A benzothiazole-cyclopentadithiophene bridged D-A-π-A sensitizer with enhanced light absorption for high efficiency dye-sensitized solar cells

Article abstract of DOI:10.1039/c4cc00577e

We demonstrate an efficient D-A-π-A sensitizer with a benzothiazole-cyclopentadithiophene moiety as the spacer in a triphenylamine organic dye for dye-sensitized solar cells. The dye has a broad visible light absorption range up to 800 nm. A power conversion efficiency >9% has been achieved using a [Co(bpy)₃]^2+/3+-based electrolyte.

Full text of DOI:10.1039/c4cc00577e

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A benzothiazole–cyclopentadithiophene bridged
D–A–p–A sensitizer with enhanced light
absorption for high efficiency dye-sensitized
solar cells†
Cite this: Chem. Commun., 2014,
50, 3965
Received 22nd January 2014,
Accepted 21st February 2014
Xingzhu Wang,a Jing Yang,a Hao Yu,b Feng Li,a Li Fan,a Wen Sun,b Yeru Liu,a
DOI: 10.1039/c4cc00577e
Zhen Yu Koh,a Jiahong Pan,a Wai-Leung Yim,c Lei Yan*b and Qing Wang*a
www.rsc.org/chemcomm
We demonstrate an efficient D–A–p–A sensitizer with a benzothiazole– To promote the performance of DSCs to a new phase, it is
cyclopentadithiophene moiety as the spacer in a triphenylamine organic however highly desirable to further enhance the light harvesting
dye for dye-sensitized solar cells. The dye has a broad visible light and at the same time retain the high efficiencies of charge
absorption range up to 800 nm. A power conversion efficiency 49% separation and collection. Thus, the development of new photo-
has been achieved using a [Co(bpy)3]2+/3+-based electrolyte.
sensitizers concomitantly possessing absorption spectra extending
into near infrared (NIR) region and high molar extinction
Dye-sensitized solar cells (DSCs) have been considered to be a coefficients, as well as matched energy levels with TiO2 and a
promising photovoltaic technology to address the ever increasing redox electrolyte is highly demanded.
energy and environmental challenges since the pioneering work by
Additional acceptor chromophores have recently been intro-
1
¨
O’Regan and Gratzel. To date, power conversion efficiencies duced in D–p–A dyes between the donor and the p-spacer, leading
(PCE) greater than 11% have been achieved using the ruthenium to a D–A–p–A architecture that facilitates intramolecular charge
(Ru)-based dyes.2 However, the relatively high cost and environ- transfer and tailors the bandgap energy for harvesting more NIR
mental concerns associated with the use of Ru dyes have prompted light.4a,9 2-Cyano-3-{6-{4-[N,N-bis(4-hexyloxyphenyl)amino]phenyl}-
strenuous efforts to develop organic dyes as an alternative choice of 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene-2-yl}acrylic acid
photosensitizers.3,4 In addition, high molar extinction coefficients (D1) has recently been reported, which contains a cyclopenta-
of organic dyes allow the use of thinner TiO2 films, which is dithiophene segment as a conjugated spacer to construct a
beneficial for enhancing charge collection and photovoltage.5,6 high absorption coefficient organic chromophore for DSCs.10
Moreover, organic dyes have versatile functional groups for tuning However, the absorption of D1 only covers the short wavelength
the electronic and optical properties. As such, many metal-free region of visible light. Here we introduce a benzothiazole–
organic dyes, especially those based on the donor–p spacer– cyclopentadithiophene (BT–CDT) moiety as a spacer into the
acceptor (D–p–A) systems, have been developed and exhibit above dye to develop a novel D–A–p–A photosensitizer D2 (Fig. 1)
relatively high performances.6,7
with much enhanced long wavelength light absorption. The
Recently, the performance of DSCs based on organic dyes has photophysical, electrochemical properties and photovoltaic char-
been further remarkably improved in conjunction with CoIII/II acteristics of the D–A–p–A dye are extensively investigated.
polypyridyl complexes as redox mediators, and a record PCE
As illustrated in Scheme S1 (ESI†), D2 was synthesized through
more than 12% has been achieved,8 which further corroborates the Suzuki coupling reaction between boronic acid substituted
organic dyes as a superior option for highly efficient DSCs. triarylamine and aldehyde precursor 2 using Pd(PPh3)4 as a catalyst,
followed by treatment with cyanoacetic acid in the presence of
piperidine as a catalyst in acetonitrile. D1 was synthesized following
a Department of Materials Science and Engineering, NUSNNI-NanoCore,
the reported procedures.10 Their chemical structures were fully
National University of Singapore, 9 Engineering Drive 1, Singapore 117575.
