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Sensitizer molecular structure-device efficiency relationship

in dye sensitized solar cells
John N. Clifford,* Eugenia Martı́nez-Ferrero, Aurélien Viterisi and
Emilio Palomares*
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Received 4th June 2010

DOI: 10.1039/b920664g

In the Dye Sensitized Solar Cell (DSSC) the dye sensitizer carries out the light harvesting function
and is therefore crucial in determining overall cell efficiency. In addition, the dye sensitizer can
influence many of the key electron transfer processes occurring at the TiO2/dye/electrolyte
interface which also determine efficiency. Dye structure can influence and drive forward electron
injection into the conduction band of the TiO2. Conversely, dye structure can help retard loss
electron transfer processes such as charge recombination of injected electrons in the TiO2 with dye
cations and also recombination of these electrons with the electrolyte. Therefore tuning dye
sensitizer light absorbing properties and control of the aforementioned electron transfer processes
through structural design of the dye sensitizer is an important avenue through which optimization
of DSSC efficiency should be pursued. In this critical review the latest work focusing on the design
of dyes for efficient DSSCs is revised (111 references).

1. Introduction economic and political stability. The fact that these resources
are still largely based on non-renewable fossil fuels such as oil
Modern society is heavily dependent upon energy resources and gas and growing public concern over the environmental
and their continued supply is crucial towards long term global damage that the use of such materials may be causing has
encouraged the development and exploitation of renewable
forms of energy. The abundance of solar energy which bathes
ICREA and Institute of Chemical Research of Catalonia, Av. Paı¨sos
Catalans 16, 43007 Tarragona, Spain. E-mail: epalomares@iciq.es, the earths surface in 120 000 terawatts of energy makes it
jnclifford@iciq.es; Fax: +34 977 920 224; Tel: +34 977 920 200 extremely attractive for harnessing as a renewable energy

Dr John N. Clifford did his PhD at Imperial College, London (UK)

under the supervision of Prof. James R. Durrant working on charge
transfer reactions in Dye Sensitized Solar Cells. After several post
doctoral positions in Italy (Dr Nicola Armaroli, ISOF, CNR,
Bologna) and Belgium (Prof. Johan Hofkens, KU Leuven) he
joined the group of Prof. Palomares at ICIQ as a Juan de la Cierva
Fellow. His current research interests include the control of the
charge transfer reactions in ‘‘molecular dye cocktails’’ on
mesoporous TiO2 to achieve efficient panchromatic response
in DSSCs.
Dr Eugenia Martı´nez-Ferrero obtained her PhD at the University
of Valencia (Spain) under the supervision of Prof. Eugenio
Coronado working on molecular materials with optical, electrical
From Left to Right: John N. Clifford, Eugenia Martı́nez-Ferrero, and magnetic properties. After a postdoctoral stay in France in the
Aurélien Viterisi, Emilio Palomares group of Prof. Clement Sánchez (UPMC, Paris) she joined the
group of Prof. Palomares at ICIQ as a Juan de la Cierva Fellow.
Her work at ICIQ has focused on optimizing the properties of several semiconductor metal oxides to enhance DSSC performance as
well as leading the work on hybrid light emitting devices (HYLEDs) in the group.
Dr Aure´lien Viterisi did his PhD at the University of Edinburgh (UK) under the supervision of Prof. David A. Leigh working on
synthetic organic and supramolecular chemistry. Aure´lien joined the group of Prof. Palomares as a post doctoral fellow under the
FP7-EU project ROBUST and his work is focused on the synthesis of molecular sensitizers and the study of supramolecular assemblies
to achieve panchromatic sensitization in DSSCs.
Dr Emilio Palomares is ICREA Professor at ICIQ. Emilio did his PhD at the Universidad Polite´cnica de Valencia under the
supervision of Prof. Hermengildo Garcı´a. He obtained a Marie Curie Post Doctoral Fellowship to work with Prof. James R. Durrant at
Imperial College London on charge transfer reactions in DSSCs. He returned to Spain to work as a Ramon y Cajal fellow at the
Instituto de Ciencia Molecular-Universidad de Valencia and after a year he moved as an independent researcher to ICIQ. He has been
recently awarded with an ERCstg fellowship (Polydot) and holds several awards including the Young Spanish Chemist Award by the
Spanish Royal Society of Chemistry.

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source. Indeed, for many this is the holy grail of renewables.

Single crystal silicon solar cells are currently the paradigm in
solar cell technology achieving over 25% conversion efficiency
in the best modules. However these cells are still too expensive
for large scale production, even taking into account the high
price of oil currently on the world market.
Dye Sensitized Solar Cell (DSSC) are currently the leading
photovoltaic alternative to silicon based solar cell technology.
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They are attractive not only because of the possibility of

