. Chmiel, N. Sakai, L.M. Herz, H.J. Snaith, Efficient ambient-air-stable solar cells with
2De3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites, Nat. Energy 2
(9) (2017) 17135.
[40] G. Grancini, C. Roldán-Carmona, I. Zimmermann, E. Mosconi, X. Lee, D. Martineau, S. Narbey, F. Oswald,
F. De Angelis, M. Graetzel, One-Year stable perovskite solar cells by 2D/3D interface engineering, Nat. Com-
mun. 8 (1) (2017) 1e8.
[41] Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan, J. Huang, Solvent annealing of perovskite-induced crystal growth for
photovoltaic-device efficiency enhancement, Adv. Mater. 26 (37) (2014) 6503e6509.
[42] Y. Wang, S. Li, P. Zhang, D. Liu, X. Gu, H. Sarvari, Z. Ye, J. Wu, Z. Wang, Z.D. Chen, Solvent annealing of PbI 2
for the high-quality crystallization of perovskite films for solar cells with efficiencies exceeding 18%, Nanoscale
8 (47) (2016) 19654e19661.
[43] C. Momblona, L. Gil-Escrig, E. Bandiello, E.M. Hutter, M. Sessolo, K. Lederer, J. Blochwitz-Nimoth, H.J. Bolink,
Efficient vacuum deposited pin and nip perovskite solar cells employing doped charge transport layers, Energy
Environ. Sci. 9 (11) (2016) 3456e3463.
[44] C.Y. Chen, H.Y. Lin, K.M. Chiang, W.L. Tsai, Y.C. Huang, C.S. Tsao, H.W. Lin, All-vacuum-deposited stoichio-
metrically balanced inorganic cesium lead halide perovskite solar cells with stabilized efficiency exceeding 11%,
Adv. Mater. 29 (12) (2017) 1605290.
[45] F. Fu, L. Kranz, S. Yoon, J. Löckinger, T. Jäger, J. Perrenoud, T. Feurer, C. Gretener, S. Buecheler, A.N. Tiwari,
Controlled growth of PbI2 nanoplates for rapid preparation of CH3NH3PbI3 in planar perovskite solar cells,
Phys. Status Solidi 212 (12) (2015) 2708e2717.
[46] T. Singh, T. Miyasaka, Stabilizing the efficiency beyond 20% with a mixed cation perovskite solar cell fabricated
in ambient air under controlled humidity, Adv. Energy Mater. 8 (3) (2018) 1700677.
[47] B.R. Sutherland, E.H. Sargent, Perovskite photonic sources, Nat. Photon. 10 (5) (2016) 295.
[48] J. Hou, X. Yin, Y. Fang, F. Huang, W. Jiang, Novel red-emitting perovskite-type phosphor CaLa1 xMgM' O6:
xEu3þ (M0 ¼ Nb, Ta) for white LED application, Opt. Mater. 34 (8) (2012) 1394e1397.
[49] T. He, Q. Huang, A. Ramirez, Y. Wang, K. Regan, N. Rogado, M. Hayward, M. Haas, J. Slusky, K. Inumara,
Superconductivity in the non-oxide perovskite MgCNi 3, Nature 411 (6833) (2001) 54e56.
[50] D.B. Mitzi, C.D. Dimitrakopoulos, L.L. Kosbar, Structurally tailored organic inorganic perovskites: optical
properties and solution-processed channel materials for thin-film transistors, Chem. Mater. 13 (10) (2001)
3728e3740.
[51] Y. Wu, J. Li, J. Xu, Y. Du, L. Huang, J. Ni, H. Cai, J. Zhang, Organiceinorganic hybrid CH 3 NH 3 PbI 3 perov-
skite materials as channels in thin-film field-effect transistors, RSC Adv. 6 (20) (2016) 16243e16249.
[52] J.A. Christians, A.R. Marshall, Q. Zhao, P. Ndione, E.M. Sanehira, J.M. Luther, In perovskite quantum dots. A
new absorber for perovskite-perovskite tandem solar cells, in: 2018 IEEE 7th World Conference on Photovoltaic
[74] W. Li, J. Li, J. Li, J. Fan, Y. Mai, L. Wang, Addictive-assisted construction of all-inorganic CsSnIBr 2 mesoscopic
perovskite solar cells with superior thermal stability up to 473 K, J. Mater. Chem. 4 (43) (2016) 17104e17110.
