Novel organic dyes with anchoring group of quinoxaline-2, 3-diol and the application in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2014.09.019

Two organic quinoxaline dyes (WQ-1 and WQ-2) with a structure of quinoxaline-2, 3-diol as the electron withdrawing and anchoring group were synthesized and applied in the dye-sensitized solar cells. Fourier transform infrared spectroscopy and two other re

Full text of DOI:10.1016/j.dyepig.2014.09.019

Accepted Manuscript
Novel organic dyes with anchoring group of Quinoxaline-2, 3-diol and the application
in dye-sensitized solar cells
Lei Wang, Xichuan Yang, Xiuna Wang, Licheng Sun
PII:
S0143-7208(14)00378-7
10.1016/j.dyepig.2014.09.019
DYPI 4532
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 3 June 2014
Revised Date: 7 September 2014
Accepted Date: 9 September 2014
Please cite this article as: Wang L, Yang X, Wang X, Sun L, Novel organic dyes with anchoring group of
Quinoxaline-2, 3-diol and the application in dye-sensitized solar cells, Dyes and Pigments (2014), doi:
10.1016/j.dyepig.2014.09.019.
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Graphical Abstract for Paper

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Novel Organic Dyes with Anchoring Group of Quinoxaline-2, 3-diol
and the Application in Dye-sensitized Solar Cells
Lei Wang, a Xichuan Yang, *, a Xiuna Wang a and Licheng Sun a, b
a State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT), DUT-KTH
Joint Education and Research Center on Molecular Devices, Dalian 116024, China.
b Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute
of Technology, 100 44 Stockholm, Sweden.
* E-mail: yangxc@dlut.edu.cn; Fax: (+86) 411-84986250
ABSTRACT: Two organic quinoxaline dyes (WQ-1 and WQ-2) with a structure of
quinoxaline-2, 3-diol as the electron withdrawing and anchoring group were synthesized and
applied in the dye-sensitized solar cells. Fourier transform infrared spectroscopy and two other
reference dyes (WQ-R1 and WQ-R2) without the hydroxyl groups were introduced to ascertain
the adsorption properties of the dye series. The effect of the new electron acceptor and anchoring
group on the performance of solar cells was investigated systematically. Under the standard light
illumination (100 mW m-2), WQ-2 got an efficiency of 2.25%, with a short circuit photocurrent
density of 5.51 mA cm-2, an open circuit voltage of 0.612 V and a fill factor of 66.74%.
KEY WORDS: DSSCs; organic sensitizers; quinoxaline-2, 3-diol; anchoring group; FTIR
INTRODUCTION
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Due to the tremendous energy demands today, dye-sensitized solar cells (DSSCs) which were
first developed by O’Regan and Grätzel in 1991[1] have drawn much attention during the past
three decades.[2-4] The main components of a DSSC are photoanode, electrolyte and counter
electrode. The photoanode is composed of conductive electrode coated with a semiconductor
material that is sensitized with dye molecules which absorb the sunlight and transfer their excited
electrons to the conduction band of the semiconductor.[8-16]
For organic sensitizers with hydroxyl as anchoring group, owing to the special direct one-step
electron injection from the dyes to TiO2 by photo-excitation of the dye-to-TiO2 charge-transfer
bands (DTCT),[17-20] the energy loss decreases which is beneficial to improve the efficiency of
DSSCs. The representative dye sensitizers often employ catechol, phenol and their derivatives as
anchoring groups in the dye structures. An etal. reported a class of thiophene-catechol light
harvesting molecules used in DSSCs and the efficiency was 0.76%.[21] Tae further improved the
efficiency to about 1.6% also with catechol as anchoring group.[22] A traditional D–π–A structure
was introduced with 2-hydroxybenzonitrile as the electron acceptor/anchoring unit in our group
and the best efficiency was about 3.4%.[23] With colourless sulfur/iodide-based hybrid
electrolyte, the sensitizers with hydroxylpyridium unit reported by Zhao et al. got an efficiency
of 2.6%.[24] The efficiency is not very high mainly because the fast electron recombination and
the narrow absorption spectra of type II sensitizers.[25]