harnessing solar energy providing a clean and abundant
energy source but also because in large scale production their
fabrication cost would be expected to be a fraction of that of
the silicon solar cell. DSSCs consist of a porous
nanocrystalline film of typically TiO2 nanoparticles (B20 nm
diameter) immobilized onto a conducting material which is
sensitized to visible light through the adsorption of a
monolayer of dye. The cell is completed by a liquid
electrolyte, typically the iodine/tri-iodide red–ox couple in
acetonitrile, and a platinum counter electrode. Fig. 1 Schematic representation of the electron transfer processes
occurring in DSSCs.
A key difference between DSSCs and silicon solar cells is
that light absorption and electron/hole transport is not
performed by the same material. In DSSCs the light liquids2 and solid state hole conducting materials.3,4 This
absorption function is fulfilled by the dye and the electron Critical Review, however, will focus principally on how
and hole transporting are fulfilled by the nanocrystalline metal optimization of DSSC performance can be achieved through
oxide and electrolyte respectively. Therefore the absorption structural design of the dye sensitizer alone. We discuss how
properties of the dye dictate the light-harvesting capacity of the design can impact upon the electron transfer processes
cell. Moreover the dye will dictate the colour of the cell, which occurring at the TiO2/dye/electrolyte interface shown in
makes this technology architecturally interesting for Fig. 1, namely electron injection, charge recombination and
incorporation into homes and buildings thereby performing the dark current. We also discuss strategies to improve DSSC
an aesthetic as well as practical function. The properties that a light-harvesting by the design of single sensitizers with broad
dye sensitizer should have are the following: absorption profiles and by sensitization with more than one
-Anchoring groups such as carboxylates or phosphonates dye (co-sensitization). Finally, we also highlight some
which are capable of covalently bonding to –OH groups on the examples where supramolecular principles have been utilized
TiO2 surface. in the design of new sensitizers.
-Optimum absorption overlap with that of the solar
spectrum for efficient light-harvesting. 2. Sensitizer control of electron transfer in DSSCs
-Correct alignment of LUMO and HOMO energy levels
with those of the TiO2 conduction band and the iodide tri- There are several excellent reviews covering the large body of
iodide red–ox electrolyte ensuring efficient electron injection experimental work on electron transfer reactions in DSSCs and
and dye regeneration. the mechanisms underpinning them.5–9 In this section these
-Capability of performing photocurrent generation over reactions are discussed, in particular how they can be tuned
prolonged periods of illumination. through molecular design of the dye sensitizer. Several studies
Fig. 1 shows the electron transfer processes occurring in a are highlighted involving most of the commonly employed
DSSC under operating conditions. Following the absorption sensitizer classes used in DSSC research today.
of a photon of energy (1) the dye sensitizer injects an electron 2.1 Marcus theory applied to DSSCs
into the conduction band of the TiO2 from the S* excited state
(2). This electron percolates through the film to the back In the study of electron transfer dynamics in DSSCs, Marcus
contact where it travels through an external circuit non-adiabatic electron-transfer theory10 can be used. A
performing work (3). Oxidized dye cations S+ are reduced quantum mechanical description of electron transfer is
by the red–ox couple (4) which is itself reduced at the platinum applied in terms of electron tunnelling from the donor to the
counter electrode (5). The possible loss mechanisms in the cell acceptor through an insulating barrier. The penetration of the
which are critical to cell performance are charge recombination electron through the barrier will fall off exponentially with
of injected electrons with the oxidized dye S+ (a) and distance r:
recombination of injected electrons with the oxidized
HAB2 = H02ebr (1)
electrolyte (b), or ‘dark current’.
2
Optimization of DSSC efficiency is being addressed in a where HAB represents the electronic coupling of the donor and
variety of ways, for example through the design of different acceptor and b is the resistance of the barrier to the penetration
metal oxide materials with different nanostructures,1 or by of the electron wavefunction. b is a property of the medium
the development of different electrolytes including ionic comprising the insulating barrier region and related to the

1636 Chem. Soc. Rev., 2011, 40, 1635–1646 This journal is c The Royal Society of Chemistry 2011
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height of this barrier. The rate of electron transfer (ket)

incorporating this tunnelling description is given by Fermi’s
Golden Rule:
!
2 ebr ðDG0 þ lÞ2
ket ¼ HAB FC ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp ð2Þ
ð4plkB TÞ 4lkB T

where FC represents the Frank–Condon factor which

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expresses the energetics of the donor and acceptor states.

