Molecular design to improve the performance of donor-π acceptor near-IR organic dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201100350

Near-dye experience: Long, flexible carbon chains in the lateral anchoring groups of the donor part of a donor-π acceptor organic dye increase the power conversion efficiency dramatically. This performance enhancement can be ascribed to the prevention of the formation of molecular aggregates on the semiconductor nanoparticles, resulting in a lower recombination rate between transported electrons and I₃^- ions. A cell based on the new dye, HY113, gives a maximum IPCE value of 93% at 660nm. Copyright

Full text of DOI:10.1002/cssc.201100350

DOI: 10.1002/cssc.201100350
Molecular Design to Improve the Performance of Donor–p Acceptor Near-
IR Organic Dye-Sensitized Solar Cells
Yan Hao,[a] Xichuan Yang,*[a] Meizhen Zhou,[a] Jiayan Cong,[a] Xiuna Wang,[a] Anders Hagfeldt,[b] and
Licheng Sun*[a]
Since Grꢀtzel and O’Brian first reported dye-sensitized solar
cells (DSCs) in 1991, much effort has been devoted towards
improving their energy conversion efficiency.[1–3] Recently, the
design and synthesis of infrared/near-infrared (IR/near-IR)-dyes
has become an important topic in the area of solar cells since
it has been recognized that the solar spectrum has a large
photon flux in the region of 500–1000 nm. Improving the ab-
sorption of solar light in this region is one of the main direc-
tions in aiming for DSC efficiencies beyond 15%.
Near-IR sensitizers are also of interest because of
their potential applications in transparent solar cells
for windows and tandem cells.
easier way, through modifying the structure of the acceptor
units. By adopting this strategy, we succeeded for the first
time in extending the absorption spectra of D–p-A sensitizers
for DSCs to the near-IR region. In particular, the dye HY103
gave a maximum IPCE of 86% at 660 nm and an overall solar-
energy-to-electricity conversion efficiency (h) of 3.7%. The low
open-circuit voltage (464 mV) limited its photovoltaic perfor-
mance.
When used in DSCs, near-IR dyes reported in the
literature, such as squaraines,[4–6] zinc phthalocya-
nines[7–10] and perylene dyes,[11–13] normally show low
incident-photon-to-current conversion efficiency
(IPCE) values and poor stability compared to donor–
p spacer–acceptor (D–p-A) organic dyes. Recently,
Ko[5] and Marder[6] independently reported several
panchromatic squaraine sensitizers, which yielded relatively
high power conversion efficiencies (PCEs) (6.29% and 6.74%)
under AM 1.5 G irradiation. However, both displayed low IPCE
values (<70%) from in the wavelength range 400–770 nm.
Also, although a promising PCE of 4.6% was achieved in a DSC
based on a structurally related zinc phthalocyanine by Taya
and co-workers,[9] the highest IPCE value was only 80% at
680 nm.
To improve this system, we report a novel D–p-A organic
dye with a lateral anchoring group: HY113. In HY113, a flexible,
long carbon chain replaces the methyl group of the donor
part of HY103. The aim of the present work is to make use of
this carbon chain to prevent the formation of molecular aggre-
gates on the semiconductor nanoparticles, thereby blocking
charge recombination at relatively high open-circuit voltages
and short-circuit photocurrent densities. HY113 performed im-
pressively, with a maximum IPCE of 93% at 660 nm and an
overall solar-energy-to-electricity conversion efficiency of 5.1%;
the highest IPCE value in the near-IR region and the highest
PCE value of a D–p-A organic dye in the near-IR region report-
ed so far for DSCs.
Recently, novel D–p-A organic dyes with two features were
reported: new chromophores for sensitization in the near-IR
region and new design of the anchoring group.[14] Compared
to conventional D–p-A dyes (that use cyanoacrylic acid as ac-
ceptor and anchoring group), the anchoring group in these
dyes was separated from the acceptor units. This change al-
lowed tuning of the highest occupied–lowest unoccupied mo-
lecular orbital (HOMO–LUMO) levels (absorption spectra) in an
The synthesis of HY113 was conducted in eight steps in
moderate yields, in a manner similar to HY103. Related syn-
thetic procedures of these dyes and their DSCs fabrication pro-
cess are described in Scheme 1 and the Supporting Informa-
tion.