Therefore one strategy to improve the performance is to modify the structure of type II organic
sensitizers, especially for the anchor group. As a common structure used as the pesticide and raw
material for industrial synthesis, quinoxaline has been applied in organic sensitizers as electron
withdrawing bridge which successfully broadens the spectra response.[26-28] In this work, one of
its derivatives quinoxaline-2, 3-diol with enediol structure was introduced as a new type of
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anchoring group in the type II dye molecules to further strengthen the electron withdrawing
property and further improve the absorption spectra. Triphenylamine was introduced as the
donor part, and the structures of WQ-1 and WQ-2 were shown in Figure 1.
EXPERIMENTAL SECTION
General Synthetic Procedure
As shown in Scheme 1, both dyes were synthesized according to the traditional methods
including amino group protection and deprotection reaction, Suzuki reaction, N-alkylation
coupling reaction. MS data were obtained with GC-TOF MS (UK), HP1100 LC/MSD (USA),
1
and LC/Q-TOF MS (UK). H NMR spectra were taken with VARIAN INOVA 400MHz (USA)
using TMS as standard. Commercial reagents and solvents were used as received without further
purification.
di-tert-butyl(4-bromo-1,2-phenylene)dicarbamate (1). 4-bromobenzene-1, 2-diamine (3.74
g, 20 mmol) and di-tert-butyl dicarbonate (9.34 g, 43 mmol) were dissolved in 100 mL ethanol
and stirred for 8 h at room temperature. The solvent was then evaporated and the crude product
was purified by column chromatography using CH2Cl2/hexane (v/v, 1/3) to get compound 1 as a
white solid (7.36 g, yield 95%). 1: 1H NMR (400 MHz, Acetone-d6) δ 8.00 (d, J = 12.7 Hz, 2H),
7.87 (s, 1H), 7.49 (d, J = 8.6 Hz, 1H), 7.26 (dd, J = 8.7, 2.3 Hz, 1H), 1.48 (s, 18H). TOF HRMS-
EI(m/z) calcd. for C16H23N2O4Br [M+] 386.0841, found 386.0838.
4–(Diphenylamino)phenylboronic acid (2). To a 100 mL two necked flask containing a
n
solution of 4–bromotriphenylamine (7.13 g, 22 mmol) in dried THF (50 mL), BuLi (10.8 mL,
27 mmol, 2.5 M) was added slowly at –78 oC under N2 while stirring. After stirring for 1 h at –78
oC, trimethyl borate (3.2 mL, 27 mmol) was added into the solution and maintained at –78 oC for
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1 h. After stirring at room temperature for another 8 h, water was added to quench the reaction
mixture and diluted HCl (2 M) was added until an acidic mixture was obtained. The reaction
mixture was poured into water and extracted with CH2Cl2 (3 × 50 mL). The combined organic
layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was
purified by column chromatography using CH2Cl2/ethyl acetate (v/v, 1/1) as eluent. The boronic
1
acid compound 2 was obtained as a white solid (4.90 g, yield 76%). 2: H NMR (400 MHz,
DMSO-d6) δ 7.88 (s, 2H), 7.67 (d, J = 8.3 Hz, 2H), 7.30 (t, J = 7.8 Hz, 4H), 7.10 – 6.98 (m, 6H),
6.88(d, J = 8.3 Hz, 2H).
di-tert-butyl(4'-(diphenylamino)-[1,1'-biphenyl]-3,4-diyl)dicarbamate (3). A mixture of
compound 1 (2.71 g, 7 mmol), compound 2 (2.08 g, 7.2 mmol), THF (80 mL), K2CO3 aqueous
solution (2 M, 5 mL), and Pd(PPh3)4 (50 mg, 0.05 mmol) was degassed and charged with N2.
o
The mixture was stirred at 70 C for 8 h. The reaction mixture was poured into water and
extracted with ethyl acetate (3 × 50 mL). The combined organic layers were dried with
anhydrous Na2SO4 and evaporated. The residue was purified by column chromatography using
1
CH2Cl2/hexane (v/v, 1/1) to afford 3 as a yellow solid (3.24 g, yield 84%). 3: H NMR (400
MHz, Acetone-d6) δ 8.00 (s, 2H), 7.87 (s, 1H), 7.62 (d, J = 8.4 Hz, 1H), 7.60 – 7.54 (m, 2H),
7.40 (dd, J = 8.4, 2.2 Hz, 1H), 7.36 – 7.28 (m, 4H), 7.14 – 7.04 (m, 8H), 1.51 – 1.50 (m, 18H).
TOF HRMS calcd. for C34H37N3O4 [M+] 551.2784, found 551.2791.
WQ-1. To a 100 mL two necked flask with compound 3 (1.66 g, 3 mmol) in 20 mL CH2Cl2
solution, 2 mL trifluoroacetic acid in 10 mL anhydrous CH2Cl2 was added slowly and stirred at
room temperature for 4 h. Then 50 mL H2O was added into the reaction solution, white solid was
o
precipitated and filtered. After dried at 50 C for 2 h, 1 equiv Oxalic acid dihydrate was added
and grinded in mortar for 20 min. The mixture was added into an autoclave with teflon lining