Though Marcus theory was developed to explain the rates of
electron transfer in non-covalently bound donor–acceptor
complexes in solution, it can also be applied to interfacial
electron transfer involving molecular species immobilized on
bulk semiconductor/metallic surfaces.11,12 In this case it is Fig. 2 Chemical structure and HOMO/LUMO frontier molecular
necessary to integrate eqn (2) over all possible donor or orbitals of organic dye JK1.
acceptor states on the electrode surface.
In eqn (2) there are four possible parameters which can vary HOMO is delocalized over the bis-dimethyl-fluorene aniline
(fixing T equal to room temperature) that can affect electron ligand whereas the LUMO is situated on the opposite end of
transfer: b, l, r and DG0. The first two, the barrier height b and the molecule with significant electron density over the –COOH
the reorganizational energy l, are not easily controllable anchoring unit indicating that photo-excitation of this
parameters. b is a function of the intervening media between sensitizer will result in migration of electron density towards
the donor and acceptor and l is largely dependent upon the the electrode surface.
polarity of the surrounding solvent environment. The second While Ru(II) bipyridyl and organic sensitizers of general type
two parameters however, r and DG0, the donor–acceptor donor–(p-bridge)–acceptor (see section 3) can effectively
distance and the difference in their energies are much more channel excited state energy towards the TiO2 surface, such
amenable to change through modifications to dye structure. directionality in the excited state for symmetrical porphyrin
The effect of these parameters on several of the electron and phthalocyanine sensitizers is problematic. This is because
transfer reactions in DSSCs is discussed in the following the HOMO and LUMO of these highly symmetrical sensitizers
sections. (e.g. ZnTPP22, Fig. 3) are effectively delocalized over the entire
conjugated p-ring system. Directionality of excited state energy
2.2 Electron injection
and improved electronic coupling between these sensitizers and
Many early studies dedicated to understanding electron TiO2 can be achieved with unsymmetrical structures such as
transfer in DSSCs were focused on electron injection from DPA–ZnP–COOH23 (Fig. 3) which when utilized in DSSCs
the photo-excited dye sensitizer into the conduction band of shows an excellent efficiency of 6% with improved Jsc. In fact
the nanocrystalline metal oxide. These and later studies the structure of DPA–ZnP–COOH rather reminds one of the
showed ultrafast electron injection kinetics occurring on donor–(p-bridge)–acceptor structural motif very common for
subpicosecond time scales for a variety of sensitizer dyes organic sensitizers today.24 In the case of DPA–ZnP–COOH
several orders of magnitude faster than emission decay one can consider the p-bridge to be in fact the porphyrin ring
lifetimes resulting in extremely efficient charge itself. Unsymmetrical phthalocyanine sensitizers have also
separation.13–20 Moreover, the kinetics of electron injection shown higher efficiencies and Jsc in DSSCs than their
were found to be non-exponential in nature, which is attributed symmetrical counterparts.25
to a variety of factors including surface heterogeneity of the The dependence of the rate of electron injection upon the
metal oxide, different sensitizer anchoring modes and injection distance and free energy parameters in agreement with Marcus
from a variety of different excited states (i.e. singlet, triplet non-adiabatic electron-transfer theory as discussed in
etc.). Ultrafast injection is often explained in terms of the section 2.1 has been investigated. Variation of the distance r
strong electronic overlap of the LUMO orbitals of the dye and between the dye excited state and the metal oxide surface was
the acceptor states on the metal oxide electrode. In N3/N719
for example, the HOMO is located mainly on the ruthenium
metal and –NCS ligands whereas the LUMO is located on the
p* orbitals of the bipyridyl ligands. As these bipyridyl ligands
are directly anchored to the electrode surface through the
–COOH anchoring groups, photo-excitation results in a shift
in excited state electronic density towards the metal oxide
surface. The most efficient organic sensitizers also display
this directional shift in excited state electron density towards
the anchoring ligands and therefore good electronic overlap
with the acceptor states of the metal oxide surface. Fig. 2 shows
the chemical structure, optimized molecular structure and Fig. 3 Chemical structure and HOMO/LUMO frontier molecular
frontier molecular orbitals for the organic dye JK1.21 The orbitals of ZnTPP and DPA–ZnP–COOH. Adapted from ref. 22 and 23.

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achieved through the use of spacer groups. Lian and co-

workers26,27 varied this distance by employing –CH2– spacers
for a series of three sensitizers Re(bpy(CH2)2n(COOH)2)CO3Cl
(n = 1–3) on nanocrystalline SnO2 films.
Re(bpy(CH2)2(COOH)2)CO3Cl (i.e. where n = 1) showed
ultrafast (o100 fs) electron injection whereas slower
injection times were found with the other two sensitizers (19
and 240 ps where n = 2 and 3 respectively) showing a clear
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dependency of electron injection upon distance. (Interestingly,