1
characterized by H NMR, 13C NMR, and MALDI-TOF MS (ESI†),
E-mail: qing.wang@nus.edu.sg; Fax: +65 6776-3604
b College of Chemistry and Key Laboratory of Low Dimensional Materials and
Application Technology of Ministry of Education, Xiangtan University,
Xiangtan Hunan Province, P. R. China 411105. E-mail: yanlei@xtu.edu.cn
c Institute of High Performance Computing, Agency for Science, Technology,
and Research, 1 Fusionopolis Way, No. 16-16 Connexis, Singapore 138632
† Electronic supplementary information (ESI) available: Experimental proce-
dures, DFT calculation and device characterization and fabrication details. See
DOI: 10.1039/c4cc00577e
and were found to be consistent with the proposed structures.
The UV-Vis absorption and emission spectra of D1 and D2 in
CH2Cl2 are shown in Fig. 1, and the corresponding data are
summarized in Table S1 (ESI†). Both dyes exhibit two major
absorption bands at 290–390 nm and 400–700 nm. The absorp-
tion at 290–390 nm is ascribed to the localized aromatic p–p*
transitions, while that at 400–700 nm originates from the
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Fig. 1 Chemical structures of D1 and D2 and their absorption and emis-
sion spectra measured in CH2Cl2.
Fig. 2 Illustration of the photoinduced electron injection for dye D1 on
TiO2 (101) (top panel) and for dye D2 on TiO2 (101) (bottom panel). The
crystal orbital analyses were carried out at the G-point of the Brillouin
zone. The electron density of selected orbitals are plotted at the isosurface
of 8 ꢀ 10ꢁ4 a.u. The calculated electron injection times are also presented.
intramolecular charge transitions. The maximum absorption
wavelengths (lmax) of D1 and D2 appear at 512 and 556 nm,
and the corresponding maximum extinction coefficients are
4.3 ꢀ 104 and 5.6 ꢀ 104 Mꢁ1 cmꢁ1, respectively, which are much charge separation upon photoexcitation. The photoinduced
higher than that of the ruthenium dye N719 (B104 Mꢁ1 cmꢁ1).11 electron injection mechanism analyzed by the G-point crystalline
The lmax of D2 is red-shifted by 44 nm as compared to D1, due to orbital calculation is summarized in Fig. 2. The result is in line
a better delocalization of electrons over the p-conjugated mole- with the commonly accepted mechanism of DSCs that the
cules when a BT–CDT moiety is used as the linker. Moreover, electron is transferred from the dye to the substrate through
when the two dyes are adsorbed onto 2.5 mm thick TiO2 films, as a a photo-excitation process, followed by an electron injection
result of deprotonation and presumably H-aggregate formation,12 process. The electron injection rate estimated by the Newns–
the absorption peaks of D1 and D2 are blue-shifted to 461 and Anderson approach is B7 and 3 fs for D1 and D2, respectively,
536 nm (Table S1 and Fig. S1, ESI†), respectively. Apparently, D1 suggesting that it is not significantly affected by the presence of
reveals a sharp hypsochromic shift by 51 nm in the absorption the benzothiazole moiety in D2. The values are also in close
spectrum, while it is only 20 nm for D2 (Fig. S1, ESI†).
accordance with other computational results for the D–p–A
To evaluate the energetics of the two dye molecules, cyclic dyes,15 suggesting that electron injection for both dyes is fast
voltammetry was performed to measure the oxidation potentials enough to ensure efficient electron injection.