[a] Y. Hao, Prof. X. Yang, M. Zhou, J. Cong, X. Wang, Prof. L. Sun
State Key Laboratory of Fine Chemicals
DUT–KTH Joint Education and Research Center on Molecular Devices
Dalian University of Technology (DUT)
2 Linggong Rd. 116024 Dalian (PR China)
Fax: (+86)411-83702185
Figure 1 shows optical absorption spectra of HY113 in
CH2Cl2 solution and on TiO2 film. The peak of HY113 in CH2Cl2
at 615 nm (e=88867mÀ1 cmÀ1) is due to the p–p* transition of
the conjugated molecule. The absorption spectrum of HY113/
TiO2 shows a peak at 650 nm, that is, red-shifted by 35 nm
compared to the dye in solution. This red-shift might be due
to interactions between the TiO2 surface and the acceptor
moiety of the dye molecule. Usually, deprotonation of the car-
boxyl group of a dye anchored on the surface of TiO2 causes a
blue-shift.[15] J-aggregation may also be involved. This observed
red-shift warrants further investigation.
E-mail: yangxc@dlut.edu.cn
sunlc@dlut.edu.cn
[b] Prof. A. Hagfeldt
School of Chemical Science and Engineering
Center of Molecular Devices, Physical Chemistry
Royal Institute of Technology (KTH)
Teknikringen 30, 10044 Stockholm (Sweden)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100350.
ChemSusChem 2011, 4, 1601 – 1605
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Scheme 1. Synthetic route towards HY113. Reagents and conditions: a) EtOH, concentrated H2SO4, reflux (overnight), 74%; b) EtOH, Pd/C (5%), 4 h, 81%;
c) 2-octanone, p-TsOH, cyclohexane, 80–908C, 8–10 h, 49%; d) Raney-Ni, H2 (1 MPa), 1308C, 99%; e) (CH3)2SO4, benzene, reflux 2 h, 79%; f) DMF/POCl3, 558C,
6 h, 55%; g) 2-dicyanomethylen-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran/2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile/2,2’-(1H-indene-1,3(2H)-diyli-
dene)dimalononitrile, pyridine, acetic acid, room temperature (overnight); h) 2.0m aqueous LiOH, EtOH, 508C, 5 h, quantitative.
Table 1. Absorption and electrochemical properties of HY113.
Absorption[a]
[b]
[c]
[d]
Dye
lmax
e
lmax onTiO2
[nm]
E0–0
[V]
Eox
EoxÀE0–0
[nm]
[mÀ1 cmÀ1
]
[V]
[V]
HY113
615
88867
650
1.922
1.098
À0.820
[a] Absorption, emission spectra were measured in CH2Cl2 solution (2ꢂ
10À5 m) at room temperature. [b] Absorption spectra on TiO2 were ob-
tained by measuring the dye adsorbed on TiO2 film in CH2Cl2. [c] E0–0 was
estimated from onset point of absorption spectra. [d] The oxidation po-
tential of the dyes were measured in CH3CN with 0.1m tetrabutylammo-
nium hexafluorophosphate (TBAPF6) as electrolyte (working electrode:
glassy carbon; reference electrode: Ag/Ag+; calibrated with ferrocene/fer-
rocenium (Fc/Fc+) as an internal reference and converted to NHE by addi-
tion of 630 mV,[17] counter electrode: Pt).
Figure 1. UV/Vis spectra of HY113 in CH2Cl2 solution (2ꢂ10À5 m; solid line)
level of the dye is much more positive than the redox poten-
tial of iodine, it is possible to optimize the molecule structure
of the dye to extend its absorption spectrum further into infra-
red region.
and on TiO2 film (dashed line).
The oxidation potential (Eox) of HY113, corresponding to its
HOMO level, was measured by cyclic voltammetry (CV) in
CH3CN solution (Table 1) at 1.098 V vs. NHE, sufficiently more
positive than the iodine/triiodide redox potential value (0.4 V
vs. NHE). This indicates that thermodynamically, the oxidized
dye formed after electron injection into the CB of TiO2 could
accept electrons from IÀ. In addition, the LUMO level of HY113
was À0.820 V, more negative than Ecb of TiO2 (À0.5 V vs.