this study and others28–32 demonstrated that intimate coupling
of the LUMO of the sensitizer and metal oxide surface is not a
necessary prerequisite for electron injection to occur). Probing
the effect of free energy DG0 upon electron injection can be
done by either modulating the Fermi level of the metal oxide or
by changing the red–ox potential of the dye sensitizer anchored
to it. The former was done by Durrant and co-workers33 by
incorporating an N3 sensitized TiO2 electrode into a three
electrode cell and applying a bias and the latter as part of the
study discussed above by Lian and co-workers26 by using a
series of different Ru(dcbpy)2X2 dyes where (X2 = 2SCN,
2CN and dcbpy) with different ground and excited state
red–ox potentials immobilized onto SnO2. In both studies
the rates of electron injection were found to be dependent
upon free energy differences.
Large p-conjugated sensitizers such as porphyrins and
phthalocyanines and organic sensitizers such as coumarines
and perylenes have a marked tendency towards aggregation in Fig. 4 Chemical structure (top) of porphyrins TCPP1 and TCPP2
solution and on the surface of TiO2 and this is one of the reasons and recombination kinetics (bottom) of these dyes on nanocrystalline
TiO2 film. Adapted from ref. 42.
why they have generally shown lower efficiencies in DSSCs.
Aggregation can quench dye excited states resulting in lower
injection yields.34–36 One way to avoid aggregation is to sensitize sensitizer structure, increasing the distance between the dye
in solutions containing coadsorbates such as chenodeoxycholic cation centre and the TiO2 surface has been shown to effect the
acid37,38 which can help break up dye aggregates on the TiO2 rate of charge recombination for a number of sensitizers.42–44
surface leading to improved cell efficiencies. However, even in For example, Durrant42 and co-workers modified the
the presence of these adsorbates aggregation can still be a porphyrin TCPP1 by substituting three of the –COOH
problem. In this case the need to employ sensitizers whose groups for triphenylamines and observed a retardation in
structures have been specifically designed to impede recombination dynamics by more than one order of
aggregation is unavoidable. Employing peripheral units magnitude (Fig. 4). Moreover, the shape of the dynamics
bearing bulky side groups such as tert-butyl can effectively change from stretched exponential to monoexponential
minimize aggregation resulting in higher Jsc (and therefore indicating that recombination has moved from a TiO2
more efficient electron injection) and an overall improvement transport limited to interfacial limited regime.
in cell efficiencies.25 Another strategy to reduce aggregation in The effect of free energy DG0 and distance r parameters upon
phthalocyanines is to anchor them in a parallel fashion to the charge recombination was also investigated by Durrant45 and
TiO2 surface using ligands which can axially co-ordinate the co-workers. Charge recombination was measured for TiO2
central metal atom. In this case it is necessary to use 5 and 6 films sensitized with a variety of sensitizers including Ru(II)
coordinate metals such as Ti and Ru.31,32 polypyridyls, phthalocyanines and porphyrins. As these dyes
have different ground state oxidation potentials (by up to
500 mV) free energy DG0 of recombination was varied. The
2.3 Charge recombination of injected electrons with dye+
different dye structures allow the distance r parameter to be
In stark contrast to the ultrafast nature of electron injection, modulated by several nanometres (estimated from HOMO/
charge recombination manifests itself as a rather slow reaction LUMO calculations). Recombination lifetimes show a clear
occurring on micro- to millisecond timescales and of a highly dependence upon distance r with no correlation between
dispersive nature.18,39 The reason for such slow recombination lifetime and free energy DG0 observed. This study underlines
has generally been ascribed to slow electron transport in the the importance of dye structure for achieving effective charge
TiO2 film due to trapping/detrapping40,41 with lifetimes separation in DSSCs: just as for electron injection where one
showing a strong dependence upon electron occupancy should consider the LUMO of the dye sensitizer to achieve
within the metal oxide.39 The effect of sensitizer molecular strong electronic coupling with TiO2, the cation centre on the
structure on recombination has been investigated in many dye sensitizer must also be considered to minimize charge
studies. Addition of secondary electron-donating groups to recombination.

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2.4 Charge recombination of injected electrons

with electrolyte+

As discussed in the previous section, charge recombination for

optimized DSSC devices is generally quite slow. In fact it is
sufficiently slow to allow for efficient regeneration of dye+ by
the iodide/tri-iodide liquid electrolyte. Of principle concern
therefore is the recombination reaction between injected
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electrons in the TiO2 electrode and the oxidised electrolyte,

