Insight into the effects of modifying chromophores on the performance of quinoline-based dye-sensitized solar cells

Article abstract of DOI:10.1016/j.saa.2017.09.002

A series of organic dyes based on quinoline as an electron-deficient π-linker, were designed and synthesized for dye sensitized solar cells (DSSC) application. These push-pull conjugated dyes, sharing same anchoring group with distinctive electron-rich donating groups such as N,N-diethyl (DEA-Q), 3,6-dimethoxy carbazole (CBZ-Q), bis(4-butoxyphenyl)amine (BPA-Q), were synthesized by Riley oxidation of [sbnd]CH₃ followed by Knoevenagel condensation of the corresponding aldehyde precursors 2a–c with cyanoacrylic acid. The optical, electrochemical, theoretical calculation and photovoltaic properties with these three dyes were systematically investigated. Compared to DEA-Q and CBZ-Q, BPA-Q possesses better light harvesting properties with regard to extended conjugate length, red-shifted intramolecular charge transfer band absorption and broaden light-responsive IPCE spectrum, resulting in a greater short circuit photocurrent density output. BPA-Q also has improved open-circuit voltage due to the apparent large charge recombination resistance. Consequently, assembled with iodine redox electrolytes, the device with BPA-Q achieved the best overall conversion efficiency value of 3.07% among three dyes under AM 1.5G standard conditions. This present investigation demonstrates the importance of various N-substituent chromophores in the prevalent D-π-A type organic sensitizers for tuning the photovoltaic performance of their DSSCs.

Full text of DOI:10.1016/j.saa.2017.09.002

Accepted Manuscript
Insight into the effects of modifying chromophores on the
performance of quinoline-based dye-sensitized solar cells
Mao Mao, Jian-Bo Wang, Xiu-Lin Liu, Guo-Hua Wu, Xia-Qin
Fang, Qin-Hua Song
PII:
DOI:
Reference:
S1386-1425(17)30710-2
doi: 10.1016/j.saa.2017.09.002
SAA 15436
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Revised date:
Accepted date:
4 June 2017
23 August 2017
3 September 2017
Please cite this article as: Mao Mao, Jian-Bo Wang, Xiu-Lin Liu, Guo-Hua Wu, Xia-
Qin Fang, Qin-Hua Song , Insight into the effects of modifying chromophores on the
performance of quinoline-based dye-sensitized solar cells, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.09.002
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ACCEPTED MANUSCRIPT
Insight into the effects of modifying chromophores on the
performance of quinoline-based dye-sensitized solar cells
Mao Mao a,*, Jian-Bo Wang b, Xiu-Lin Liu b, Guo-Hua Wu c, Xia-Qin Fang d, Qin-Hua Song b,*
School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing
a
210044, PR China
b Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China
c College of Science and Technology, Nihon University, 1-18-14 Kanda Surugadai, Chiyodaku, Tokyo
101-8308, Japan
d
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of
Sciences, Hefei 230031, PR China
Abstract
A series of organic dyes based on quinoline as an electron-deficient π-linker, were designed and
synthesized for dye sensitized solar cells (DSSC) application. These push-pull conjugated dyes, sharing
same anchoring group with distinctive electron-rich donating groups such as N,N-diethyl (DEA-Q),
3,6-dimethoxy carbazole (CBZ-Q), bis(4-butoxyphenyl)amine (BPA-Q), were synthesized by Riley
oxidation of −CH3 followed by Knoevenagel condensation of the corresponding aldehyde precursors
2a-c with cyanocrylic acid. The optical, electrochemical, theoretical calculation and photovoltaic
properties with these three dyes were systematically investigated. Compared to DEA-Q and CBZ-Q,
BPA-Q possesses better light harvesting properties with regard to extended conjugate length,
red-shifted intramolecular charge transfer band absorption and broaden light-responsive IPCE spectrum,
* Corresponding authors. Fax: +86-551-63601592
E-mail address: mmao@nuist.edu.cn (M. Mao), qhsong@ustc.edu.cn (Q. Song)
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resulting in a greater short circuit photocurrent density output. BPA-Q also has improved open-circuit
voltage due to the apparent large charge recombination resistance. Consequently, assembled with
iodine redox electrolytes, the device with BPA-Q achieved the best overall conversion efficiency value
of 3.07% among three dyes under AM1.5 G standard conditions. This present investigation
demonstrates the importance of various N-substituent chromophores in the prevalent D-π-A type
organic sensitizers for tuning the photovoltaic performance of their DSSCs.
Keywords: Dye-sensitized solar cells, organic sensitizer, quinoline, donor-π-acceptor,
electron-deficient
1. Introduction
As a result of depleting fossil fuel reserve, global warming and related issues, there is tremendous
growth of interest in environmental friendly and renewable energy sources −− especially solar energy
and solar cells [1–3]. In particular, among the devices that can convert sunlight into electricity,
dyes-sensitized solar cells (DSSCs) are increasing popular because of their impressive efficiency, low
manufacturing cost, flexibility in terms of colours and appearance, facile manufacture, and good
stability [3–7]. Research on DSSCs has been very active since they were epoch-making breakthrough
by O’Regan and Grätzel in 1991 [1].