[75] L.-J. Chen, C.-R. Lee, Y.-J. Chuang, Z.-H. Wu, C. Chen, Synthesis and optical properties of lead-free cesium tin
halide perovskite quantum rods with high-performance solar cell application, J. Phys. Chem. Lett. 7 (24) (2016)
5028e5035.
[76] A.K. Jena, A. Kulkarni, T. Miyasaka, Halide perovskite photovoltaics: background, status, and future prospects,
Chem. Rev. 119 (5) (2019) 3036e3103.
[77] M.-G. Ju, J. Dai, L. Ma, X.C. Zeng, Lead-free mixed tin and germanium perovskites for photovoltaic application,
J. Am. Chem. Soc. 139 (23) (2017) 8038e8043.
[78] M.I.H. Ansari, A. Qurashi, M.K. Nazeeruddin, Frontiers, opportunities, and challenges in perovskite solar cells:
a critical review, J. Photochem. Photobiol. C Photochem. Rev. 35 (2018) 1e24.
[79] C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic
cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (15)
(2013) 9019e9038.
[80] N. Pellet, P. Gao, G. Gregori, T.Y. Yang, M.K. Nazeeruddin, J. Maier, M. Grätzel, Mixed-organic-cation Perov-
skite photovoltaics for enhanced solar-light harvesting, Angew. Chem. Int. Ed. 53 (12) (2014) 3151e3157.
[81] Z. Li, M. Yang, J.-S. Park, S.-H. Wei, J.J. Berry, K. Zhu, Stabilizing perovskite structures by tuning tolerance
factor: formation of formamidinium and cesium lead iodide solid-state alloys, Chem. Mater. 28 (1) (2016)
284e292.
[82] L.N. Quan, M. Yuan, R. Comin, O. Voznyy, E.M. Beauregard, S. Hoogland, A. Buin, A.R. Kirmani, K. Zhao,
A. Amassian, Ligand-stabilized reduced-dimensionality perovskites, J. Am. Chem. Soc. 138 (8) (2016) 2649e2655.
[83] F. Hao, C.C. Stoumpos, R.P. Chang, M.G. Kanatzidis, Anomalous band gap behavior in mixed Sn and Pb pe-
rovskites enables broadening of absorption spectrum in solar cells, J. Am. Chem. Soc. 136 (22) (2014)
8094e8099.
[84] E. Mosconi, A. Amat, M.K. Nazeeruddin, M. Grätzel, F. De Angelis, First-principles modeling of mixed halide
organometal perovskites for photovoltaic applications, J. Phys. Chem. C 117 (27) (2013) 13902e13913.
[85] T.J. Jacobsson, M. Pazoki, A. Hagfeldt, T. Edvinsson, Goldschmidt's rules and strontium replacement in lead
halogen perovskite solar cells: theory and preliminary experiments on CH3NH3SrI3, J. Phys. Chem. C 119
(46) (2015) 25673e25683.
[86] J. Ali, Y. Li, P. Gao, T. Hao, J. Song, Q. Zhang, L. Zhu, J. Wang, W. Feng, H. Hu, Interfacial and structural mod-
ifications in perovskite solar cells, Nanoscale 12 (10) (2020) 5719e5745.
[87] A. Karavioti, E. Vitoratos, E. Stathatos, Improved performance and stability of hole-conductor-free mesoporous
perovskite solar cell with new amino-acid iodide cations, J. Mater. Sci. Mater. Electron. (2020) 1e9.
[88] Y. Xiang, J. Zhuang, Z. Ma, H. Lu, H. Xia, W. Zhou, T. Zhang, H. Li, Mixed-phase mesoporous TiO 2 film for
high efficiency perovskite solar cells, Chem. Res. Chin. Univ. 35 (1) (2019) 101e108.
[89] J.J. Choi, X. Yang, Z.M. Norman, S.J. Billinge, J.S. Owen, Structure of methylammonium lead iodide within
mesoporous titanium dioxide: active material in high-performance perovskite solar cells, Nano Lett. 14 (1)
(2014) 127e133.
[90] T. Leijtens, G.E. Eperon, S. Pathak, A. Abate, M.M. Lee, H.J. Snaith, Overcoming ultraviolet light instability of
sensitized TiO 2 with meso-superstructured organometal tri-halide perovskite solar cells, Nat. Commun. 4 (1)
(2013) 1e8.