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under N2 and heated at 170 C for 6 h. After cooled into room temperature, the solid was
dissolved and purified by column chromatography using CH2Cl2/CH3OH (v/v, 50/1) to afford
1
WQ-1 as a pale yellow solid (0.24 g, yield 21%). WQ-1: H NMR (400 MHz, DMSO-d6) δ
11.96 (s, 2H), 7.50 (d, J = 8.1 Hz, 2H), 7.33 (t, J = 7.4 Hz, 6H), 7.18 (d, J = 8.2 Hz, 1H), 7.05 (d,
J = 8.1 Hz, 8H). TOF HRMS calcd. for C26H19N3O2 [M+] 405.1477, found 405.1481.
N, N-diphenyl-4-(thiophen-2-yl) aniline (4). 4–bromotriphenylamine and thiophen-2-yl
1
boronic acid were reacted as reaction c to get the white compound 4 (yield 81%). 4: H NMR
(400 MHz, Acetone-d6) δ 7.60 – 7.55 (m, 2H), 7.40 – 7.35 (m, 2H), 7.35 – 7.29 (m, 4H), 7.12 –
7.02 (m, 9H). TOF HRMS calcd. for C22H17NS [M+] 327.1082, found 327.1072.
N, N–diphenyl–4–(5–(4, 4, 5, 5–tetramethyl–1, 3, 2–dioxaborolan–2–yl)thiophen–2–yl)
n
aniline (5). To a solution of 2 (0.794 g, 2.43 mmol) in THF (50 mL), BuLi (1.17 mL, 2.5 M in
o
o
n-hexane) was added dropwisely at –78 C under N2. The reaction was kept at –78 C for 1 h.
Then 2–isopropoxy–4, 4, 5, 5–tetramethyl–1, 3, 2–dioxaborolane (0.456 g, 2.45 mmol) was
added, the mixture was allowed to warm to room temperature and stirred overnight. The solution
was extracted with ether, and the combined organic layers were dried over anhydrous Na2SO4
and evaporated to dryness. The residue was purified by chromatography on silica gel eluting
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with hexane/ethyl acetate (v/v, 20/1) to afford 5 as a yellow solid (0.77 g, 70%). 5: H NMR
(400MHz, CDCl3) δ 7.63 – 7.52 (m, 1H), 7.39 – 7.28 (m, 5H), 7.19 – 7.03 (m, 10H), 1.38 (s,
12H). GC/TOF HRMS-EI (m/z): [M]+ calcd. for C28H28NO2SB, 453.1934; found, 453.1939.
di-tert-butyl (4-(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-1,2-phenylene)dicarbamate
(6). Compound 5 and compound 1 were reacted as reaction c to get the yellow solid 6 (yield
70%). 6: 1H NMR (400 MHz, Acetone-d6) δ 8.02 (s, 2H), 7.93 (s, 1H), 7.65 – 7.59 (m, 3H), 7.45
(dd, J = 8.4, 2.2 Hz, 1H), 7.38 (d, J = 2.2 Hz, 3H), 7.33 (dd, J = 8.5, 7.4 Hz, 3H), 7.14 – 7.08 (m,
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6H), 7.06 (d, J = 8.7 Hz, 2H), 1.51 (d, J = 4.8 Hz, 18H). TOF HRMS calcd. for C38H39N3O4S
[M+] 633.2661, found 633.2670.
1
WQ-2: WQ-2 was gained by compound 6 by a similar reaction as WQ-1. WQ-2: H NMR
(400 MHz, DMSO-d6) δ 11.96 (d, J = 26.2 Hz, 2H), 7.59 (d, J = 8.7 Hz, 2H), 7.43 (d, J = 1.8
Hz, 1H), 7.42 – 7.36 (m, 2H), 7.34 (t, J = 7.9 Hz, 5H), 7.16 (d, J = 8.4 Hz, 1H), 7.08 (dd, J =
12.7, 7.4 Hz, 6H), 6.99 (d, J = 8.7 Hz, 1H). TOF HRMS calcd. for C30H21N3O2S [M+] 487.1354,
found 487.1366.
Preparation of Dye-sensitized Solar Cells
The DSSCs were fabricated as follows. A double-layer TiO2 photoelectrode (thickness 14 ꢀm,
area 6 × 6 mm) was used as a working electrode. Fluorine-doped tin oxide (FTO) glass plates
(Pilkington-TEC8) were cleaned using an ultrasonic bath by detergent and ethanol each for 30
min. A layer of 10 nm TiO2 paste (T/SP, SALARONIX) was coated on the FTO glass by screen
o
printing and then dried for 3 min at 130 C. This procedure was repeated for 6 times to get a 10
ꢀm film, then a 4 ꢀm layer of 300 nm (DHS-SLP1, Heptachroma, China) TiO2 paste was screen-
printed as scattering layer. The double-layer TiO2 electrodes were gradually heated under an air
flow to 500 °C and maintained for 30 min. The sintered film was further treated with 40 mM
TiCl4 aqueous solution at 70 ºC for 30 min, then washed with water and ethanol, and annealed at
500 ºC for 30 min. After the film was cooled to 80 °C, it was immersed into a 2 × 10-4 M dye
solution in a mixed solvent of ethanol and dichloromethane (v/v, 4/1) under dark for 10 h. Co-
sensitization was done in a solution with 2 × 10-4 M dye and 0.5 equiv, 1.0 equiv CDCA in the
solution, respectively. The sensitized TiO2 electrode was then rinsed with the ethanol and dried.
The hermetically sealed cells were fabricated by assembling the dye-loaded film as the working
electrode and sintered Pt/FTO counter electrode (Heptachroma, China) separated with a hot-melt