Notoriously, the design of DSSCs generally consists of four components: a dye, a nanocrystalline
photoanode, a counter electrode, and a redox-shuttle. As the pivotal components in determining the
light-harvesting properties and controlling electron-transfer processes at the TiO2-dye-electrolyte
interfaces in DSSCs, photosensitizing dyes have been widely exploited over the last two decades and
achieved substantial improvements in ultimate power output [8–11]. Numerous studies have been
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reported on structure-property relationship of metal-bearing (typically Ru and Zn complexes) and
metal-free pure organic dyes. Up to now, through intensive studies, the highest power conversion
efficiency of DSSCs based on Zn-porphyrin complexs and dithienopicenocarbazole dyes with cobalt
redox mediator has reached a record of 13% under standard conditions [12,13]. The metal-free dyes,
nonetheless, have exhibited several advantages as promising class of sensitizer including the flexible
structural modulation and large molar extinction coefficients as compared to the organometallic
complexes [14,15]. Nowadays, how to improve photovoltaic performance with a simple organic dye
structure is still a challenge for DSSCs. Use of appropriate electron-donating groups is of particular
importance in the design approach of sensitizers as they evidently tune optical, orbital energy level and
electrochemical properties further influence the performance of the resultant dyes. To efficiently
facilitate intramolecular charge transfer (ICT) from donor to the acceptor and reduce charge
recombination upon photoirradiation, the typical donor–π–acceptor (D–π–A) configuration with
electronic push-pull effects has been a suitably exploited architecture for molecular design. The
diphenylamine, one of the most prospective candidates in D–π–A dyes, has been extensively studied
owing to its excellent electron-donating ability and photostability [16]. Indeed, organic dyes bearing the
diphenylamine derivatives produce some of the best efficiencies of organic chromophores reported to
date for DSSC applications [12]. Carbazole is structurally analogous to the diphenylamine unit except
that an additional C−C bond that links two phenyl rings [17–19]. On the other hand, it should be noted
that scientists have devoted themselves to exploring new rigidified π linker to optimize molecule’s
photovoltaic performances, focusing especially on the study of electron-withdrawing units which can
extend the absorption spectra via decreasing the highest occupied molecular orbital (HOMO)-lowest
unoccupied molecular orbital (LUMO) gap of dye molecules [20]. For example, quinoxaline [21,22],
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thiazolothiazoles [23], benzothiadiazole [24,25], benzotriazole [26], diphenylquinoxaline [27], and
diketopyrrolopyrroles [28,29] have been used as building blocks in functional materials. Quinoline, as a
electron-withdrawing unit with a rigid planar structure, is promising to be useful in DSSCs as
metal-free photosensitizers [30–34]. The unsaturated nitrogen atom on the quinoline ring enhances its
electron affinity.
In the present work, we have designed and synthesize a series of D–π–A dyes with various donors as
photosensitizers for gaining further insight into quinoline system (Chart 1). All designed
quinolin-framed sensitizers were functionalized with the cyanoacetic acid anchoring group that
functions to covalently link the sensitizer onto the mesoporous TiO2 surface. DEA-Q is a small
molecule size sensitizer featuring a simple N,N-diethyl group as an donor. The DSSCs fabricated with
DEA-Q displayed a relatively low power conversion efficiency of 0.72%. Based on DEA-Q, we
attempted to insert diarylamine chromophores such as 3,6-dimethoxy carbazole and
bis(4-butoxyphenyl)amine instead dialkyamine unit to construct another two dyes named CBZ-Q
(0.75%) and BPA-Q (3.07%), respectively. Both of them demonstrated enhanced efficiencies compared
with DEA-Q. Interestingly, with a small change in the molecular design, BPA-Q based DSSCs
exhibited a great enhancement in photocurrent compared to CBZ-Q, which can be ascribed to the
higher molar absorption coefficient with a red shift in the absorption band. The impacts of introducing
different electron donors N,N-diethyl, 3,6-dimethoxy carbazole, bis(4-butoxyphenyl)amine on the
quinoline’s light absorptions, frontier molecular orbital energy levels, electrochemical properties and
photovoltaic parameters will be scrutinized via joint experimental measurements and theoretical
calculations.
Chart 1 here
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2. Experimental
2.1. Materials and reagents
All reagents and materials in the experiments were purchase from J&K, Aladdin, TCI and
Sigma-Aldrich and used without further purification. Solvents for measurements of spectroscopy are
high performance liquid chromatography (HPLC) grade. All chemical reactions were carried out under
N2 atmosphere and monitored by thin layer chromatography. The starting materials
6-bromo-2-methylquinoline,
6-(diethylamino)quinoline-2-carbaldehyde
(2a),
and
6-(bis(4-butoxyphenyl)amino)quinoline-2-carbaldehyde (2c) were prepared according to reported
literatures [34–36].
2.2. Analytical instruments and measurements
1
Melting points were uncorrected. H and 13C NMR spectra were measured with a Bruker AV
spectrometer operating at 300 and 75 MHz, respectively and chemical shifts were reported in ppm
using tetramethylsilane (TMS) as the internal standard. Mass spectra were obtained with a Micromass
GCT-TOF or Thermo LTQ Orbitrap mass spectrometers. FTIR spectra were carried out on a Bruker
Vector22 infrared spectrometer. UV-Vis absorption and fluorescence emission spectra were recorded at
room temperature with a Shimadzu UV-2450 PC UV/Vis spectrometer and a Shimadzu RF-5301PC
fluorescence spectrometer, respectively. Cyclic voltammetry was carried out on a CHI620D
multipurpose electrochemical analyzer in CH2Cl2 solution with 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) as supporting electrolyte. A glassy carbon, a Pt wire, a saturated
calomel were employed as working, counter, and reference electrode, respectively.
Ferrocene/ferrocenium (+ 0.63 V vs. normal hydrogen electrode (NHE)) was used as an external
standard.
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2.3. Fabrication of dye-sensitized solar cells
The nanocrystalline titanium dioxide (TiO2) electrode and the platinum (Pt) cathode were prepared
according to our previous report [37–38]. Nanocrystalline electrodes are about 12 μm thickness
[determined by a profilometer (XP-2, AMBIOS Technology Inc.)]. A solution of 0.3 mM of the dye in
dry CHCl3 was prepared, and the TiO2 electrodes were immersed in the solution for 12 h at the room
temperature. After rinsing with CHCl3 and EtOH to remove the unbound dye, the electrodes were
placed under vacuum for 2 h for further drying. The dye-adsorbed TiO2 electrodes was assembled with
a thermally Pt counter electrode by use of a hot-melt Surlyn film as a spacer to produce a
sandwich-type
electrochemical
cell.
