Fine-tuning the electronic structure of organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1021/ol301730c

A series of metal-free organic dyes exploiting different combinations of (hetero)cyclic linkers (benzene, thiophene, and thiazole) and bridges (4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) and benzodithiophene (BDT)) as the central π-spacers were synthesized and characterized. Among them, the sensitizer containing the thiophene and CPDT showed the most broad incident photon-to-current conversion efficiency spectra, resulting in a solar energy conversion efficiency (η) of 6.6%.

Full text of DOI:10.1021/ol301730c

ORGANIC
LETTERS
2012
Vol. 14, No. 17
4330–4333
Fine-tuning the Electronic Structure of
Organic Dyes for Dye-Sensitized Solar Cells
€
Peng Gao, Hoi Nok Tsao, Michael Gratzel, and Mohammad K. Nazeeruddin*
Laboratory for Photonics and Interfaces, School of Basic Sciences, Swiss Federal
Institute of Technology, 1015 Lausanne, Switzerland
mdkhaja.nazeeruddin@epfl.ch
Received June 23, 2012
ABSTRACT
A series of metal-free organic dyes exploiting different combinations of (hetero)cyclic linkers (benzene, thiophene, and thiazole) and bridges
(4H-cyclopenta[2,1-b:3,4-b0]dithiophene (CPDT) and benzodithiophene (BDT)) as the central π-spacers were synthesized and characterized.
Among them, the sensitizer containing the thiophene and CPDT showed the most broad incident photon-to-current conversion efficiency spectra,
resulting in a solar energy conversion efficiency (η) of 6.6%.
In recent decades, increasing attention has been paid to
the development of dye-sensitized solar cells (DSCs) as a
source of renewable energy,1 mainly due to their poten-
tially low-cost fabrication, possibility of transparency. and
color selectivity, which can be integrated into building and
automobile applications.2 In order to increase power con-
version efficiency and decrease the fabrication cost, ex-
tensive research has been focused on the development
of new metal-free organic dye sensitizers.3 In particular,
donor-π-acceptor (D-π-A) dyes are seen as one of the
most promising organic dyes, owing to their efficient
intramolecular charge transfer (ICT) characteristics.
Moreover most organic sensitizers are easy to synthesize
and generally possess high molar extinction coefficients.4
Heterocyclic compounds such as thiophene and thiazole
are widely used as building blocks in organic opto-
electronics.5 To date, the molecular architecture of most
donor fragments of the dyes is triphenylamine (TPA) in
which a benzene ring typically serves as the linker. How-
ever, thiophene, which is a relatively more electron-rich
heterocycle, and thiozole, which in contrast is relatively
electron-deficient, are seldom employed to connect the
nitrogen of the diarylamine with other π-bridges.6
Regarded as a fused-ring analogue of 3-alkylthiophene
and a structural analogue of fluorene, CPDT has attracted
considerable research interest.7 Organic dyes using the
€
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M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.;
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Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (c)
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Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (d) Neale, N. R.;
Kopidakis, N.; van de Lagemaat, J.; Gratzel, M.; Frank, A. J. J. Phys.
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C.; Shinpo, A.; Hara, K. J. Phys. Chem. C 2007, 111, 7224. (f) Qin, H.;
Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Nazeeruddin, M. K.;
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r
10.1021/ol301730c
Published on Web 08/10/2012
2012 American Chemical Society

alkyl-functionalized CPDT as the conjugated bridge showed
an extremely high molar absorption coefficient and a high
power conversion efficiency of 8.95% in liquid cell and 6% in
a solid-state DSC.8 BDT, the dithiophene analog of phenan-
threne, has several isomers, of which benzo[1,2-b:4,5-b0]-
dithiophene and benzo[2,1-b:3,4-b0]dithiophene have already
been widely utilized in the construction of conjugated
copolymers9 and D-π-A dyes10 for photovoltaic applica-
tions. Another isomer of BDT, benzo[1,2-b:4,3-b0]-
dithiophene, however, to the best of our knowledge, has
hardly been investigated in the DSC field.
cyanoacrylic acid as the electron acceptor. The synthesized
organic dyes (C220, G54, G55, G104, G107, and G108) are
shown in Figure 1, with the synthetic methodology of the
five target molecules being highlighted in Scheme S1. The
C220 sensitizer is synthesized according to the reported
procedure for comparison.8b
Figure 2. UVꢀvis absorption spectra of all the dyes in CH2Cl2
solution.