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Surlyn 1702 film (60 ꢀm, Dupont). Electrolyte consisting of 0.3 M 1, 2-dimethyl-3-n-
propylimidazolium iodide (DMPII), 0.25 M LiI, 0.01 M I2, 0.05 M tetrabutyl ammonium iodide
(TBAI), 0.1 M 4-terbutyl pyridine in acetonitrile/valeronitrile (AN/VN = 85/15, v/v) was
injected into the cell and sealed with a cover glass.
Prepared solar cells were characterized by current–voltage characteristics (J-V) and incident
photon-to-current conversion efficiency (IPCE). Current–voltage characteristics were carried out
with an AM 1.5G solar simulator (Newport, UK). The incident light intensity was 1000 W m-2
calibrated with a standard Si solar cell. For the J–V curves, the active area was 0.1256 cm-2. The
measurement of the incident photon-to-current conversion efficiency (IPCE) was performed by a
Hypermono-light (SM-25, Jasco Co. Ltd., Japan). The Jsc was calibrated by integrating the IPCE
value tuned light density of AM 1.5 against wavelength.
RESULTS AND DISCUSSION
The UV-Vis absorption spectra of WQ-1 and WQ-2 were collected and displayed in Figure 2.
Considering the dissolving property of the dye series, a mixed solvent of ethanol and
dichloromethane (v/v, 4/1) was chosen in this work. The UV-Vis absorption spectra of the WQ
dye solution and the dye-adsorbed transparent TiO2 films (T/SP, SOLARONIX, doctor-blade
printed on a clean glass, sensitized 1 h in dye bath) were recorded on a HP8453 (USA)
spectrophotometer. In the solution, owing to the simple conjugation system of the dye series, the
absorption spectra of both dyes were cutted off at 450 nm. Compared with other similar dye
structures, existence of the heterocyclic ring in the acceptor unit increased the electron
withdrawing property and the absorption maxima was red-shifted further.[23] For WQ-1, the
maximum absorption peak was at 346 nm, after the introduction of thiophene to increase the
conjugation system and ease the spatial tension, the absorption of WQ-2 in the solution was red-
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shifted for about 35 nm and the extinction coefficient increased to 2.81×104 M-1 cm-1. After
adsorbed on the TiO2 film, although the peak for WQ-1 is affected by the noise of blank spectra
of TiO2 film, the absorption peaks became broader clearly and were extended to about 500 nm.
The color of dyed films was apparently darker than that of the dye solution which was also
showed in the inset photograph. The interaction between dye molecules and TiO2 surface leads to
the aggregation and the form of DTCT band, and both consequently, broaden the absorption
spectra that have been gained. [20, 26]
Chenodeoxycholic acid (CDCA) has been broadly employed as a co-adsorbent to make the
arrangement of dye molecules on the TiO2 surface more orderly.[29, 30] To further investigate
adsorption properties, UV-Vis spectra of the WQ dyes on the film co-adsorbed with CDCA were
also collected and showed in Figure 3. The absorption peaks of both dyes were red-shifted with
the addition of CDCA in the dye solution. Previous studies showed after DTCT bands were
formed by adsorbed on the TiO2, the LUMO levels were decreased accordingly, which caused
the red-shift of the UV-Vis spectra. Through the increase of CDCA, the aggregation was
suppressed continuously, the peaks were narrowed [31-33] and this trend became more obvious.
Electrochemical redox potentials were showed in Figure 4. The energy levels of WQ dyes
were determined by cyclic voltammetry (CV) and the corresponding data were also collected in
Table 1. The data were obtained using a three-electrode cell by the electrochemistry workstation
(CHI630, CHINA). The working electrode was a glass carbon disk electrode, the auxiliary
electrode was a Pt wire, and Ag/Ag+ was used as the reference electrode. Tetrabutylammonium
hexafluorophosphate was used as supporting electrolyte in CH2Cl2/CH3CH2OH (v/v, 1/4). The
ferrocenium/ferrocene (Fc/Fc+) redox couple was used as an internal potential reference also
showed in Figure 4. Table 1 showed photo-electrical property of the quinoxaline-2, 3-diol dyes.
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The ground state oxidation potentials vs. NHE were calibrated by addition of 440 mV to the