The
electrolyte
solution
containing
0.6
M
1,2-dimethyl-3-n-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2 and 0.5 M
tert-butylpyridine (TBP) in CH3CN was injected to the sealed device from the pre-cut channel on the
counter electrode. Subsequently, the hole was sealed by a Surlyn film and a cover glass. To prevent
inflated photocurrents arising from stray light, a black metal mask surrounded the active area during all
measurements. The total covered active electrode area of DSSC was 0.25 cm2 (0.5 cm × 0.5 cm). The
photocurrent-voltage (J-V) characteristics of the solar cells were carried out using a Keithley 2420 3A
source meter controlled by Test-point software under solar simulator (solar AAA simulator, oriel USA,
calibrated with a standard crystalline silicon solar). The monochromatic incident photo-to-current
conversion efficiency (IPCE) of the DSSCs was measured using a 300 W Xe lamp light source with
monochromatic incident light. Electrochemical impedance spectroscopy (EIS) measurements of the
DSSCs were carried out using an AUTOLAB PGSTAT 302 N analyzer in the frequency region from 50
mHz to 1000 kHz.
2.4. Synthesis procedures and characterization data of new compounds
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2.4.1. 3,6-dimethoxy-9-(2-methylquinolin-6-yl)-9H-carbazole (1b)
A mixture of 6-bromo-2-methylquinoline [35] (0.44g, 2mmol), 3,6-dimethoxy-9H-carbazole (0.56g,
2.5mmol), CuI (76 mg, 0.4mmol), 1,10-phenanthroline (72 g, 0.4mmol) and t-BuOK (0.28g, 2.5mmol)
in toluene (15ml) was stirred and heated to reflux under N2 for 12h. After cooling to room temperature,
the solvent was removed by rotary evaporation, and the crude product was purified by column
chromatography on silica gel using dichloromethane and petroleum ether (PE) as eluent to give
compound 1b (0.43 g, 58%) as a yellow solid. Rf = 0.33 (EtOAc/PE = 1:9). m.p. 154–156 oC. 1H NMR
(CDCl3, 300 MHz, ppm) δ 8.23 (d, J = 9.0 Hz, 1 H, ArH), 8.11 (d, J = 8.4 Hz, 1 H, ArH), 7.94 (d, J =
2.1 Hz, 1 H, ArH), 7.87 (dd, J = 8.7 Hz, J = 2.1 Hz, 1 H, ArH), 7.58 (d, J = 2.4 Hz, 2 H, ArH), 7.38 (d,
J = 8.7 Hz, 3 H, ArH), 7.05 (dd, J = 9.0 Hz, J = 2.4 Hz, 2 H, ArH), 3.96 (s, 6 H, OCH3), 2.82 (s, 3 H,
CH3). 13C NMR (CDCl3, 75 MHz, ppm) δ 159.3, 154.1, 146.3, 136.2, 135.8, 135.0, 132.7, 130.2, 129.3,
128.4, 127.5, 123.9, 123.7, 122.7, 119.2, 115.1, 110.5, 102.9, 56.0, 25.2. IR (KBr, cm–1): 3048, 2948,
1592, 1486, 1375, 829. HRMS (EI) calcd for M+ C24H20N2O2: 368.1525, found 368.1533.
2.4.2. 6-(3,6-dimethoxy-9H-carbazol-9-yl)quinoline-2-carbaldehyde (2b)
A portion of dioxane solution (7 mL) dissolved with 1b (0.74 g, 2.0 mmol) was added to dioxane (3
mL) suspended with SeO2 (0.33 g, 3.0 mmol) at about 60 oC with certain min under N2 atmosphere and
the mixture was heated at 80 oC for 8 h. After cooling, the mixture was extracted with ethyl acetate and
washed by saturated solution of NaCl. Solvent was removed and 2b (0.32 g, 42%) was obtained as
yellow solid via a silica gel chromatography column using mixtures of CH2Cl2 and PE as eluent. Rf =
1
0.67 (10% EtOAc/PE = 1:9). m.p. 210–212 oC. H NMR (CDCl3, 300 MHz, ppm) δ 10.28 (s, 1 H,
CHO), 8.45 (d, J = 9.6 Hz, 1 H, ArH), 8.35 (d, J = 9.6 Hz, 1 H, ArH), 8.13–8.05 (m, 3 H, ArH), 7.58
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(d, J = 2.4 Hz, 2 H, ArH), 7.53 (d, J = 9.0 Hz, 2 H, ArH), 7.07 (dd, J = 9.0 Hz, J = 2.4 Hz, 2 H, ArH),
3.97 (s, 6 H, CH3). 13C NMR (CDCl3, 75 MHz, ppm) δ 193.3, 154.5, 152.5, 146.2, 138.8, 136.8, 135.6,
132.2, 130.8, 129.2, 124.3, 123.2, 118.1, 113.3, 110.5, 103.1, 56.0. IR (KBr, cm–1): 2997, 2831, 1698,
1617, 1469, 1206, 770 cm-1. HRMS (EI) calcd for M+ C24H18N2O3: 382.1317, found 382.1333.
2.4.3. (E)-2-cyano-3-(6-(diethylamino)quinolin-2-yl)acrylic acid (DEA-Q)
To a stirred solution of compound 2a [36] (50 mg, 0.09 mmol) and cyanoacetic acid (76 mg, 0.9
mmol) in CHCl3 (15ml) was added EtOH (5ml) and piperidine (50 μl). The reaction mixture was
refluxed for 12 h under nitrogen atmosphere. After removing the solvent, the residue was purified by
chromatograph using CH2Cl2 and CH3OH (10:1) as eluent to give DEA-Q (35 mg, 50%) as red solid.