The UVꢀvis absorption spectra of the dyes in CH2Cl2
solutions are depicted in Figure 2. Their absorption,
electrochemical properties, and frontier orbital energy
levels are summarized in Table 1. The electronic absorp-
tion spectra reveal two major characteristics depending
on the substituted π-bridges. Specifically, the four dyes
(G54, G104, and C220) containing CPDT as a bridge
exhibit a single prominent band with the absorption max-
imum around ca. 530ꢀ570 nm, while the others (G55, G107,
and G108) with BDT as the bridge have two absorption
bands at ca. 370ꢀ410 nm and at ca. 450ꢀ510 nm. Such
absorption characteristics indicate that the electron density
on CPDT is richer than that of BDT and the absorption
bands can be ascribed to the πꢀπ* transition and intra-
molecular charge transfer (ICT) transitions of the D-π-A
conjugated backbone. Apparently, in either case of the
bridges (CPDT or BDT), the absorption maxima of ICT
transitions are generally red-shifting in the sequence thiazol
< phenyl < thienyl. The phenomenon is in line with the
electronic richness of the relative (hetero)cycles, and due to
the fortified quinoidal character of thiophene, the adoption
of the thiophene linker endows the corresponding dye with
the smallest band gap.11 This result clearly indicates that
modulation of the electron density of D-π-A dyes can be
achieved by altering the electronic nature of the π-bridges.
Cyclic voltammetry (CV) is employed to estimate the
first oxidation potential (Eox), which corresponds to the
HOMO levels of the dyes (Table 1, SI Figure S1). Nor-
mally, two oxidation waves are observed on the voltam-
mograms. The first oxidation waves at lower oxidation
potentials are generally from the contribution of triaryla-
mine, whereas the second ones, with higher oxidation
Figure 1. Structure of the synthesized dyes.
Judiciously varying the conjugating π-spacer between
the donor and acceptor fragments has been the most
popular approach to structural modification for modulat-
ing the frontier orbital energy levels and for generating
dyes with broad and intense absorption. Therefore, this
work elaborates on the use of various (hetero)cyclic linkers
(benzene, thiophene, and thiazole) combined with differ-
ent conjugated π-bridges (CPDT and BDT) to evaluate the
impact on the overall efficiency while maintaining
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Li, G. J. Am. Chem. Soc. 2009, 131, 15586. (c) Huo, L. J.; Guo, X.;
Zhang, S. Q.; Li, Y. F.; Hou, J. H. Macromolecules 2011, 44, 4035. (d)
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Chem., Int. Ed. 2010, 49, 1500. (e) Liang, Y. Y.; Xu, Z.; Xia, J.; Tsai,
S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. P. Adv. Mater. 2010, 22, E135. (f)
Price, S.; Stuart, A.; Yang, L.; Zhou, H.; You, W. J. Am. Chem. Soc.
2011, 133, 4625. (g) Wang, M.; Hu, X.; Liu, P.; Li, W.; Gong, X.; Huang,
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Sun, J.; Katz, H. E. J. Am. Chem. Soc. 2010, 132, 5394. (i) Zhou, H.;
Yang, L.; Stuart, A.; Price, S.; Liu, S.; You, W. Angew. Chem., Int. Ed.
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Org. Lett., Vol. 14, No. 17, 2012
4331

The DSC performances of all the dyes are evaluated
under AM1.5 G irradiation at 100 mW cmꢀ2 (details of the
devices’ fabrication and testing are described in the Sup-
porting Information). Hereby, the focus is on the influence
of the two different bridging units CPDT and BDT,
together with the three varying linker groups, on the
DSC characteristics. First, the effect of the BDT in com-
parison to CPDT is highlighted in terms of the changes in
Table 1. Optical and Electrochemical Properties of the Dyes
absorptiona oxidation potential datac
ε (Mꢀ1 Eox (V)
EoxꢀE0ꢀ0 HOMO/
λmax
cmꢀ1
)
)
(vs
E0ꢀ0
(V) (vs
NHE)
LUMO
(eV)e
dye
(nm)
(λmax
NHE)
(V)d
G54
565
375
45468
38688
0.635
0.86
0.96
0.75
1.08
0.815
1.79
2.18
1.97
1.95
2.15
1.87
ꢀ1.15
ꢀ1.32
ꢀ1.01
ꢀ1.2
ꢀ5.14/
ꢀ3.35
ꢀ5.36/
ꢀ3.18
ꢀ5.46/
ꢀ3.49
ꢀ5.25/
ꢀ3.3
VOC. When scrutinizing the dyes with the same linkers
(C220 vs G55 for the phenyl, G54 vs G107 for the thio-
phene, and G104 vs G108 for the thiazole linkers), it is
apparent that the VOC increases by more than 40 mV in the
presence of BDT (Table 2 and Figure 3a). This observation
is reflected by the corresponding VOC shifts toward higher