-
potentials vs. Fc/Fc+, which were sufficiently positive compared to the redox potential of the I-/I3
couple (0.35 V vs. NHE) as an electrolyte. Additionally the estimated LUMO levels of both dyes
were much more negative than the conduction-band edge of TiO2 (–0.5 V vs. NHE), ensuring an
efficient electron injection process from the excited state of dye molecules into the conduction
band of TiO2. The expansion of π-conjugated system of WQ-2 decreased the energy gap
between HOMO and LUMO. Furthermore, the large difference between the TiO2 conductive
band and the LUMO levels of the dyes suggested that 4-tert-butylpyridine could be used in the
electrolyte to yield a higher photo-voltage.
The energy state and electron distribution were calculated to gain insight into the
photoelectrical properties of WQ dye series. The structural and electronic properties of the dyes
were calculated with DFT (density functional theory) at the B3LYP/6-31G (d) level in gas phase
using the Gaussian 09 program package, TD-SCF methods were introduced to calculate the
energy of excitation state. The optimized geometries and isodensity plots of the frontier orbitals
of the chromospheres were shown in Table S1. Indicated by the calculation results, the HOMO
was mostly located on the triphenylamine and π-bridge moiety, the LUMO was mostly located
on the quinoxaline-2, 3-diol moiety. It was also clear that the HOMO and LUMO localized more
reasonable for WQ-2 owing to the introduction of thiophene. Compared with the unbounded
states, the calculations of electron densities showed that the photostimulated exciton (LUMO
density) was localized more on the Ti-hydroxyl chelate site in the adsorbed WQ dyes, which was
a proof of DTCT band.[20, 21] The energy calculation result also proved the narrow down of
energy levels after the adsorption of WQ dyes, which will cause the absorption red-shifted and is
in accordance with the experimental result.
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The FTIR spectra of dye powder and the dye-loaded TiO2 powder were collected by the FTIR
equipment (NEXUS, America) and the results were showed in Figure 5. The TiO2 films were
immersed into the dye solutions overnight in the dark, dried and scraped off the glass to form the
samples. To collect the spectra of dye powder, KBr was chosen to be a blank. For the dyed TiO2
powder, the spectra were recorded with TiO2 as a blank. As we have known, quinoxaline-2, 3-
diol had two resonant structures—“keto” form and “enolic” form. After the synthesis was
finished, no further treating method was introduced, and the NMR data illustrated the structure of
the dye series were in the “enolic” form. Confirmed by checking the library of FTIR spectra,[34]
here FTIR showed again the same result, as the stretching vibration of hydroxyl group appeared
at about 3500 cm-1, the peak at 1270 cm-1 was the stretching vibration of C-O bond in hydroxyl
group. Peaks at about 1690 cm-1, 1590 cm-1, 1470 cm-1 were assigned to the skeleton vibration of
aromatic structures. There was little change for the benzene skeleton after adsorbed on the TiO2,
but for the C=N stretching vibration peaks at 1690 cm-1, the wavenumber was decreased and
shifted to around 1620 cm-1, which could indicate the existence of relation between the nitrogen
and TiO2,[35-38] or it was caused by the change of electronic effect after adsorption of O-H on
TiO2.
To determine which interaction caused this change, we also synthesized two other reference
dyes WQ-R1 and WQ-R2 without the hydroxyl groups, the structures were showed in Figure 6
and the synthesis details were summarized in Scheme S1 in the supporting information.
Unexpectedly, although both dyes with or without steric hindrance displayed better photo
absorption properties in the solution, there was no adsorption phenomena observed on the TiO2
film by UV-Vis equipment after immersing the TiO2 film into the reference dye solution for
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more than 48 h. So the shifts of C=N vibration peaks were caused mainly by change of electronic
effect and WQ dyes were adsorbed by the hydroxyl groups.
The dye series were fabricated into the DSSCs then to investigate the photovoltaic properties.