Rf = 0.18 (CH3OH/CH2Cl2 = 1:9). m.p. 187–189 oC. 1H NMR (DMSO, 300 MHz, ppm) δ 8.31 (s, 1 H,
CH), 8.12 (d, J = 8.4 Hz, 1 H, ArH ), 7.86–7.82 (m, 2 H, ArH), 7.52 (dd, J = 9.6 Hz, J = 2.7 Hz, 1 H,
13
ArH), 6.93 (d, J = 2.4 Hz, 1 H, ArH), 3.57–3.50 (m, 4 H, CH2), 1.19 (t, J = 6.9 Hz, 6 H, CH3). C
NMR (DMSO, 75 MHz, ppm) δ 163.9, 152.8, 147.6, 144.3, 141.3, 133.5, 131.2, 131.0, 124.0, 120.3,
116.2, 104.1, 102.7, 44.3, 12.7. IR (KBr, cm–1): 2969, 2216, 1608, 1500, 1353, 821. HRMS (EI) calcd
for (M–CO2)+ C16H17N3 251.1422, found 251.1426.
2.4.4. (E)-2-cyano-3-(6-(3,6-dimethoxy-9H-carbazol-9-yl)quinolin-2-yl)acrylic acid (CBZ-Q)
According to the synthetic procedure of dye DEA-Q, CBZ-Q was prepared from compound 2b and
cyanoacetic acid in a yield of 58% as a red solid. Rf = 0.31 (CH3OH/CH2Cl2 = 1:9). m.p. 234–235 oC.
1H NMR (DMSO, 300 MHz, ppm) δ 8.66 (d, J = 8.7 Hz, 1 H, ArH), 8.44 (s, 1 H, CH), 8.27–8.33 (m, 2
H, ArH), 8.11 (d, J = 8.7 Hz, 2 H, ArH), 7.88 (d, J = 2.1 Hz, 2 H, ArH), 7.52 (d, J = 8.7 Hz, 2 H, ArH),
7.08 (dd, J = 8.7 Hz, J = 2.1 Hz, 2 H, ArH), 3.90 (s, 6 H, OCH3). 13C NMR (DMSO, 75 MHz, ppm) δ
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162.9, 154.3, 152.4, 147.3, 146.0, 137.2, 136.7, 135.2, 131.3, 128.9, 128.7, 124.0, 123.6, 122.5, 118.2,
115.6, 110.9, 103.7, 55.9. IR (KBr, cm–1): 2924, 2855, 2222, 1615, 1489, 1381, 1203. HRMS (EI)
calcd for (M–CO2)+ 405.1477, found 405.1508.
2.4.5. (E)-3-(6-(bis(4-butoxyphenyl)amino)quinolin-2-yl)-2-cyanoacrylic acid (BPA-Q)
According to the synthetic procedure of dye DEA-Q, BPA-Q was prepared from compound 2c [34]
and cyanoacetic acid in a yield of 60% as a black solid. Rf = 0.44 (CH3OH/CH2Cl2 = 1:9). m.p.
o
171–173 C. 1H NMR (300 MHz, DMSO[D6], ppm): δ 8.14 (s, 1 H, CH), 8.12 (d, J = 9.0 Hz, 1 H,
ArH), 7.88 (d, J = 8.7 Hz, 1 H, ArH), 7.81 (d, J = 9.0 Hz, 1 H, ArH), 7.33–7.29 (m, 1 H, ArH),
7.16–7.12 (m, 4 H, ArH), 7.00–6.96 (m, 5 H, ArH), 3.97 (t, J = 6.3 Hz, 4 H, CH2), 1.73–1.66 (m, 4 H,
CH2), 1.48–1.41 (m, 4 H, CH2), 0.94 (t, J = 7.2 Hz, 6 H, CH3). 13C NMR (75 MHz, DMSO[D6], ppm):
δ 163.4, 156.3, 148.7, 148.2, 143.2, 139.2, 134.8, 130.3, 129.6, 127.7, 124.5, 122.5, 115.9, 111.1, 67.5,
40.0, 18.9, 14.0. IR (KBr, cm–1): 2956, 2869, 2219, 1608, 1505, 1377, 1240, 829. HRMS (EI) calcd for
(M–CO2)+ C32H33N3O2: 491.2573, found 491.2544.
3 Results and discussion
3.1 Synthesis
The classical electron-rich N-substituted donors such as N,N-diethyl, 3,6-dimethoxy carbazole,
bis(4-butoxyphenyl)amine were used in the quinoline system to study the effect of different electro-rich
donor moieties in their optical, redox and the subsequent photovoltaic properties. The synthetic routes
of the three dyes are shown in Fig. 1. The precursor aldehydes 2a-c were prepared by Riley oxidation
of 6-substituent 2-methylquinoline (1a-c) with commercially available SeO2. The cyanoacetic acid
anchoring group was added to compounds 2a-c by a Knöevenagel reaction to yield final products
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DEA-Q, CBZ-Q, and BPA-Q in fair to excellent yields. The intermediate products and final dyes were
1
structurally characterized by H and 13C NMR spectroscopies as well as high-resolution mass
spectrometry (HRMS), and their corresponding characterization data are summarized in the
Experimental section.
Fig. 1 here
3.2 Photophysical properties of the dyes in solution and on TiO2 film
The effect of various donors substituent of quinoline spacer on absorption and emission spectra of
DEA-Q, CBZ-Q and BPA-Q has been studied by UV-Vis spectroscopy and fluorescence spectroscopy,
and the corresponding characteristic data are summarized in Table 1. Fig. 2a displays the molar
extinction coefficients change of the dye solution with the concentration 10–5 M. All three dyes exhibit
two major absorption bands at the range of 280−620 nm. The narrow absorption band around 310 nm is
attributed to the localized aromatic π−π* transitions of the conjugated system. The major absorption
band in the visible region (400−620 nm) is assigned to the intramolecular charge-transfer (ICT)
transition. As tabulated in Table 1, the ICT peak (absλmax) in the low-energy absorption band of DEA-Q
is observed at 459 nm. The replacement of N,N-diethyl in DEA-Q by 3,6-dimethoxy carbazole in
CBZ-Q for prolonging π-conjugation length results in a hypsochromic shift of 7 nm in the ICT peak,
with a maximum absorption band at 452 nm. Remarkably, further replacing the carbazole group in
CBZ-Q with the tert-butoxy modified diphenylamine as the donor chromophore to give dye BPA-Q
(466 nm), leads to a significant red-shift of 14 nm in the ICT peak.
