Tetrahedron Letters
Novel D–p–A type II organic sensitizers for dye sensitized solar cells
Shi-Feng Li a, Xi-Chuan Yang a, , Ming Cheng a, Jiang-Hua Zhao a, Yu Wang a, Li-Cheng Sun a,b,
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a State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, China
b Department of Chemistry, Organic Chemistry, Royal Institute of Technology (KTH), Stockholm 10044, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 February 2012
Revised 3 April 2012
Accepted 12 April 2012
Available online 20 April 2012
Four organic donor–p–conjugated-acceptor (D–p–A) type II dyes with different thiophene linkers are
reported for dye sensitized solar cells (DSSCs). For the first time, a donor (triphenylamine) was introduced
in type II sensitizers, and 2-hydroxybenzonitrile as acceptor/anchoring moiety was covalently linked TiO2
particles. The dye LS203 in this series gives the best solar energy conversion efficiency of 3.4%, with
Jsc = 7.4 mA cmÀ2, Voc = 0.67 V, FF = 0.69, the maximum IPCE value reaches 66.9%.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Photovoltaic devices
Solar cells
Due to the increase in solar energy conversion efficiency and
decrease in cost, dye sensitized solar cells (DSSCs) have drawn
much attention during the past two decades.1,2 In most of tradi-
dyes, LS203 gives the best solar energy conversion efficiency of
3.4%, the maximum IPCE value reaches about 66.9%.
The absorption spectra of the four dyes dissolved in CH2Cl2
(Fig. 2a) and adsorbed on TiO2 (Fig. 2b) are displayed in Figure 2,
and the photophysical data are collected in Table 1. In CH2Cl2 solu-
tion, the absorption maxima are between 351 and 376 nm. When
the four dyes were absorbed on the TiO2, the absorption maxima
do not have significant changes, but the absorption peaks become
much wider and the cut-off absorptions of the four dyes changed
from 400 nm dissolved in CH2Cl2 to more than 450 nm absorbed
on TiO2. That brings about a surprising change of the colors of
the dyes from the CH2Cl2 solutions to the TiO2 absorbed films
showed in Figure 3.
The first oxidation potentials (Eox) of the four dyes were mea-
sured by cyclic voltammetry (CV) in CH2Cl2 solution; the data were
collected in Table 1. It shows that the HOMO levels of the four dyes
were sufficiently more positive than the iodine/triiodide redox po-
tential value (0.4 V vs NHE), indicating that the oxidized dyes
formed after electron injection into the CB of TiO2 could accept
electrons from I- thermodynamically.17,18 In addition, the LUMO
levels of the four dyes were more negative than the Ecb of TiO2
(À0.5 V vs NHE) indicating that the dyes all have enough driving
force for electron injection.17
tional organic D–p–A dyes, cyanoacrylic acid group was always
introduced as acceptor and anchoring group, which has limited
much more excellent dyes that were developed.2 It is well known
that the traditional DSSCs (Type I DSSCs) are the devices in which
electrons are injected from the adsorbed dyes by photoexcitation
of the dyes followed by electron injection from the excited dyes
to TiO2.3,4 Type II DSSCs in which electrons are injected not only
by electron injection from the excited states of dyes to TiO2, but
also by direct one-step electron injection from the dyes to TiO2
by photoexcitation of the dye-to-TiO2 charge-transfer bands.5–13
The DSSCs employing catechol or its derivatives in the dye struc-
tures have been considered as typical examples of Type II DSSCs.
An et al. reported a class of Type II thiophene-catechol sensitizers
used in DSSCs.14 However, the reported solar energy-to-electricity
conversion efficiencies of Type II DSSCs did not exceed 1.6%.5,14
In this Letter we report four novel Type II organic dyes LS201,
LS202, LS203, and LS204 with Donor–p–Acceptor design in struc-
ture (Fig. 1). Triphenylamine was introduced as a donor part in
Type II sensitizer, and catechol was used instead by 2-hydrox-
ybenzonitrile as the electron acceptor/anchoring group. Compared
with the traditional organic D–
p
–A dyes with cyanoacrylic acid
The J–V characteristic curves are shown in Figure 4a, and all
data are summarized in Table 2. The best solar cell we have
achieved is sensitized by LS203 dye, which has 3.4% efficiency un-
der AM 1.5 solar simulator illumination with a short circuit Jsc of
7.4 mA cmÀ2 and an open circuit voltage Voc of 0.67 V. The solar
cells sensitized by LS201, LS202, and LS204 have efficiencies of
2.8%, 3.1%, and 2.6%, respectively. All the dyes show better cell
performance compared to the compound Cat-v-Q.5 From the data,
system, the electron donor, -spacer and electron acceptors were
p
linked by C–C single bonds without the involvement of any vinyl
group, which would hopefully increase the stability of DSSCs.15,16
This is a new strategy to design Type II sensitizers. Among the four
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Corresponding authors. Tel.: +86 411 88993886; fax: +86 411 83702185.
it is clear to see that introducing alkyl group to the
p-spacer is
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.