The J-V curves of both dyes were collected and showed in Figure 7, all the data were
summarized in Table 2. The efficiency for WQ-1 and WQ-2 were 1.18% and 1.58%
respectively, mainly because of the low short circuit photocurrent density (Jsc). CDCA were
introduced to co-sensitize with the WQ dye, and the consistency of CDCA in the sensitizing
solution was changed to optimize the performance. When the molar ratio was 1/0.5
(Dye/CDCA), both dyes gained the best efficiency. Under the optimal condition, for WQ-1 the
highest open circuit voltage (Voc) was 0.662 V, because of the narrow light absorption range, a
Jsc of 4.10 mA cm-2 was gained, with a fill factor (FF) of 69.5%, the eventual efficiency was
1.89%. WQ-2 got a higher Jsc of 5.51 mA cm-2 owing to the extended light absorption response,
although Voc decreased to 0.612 V, a better efficiency of 2.25% was gained eventually. The
further increase of CDCA didn’t improve the performance because competitive adsorption
between dye molecules and CDCA was not beneficial for the solar cells. Figure 6b showed the
IPCE curves of the solar cells co-adsorbed by WQ dye and CDCA. For the comparatively
narrow absorption range, IPCE responses were cutted at about 600 nm, and the summit of IPCE
curves was about 73% got by WQ-2 at 420 nm. Considering the comparatively low extinction
efficient of WQ dyes, the electron injection efficiency was high.
The electrochemical impedance spectroscopy (EIS) also was employed to study the interfacial
charge transfer processes in DSSCs based on the WQ dyes. EIS tests were carried out using an
impedance/gain-phase analyzer (Zennium Zahner，German) with forward bias under dark. The
spectra were scanned in a frequency range of 10-2-106 Hz at room temperature. The alternate
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current (AC) amplitude was set at 10 mV, under dark conditions with an applied bias voltage of -
0.680 V. This result was collected in Figure 8. The intermediate frequency semicircle in the
Nyquist plot represents the charge transfer phenomena between the TiO2 surface and the
electrolyte.[39] This recombination resistance decreased in the order of WQ-1 > WQ-2.
Additionally the electron recombination lifetimes (τ) were calculated from the angular frequency
(ωmin) at the intermediate–frequency peak in bode phase plot using τ = 1/ωmin. The calculation
results showed the same order as the recombination resistance. Co-sensitization of CDCA on the
surface of TiO2 further helped to increase the recombination resistance of the solar cells, for WQ
dyes, the addition of 0.5 equiv CDCA was the best condition in Figure 7, which was in
consistence with performance of the solar cells.
CONCLUSION
Two organic dye sensitizers with quinoxaline-2, 3-diol were synthesized and applied into the
dye-sensitized solar cells. As a new electron acceptor and anchoring group, quinoxaline-2, 3-diol
showed a good electron withdrawing property and the absorption of dyes red-shifted compared
with the other similar dye molecules without heterocyclic ring. FTIR study showed that WQ
dyes were adsorbed by hydroxyl groups, which was also proved by the reference dyes without
hydroxyl groups. Overall the WQ dyes showed comparatively good performance given their
simple structures. Under the standard light illumination (AM 1.5G, 100 mW cm-2), WQ-2 got an
efficiency of 2.25%, with Jsc = 5.51 mA cm-2, Voc = 0.612 V, FF = 66.74% and the IPCE value
reached 73%.
APPENDIX Supporting Information
Supporting information includes the synthetic routes of reference dyes and the gaussian
caculation result of the WQ dyes.
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ACKNOWLEDGEMENT
We gratefully acknowledge the financial support of this work from China Natural Science
Foundation (Grant 21276044, 21120102036, 91233201, 51372028), the National Basic Research
Program of China (Grant No. 2014CB239402), the Swedish Energy Agency, K&A Wallenberg
Foundation.
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Captions for Figures, Schemes and Tables
Table 1 Photophysical and electrochemical properties of WQ-1, WQ-2 dyes
Table 2 Photovoltaic performance of DSSCs based on WQ-1, WQ-2 dyes
16