NHE),[16] indicating that HY113 has enough driving force for
electron injection from its excited state. Because the HOMO
The IPCE of a cell based on HY113 is shown in Figure 2. A
device based on HY113 gave high IPCE values between
480 nm and 800 nm, with a highest value of 93% at 660 nm;
the highest IPCE value for near-IR dyes in DSCs reported so far.
The relatively high IPCE values in the 600–800 nm range are
particularly intriguing because of their potential applications in
solid-state solar cells and possible tandem systems.
The DSC was fabricated in CH2Cl2 (2ꢂ10À4 m) with cheno-
deoxycholic acid (2ꢂ10À4 m) as co-adsorbent. It shows an im-
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ChemSusChem 2011, 4, 1601 – 1605

based on HY113 was increased by 1.5 mAcmÀ2 and the Voc was
also increased by 55 mV. As a result, the solar-energy-to-elec-
tricity conversion efficiency of the DSC based on HY113 was
considerably increased, from 3.7% to 5.1% (i.e., by about
35%). This result corroborates our initial design of the dye
structure. The long flexible carbon chain in HY113 may func-
tion as barrier to block the charge recombination between
electrons in the conduction band of TiO2 and the oxidized dye.
The IPCE at maximum absorption reached 93%, and integra-
tion of the curve over the spectrum gave a short-circuit current
J
sc of 13.35 mAcmÀ2, in agreement with the measured value.
Figure 4 shows electrochemical impedance spectra (EIS)[18–21]
for devices based on HY103 and HY113 under forward bias
(À0.6 V) in the dark. The larger semicircles in the Nyquist plots
Figure 2. Spectral response of the photocurrent of DSC based on HY113
dye. The left ordinate shows the IPCE as a function of the wavelength of
*
monochromatic light ( ). The right ordinate shows the integral of the IPCE
with AM 1.5G solar emission (ASTM G173-03 Reference Spectra Derived
*
from SMARTS v. 2.9.2) against wavelength ( ).
Figure 3. Photovoltaic performance of a DSC based on HY113.
pressive performance, with 5.1% efficiency under simulated
solar irradiation (Figure 3 and Table 2) with a short-circuit cur-
rent Jsc of 13.35 mAcmÀ2, an open-circuit voltage (Voc) of
519 mV, and a fill factor (FF) of 0.73. To the best of our knowl-
edge, this is the highest efficiency ever reported for organic
near-IR dye-sensitized TiO2 solar cells. Compared to the cell
based on the dye HY103 reported earlier, the Jsc of the device
Figure 4. Electrochemical impedance spectra, scanned from 10À2 to 107 Hz
at room temperature, for devices based on HY103 and HY113. a) Nyquist
plots, and b) Bode phase plots. The cells were measured at À0.6 V in the
dark. The alternate current (AC) amplitude was set at 10 mV.
Table 2. Photovoltaic performance of DSCs based on HY103 and HY113
dyes.[a]
Dye
Jsc
Voc
[mV]
Fill factor
h
[mAcmÀ2
]
[%]
are attributed to the charge-transfer processes occurring at the
TiO2/dye/electrolyte interface. Evidently, the charge-transfer re-
sistance at the interface of the device based on HY113 was de-
creased in comparison to the device based on HY103. The ad-
dition of the long carbon chain (HY113) shifts the mid-frequen-
cy peak to lower frequencies, as seen in Figure 4b. The calcu-
HY103
HY113
11.76
13.35
464
519
0.67
0.73
3.7
5.1
[a] Irradiated light: AM 1.5 G (100 mWcmÀ2); working area: 0.159 cm2;
electrolyte: 0.6m 1,2-dimethyl-3-n-propylimidazolium iodide (DMPII),
0.5m LiI, 0.02m I2, 0.1m tetrabutyl ammonium iodide (TBAI) in acetoni-
trile/valeronitrile=85:15.