In general, the electron transport capacity and charge transfer properties of molecules are determined
by two factors, i.e., the ability of donor giving electrons, and the degree of conjugation between
N-substituted donor and quinoline. The red-shifting of the ICT absorption band from CBZ-Q to
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BPA-Q is mainly related to the larger torsion angle between the carbazole and quinoline (vide infra),
resulting in unfavorable charge transfer from the donor unit to the acceptor unit. On the other hand, the
electron donating capacity of diethylamine is stronger than biphenyl amine, nevertheless, biphenyl
amine is more favorable for conjugate extension (λonset : 610 nm for BPA-Q vs. 570 nm for DEA-Q).
As a result, the trend of absλmax appear to be in order of BPA-Q > DEA-Q > CBZ-Q.
From Fig. 2a, we can see that the absorption shapes are almost similar, the CBZ-Q (1.01 × 104 M–1
cm–1) has relatively lower molar extinction coefficient as compared to both BPA-Q (1.81 × 104 M–1
cm–1) and DEA-Q (2.02 × 104 M–1 cm–1) at the visible region. This could be ascribed to the lower
oscillator strength of CBZ-Q according to quantum-chemical calculations (Table 2) [39,40].
Upon adsorption onto a TiO2 film, DEA-Q, CBZ-Q and BPA-Q also display good absorption at the
light range of 400−600 nm (Fig. 2b). It was observed that the absorption maxima of all three dyes show
obviously hypsochromic shifts in their ICT characteristic bands, which may be ascribed to the
deprotonation of –COOH anchoring group for the strong intermolecular interaction between the dyes
and the TiO2 surface [41]. Meanwhile, there is a variation tendency similar to that occurring in solution
with a gradual bathochromic shift in the ICT peak can be observed from CBZ-Q (438 nm), DEA-Q
(440 nm) to DPA-Q (460 nm). The introduction of the bis(4-butoxyphenyl)amine into the quinoline
system combined with a cyanoacetic acid acceptor is expected to enhance the conjugation, thus
facilitating the ICT process in the push-pull system and possessing stronger light-harvesting capability
both in solution and onto TiO2 films.
Fig.3 shows the normalized absorption and emission spectra of the three dyes. The fluorescent
emission spectra are obtained by excitation at the corresponding wavelength of maximum absorption in
CHCl3 solution, and the emission peaks (fluorλmax) are observed at 566 nm, 550 nm and 554 nm for
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DEA-Q, CBZ-Q and BPA-Q, respectively. The calculated values of Stokes shifts (∆ῡ) are collected in
Table 1, being 4.12 × 103 cm–1 for DEA-Q, 3.94 × 103 cm–1 for CBZ-Q, 3.41 × 103 cm–1 for BPA-Q.
The smaller ∆ῡ for BPA-Q compared to those of the other two dyes implies that a smaller energy
required for geometrical reorganization of quinolin dyes at photoexcited state [42–44]. The excitation
transition energy (E0−0) values are calculated from:
E0−0 = 1240 / λint,
(1)
where λint is the intersection wavelength of the normalized absorption and emission curves. The
corresponding E0−0 values of DEA-Q, CBZ-Q and BPA-Q are 2.34, 2.48 and 2.38 eV, respectively
(Table 1).
Fig. 2 here
Table 1 here
Fig. 3 here
3.3 Electrochemical properties of the dyes
In DSSCs, it is essential to use rational molecular design to obtain promising photosensitizers with
well-matched energy levels. The HOMO and LUMO levels of the sensitizers, critically determining
dye-regeneration and photo-generated electron-injection, play a pivotal role in the ultimate power
conversion efficiencies (PCEs) in DSSCs. Herein, to thermodynamically research the feasibility of dye
regeneration and charge injection for material combination of the devices, the electrochemical
properties of three dyes, DEA-Q, CBZ-Q and BPA-Q, were analyzed by cyclic voltammograms (CV)
in 0.1 M TBAPF6/CH2Cl2 solutions (Fig. 4). The saturated calomel reference electrode (SCE) was
calibrated using the Fc/Fc+ redox couple as an external standard, and the ground-state oxidation
potential (E1/2) of the Fc/Fc+ couple was found to be 0.42 V vs. the SCE reference electrode. The
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oxidation potentials vs. the NHE were calibrated by the addition of 0.63 V to the potentials vs. Fc/Fc+.
The energy level data including E1/2, E0−0, and excited-state reduction potential E(S+/S*) are listed in
Table 1.
The HOMOs of DEA-Q, CBZ-Q and BPA-Q corresponding to the E1/2 were measured to be 1.05,
1.20 and 0.99 V (vs. NHE), respectively. The upshift of E1/2 value is usually accompanied by the
observed red-shift of the ICT absorption band. The LUMOs of DEA-Q, CBZ-Q and BPA-Q
corresponding to the E(S+/S*), calculated from the equation E(S+/S*) = E1/2 – E0–0, were –1.29, –1.28 and
–1.39 V (vs. NHE) , respectively. In principle, energy values for the I–/I3– redox couple and the
conduction band (CB) of TiO2 semiconductor in DSSCs are 0.4 V and –0.5 (vs. NHE), respectively,
and according to previously reported literature the minimum driving force requirement for dye
regeneration by the electrolyte and excited electron injection into TiO2 is about 0.3 V [45]. As shown in
the energy diagram of HOMO and LUMO levels (Fig. 5), all dyes theoretically possesses sufficient
HOMO and LUMO values for dye-regeneration and electro-injection with a driving force of more than
0.5 V, thus the relevant energies of the dyes that govern the interfacial chemistry are deemed
appropriate for sensitization, that is to say, there should be no energetic barrier for efficient electron
dye-regeneration and injection.