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Scheme 1 Synthetic routes of WQ dyes
Figure 1 The structures of dyes WQ-1, WQ-2
Figure 2 The absorption spectra of WQ-1, WQ-2 in ethanol and dichloromethane (v/v, 4/1)
solution (a) and on the TiO2 films (b); inset is the photograph of the solutions and films (c)
Figure 3 The absorption spectra of WQ-1 (a), WQ-2 (b) on the films co-adsorbed with CDCA
Figure 4 CV curves of WQ dyes
Figure 5 FTIR spectra of WQ-1 and WQ-2 dye powder and dye on the TiO2
Figure 6 The structures of reference dyes WQ-R1, WQ-R2
Figure 7 The J-V curves (a) and IPCE spectra (b) of DSSCs based on WQ-1, WQ-2 dyes
Figure 8 The Nyquist (a) and Bode (b) plots of DSSCs based on WQ-1 and WQ-2
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Table 1. Photophysical and electrochemical properties of WQ-1, WQ-2 dyes
HOMO a
ε
λmax on
LUMO
b
λmax
E0-0
TiO2
(×104
(V vs.
(V vs.
Dye
(nm)
(V)
M-1cm-1)
(nm)
350
370
NHE)
NHE)
346
382
2.61
2.81
0.824
0.776
2.93
2.74
-2.10
-1.96
WQ-1
WQ-2
a
The oxidation potentials of the dyes were measured and converted to NHE
b
by addition of 440 mV. E0–0 was determined from intersection of the tangent
of absorption on TiO2 film and the X axis by 1240/λ.

