Structure-property relationship of naphthalene based donor-π-acceptor organic dyes for dye-sensitized solar cells: Remarkable improvement of open-circuit photovoltage

Article abstract of DOI:10.1039/c2jm34363k

Four new donor-π-acceptor organic dyes (YF01-04), containing naphthalene-substituted amines as an electron donor and cyanoacrylic acid as an electron acceptor, were designed and synthesized, and their photophysical properties and dye-sensitized solar cells (DSCs) performances were characterized. Dyes YF02 and YF04, with 2,6-disubstituted naphthalene frameworks, were superior than their analog dyes YF03 and YF01, having 1,2-disubstituted naphthalene moiety, in incident-photo-to-current conversion efficiency (IPCE) and total solar-to-electric conversion efficiency (η). The DSCs based on YF02, comprised of diphenylamine moiety as the donor, produced the highest η of 5.29% compared to 4.03% of the analog dye YF04, which has pyrrolidine as the donor. Remarkably, a high open-circuit photovoltage (V _oc) of 0.799-0.807 V was achieved in the cases of YF02-03, which have diphenylamine-donors. To better understand the structure-property relationship for DSCs application, molecular modelling was performed on YF01-04 and vertical electronic excitations were calculated using long-range corrected energy functional WB97XD and CAM-B3LYP at the basis set level DGDZVP, which were in excellent agreement with the experimental results. Moreover, the equilibrium molecular geometries of dyes YF01-04 were calculated at the density function theory (DFT) level using the hybrid energy functional B3LYP and basis set DGDZVP. The torsion angles () between the naphthalene moiety and diphenylamine donor in YF02 and YF03 were more twisted than that of the pyrrolidine-donor dyes YF01 and YF04, precluding efficient intermolecular π-π charge transfer, which translated into high V_oc. Compared to the reference dye TA-St-CA, which is based on diphenylamine as an electron donor linked to a phenyl ring, YF02 achieved higher V_oc, which indicated that naphthalene substituted with diphenylamine is more efficient in retarding charge recombinations.