Fig. 4 here
Fig. 5 here
3.4 Density functional theory calculations
Density functional theory (DFT) calculations were employed using the hybrid B3LYP functional and
6-31G (d) basis set to compare the electronic properties and geometries of DEA-Q, CBZ-Q and
BPA-Q. All structural optimization and energy calculation were performed using the Gaussian 09
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program package [46]. The optimized configurations and frontier orbitals electron distributions of
molecules are shown in Fig. 6. To clarify the impact of N-substituent on the optical properties in
quinoline sensitizers, the electronic transitions at the optimized structures also have been calculated by
time-dependent DFT (TD-DFT) calculations with considering the CHCl3 solvent effects. The maximum
absorption wavelengths, oscillator strength (f) and corresponding transition assignment are listed in
Table 2.
It is been well known that carbazole is a structurally analogous to the diphenylamine unit, with the
exception of a C–C single bond that links two aromatic rings. This bond serves to increase the planarity
of the two phenyl rings thereby imparts a remarkably different electronic structure relative to the
diphenylamine unit. As shown in Fig. 6, quantum chemical calculations suggest that the additional C–C
single bond of CBZ-Q holds the carbazole unit almost perpendicular to the quinoline-bridge therefore
disrupting the conjugation of the HOMO on the donor unit and the LUMO that is localized to the
cyanoacrylic acid. From the results of the TD-DFT calculations in Table 2, the HOMO to LUMO
transition basically contributes to the characteristic ICT absorption band. The relatively better
coplanarity of the quinoline bridge with the pinwheel orientation of the diphenylamine group for
BPA-Q yields a HOMO delocalization over almost the whole molecule skeleton, whereas the HOMO
level of CBZ-Q is predominantly distributed on carbazole donor unit, indicating that replacing
carbazole with diphenylamine unit in the quinoline system effectively elongates the conjugation length
and induces the electronic coupling between the terminal amino group and quinoline core.
These differences in the spatial distribution of the frontier molecular orbital also help explain the
much higher extinction coefficients recorded for BPA-Q (1.81 × 104 M–1 cm–1) relative to CBZ-Q
(1.01 × 104 M–1 cm–1) derived from the UV-visible experimental spectra (Fig. 3). In addition, the f
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calculated for the aforementioned transition is 0.62 for DEA-Q, 0.26 for CBZQ and 0.56 for BPA-Q,
respectively, which is consistent with the trend of molar extinction coefficients (Table 2). The restricted
geometry of CBZQ also causes the HOMO to LUMO transition that is expected at 619 nm but is
scanty 452 nm by experimentally observed in the CHCl3 solution.
Fig. 6 here
Table 2 here
3.5 Photovoltaic performances
The photovoltaic performance of these dyes as sensitizers for DSSCs was tested using 0.6 M
DMPImI, 0.1 M LiI, 0.05 M I2 and 0.5 M TBP in acetonitrile as the redox electrolyte under a standard
simulated solar irradiation (AM 1.5G, 100 mW cm-2). As for the dye bath, 0.3 mM of dye was
dissolved in the CHCl3 without any co-adsorbent. The J−V profiles are provided in Fig. 7, and the
IPCE spectra as a function confirming the variation in short circuit current for DSSCs are recorded in
Fig. 8. The photovoltaic conversion efficiency (η), is calculated through the following equation:
η = Jsc × Voc × FF / P
(2)
Here, Jsc is short-circuit current density, Voc is open-circuit voltage, FF is the fill factor, and P is the
intensity of the incident light. The detailed parameters are summarized in Table 3. The BEA-Q
dye-based devices yielded a moderate efficiency of 3.07% (~ 47% of the PCE generated by the
reference DSSCs using N3 dye) with a remarkable Jsc of 6.13 mA cm−2, a Voc of 0.68 V and a FF of
0.73. As listed in Table 3, the results are significantly higher than the 0.72% (Jsc = 2.46 mA cm−2, Voc =
0.53 V, FF = 0.55) and 0.75% (Jsc = 2.09 mA cm−2, Voc = 0.58 V, FF = 0.62) recorded for the DEA-Q
and CBZ-Q based devices under the same conditions. Clearly, the change of the donor unit has a
significant influence on the device characteristics.
After careful analysis of measurement results, it has been exhibited that high η of BPA-Q mainly
arises from its increased Jsc. With respect to the device sensitized by DEA-Q with alkyl amine donor,
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CBZ-Q bearing aromatic amine group show the decreased Jsc by 0.37 mA cm−2, probably as a result of
the unfavorable charge migration in the molecular structure. In spite of this, a slightly increase of 50
mV in the Voc can be seen. By incorporating diphenylamine instead of carbazole into the skeleton of
quinoline-framed dyes, a stark improvement in the Jsc of 4.04 mA cm−2 are obtained for BPA-Q
sensitized cell ultimately, accompanied by a Voc gain about 100 mV. The Jsc of BPA-Q is higher than
those of DEA-Q and CBZ-Q by ~149% and 193%, respectively. As is known to all, the integral of the
IPCE spectrum is generally associated with the Jsc and the IPCE as a function of wavelength can be
expressed as follows:
IPCE = (1240 × Jsc) 100 / (λ × Pin) (in%)
(3)
where λ stands for a specific wavelength and Pin is a fixed input power. To better understand the
reasons for the photocurrent difference, IPCE spectra measurements for DEA-Q, CBZ-Q and BPA-Q
sensitized devices should be carried out. As discussed in the previous section, for one thing, the simple
N,N-diethyl group on DEA-Q is not an ideal group to extend the conjugated length of molecular; for
another thing, the carbazole on the CBZ-Q is almost perpendicular to quinoline group, resulting in
serious interference with the electron delocalization over the whole molecular (Fig. 6). Accompanied
by the better conjugated backbone, sensitizer BPA-Q is beneficial for collecting photons from longer
wavelength, and is supposed to present an extended IPCE response with respect to the other two.