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octanone (89 mL, 0.57 mol) was added at 80~908C for 8~10 h.
The resulting water was removed by co-boiling with cyclohexane.
Sodium carbonate (0.55 g) in water (20 mL) was poured into the
reaction mixture. The reaction mixture was stirred overnight at RT
for 1 h. The organic layer was washed with water and dried over
magnesium sulfate. The solvent was removed by rotary evapora-
tion. The residue was purified by column chromatography (silica
gel, acetic ester/hexane=1:20) to give 3 as colorless oil. (20.72 g,
25%). 1H NMR (400 MHz, CDCl3, ppm): d=6.97 (d, J=7.7 Hz, 1H),
6.42 (d, J=7.6 Hz, 1H), 6.26 (s, 1H), 5.15 (s, 1H), 4.13 (q, J=7.1 Hz,
2H), 3.54 (s, 1H), 2.79 (t, J=7.8 Hz, 2H), 2.57 (t, J=8.0 Hz, 2H), 2.31
(m, 2H), 1.51 (m, 5H), 1.29 (m, 19H), 0.88 ppm (dd, J=15.7, 6.7 Hz,
6H). HRMS-EI (m/z): [M]+ calcd. for C27H43NO2, 413.3294; found,
413.3284.
lated EIS with variation of the electron lifetime shows qualita-
tively the same behavior. The electron lifetime (t) in TiO2 film
for DSC sensitized by the dye HY113 is 42 ms, which is longer
than that for the DSC sensitized by the dye HY103 (32 ms). It
can be concluded that the charge recombination is suppressed
for the device based on the HY113, increasing both the elec-
tron lifetime in TiO2 and the photovoltage. This result is in ac-
cordance with the increased Voc and Jsc of cells based on
HY113.
Figure 5 shows the amounts of HY103 and HY113 dye ad-
sorbed on TiO2 films sensitized in CH2Cl2 solvent.[22] The ab-
sorbed amounts of dye are 9.02ꢂ10À5 mmcmÀ2 and 3.645ꢂ
10À5 mmcmÀ2 for devices sensitized by HY103 and HY113, re-
Ethyl 3-(2,4-dihexyl-2-methyl-1,2,3,4-tetrahydroquinolin-7-yl)propa-
noate (4): To a solution of 3 (12.05 g) in absolute ethanol was
added a 50% (w/w) slurry of Raney nickel in ethanol (1 g). The re-
action was hydrogenated over H2 gas (1 MPa) at 1308C. The result-
ing solution was filtered carefully over celite and washed with eth-
anol. The solvent was removed in vacuo and the residue 4 (12 g,
~99%) was used for the next reaction without further purification.
1H NMR (400 MHz, CDCl3): d=7.09 (d, J=7.8 Hz, 1H), 6.52 (d, J=
7.7 Hz, 1H), 6.35 (s, 1H), 4.14 (q, =7.1 Hz, 2H), 2.80 (m, 3H), 2.57
J
(t, J=8.0 Hz, 2H), 1.74 (dd, J=14.3, 11.1 Hz, 2H), 1.61 (m, 5H), 1.27
(m, 21H), 0.88 ppm(dd, J=11.0, 6.3 Hz, 6H). HRMS-EI (m/z): [M]+
calcd. for C27H45NO2, 415.3450; found, 415.3460.
Ethyl 3-(2,4-dihexyl-1,2-dimethyl-1,2,3,4-tetrahydroquinolin-7-yl)pro-
panoate (5): 4 (4.15 g, 10 mmol) and (CH3)2SO4 (0.76 g, 6 mmol)
were dissolved in benzene. The mixture was refluxed overnight.
10% NH4OH (10 mL) was added at 708C and stirred for 1 h. The or-
ganic layer was dried over magnesium sulfate. The solvent was re-
moved by rotary evaporation and the residue was purified by chro-
matography (silica gel, dichloromethane/hexane=1:2) to provide 5
as colorless oil. 1H NMR (400 MHz, CDCl3): d=7.04 (d, J=7.6 Hz,
1H), 6.48 (d, J=7.5 Hz, 1H), 6.41 (s, 1H), 4.14 (q, J=7.1 Hz, 2H),
2.88 (t, J=7.8 Hz, 2H), 2.76 (s, 3H), 2.61 (m, 3H), 1.65 (dd, J=14.5,
11.0 Hz, 2H), 1.49 (m, 5H), 1.27 (m, 21H), 0.88 ppm(dd, J=11.2,
6.4 Hz, 6H). HRMS-EI (m/z): [M]+ calcd. for C28H47NO2, 429.3607;
found, 429.3613.