or the ‘‘dark current’’. Voc of optimized DSSCs could be as
high as 0.92 V, as noted recently by Snaith,46 however
optimized devices typically only achieve values of
700–800 mV. One key reason for this disparity is caused by
the leakage of injected electrons into the electrolyte and is key
to achieving higher efficiencies for these devices.
The molecular structure of the dye sensitizer can play a key Fig. 5 Chemical structures of Ru(II) polypyridyl sensitizers TG6, K19,
role in minimizing the dark current. It can act as a barrier AR20, AR24a and AR27a.
impeding this reaction by forming a compact monolayer on the
TiO2 surface.47 Furthermore, amphiphilic dyes including difference in iodine binding of the two sensitizers with the
Z90748–50 which contain long alkyl chains can block the oxygen atom of K19 inducing a lower binding constant than
approach of the charged oxidized electrolyte towards the that of the sulfur atom of TG6. In another study by Reynal
TiO2 surface. DSSCs made from Z907 show improved Voc et al.62 the presence of electron-donating –NH2 in AR24a and
indicating dark current suppression. Alkyl chains have also electron-withdrawing –NO2 in AR27a (Fig. 5) has a
been incorporated into the structures of other dye sensitizers to detrimental effect upon electron lifetime, cell voltage and
improve Voc.51–56 ultimately cell efficiency for DSSC devices made with these
A curious phenomena is that DSSCs made from highly dyes in comparison to devices made with the reference dye
conjugated dye sensitizers such as porphyrins, AR20.
phthalocyanines and organic sensitizers generally show
poorer Voc indicating that dark current is a significant
3. Sensitizer state-of-the-art
problem limiting the efficiency of these devices. Measurement
of electron lifetimes in functioning DSSCs at open circuit can Since the demonstration by Grätzel and O’Regan63 in 1991 of
be used as a means of rationalizing Voc for different devices.57 the first efficient DSSC of 7% based on nanocrystalline TiO2,
Mozer and Mori58 compared electron lifetimes of DSSCs made the best cell performances have continued to be recorded with
with N719 and Zn porphyrin sensitizers and the shorter Ru(II) polypyridyl dyes.64 The 10% efficiency barrier was
electron lifetimes for the latter were used to explain the broken by N365 (or its di-tetrabutyl-ammonium salt
roughly 150 mV lower Voc for these devices. O’Regan and equivalent N719)66 and indeed this was the dye par
co-workers59 compared electron lifetimes of DSSCs of N719 excellance for many years in the field. Ru(II) polypyridyl
and Ru phthalocyanine and the electron lifetimes were also complexes have continued to evolve, becoming ever more
shown to be much shorter for the devices based on the highly structurally advanced in order to address some of their
conjugated Ru phthalocyanine. Mori and co-workers60 drawbacks when employed in DSSCs. Such drawbacks
compared electron lifetimes in devices for a series of eight include their limited absorbance at longer wavelengths for
different organic sensitizers including donor–(p-bridge)–acceptor which N749,67,68 or ‘‘black dye’’ (see section 4) was
type dyes and indoline dyes compared to devices made with Ru(II) developed, or their long term stability for which amphi-
polypyridyl dyes N719 and N749 (black dye). In all cases the philic dyes containing long alkyl chains such as Z90749,50 (see
electron lifetimes were much shorter for the organic dye based section 2.4) were developed. Another key issue is their limited
devices than either the N719 or N749 based devices. extinction coefficients (N719 has an e of 13 900 M1 cm1 at
The correlation between lower Voc and fast electron lifetimes 541 nm).66 This requires that devices are made using TiO2 films
in functioning devices appears clear, however the reasons for over 8 mm thick to efficiently capture all of the incident light.
this and the role dye structure plays in determining it is still Thicker films result in both lower Voc (due to increased dark
under discussion. Even seemingly innocuous changes to current) and lower fill factor (due to an increase in electrolyte
sensitizer structure can have an enormous effect on cell resistance). To this end, a number of polypyridyl amphiphilic
voltage as demonstrated in two recent studies. O’Regan61 heteroleptic sensitizers such as K19,69 K77,70 C101,71 CYC-B1
and co-workers measured electron lifetimes and cell (Z991)72 and CYC-B1173 have been developed for use in
efficiencies for DSSC devices made with two Ru(II) DSSCs (Fig. 6). These dyes which incorporate thiophenes or
polypyridyl dyes, TG6 and K19 (see Fig. 5) whose structures phenylenevinylenes into the ancillary bipyrpidyl ligands show
differ by only two atoms (sulfur and oxygen). The re–dox and higher e values (24 200 M1 cm1 at 554 nm for CYC-B11), in
absorption properties of these sensitizers are essentially the addition to a shift in the onset of photocurrent wavelength to
same, however, devices made with TG6 show faster electron the red. Indeed C101 is currently one of the best dye sensitizers
lifetimes and inferior Voc. This difference was explained by the used in DSSCs with efficiencies of over 11% recorded.

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Fig. 8 Top: scheme depicting the donor-(p-bridge)-acceptor general

structure. Bottom: chemical structure of C219 organic dye.

NKX-2677 (Fig. 7) and by 2006 Ito and co-workers 80 had

achieved the extremely impressive efficiency of 9% with the
indoline dye D149 (Fig. 7).
Many more recent organic sensitizers have achieved
efficiencies which closely rival those of Ru(II) polypyridyls in
DSSCs.21,54,55,81–84 Indeed a recent study involving the dye
C219 (Fig. 8) showed an efficiency of over 10%.85 These
sensitizers all have the same structural motif in common,
Fig. 6 Chemical structure of Ru(II) polypyridyl sensitizers C101, K19,
K77, CYC-B1 and CYC-B11.
namely donor–(p-bridge)–acceptor where red–ox and
absorption properties can be tuned by the judicious selection
Organic sensitizers24 are an attractive alternative to Ru(II) of the individual units. The most common donors used are
polypyridyls as they do not contain any toxic or costly metal arylamines while the acceptor is usually the cyanoacrylate
and their properties are rather easily tuned by facile structural group. The p-bridge often consists of one or more thiophene
modification. In addition, they generally have much higher units. Molecular orbital calculations for these sensitizers have
extinction coefficients when compared to Ru(II) polypyridyls, shown that the HOMO is located on the arylamine donor unit
often higher than 100 000 M1 cm1, making them excellent whereas the LUMO is centred on the cyanoacrylate anchoring
for use in solid state DSSCs utilising hole transporting group resulting in efficient electron injection into TiO2.
materials such as P3HT3 or OMeTAD4 in which thinner Stability tests have found these sensitizers to be extremely
device architectures are required. However there are a promising for use in DSSCs.54,84
number of limitations regarding organic dyes including Porphyrins and phthalocyanines86 are also interesting
narrow absorption bands, aggregation, poor absorption in candidates as sensitizers for DSSCs. They are highly robust,
the red and poor stability. DSSCs utilising organic sensitizers being both photo- and electrochemically stable. Moreover they
initially showed very poor performances. However studies by are excellent light-harvesters with high extinction coefficients.
Arakawa and Hara74–77 involving coumarine dyes and Porphyrin absorption is centred mainly on the intense Soret
Horiuchi and Uchida78,79 using indoline dyes showed their band at 400 nm and moderate Q-bands at 600 nm.
promise so that by 2005 DSSC devices were already showing Phthalocyanines display an intense Q-band located at around
efficiencies of almost 8% with, for example, the sensitizer

Fig. 7 Chemical structures of coumarin NKX-2677 and indoline

D149. Fig. 9 Chemical structure of dyes Zn–TP–(COOH)2, TT1, and SQ1.