As shown in Fig. 8, all dyes convert sunlight into photocurrent in the UV-visible range and the
onsets of the IPCE spectra of these sensitizers-based DSSCs are 600 nm (DEA-Q), 611 nm (CBZ-Q)
and 700 nm (BPA-Q), respectively. The IPCE action curves become broader from DEA-Q to BPA-Q,
which is in good agreement with their respective light-harvesting of the dye-loaded TiO2 films (Fig. 2b).
The DEA-Q based DSSCs has relatively low IPCE and gave over only 20% IPCE values from 400 to
510 nm and displays the maximum value of 30% at around 460 nm. Meanwhile, the broader IPCE
spectrum is observed for the device based on CBZ-Q than that of DEA-Q, owing to its broader
absorption after being adsorbed on the TiO2 surface. Nevertheless, the relatively lower molar extinction
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coefficient in the visible region (Table 1) and the smaller dye loading (Table 3) impairs the light
harvesting capacity, consequently, the IPCE of CBZ-Q is inferior to that of DEA-Q in most regions of
the spectra, especially, in the range of 410–530 nm. Hence, it can be predicted that DEA-Q based
device should present higher Jsc than that of CBZ-Q. Exhilaratingly, exceeding 40% IPCE value can be
observed for BPA-Q based DSSC in a broad region ranged from 400 to 575 nm, accompanying with a
maximum IPCE value of 55% at 462 nm. In comparison with the diethylamine and carbazole donors,
dye bearing diphenylamine show relatively broader IPCE spectra with higher plateaus, and the IPCE
tendency of these dyes conforms well to the order of their Jsc values: BPA-Q > DEA-Q > CBZ-Q.
This outcome may be comprehended in three reasons: firstly, BPA-Q possesses the red-shifted and
broaden absorption profile both in solution and on TiO2 film (Fig. 2) caused by the concerted
contribution from intensified charge separation and conjugation between donor and acceptor (Fig. 6
and Table 2) associated with the improved electron-donating ability of the diphenylamine group.
Secondly, the larger dihedral angle in CBZ-Q could impede more efficient photoinduced electron
transfer from the dye to the TiO2 conduction band than the smaller dihedral angle in BPA-Q. The
higher absolute value of LUMO level of BPA-Q sufficiently guarantees the larger difference between
the excited state and the CB of TiO2, indicating a stronger driving force for electron injection and more
charge collection on TiO2 (Fig. 5). Thirdly, the dye BPA-Q has an augmented dye loading amount than
the other two dyes. The photoelectric performance of DSSCs can be enhanced by proper modification
of organic dyes using various donors. The highest and broadest IPCE response of BPA-Q explained its
highest Jsc value from the J-V measurement, which strongly indicates bis(4-butoxyphenyl)amine unit is
beneficial to obtain the light-harvesting.
Fig. 7 here
Table 3 here
Fig. 8 here
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Electrochemical impedance spectroscopy (EIS) was performed to elucidate the origin of the higher
Voc values of BPA-Q based cell. Generally, the Voc of a DSSC is relevant to the charge transfer
processes at the TiO2-dye-electrolyte interface [47]. The Nyquist plots (i.e. minus the imaginary part of
the impedance (–Z’’), versus the real part of the impedance (Z’), when sweeping the frequency) for
these sensitizers based DSSCs under a forward bias of –0.60 V in the dark are shown in Fig. 9.
Typical Nyquist diagrams for DSSCs consist of three regimes. The first semicircle in highest frequency
range represents charge transfer resistance at the redox electrolyte/Pt counter electrode interface. The
second large semicircle at the intermediate frequency is ascribed to the recombination resistance across
the TiO2-dye-redox electrolyte interface (Rct). The third semicircle in the low frequency region stands
for the impedance of Warburg ion diffusion in the electrolyte, which is too small and overlaps with the
second large one.
A larger Rct means more efficiently charge recombination is blocked, indicating a smaller dark
current and higher Voc, and a longer electron lifetime is also usually accompanied by a slower charge
recombination of electrons in TiO2 and electron acceptors in the electrolyte [48–50]. One sees that the
radius decreases in the order of BPA-Q > CBZ-Q > DEA-Q. Calculated by the Z-view software and
the inset model, the corresponding resistance of DEA-Q, CBZ-Q and BPA-Q are 25, 57 and 109 Ω,
respectively, implying attenuating charge recombination process at the TiO2-dye-electrolyte interface
(DEA-Q > CBZ-Q > BPA-Q). For the quinoline based DSSCs, BPA-Q based device obtains the most
favorable results of charge recombination hindrance leading to the best experimental results of the Voc
(0.68 V). On the contrary, the DEA-Q based device shows the poorest resistance values during charge
transfer process, not surprisingly, the lowest Voc of 0.53V is obtained. The rod structure of DEA-Q
does not effectively keep the electrolyte from contacting the semiconductor surface [9]. Therefore, it is
conjectured that pyramidal geometry of donor together with higher dye amount for BPA-Q may be the
intrinsic reason for increased charge recombination resistance. Additionally, the double butoxy chains
introduced on the diphenylamine unit improve the solubility of the dye as well as potentially suppress
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the dark current by blocking the electrolytes from close contact with the TiO2 surface [51–53].