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Table 2 Photovoltaic performance of DSSCs based on WQ-1, WQ-2 dyes
Voc
Jsc
(mA/cm2)
2.78
FF
η
CDCA
Dye
(mV)
(%)
(%)
0
0.615
0.662
0.586
0.580
0.612
0.610
69.0
69.5
69.1
68.40
66.74
67.30
1.18
1.89
1.29
1.58
2.25
1.99
0.5 equiv
1.0 equiv
0
4.10
WQ-1
3.19
3.99
0.5 equiv
1.0 equiv
5.51
WQ-2
4.86

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N
N
N
N
OH
OH
S
OH
OH
N
N
WQ-2
WQ-1
Figure 1 The structures of dyes WQ-1, WQ-2

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(a) di-tert-butyl dicarbonate, r.t., 8 h; (b) (1) nBuLi, THF, -78 oC, 1 h; (2) B(OMe)3, -78 oC, 1 h; (3)
HCl, 0.5 h; (c) 1, di-tert-butyl (4-bromo-1,2-phenylene)dicarbamate, Pd(PPh3)4, K2CO3 (2 M),
o
THF, 70 C, overnight; (d) (1) trifluoroacetic acid, CH2Cl2, r. t. , 4 h; (2) oxalic acid dihydrate,
170 oC, 6 h; (e) 2-thiopheneboronic acid, Pd(PPh3)4, K2CO3 (2 M), THF, 70 oC, overnight; (f) (1)
o
o
nBuLi, THF, -78 C, 1 h; (2) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, -78 C, 1 h;
(g) 1, trifluoroacetic acid, CH2Cl2, r.t., 4 h; (h) (1) trifluoroacetic acid, CH2Cl2, r. t. , 4 h; (2) oxalic
acid dihydrate, 170 oC, 6 h.
Scheme 1 Synthetic routes of WQ dyes

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Figure 2 The absorption spectra of WQ-1, WQ-2 in ethanol and dichloromethane (v/v, 4/1) solution
(a) and on the TiO2 films (b); inset is the photograph of the solutions and films (c)

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Figure 3 The absorption spectra of WQ-1 (a), WQ-2 (b) on the films co-sensitized
with CDCA

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Figure 4 CV curves of WQ dyes

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Figure 5 FTIR spectra of WQ-1 and WQ-2 dye powder and dye on the TiO2

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Figure 6 The structures of reference dyes WQ-R1, WQ-R2

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Figure 7 The J-V curves (a) and IPCE spectra (b) of DSSCs based on WQ-1, WQ-2
dyes

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Figure 8 The Nyquist (a) and Bode (b) plots of DSSCs based on WQ-1 and WQ-2