[91] T. Leijtens, B. Lauber, G.E. Eperon, S.D. Stranks, H.J. Snaith, The importance of perovskite pore filling in orga-
nometal mixed halide sensitized TiO2-based solar cells, J. Phys. Chem. Lett. 5 (7) (2014) 1096e1102.
[92] H.J. Snaith, A. Abate, J.M. Ball, G.E. Eperon, T. Leijtens, N.K. Noel, S.D. Stranks, J.T.-W. Wang,
K. Wojciechowski, W. Zhang, Anomalous hysteresis in perovskite solar cells, J. Phys. Chem. Lett. 5 (9)
(2014) 1511e1515.
[93] M. Kim, G.-H. Kim, T.K. Lee, I.W. Choi, H.W. Choi, Y. Jo, Y.J. Yoon, J.W. Kim, J. Lee, D. Huh, H. Lee,
S.K. Kwak, J.Y. Kim, D.S. Kim, Methylammonium chloride induces intermediate phase stabilization for effi-
cient perovskite solar cells, Joule 3 (9) (2019) 2179e2192.
[94] H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Interface engineering
of highly efficient perovskite solar cells, Science 345 (6196) (2014) 542e546.
[95] N.J. Jeon, H. Na, E.H. Jung, T.-Y. Yang, Y.G. Lee, G. Kim, H.-W. Shin, S.I. Seok, J. Lee, J. Seo, A fluorene-
terminated hole-transporting material for highly efficient and stable perovskite solar cells, Nat. Energy 3 (8)
(2018) 682e689.
[118] W.S. Yang, B.-W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, Iodide
management in formamidinium-lead-halideebased perovskite layers for efficient solar cells, Science 356
(6345) (2017) 1376e1379.
[119] B. Philippe, B.-W. Park, R. Lindblad, J. Oscarsson, S. Ahmadi, E.M. Johansson, H.k. Rensmo, Chemical and elec-
tronic structure characterization of lead halide perovskites and stability behavior under different exposures. A
photoelectron spectroscopy investigation, Chem. Mater. 27 (5) (2015) 1720e1731.
[120] Y. Rong, L. Liu, A. Mei, X. Li, H. Han, Beyond efficiency: the challenge of stability in mesoscopic perovskite
solar cells, Adv. Energy Mater. 5 (20) (2015) 1501066.
[121] T. Supasai, N. Rujisamphan, K. Ullrich, A. Chemseddine, T. Dittrich, Formation of a passivating CH3NH3PbI3/
PbI2 interface during moderate heating of CH3NH3PbI3 layers, Appl. Phys. Lett. 103 (18) (2013) 183906.
[122] A. Krishna, A.C. Grimsdale, Hole transporting materials for mesoscopic perovskite solar cellsetowards a
rational design? J. Mater. Chem. 5 (32) (2017) 16446e16466.
[123] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N.-G. Park, 6.5% efficient perovskite quantum-dot-sensitized solar
cell, Nanoscale 3 (10) (2011) 4088e4093.
[124] S. Shao, J. Liu, J. Bergqvist, S. Shi, C. Veit, U. Würfel, Z. Xie, F. Zhang, In situ formation of MoO3 in PEDOT: PSS
matrix: a facile way to produce a smooth and less hygroscopic hole transport layer for highly stable polymer
bulk heterojunction solar cells, Adv. Energy Mater. 3 (3) (2013) 349e355.
[125] F. Hou, Z. Su, F. Jin, X. Yan, L. Wang, H. Zhao, J. Zhu, B. Chu, W. Li, Efficient and stable planar heterojunc-
tion perovskite solar cells with an MoO 3/PEDOT: PSS hole transporting layer, Nanoscale 7 (21) (2015)
9427e9432.
[126] S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T.C. Sum, Y.M. Lam, The origin of high
efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells, Energy Envi-
ron. Sci. 7 (1) (2014) 399e407.
[127] K. Mahmood, S. Sarwar, M. Mehran, Current status of electron transport layers in perovskite solar cells: ma-
terials and properties, RSC Adv. 7 (28) (2017) 17044e17062.
[128] H. Back, G. Kim, J. Kim, J. Kong, T.K. Kim, H. Kang, H. Kim, J. Lee, S. Lee, K. Lee, Achieving long-term stable
perovskite solar cells via ion neutralization, Energy Environ. Sci. 9 (4) (2016) 1258e1263.