(ED/D+), which correspond to the HOMO level potentials of the
The j–V characteristics of DSCs sensitized with D1 and D2 on
dyes. As summarized in Table S1 (ESI†), the half-wave potentials 7.0 mm TiO2 films (without scattering layer) employing a [Co(bpy)3]2+/3+
of D1 and D2 are 1.02 and 1.09 V (vs. NHE), respectively, based electrolyte are shown in Fig. S3 and Table S2 (ESI†). The D1
indicating that the HOMO levels of both dyes are significantly cells exhibited a maximum PCE of 6.53% ( jsc = 11.8 mA cmꢁ2, Voc
=
more positive than that of [Co(bpy)3]2+/3+ (0.57 V vs. NHE),7c and 768 mV, and FF = 0.72). Under the same conditions, the cells
ensuring fast regeneration of the dyes. The excited state potential sensitized with D2 exhibited a PCE of 7.99% ( jsc = 15.2 mA cmꢁ2
,
(ED*/D+) reflecting the LUMO level of the dye can be derived from Voc = 786 mV, and FF = 0.67). The superior PCE of D2 is attributed to
the ground-state oxidation potential and the zero–zero excitation the relatively high jsc stemming from its broader absorption
energy (E0–0) determined from the intersection of the normalized and enhanced molar extinction coefficient in the visible region.
absorption and emission spectra.13 The ED*/D+ of D1 and D2, The drastic enhancement of jsc for D2 over D1 was also consistently
determined to be ꢁ1.01 and ꢁ0.86 V vs. NHE, respectively, is seen in the incident photon-to-electron conversion efficiency (IPCE)
more negative than the conduction band of TiO2 at approximately spectra (Fig. S2, ESI†).
ꢁ0.50 V vs. NHE, indicating that the driving force is sufficiently
In order to further improve the performance of D2, TiO2
electrodes consisting of mesoporous TiO2 layers and scattering
large for effective electron injection from the two dyes.14
The density functional theory (DFT) calculations were carried particles (4.8 mm transparent + 2 mm scattering layers) were
out to provide more insight into the electronic properties of the employed. The optimized PCE of D2 reaches 9.01% with a jsc of
dyes. The electron distributions in HOMO and LUMO levels of 17.0 mA cmꢁ2 and a Voc of 749 mV. The IPCE spectrum of the
the two dyes are shown in Fig. S2 (ESI†). The HOMOs of both D1 cell is shown in Fig. 3, with the onset wavelength extending to
and D2 are fairly delocalized over the TPA-donor and spacer 780 nm and external quantum efficiency generally above 80%
moieties and the LUMOs are extended over the cyanoacetic unit in the range of 400–650 nm. This matches well with its broader
and part of the spacer. Such a spatially well-separated orbital absorption spectrum and enhanced molar extinction coeffi-
distribution is highly desirable for efficient intramolecular cient. Interestingly, the PCE of the D2 cell was even raised to
3966 | Chem. Commun., 2014, 50, 3965--3968
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0
EDR rate constants (krg + kedr0), while that in redox inactive
inert cells is 19.6 sꢁ1 (kedr0). Regeneration efficiency (Zreg) of D2
0
is calculated to be more than 99.9% with krg0/(krg + kedr0).
In summary, we report a new D–A–p–A organic dye – D2. Due
to the presence of a strong electron-withdrawing benzothiadia-
zole unit in the p-bridge, D2 exhibits much enhanced absorp-
tion of long wavelength photons compared with the reference
D–p–A dye. Computational and experimental studies reveal that
D2 has ultrafast charge injection in TiO2 upon photoinduced
excitation and almost unity regeneration efficiency in a [Co(bpy)3]2+/3+
-
based electrolyte in DSCs. As a result, the cells achieved a power
conversion efficiency 49% under AM1.5 1 sun illumination, which is
much superior to the D–p–A dye under similar conditions. These
results suggest that rational structural engineering of organic dyes in
conjunction with a favorable redox mediator could concomitantly
improve the jsc and Voc of DSCs and eventually lead to much
enhanced power conversion efficiency.
Fig. 3 J–V characteristics of a DSC sensitized with D2. The inset shows
the IPCE spectrum of the cell. Light intensity for the J–V measurement is
AM1.5G 100 mW cmꢁ2
.
This research was supported by the National Research
Foundation Singapore under its Competitive Research Program
(CRP Award No. NRF-CRP4-2008-03) and Ministry of Education
Tier 2 research grant (MOE2011-T2-2-130). The authors would also
like to thank the NSFC (Nos. 91333206; 20974091; 50803051),
the Natural Science Foundation of Hunan Province of China
(No.10JJ1002) and the Foundation of the Hunan Provincial Educa-
tion Department (10B107) for financial support.
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