values when plotted against capacitance (Figure 4a). For
instance, in comparing C220 with G55, the capacitance for
the latter is shifted toward higher VOC (Figure 4a), indicat-
ing a higher TiO2 conduction band ultimately resulting
in an elevated VOC for the G55 cell. The same trend is
observed for G54 vs G107 and G104 vs G108 (Figure 4a),
emphasizing the fact that the BDT unit triggers an upward
shift of the TiO2 conduction band, which is beneficial for
enhancing the VOC. Since the same TiO2 layers are em-
ployed for all cells and all dyes investigated, it is highly
unlikely that changes in TiO2 trap states are the cause for
the observed variations in conduction bands. In terms of
VOC, the effect of the linker group is even more dramatic
than replacing BDT with the CPDT acceptor. For the
same π-bridge, the incorporation of thiophenes or thia-
zoles instead of the common phenyl as linkers severely
reduces the VOC more significantly than 100 mV (Table 2,
Figure 3a) even though the difference between thiophene
and thiazole is not as major. This behavior is also reflected
by the shift in VOC in the dependence of capacitance as
illustrated by the photovoltage decay measurements
(Figure 4a). Overall, the combination of the BDT bridge
and the phenyl linker poses a powerful system in achieving
an outstanding VOC, surpassing 800 mV, one of the highest
values reported for the I3ꢀ/Iꢀ redox couple.
G55
459 23212
G104
G107
G108
C220
539
406
50567
41683
502 21258
381
42430
ꢀ1.07
ꢀ1.05
ꢀ5.58/
ꢀ3.43
ꢀ5.32/
ꢀ3.45
458 25540
552
53605b
a Measured in 2 ꢁ 10ꢀ6 M of CH2Cl2 solutions at room temperature.
b Slightly lower than the value measured in chloroform.8a c Measured in
CH2Cl2 containing 0.1 M of tetrabutylammonium tetrafluoroborate
(TBABF4) electrolyte (working electrode: glassy carbon; counter elec-
trode: Pt; reference electrode: Pt; calibrated with ferrocene/ferrocenium
(Fc/Fcþ) as an internal reference and converted to NHE by addition of
630 mV).12 d Estimated from onset wavelength in absorption spectra.
e NHE vs the vacuum level is set to 4.5 V.13
potentials and quasi-reversible behavior, are from the
CPDT and BDT π-bridge. It is obvious that the Eox varies
in accord with the electron density of the conjugated
π-bridges. Generally, the CPDT based dyes have a lower
first oxidation potential than that of the BDT based dyes,
whereas the thiophene linked molecules exhibit the lowest
oxidation potential among the three kinds of (hetero)cyclic
linkers. The zeroꢀzero energy (E0ꢀ0), estimated from the
onset wavelength of the absorption spectra together with
the HOMO level energy, is used to calculate the LUMO
levelsof thesedyes(seeTable1). The thusobtainedLUMO
valuesare muchmorenegativethanthe Fermi level ofTiO2
(ꢀ0.5 V vs NHE),9b allowing efficient electron injection
from the excited dyes into the TiO2 electrode. All the dyes
exhibitmore positiveHOMO levels thanthat of the iodide/
triiodide redox couple (0.4 V vs NHE) guaranteeing
efficient dye regeneration.
Table 2. Photovoltaic Performance Data of the Dyes
Jsc
(mA/cm2)
Voc
FF
η
(%)
To gain further insight into the molecular structure and
frontier molecular orbitals of these dyes, the geometries of
the dyes are optimized by density functional theory (DFT)
calculations at the B3LYP.6-31G(d) level. SI Figure S2
displays the relative energies and electron distributions of
the HOMO and LUMO of the dyes. HOMOs are mainly
delocalized from the diarylamino donor to the heterocyclic
π-spacer, whereas LUMOs show localized electron distri-
butions though the cyanoacrylic acid and its adjacent
π-spacer. This separation of electrons will ensure efficient
electron injection from the dye to the TiO2 film.
dye
combination
(mV)
(%)
C220
G55
benzene-CPDT
benzene-BDT
thiophene-CPDT
thiophene-BDT
thiazole-CPDT
thiazole-BDT
15.4
9.4
756
810
622
671
603
683
0.76
0.74
0.71
0.70
0.75
0.78
8.8
5.6
6.6
4.8
5.4
5.1
G54
14.9
10.1
11.8
9.6
G107
G104
G108
Even though the BDT has the advantage of increasing
the VOC, the absorption however is blue-shifted compared
to the CPDT unit as already shown by the UVꢀvis ab-
sorption spectra. This phenomenon is mirrored in the
incident photon-to-current conversion efficiency (IPCE)
of the corresponding dyes (Figure 3b). Consequently, all
(12) Mei, J.; Graham, K. R.; Stalder, R.; Reynolds, J. R. Org. Lett.