[129] Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J.M. Pringle, U. Bach, L. Spiccia, Y.-B. Cheng, Degradation observations
of encapsulated planar CH 3 NH 3 PbI 3 perovskite solar cells at high temperatures and humidity, J. Mater.
Chem. 3 (15) (2015) 8139e8147.
[130] T.Y. Yang, G. Gregori, N. Pellet, M. Grätzel, J. Maier, The significance of ion conduction in a hybrid organice
inorganic lead-iodide-based perovskite photosensitizer, Angew. Chem. Int. Ed. 54 (27) (2015) 7905e7910.
[131] J.T. Tisdale, E. Muckley, M. Ahmadi, T. Smith, C. Seal, E. Lukosi, I.N. Ivanov, B. Hu, Dynamic impact of elec-
trode materials on interface of single-crystalline methylammonium lead bromide perovskite, Adv. Mater. Inter-
face 5 (18) (2018) 1800476.
[132] H. Wei, Y. Fang, P. Mulligan, W. Chuirazzi, H.-H. Fang, C. Wang, B.R. Ecker, Y. Gao, M.A. Loi, L. Cao, Sen-
sitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals, Nat. Photon. 10 (5)
(2016) 333.
[133] A. Guerrero, J. You, C. Aranda, Y.S. Kang, G. Garcia-Belmonte, H. Zhou, J. Bisquert, Y. Yang, Interfacial degra-
dation of planar lead halide perovskite solar cells, ACS Nano 10 (1) (2016) 218e224.
[134] K. Domanski, J.-P. Correa-Baena, N. Mine, M.K. Nazeeruddin, A. Abate, M. Saliba, W. Tress, A. Hagfeldt,
M. Grätzel, Not all that glitters is gold: metal-migration-induced degradation in perovskite solar cells, ACS
Nano 10 (6) (2016) 6306e6314.
[135] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, A hole-conductorefree, fully print-
able mesoscopic perovskite solar cell with high stability, Science 345 (6194) (2014) 295e298.
[136] Y. Yuan, Q. Wang, Y. Shao, H. Lu, T. Li, A. Gruverman, J. Huang, Electric-field-driven reversible conversion
between methylammonium lead triiodide perovskites and lead iodide at elevated temperatures, Adv. Energy
Mater. 6 (2) (2016) 1501803.
[137] T. Zhang, S.H. Cheung, X. Meng, L. Zhu, Y. Bai, C.H.Y. Ho, S. Xiao, Q. Xue, S.K. So, S. Yang, Pinning down the
anomalous light soaking effect toward high-performance and fast-response perovskite solar cells: the ion-
migration-induced charge accumulation, J. Phys. Chem. Lett. 8 (20) (2017) 5069e5076.
[138] H.-S. Kim, N.-G. Park, Parameters affecting IeV hysteresis of CH3NH3PbI3 perovskite solar cells: effects of
perovskite crystal size and mesoporous TiO2 layer, J. Phys. Chem. Lett. 5 (17) (2014) 2927e2934.
[161] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza,
H.J. Snaith, Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite
absorber, Science 342 (6156) (2013) 341e344.
[162] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for colorful, efficient, and stable
inorganiceorganic hybrid nanostructured solar cells, Nano Lett. 13 (4) (2013) 1764e1769.
[163] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Grätzel, Sequential depo-
sition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (7458) (2013) 316e319.
[164] M. Liu, M.B. Johnston, H.J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition,
Nature 501 (7467) (2013) 395e398.
[165] J.-H. Im, J. Chung, S.-J. Kim, N.-G. Park, Synthesis, structure, and photovoltaic property of a nanocrystalline 2H
perovskite-type novel sensitizer (CH 3 CH 2 NH 3) PbI 3, Nanoscale Res. Lett. 7 (1) (2012) 353.
[166] S. Pang, H. Hu, J. Zhang, S. Lv, Y. Yu, F. Wei, T. Qin, H. Xu, Z. Liu, G. Cui, NH2CH NH2PbI3: an alternative
organolead iodide perovskite sensitizer for mesoscopic solar cells, Chem. Mater. 26 (3) (2014) 1485e1491.
[167] K. Mahmood, B.S. Swain, A.R. Kirmani, A. Amassian, Highly efficient perovskite solar cells based on a nano-
structured WO 3eTiO 2 coreeshell electron transporting material, J. Mater. Chem. 3 (17) (2015) 9051e9057.