1640 Chem. Soc. Rev., 2011, 40, 1635–1646 This journal is c The Royal Society of Chemistry 2011
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700 nm due to the smaller band gap for these highly conjugated electron donation to Ru causing the red shift of the MLCT
p-aromatic systems. As discussed in section 2.2, porphyrins bands by decreasing the p* level of the 4,4 0 ,4 0 0 -tricarboxy-
and phthalocyanines display certain limitations for use in 2,2 0 : 6 0 ,2 0 0 -terpyridine ligand and an increase in the energy of
DSSCs such as an inherent lack of directionality in the the t2g metal orbital. The resulting photovoltaic devices using
excited state for symmetrical structures and a tendency N749 exhibit impressive near-IR photoresponse with an
towards aggregation. Achieving better excited state absorption threshold of 920 nm and a plateau higher than
directionality can be overcome by breaking the symmetry of 70% between 400 and 700 nm. However the overall efficiencies
these large planar p-aromatic systems and designing of these devices are not significantly better than cells made with
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unsymmetrical complexes such as Zn–TP–(COOH)287 and N719. More recently a new Ru(II) polypyridyl complex with
TT125 (Fig. 9) which are currently two of the most efficient the ligand 2,6-bis-(4-carboxyquinolin-2-yl)pyridine has shown
porphyrin and phthalocyanine dyes utilized in DSSCs with 35% IPCE at 900 nm.89 Although overall efficiencies of devices
efficiencies of 7.1% and 3.5% respectively. In addition, made with this sensitizer are lower than for N749, to date this is
substitution of Zn–TP–(COOH)2 at the b position of the the highest IPCE value reported in the near IR region for a
porphyrin ring with the conjugated diethenyl linker and Ru(II) polypyridyl dye. Panchromatic organic sensitizers are
carboxylic acid groups results in a red shift in the Soret and also being developed, for example the phenoxazine dye
Q-bands giving better absorption in the UV-visible. The TH30490 (Fig. 10) which contains a thiophene p-bridge that
staggered aryl substituents at the a positions of the extends the absorption spectrum allowing for a broad IPCE
porphyrin ring of Zn–TP–(COOH)2 and the bulky tert-butyl from 300 to 920 nm with a maximum of 67% at 580 nm.
groups on TT1 help to minimize aggregation in these Although the final efficiency is only 3%, the extension of its
complexes when bound to TiO2. light harvesting capability to the near-IR is very promising.
Near-IR absorbing squarine dyes such as SQ188 (Fig. 9) are An altogether different approach to achieve panchromatic
also under investigation as sensitizers for DSSCs. Similar to absorption of DSSCs involves the co-sensitization of two or
phthalocyanines they also show intense absorption bands but more dyes with complementary absorption spectra.
efficiencies have so far not greatly exceeded those of the Porphyrins, phthalocyanines, naphthalocyanines, cyanines
phthalocyanines such as TT1. and squarines all display intense absorption bands at lower
energies making them excellent candidates for co-sensitization
studies since they show an optical window over a large region
4. Towards panchromatic sensitization of the visible spectrum allowing them to be combined with
As already mentioned in section 1, one of the properties that an sensitizers that absorb in this part of the spectrum. Despite the
ideal dye sensitizer should possess is a broad absorption band many studies present in the literature and though co-sensitized
with optimum overlap with that of the solar spectrum. The DSSC devices generally show improved light-harvesting, they
most efficient Ru(II) polypyridyl sensitizers are noticeably poor also usually show poorer overall efficiencies when compared to
light-harvesters at longer wavelengths. For example, the reference devices made from the individual sensitizers alone.
incident-photon-to-current efficiency (IPCE) of C101 ranges One reason for this seems rather straightforward in that there
from 400 to 800 nm, only exceeding 80% from 480 to 660 nm.71 is only a finite number of anchoring sites on the TiO2 surface so
The development of panchromatic single sensitizers is however any improvement in IPCE in one part of the spectrum by the
rather challenging as absorption of lower energy photons introduction of a given sensitizer may be offset by poorer
towards the near-IR means the sensitizer band gap will have performance in IPCE in another part due to the removal of
to be sufficiently small. Such a small band gap may the other sensitizer dye(s). Another reason for poor
compromise other properties of the dye such as, for example, performance of co-sensitized DSSCs is the deactivation of
ground and excited state red–ox properties which are already dye excited states due to energy or electron transfer processes
tuned for efficient electron injection into the TiO2 and between the different sensitizers.
regeneration of the oxidized form of the sensitizer by the In spite of this, there are some examples of successful
iodide/tri-iodide re–dox couple. combinations of dyes resulting in both increased light
The first notable attempt to develop a panchromatic harvesting and improved cell efficiencies. In a study by
sensitizer for DSSCs was the Ru(II) polypyridyl dye Torres and co-workers91 involving the phthalocyanine TT1
N749,67,68 or ‘‘black dye’’ (Fig. 10) containing a carboxylated and the organic dye JK2, co-sensitized devices yielded an
terpyridyl ligand and three thiocianate groups. The three overall cell efficiency of 7.74%, which was higher than the
thyocianato anionic ligands stabilize the excited states by reference devices, with photoresponse extending up to 750 nm
and with an IPCE of 72% at 690 nm corresponding to the
Q-band of TT1. Wang and Zhang92 combined three organic
sensitizers absorbing in different regions of the UV-visible: a
yellow merocyanine dye (lmax at 380 nm), a red hemicyanine
dye (lmax at 535 nm) and a blue squarylium cyanine dye (lmax
at 642 nm). Co-sensitized devices showed a broad absorption
from 350 to 750 nm giving an overall efficiency of 6.5%, which
was higher than reference devices made with each individual
sensitizer only. Co-sensitization of these three dyes resulted in
Fig. 10 Chemical structure of dyes TH304 and N749. reduced aggregation and improvements in injection efficiencies