2010, 12, 660.
€
(13) Burgi, L.; Turbiez, M.; Pfeiffer, R.; Bienewald, F.; Kirner, H.-J.;
Winnewisser, C. Adv. Mater. 2008, 20, 2217.
4332
Org. Lett., Vol. 14, No. 17, 2012

Figure 3. (a) JꢀV curves for DSSCs based on the dyes under
illumination of AM 1.5 G simulated sunlight (100 mW cmꢀ2).
(b) IPCE spectra of the same DSSCs.
Figure 4. (a) Dependence of capacitance on open circuit poten-
tial VOC as measured by voltage decay and (b) dependence of
TiO2 electron lifetimes on capacitance as recorded from photo-
current decay measurements for the investigated dyes.
sensitizers containing BDT exhibit a JSC of only ∼10 mA
cmꢀ2, whereas all other compounds consisting of CPDT
result in better cell performances with a much higher JSC
(Table 2), even though all of the BDT based dyes yield more
enhanced electron lifetimes than the CPDT counterparts as
evidenced by photocurrent decay measurements (Figure 4b).
Having elucidated the effect of the bridging units on the
studied the impact of various conjugated π-spacers on
the device performance in DSCs. The power-conversion
efficiency of DSCs is higher when CPDT and/or phenyl are
used as the π-conjugated spacer. The introduction of
CPDT instead of BDT contributed to improving the cell’s
Jsc but deteriorating the Voc, while the use of phenyl rather
than thiophene and thiazole mainly resulted in an in-
creased Voc, respectively. Amongthe synthesizednewdyes,
G54 containing CPDT and thiophene yielded the best
conversion efficiency of 6.6% due to the relatively highest
Jsc. On the other hand, although with much poorer ab-
sorption ability, BDT and phenyl bridged G55 showed the
highest Voc and longest electron lifetime, which thus
dramatically improved the DSC performance (5.6%).
These results strongly suggest that metal-free organic dyes
could be tuned by appropriate control of the energy levels
ofπ-conjugated spacers through systematicstructuremod-
ification of the π-system, leading to further development of
highly efficient organic dyes.
JSC, it is also crucial to understand which role the linkers
playinthisaspect. Takingintoconsideration the BDT dyes
G55, G107, and G108, it is apparent that the JSC increases
with the linkers thiophene > thiazole > phenyl, even though
those changes are not significant (Table 2). On the other
hand, the sensitizers containing CPDT show a decline in
J
SC according to the trend phenyl > thiophene > thiazole.
Hereby, the linkers impinge a more dramatic impact on the
SC, particularly for the thiazole case where the JSC is
J
reduced by more than 3 mA cmꢀ2 (Table 2). This behavior
is simply the result of the corresponding blue shifts as
indicatedbythe IPCE (Figure3b). Basedon these findings,
the CPDT seems to be more prone to the nature of the
linkers than the BDT bridge.
Acknowledgment. This work was supported by EU FP7
Project ENERGY-261920 “ESCORT”. M.K.N. thanks
the World Class University program, Photovoltaic Mate-
rials, Department of Material Chemistry, Korea Univer-
sity, Chungnam, 339-700, Korea, funded by the Ministry
of Education, Science and Technology through the Na-
tional Research Foundation of Korea (No. R31-2008-000-
10035-0).
Inaddition, the linkers alsohave aneffectonthe electron
lifetimes in the TiO2 film. As the photocurrent decay
measurements indicate (Figure 4b), for both the BDT and
CPDT bridges, the phenyl linker leads to the longest lifetimes,
whereas the thiophene and thiazole units shorten the life-
times. There are no notable differences in lifetimes between
those two linkers themselves. Furthermore, as compared to
the CPDT with the same linkers, the BDT sensitizer based
TiO2 layers exhibit prolonged lifetimes as well. Overall, the
BDT bridge is very appealing in terms of both the observed
upper shift of the TiO2 conduction band desired for a high
Supporting Information Available. Experimental de-
tails, DSSCs fabrication, and characterization details for
new compounds. The material is available free of charge
via the Internet at http://pubs.acs.org.
VOC and the longer lifetimes of the electrons.
In summary, we have synthesized a series of metal-free
The authors declare no competing financial interest.
organic sensitizers based on the D-π-A structure and
Org. Lett., Vol. 14, No. 17, 2012
4333