In brief, the best photovoltaic performance of dye BPA-Q can be attributed to its greatly enhanced
IPCE response and significantly large charge recombination resistance on the basis of the above studies,
yielding more efficient photoelectric conversion than that of DEA-Q and CBZ-Q.
Fig. 9 here
The relatively better performance of BPA-Q based device arouses our interest to further optimize its
performances by the co-adsorption strategy. Consequently, chenodeoxycholic acid (CDCA) was
incorporated in the CHCl3 bath during the sensitization process to hinder the formation of sensitizer
aggregation and insulate TiO2 films from electrolyte oxidant [54–61]. After the preliminary attempt,
BPA-Q with 2 mM CDCA led to a superior  of 3.15% with a Jsc of 6.24 mA cm–2, a Voc of 0.69 V and
an FF of 0.73 under standard conditions. As compared to the DSSC based on BPA-Q in the absence of
the CDCA, the Jsc and Voc slightly increases which might be attributed to dissociation of aggregation
–
and the enhancement of the blocking of I3 upon addition of CDCA, which ameliorates the electron
injection efficiency [58–60]. However, the dye loading on TiO2 film is reduced by co-adsorption of 2
mM CDCA (2.65 × 10-7 mol cm–2 in absence of CDCA versus 2.12 × 10-7 mol cm–2 with 2 mM CDCA)
due to the occupation of CDCA on TiO2 surface. We also tried to introduce 20 mM CDCA, regrettably,
the performance of DSSCs (η = 1.23%, Jsc = 4.40 mA cm−2, Voc = 0.42 V, FF = 0.65) dropped rapidly,
owing to an increasing loss of light harvesting molecules (dye loading 0.90 × 10-7 mol cm–2) which
limited the photoinduced collection [61]. Similar trend was also observed for the other two dyes
substituted with different donor group on addition of CDCA. Different proportions of electrolyte
(DMPImI / LiI/ I2 / TBP) were also investigated for BPA-Q based device, however, the PCE has not
improved significantly. There is still plenty of room for improvement, and other electrolyte, such as
Cobalt electrolytes instead of iodide source will be studied for fabricating more efficient DSSCs in our
future work.
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4. Conclusions
In conclusion, three efficient organic dyes DEA-Q, CBZ-Q and BPA-Q based on electron-deficient
quinoline have been designed and synthesized for DSSCs application. These D-π-A type dyes consist
of different donor such as N,N-diethyl (DEA-Q), 3,6-dimethoxy carbazole (CBZ-Q),
bis(4-butoxyphenyl)amine (BPA-Q) and same cyanoacrylic acid anchoring group. The optical and
electrochemical properties combined with the theoretical calculation confirmed that the
bis(4-butoxyphenyl)amine serving as the donor is beneficial to higher Jsc by means of improved
light-harvesting property as a result of the notable bathochromic shift of ICT absorption wavelength
and increased molar absorption coefficient compared to that of N,N-diethyl and 3,6-dimethoxy
carbazole units. Meanwhile, the relatively better coplanarity of the quinoline bridge with the pinwheel
orientation of the diphenylamine for BPA-Q is favor of effective π-conjugation throughout the whole
molecule skeleton, which can facilitate the electron transfer from the donor to acceptor. BPA-Q based
device also obtains the most favorable results of charge recombination hindrance and leads to the best
experimental results of the Voc. As a result, among the three dyes, the BPA-Q based DSSCs under AM
1.5G illumination exhibited a highest conversion efficiency of 3.07% (Jsc = 6.13 mA cm−2, Voc = 0.68 V,
FF = 0.73), which reached 47% of N3 cells under the same conditions. These established
structure-property relationships give the deep insight on the impact of various N-substituent donors in
prevalent D-π-A conjugated organic sensitizers on device performances.
Acknowledgment
We are grateful for financial support from the National Natural Science Foundation of China (Grant
Nos. 21272224 and 21406189) and Anhui Provincial Natural Science Foundation (Grant No.
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1708085MB33).
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Table 1. Summary of photophysical and electrochemical properties for DEA-Q, CBZ-Q and BPA-Q.
abs
a
fluor
b
d
e
f
Dye
λ
λ
∆ῡ c
λmax
E1/2
E0–0
E(S+/S*) g
(V vs. NHE)
max
max
(nm)
(nm)
566
550
(103 cm–1)
(nm)
441
438
(V vs. NHE)
(eV)
2.34
2.48
459 (2.02)
452 (1.01)
4.12
3.94
1.05
–1.29
–1.28
DEA-Q
CBZ-Q
1.20
466 (1.81)
554
3.41
460
0.99
2.38
–1.39
BPA-Q
a
Corresponds to maximum wavelength of lowest-energy absorption band in CHCl3 solutions; ε in units of 104 M–1
cm–1 indicated in parentheses. Maximum wavelength of emission band after excitation at
b
abs
c
λ
.
Stokes shift
max
calculated with the relationship: ∆ῡ =1/ absλmax–1/ fluorλmax. d Absorption maximum on 4 μm TiO2 films. e Measured in
CH2Cl2 solutions with 0.1 M TBAPF6 (calibrated with ferrocene as an external reference and converted to the NHE by
addition of 0.63 V). f E0–0 estimated from the intersection wavelength (λint) of the normalized absorption and emission
curves in CHCl3 by 1240/λint. g calculated with the relationship: E(S+/S*) = E1/2 – E0–0
.
Table 2 Maximum absorption wavelengths, orbital energies, oscillator strength (f) and assignment of
DEA-Q, CBZ-Q and BPA-Q in CHCl3 solution optimized at B3LYP/61G(d) level.
Dye
λmax
(nm)
493
Energy level
(eV)
f
Transition
assignment a
H—›L (70%)
2.51
0.62
0.26
0.56
DEA-Q
CBZ-Q
BPA-Q
619
523
2.00
2.37
H—›L (71%)
H—›L (70%)
a H —› HOMO; L —› LUMO.
29