This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 1635–1646 1641
 View Article Online

and photocurrent. Ogura and co-workers93 co-sensitized using

the black dye N749 and the indoline dye D131 with devices
achieving a power conversion efficiency of 11%, higher
than the reference devices and indeed the highest for any
co-sensitized DSSC. Keeping the different dye species
isolated from one another by selectively positioning them on
the TiO2 electrode was the aim of two recent co-sensitization
studies. Hayese and co-workers94 co-sensitized nanocrystalline
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TiO2 with black dye N749 and the organic dye NK3705. Under
pressurized CO2 conditions and controlled time, N749 uptake
occurs faster than the normal dipping sensitization procedure,
and so the dye occupies the upper microns of the TiO2 film.
Subsequent dipping with NK3705 sensitizes the dye-free lower
microns of the film next to the glass contact, producing a
Fig. 11 Schematic view of the energy transfer relay based DSSC using
bilayer structure. The resulting device shows a broad IPCE
the perylene PTCDI and phthalocyanine dye TT1.
curve from 300 to 900 nm, exceeding 70% from 400 to 650 nm,
and an improved overall efficiency of 9.16% when compared to
the reference devices. In another study Park and co-workers95 Siegers and co-workers97 used just such a strategy by
applied a column chromatography method to achieve similar employing a dyad sensitizer which consisted of the organic
selective positioning of dyes on nanocrystalline TiO2. The dye Fluorol 7GA acting as the relay dye which is covalently
insertion of polystyrene into the pores of the TiO2 retards linked to the Ru(II) polypyridyl complex [Ru(dcbpy)2(acac)]Cl
the flow rate of the mobile phase (the dye solution for that is itself attached to the TiO2 surface. Devices made with
adsorption and an aqueous solution of NaOH and this dyad sensitizer show enhanced IPCE spectra when
polypropyleneglycol for desorption) and thus, allows the compared to reference cells consisting of the polypyridyl
selective positioning of three dyes (an organic dye P5, N719 complex only. However only a negligible increase in overall
and N749) at different depths of the film. The resulting triple- device efficiency was observed. More recently McGehee and
dye-layer devices show a broad IPCE from 400 to 840 nm and coworkers98 presented a novel design where the higher energy
improved overall efficiencies of 4.8% when compared to photons are absorbed by the perylene dye PTCDI which is
reference devices. dissolved in the cell electrolyte and undergoes FRET to the
The problem of limited anchoring sites for sensitizers on the phthalocyanine TT1 which is anchored to the TiO2 surface
TiO2 electrode is a significant one for co-sensitization studies. (Fig. 11). The presence of the relay dye in the electrolyte allows
Several strategies have been used to overcome this. One for a full monolayer of TT1 to anchor to the TiO2 electrode.
method is based on the controlled sensitization of TiO2 in Devices made in this way show improved IPCE spectra when
which dye sensitized films are covered with a layer of Al2O3 compared to reference devices with an increase in overall cell
and onto which a secondary monolayer of dye is deposited.88,96 efficiency. The same concept has been applied in solid state
The layer of Al2O3 has duel functionality: it allows for solar cells with N877 as the donor and SQ1 as the acceptor.99
increased dye adsorption onto the TiO2 electrode and with
the selection of suitable sensitizers with suitable re–dox
properties it also facilitates the formation of a hole-transfer 5. Supramolecular sensitizers
cascade in which holes are shuttled from the inner dye layer to
the outer dye layer, increasing the distance between charge Supramolecular interactions involving sensitizer dyes
separated species and retarding charge recombination. The immobilized onto nanocyrstalline TiO2 electrodes can be
best results have been obtained using the sensitizer JK2 and a exploited in DSSCs offering further exciting possibilities for
squarine dye SQ1 in the configuration TiO2/JK2/Al2O3/SQ188 optimization of these devices. Despite this however, there are
whose IPCE extends from 350 to 700 nm and reaches 85% at surprisingly few examples in the literature. The few studies
453 nm (absorption band of JK2) and 79% at 660 nm conducted can be divided into the following categories: (i) dye
(absorption band of SQ1) and an overall cell efficiency of sensitizer encapsulation by macrocycles, (ii) host–guest ion
8.65% which is higher than the reference devices. binding dye sensitizers and (iii) self assembly of dye
Another strategy which tries to solve the problem of limited monolayers onto the TiO2 surface via supramolecular
space on the TiO2 surface involves Förster resonance energy interactions.
transfer (FRET) from a ‘‘relay’’ dye to a secondary
5.1 Rotaxane-encapsulated dye structures
‘‘sensitizing’’ dye which utilizes this energy to inject electrons
into the TiO2 (Fig. 11). This requires the strong overlap of the Haque and co-workers100 employed an azobenzene based dye
emission spectrum of the relay dye with the absorption which was threaded through an a-cyclodextrin ring (Fig. 12a).
spectrum of the sensitizing dye for effective FRET. In this The formation of this rotaxane was carried out in solution and
way panchromatic sensitization is possible with the relay dye the entire structure was then immobilized onto nanocrystalline
absorbing photons at shorter wavelengths (the blue/green part TiO2 film through the cyclodextrin macrocycle. Transient
of the spectrum) and transferring this energy to the sensitizer absorption spectroscopy studies demonstrated that charge
dye that absorbs in the red/near-IR region of the spectrum. recombination is significantly retarded in TiO2 films