[168] A.K. Chandiran, M. Abdi-Jalebi, A. Yella, M.I. Dar, C. Yi, S.A. Shivashankar, M.K. Nazeeruddin, M. Grätzel,
Quantum-confined ZnO nanoshell photoanodes for mesoscopic solar cells, Nano Lett. 14 (3) (2014) 1190e1195.
[169] G.S. Han, H.S. Chung, B.J. Kim, D.H. Kim, J.W. Lee, B.S. Swain, K. Mahmood, J.S. Yoo, N.-G. Park, J.H. Lee,
Retarding charge recombination in perovskite solar cells using ultrathin MgO-coated TiO 2 nanoparticulate
films, J. Mater. Chem. 3 (17) (2015) 9160e9164.
[170] L. Wang, W. Fu, Z. Gu, C. Fan, X. Yang, H. Li, H. Chen, Low temperature solution processed planar hetero-
junction perovskite solar cells with a CdSe nanocrystal as an electron transport/extraction layer, J. Mater.
Chem. C 2 (43) (2014) 9087e9090.
[171] Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan, J. Huang, Large fill-factor bilayer iodine perovskite solar cells
fabricated by a low-temperature solution-process, Energy Environ. Sci. 7 (7) (2014) 2359e2365.
[172] L.-C. Chen, J.-C. Chen, C.-C. Chen, C.-G. Wu, Fabrication and properties of high-efficiency perovskite/PCBM
organic solar cells, Nanoscale Res. Lett. 10 (1) (2015) 312.
[173] C. Li, F. Wang, J. Xu, J. Yao, B. Zhang, C. Zhang, M. Xiao, S. Dai, Y. Li, Z.a. Tan, Efficient perovskite/fullerene
planar heterojunction solar cells with enhanced charge extraction and suppressed charge recombination, Nano-
scale 7 (21) (2015) 9771e9778.
[174] W. Ke, G. Fang, J. Wan, H. Tao, Q. Liu, L. Xiong, P. Qin, J. Wang, H. Lei, G. Yang, Efficient hole-blocking layer-
free planar halide perovskite thin-film solar cells, Nat. Commun. 6 (1) (2015) 1e7.
[175] M.A. Green, E.D. Dunlop, J. Hohl-Ebinger, M. Yoshita, N. Kopidakis, A.W. Ho-Baillie, Solar cell efficiency ta-
bles (version 55), Prog. Photovoltaics Res. Appl. 28 (2019) (NREL/JA-5900-75827).
[176] T. Miyasaka, Lead halide perovskites in thin film photovoltaics: background and perspectives, Bull. Chem. Soc.
Jpn. 91 (7) (2018) 1058e1068.
[177] H.D. Kim, H. Ohkita, Potential improvement in fill factor of lead-halide perovskite solar cells (solar RRL 7∕
2017), Solar RRL 1 (6) (2017) 1770121.
[178] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, High-performance photovoltaic perovskite
layers fabricated through intramolecular exchange, Science 348 (6240) (2015) 1234e1237.
[179] Q. Jiang, Z. Chu, P. Wang, X. Yang, H. Liu, Y. Wang, Z. Yin, J. Wu, X. Zhang, J. You, Planar-structure perov-
skite solar cells with efficiency beyond 21%, Adv. Mater. 29 (46) (2017) 1703852.
[180] K.A. Bush, A.F. Palmstrom, J.Y. Zhengshan, M. Boccard, R. Cheacharoen, J.P. Mailoa, D.P. McMeekin,
R.L. Hoye, C.D. Bailie, T. Leijtens, 23.6%-efficient monolithic perovskite/silicon tandem solar cells with
improved stability, Nat. Energy 2 (4) (2017) 1e7.
[181] W. Zhu, C. Bao, F. Li, T. Yu, H. Gao, Y. Yi, J. Yang, G. Fu, X. Zhou, Z. Zou, A halide exchange engineering for
CH3NH3PbI3 xBrx perovskite solar cells with high performance and stability, Nano Energy 19 (2016) 17e26.
[182] J. You, L. Meng, T.-B. Song, T.-F. Guo, Y.M. Yang, W.-H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen,
Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nano-
technol. 11 (1) (2016) 75.
[183] G. Niu, W. Li, F. Meng, L. Wang, H. Dong, Y. Qiu, Study on the stability of CH 3 NH 3 PbI 3 films and the effect
of post-modification by aluminum oxide in all-solid-state hybrid solar cells, J. Mater. Chem. 2 (3) (2014)
705e710.