Figure 5. Adsorbed amount of dyes HY103 and HY113 on TiO2 films sensi-
tized in CH2Cl2 solvent.
spectively. That is, a higher amount of dye HY103 absorbed,
but the device had a lower Jsc than the one with HY113. This
result reveals that HY103 has stronger tendency to aggregate
on TiO2. The flexible long carbon chains in the dye HY113 mini-
mize molecular aggregation, on the semiconductor nanoparti-
cles, leading to the higher Jsc and Voc values.
Ethyl 3-(6-formyl-2,4-dihexyl-1,2-dimethyl-1,2,3,4-tetrahydroquino-
lin-7-yl)propanoate (D1): POCl3 (0.49 mL, 5.4 mmol) was added
dropwise with stirring to a solution of 5 (2.27 g, 5.3 mmol) in fresh
distilled DMF (1.3 mL, 16.7 mmol) at 15~208C under N2. The mix-
ture was held at 558C for 6 h and then cooled, ice (100 g) was
added, and then 5n NaOH to pH 6. The mixture was extracted fur-
ther with dichloromethane (3ꢂ20 mL) and water. The combined
organic layers were dried over MgSO4. The solvent was removed
by rotary evaporation and the residue was purified by column
chromatography (silica gel, dichloromethane) to provide D1: light
yellow oil (yield 89%). HRMS-EI (m/z): [M]+ calcd. for C29H47NO3,
457.3556; found, 457.3567.
In conclusion, we demonstrate that D–p-A near-IR organic
dyes with lateral anchoring groups are efficient sensitizers. The
dye HY113 gives maximum IPCE value of 93% at 660 nm and
an overall solar-energy-to-electricity conversion efficiency of
5.1%. These are the highest IPCE value of a near-IR organic
dye and the highest PCE value of a D–p-A near-IR organic dye
reported in DSCs. The stability of the dyes in photovoltaic devi-
ces is good, with increased PCE in a month under the usual
sunlight at room temperature. Further structural optimization
of dyes to reduce the aggregation on TiO2 and to direct the
charge distribution in the excited states of the dyes is likely to
yield more efficient DSCs. Also, further accurate stability analy-
sis is in progress.
(E)-3-(6-(2-(4-cyano-5-(dicyanomethylene)-2,2-dimethyl-2,5-dihydro-
furan-3-yl)vinyl)-2,4-dihexyl-1,2-dimethyl-1,2,3,4-tetrahydroquinolin-
7-yl)propanoic acid (HY113): A mixture of D1 (1.12 g, 2.45 mmol),
2-dicyanomethylen-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran
(0.48 g, 2.40 mmol), pyridine (20 mL), and several drops of acetic
acid was stirred at room temperature overnight. Pyridine was dis-
tilled out under vacuum. The resulting coarse product was used in
the next step without further purification. The residue ester was
hydrolyzed in 2m LiOH in equal volume of ethanol and water by
heating at 508C for 5 h. The reaction mixture was diluted by water
Experimental Section
Ethyl 3-(2,4-dihexyl-2-methyl-1,2-dihydroquinolin-7-yl)propanoate
(3): To a solution of ethyl 3-(3-aminophenyl)propanoate 2 (38.6 g,
0.2 mol) and toluenesulfonic acid (1.9 g) in cyclohexane (20 mL), 2-
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6.90 (d, J=15.5 Hz, 1H), 6.71 (s, 1H), 3.11 (t, J=7.8 Hz, 2H), 3.04 (s,
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We gratefully acknowledge financial support of this work by the
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Keywords: aggregation · dyes · photophysics · sensitizers ·
solar cells
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Received: July 9, 2011
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Published online on October 28, 2011
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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