1642 Chem. Soc. Rev., 2011, 40, 1635–1646 This journal is c The Royal Society of Chemistry 2011
Published on 12 November 2010. Downloaded by University of Wollongong on 11/2/2018 9:31:18 AM. View Article Online

Fig. 13 Chemical structure of host–guest ion binding sensitizers K51,

1-a-CD and PMI-1.

cations in the electrolyte. When these cations are adsorbed

onto the electrode surface they shift the band edge positively
and this will manifest itself in lower device Voc. Planells and
co-workers102 aimed at circumventing this problem by
designing the perylene dye PMI-1 which incorporates a Li+
coordinating crown ether into its structure (Fig. 13). Because
this crown ether is positioned on the opposite end of the
sensitizer structure with respect to the anchoring unit,
the Li+ ions are trapped well away from the TiO2 surface.
1
H-NMR and mass spectrometry studies revealed that the
crown ether in PMI-1 did in fact complex Li+ ions resulting
in increased Voc for DSSC devices made with this sensitizer. In
a related study involving solid-state DSSCs employing
OMeTAD as the hole-transporting material, Snaith and
co-workers103 used the Ru(II) bipyridyl complex K51
(Fig. 13) containing polyethylene glycol chains to coordinate
Li+ ions. DSSC devices made with this sensitizer showed an
impressive 125 mV increase in Voc and an improved cell
Fig. 12 Rotaxane-encapsulated dye structures composed of a efficiency (3.8%) compared to reference devices (3.2%).
cyclodextrin ring and (a) an azobenzene based dye and (b) JK2. Falaras and Pikramenou104 designed the supramolecular
host 1-a-CD consisiting of a Ru(II) tris-bipyridyl core with
sensitized with the encapsulated dye compared to films an appended a-cyclodextrin ring (Fig. 13). In this case the
sensitized with non-encapsulated reference dye indicating function of the cyclodextrin is to complex I/I3 from the
that the cyclodextrin acts as an insulating shield between electrolyte within the macrocycle cavity thereby facilitating
injected electrons in the TiO2 and the oxidised dye. Ko and efficient regeneration of dye cations by the red–ox electrolyte.
co-workers101 also used this strategy to control interfacial This dye was used in solid-state DSSCs employing a composite
electron transfer at the TiO2 surface. However in this study polymer electrolyte. Devices showed increased Jsc, Voc and
nanocrytalline TiO2 films were first pretreated with a-, b- and overall efficiency (1.64%) with respect to reference devices
g-cyclodextrin followed by complexation with the organic dye (1.17%). The ability of 1-a-CD to complex the red–ox couple
JK2 on the film surface resulting in the formation of a rotaxane leading to improved regeneration was confirmed using solar
stoppered by the arylamino moiety at one extremity and by the cell devices containing low I/I3 concentrations. Under these
TiO2 particle at the other (Fig. 12b). DSSC devices made with conditions devices containing 1-a-CD still showed higher Voc
b-cyclodextrin encapsulated JK2 showed increased Jsc, Voc and values in comparison to devices made with the reference
an improvement in overall efficiency (8.65%), when compared sensitizer.
to reference devices made with JK2 only (7.42%). The
5.3 Dye self-assembly via supramolecular interactions
increased Voc and efficiency were rationalised in terms of
retardation of interfacial charge recombination processes and Biomimicry of energy and electron transfer processes found in
this was confirmed by photovoltage transient and nature such as those in photosynthetic reaction centres has
electrochemical impedance measurements. received much attention.105 There are many examples of self-
assembled donor–acceptor supramolecular complexes in solu-
tion involving dyes such as porphyrins and phthalocyanines as
5.2 Host–guest ion binding dye sensitizers
the donor.106,107 Supramolecular complexes have also been
It has been widely shown that the conduction band edge of immobilized onto electrode surfaces.108–110 For example, in a
nanocrystalline TiO2 is extremely sensitive to the presence of recent paper Imahori and co-workers110 were able to assemble

This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 1635–1646 1643
 View Article Online

Acknowledgements
We would like to thank financial support from ICIQ, ICREA
and Spanish MICINN for projects CONSOLIDER-HOPE
0007-2007 and CTQ-2007-60746-BQU projects. EP thanks
the European Research Council for the ERC starting grant
Polydot and the EU for the FP7-ROBUST project. JNC
thanks the Spanish MICINN for the Juan de la Cierva
Published on 12 November 2010. Downloaded by University of Wollongong on 11/2/2018 9:31:18 AM.

Fellowship.

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