Full text of DOI:10.1039/c2jm34363k

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ISSN 0959-9428
PAPER
M. Vallet-Regí et al.
Supramolecular mechanisms in the synthesis of mesoporous magnetic
nanospheres for hyperthermia
0959-9428(2012)22:1;1-2
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ARTICLE TYPE
Structure-Activity Relationship of Naphthalene Based Donor-π-
Acceptor Organic Dyes for Dye-Sensitized Solar Cells: Remarkable
Improvement of Open-Circuit Photovoltage
Yang Fan,a Md. Akhtaruzzaman,*b Ashraful Islam,c Tienan Jin,*a Ahmed El-Shafei,*d Chuanjiang Qin,c
Liyuan Han,c Yoshinori Yamamoto,ae Khalid A. Alamry,e Samia A. Kosa,e Mahmoud A. Husseine and
Abdullah Mohamed Asirie
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Four new donor-π-acceptor organic dyes (YF01-04) containing naphthalene-substituted amines as an
10 electron donor and cyanoacrylic acid as an electron acceptor were designed and synthesized, and their
photophysical properties and dye-sensitized solar cells (DSCs) performances were characterized. Dyes
YF02 and YF04 with 2,6-disubstituted naphthalene framework were superior to their analog dyes YF03
and YF01 having 1,2-disubstituted naphthalene moiety in incident-photo-to-current conversion efficiency
(IPCE) and total solar-to-electric conversion efficiency (η). The DSCs based on YF02 comprised of
15 diphenylamine moiety as the donor produced the highest η of 5.29% compare to 4.03% of the analog dye
YF04 which has pyrrolidine as the donor. Remarkably, a high open-circuit photovoltage (Voc) of 0.799-
0.807 V was achieved in the case of YF02-03, which has diphenylamine-donor. To better understand the
structure-activity relationship for DSCs application, molecular modelling were performed on YF01-04
and vertical electronic excitations were calculated using long-range corrected energy functional WB97XD
20 and CAM-B3LYP at the basis set level DGDZVP, which were in excellent agreement with the
experimental results. Moreover, the equilibrium molecular geometries of dyes YF01-04 were calculated
at the DFT level using the hybrid energy functional B3LYP and basis set DGDZVP. The torsion angles
(θ) between the naphthalene moiety and diphenylamine donor in YF02 and YF03 were more twisted than
that of the pyrrolidine-donor dyes YF01 and YF04, precluding efficient intermolecular π-π charge
25 transfer, which translated into high Voc. Compared to the reference dye TA-St-CA, which is based on
diphenylamine as an electron donor linked to a phenyl ring, YF02 achieved higher Voc, which indicated
that naphthalene substituted with diphenylamine is more efficient in retarding charge recombinations.
intramolecular
charge-transfer
characteristics.
Recently,
tremendous research efforts have been invested to improve the
efficiency of solar energy conversion. In principle, high
performance DSCs require organic dyes to produce high short-
50 circuit photocurrent (Jsc), high open-circuit photovoltage (Voc),
and high fill factor (FF).4 Generally, the efficiency of the DSC is
mainly determined by Jsc and Voc. Jsc is usually correlated to the
absorption ability and electron injection efficiency of organic
dyes, while the effect of Voc is considered to be more complicated
55 than Jsc. Therefore, in order to further improve the efficiency of
DSCs, fundamental understanding of structure-activity
relationship is needed to reduce the charge recombination and
increase the electron injection efficiency.5 In addition, charge
recombination and electron injection efficiency are closely
60 correlated with the Voc, which means that Voc becomes the key
factor among many factors responsible for enhancing the
performance of DSCs. One strategy to improve the Voc is to
decrease the molecular -stacked aggregation of dyes on the TiO2
surface because strong dye aggregation can lead to self-
Introduction
Dye-sensitized solar cells (DSCs) are one of the most promising
30 next-generation photovoltaic cells due to their versatile, energy-
saving, and environmentally friendly nature in comparison with
the traditional silicon-based solar cells.1 At present, the state-of-
the-art DSCs based on ruthenium(II)–polypyridyl complexes as
the active material have an overall power conversion efficiency
35 (η) approaching 12% under standard (Global Air Mass 1.5)
illumination.2 However, the rarity and high cost of the ruthenium
metal may limit their development for large-scale applications.
Metal free organic dyes have attracted considerable attention in
recent years due to their high molar extinction coefficients,
40 flexible structural modifications and low costs. In the past few
years, many researchers have focused on developing metal-free
organic sensitizers, and some of these endeavours have achieved
decent efficiencies.3 Among them, the donor-(π-spacer)-acceptor
(D-π-A) conjugated system is the basic structure for designing the
45 organic sensitizers due to their effective photo-induced
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Journal of Materials Chemistry
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35 General information for materials synthesis
1H NMR and 13C NMR spectra were recorded on JEOL JMTC-
1
270/54/SS (JASTEC, 400 MHz) spectrometers. H NMR spectra
are reported as follows: chemical shift in ppm (δ) relative to the
chemical shift of CDCl3 at 7.26 ppm, integration, multiplicities (s
40 = singlet, d = doublet, t = triplet, q = quartet and m = multiple),
and coupling constants (Hz). 13C NMR spectra reported in ppm
(δ) relative to the central line of triplet for CDCl3 at 77.00 ppm.
High-resolution mass spectra were obtained on a BRUKER
APEXIII spectrometer. Column chromatography was carried out
45 employing Silica gel 60N (spherical, neutral, 40~100 µm,
KANTO Chemical Co.). Analytical thin-layer chromatography
(TLC) was performed on 0.2 mm precoated plate Kieselgel 60
F254 (Merck). All other reagents and solvents commercially
available were used without further purification unless otherwise
50 noted.
Figure 1. Organic dyes TA-St-CA and AK01.
quenching and reduction of electron injection resulting in a low
Voc.3b,6 In our recent DSCs study based on AK01 dye7 compared
to TA-St-CA dye (Figure 1),8 it was demonstrated that AK01
showed higher overall conversion efficiency (η) than TA-St-CA
(6.20% vs 5.41%). The Voc of AK01 was much lower than TA-
St-CA (0.64 V vs 0.718 V),7 the higher efficiency of AK01 was
mainly ascribed to the higher Jsc. Considering the structures of
5
Synthesis of compounds YF01-04
A mixture of the corresponding aldehyde derivatives (1.2 mmol),
2-cyanoacetic acid (306 mg, 3.6 mmol) and ammonium acetate
(46 mg, 0.6 mmol) was dissolved in 10 mL acetonitrile and 10
55 mL acetic acid, and the mixture was stirred for 12 hours in Ar
under reflux condition. After evaporation of the solvent, the
residue was purified by silica-gel column chromatography to give
compounds YF01-04.
10 triphenyl amine (TA-St-CA) and indoline moiety (AK01), it is
likely that the higher Voc of TA-St-CA is due to large torsion
angels in the molecular structure of TA-St-CA. In an extension to
our continuous effort for enhancing the Voc,9 we report herein a
novel series of D-π-A sensitizers YF based on naphthalene-linked
15 pyrrolidine or diphenylamine as an electron donor, oligo-
phenylenevinylene as a π-linkage, and the cyanoacrylic acid as an
electron acceptor/anchor group (Figure. 2) for nanocrystalline
TiO2 solar cells. DSCs based on the novel dyes YF02 and YF04
comprised of a naphthalene-linked diphenylamine as a donor
20 showed a high Voc of ~0.8 V. To study the influence of
naphthalene-amine moiety and its substitution position on the
photophysical, photoelectrochemical properties, and solar-cell
performance, the pyrrolidine-naphthalene based dyes, YF01 and
YF03, were also synthesized. Molecular orbital calculations at
25 the DFT level were performed, and the equilibrium molecular
geometries of the YF and TA-St-CA were calculated to better
understand the structure-activity relationship, and in particular the
interrelationship between the torsion angels (θ1-6) and Voc. Figure
2 shows the molecular structures of dyes YF01-04, where θ1-θ6
30 refer to the locations of torsion angels of interest.
60
Dye YF01. Yield 92%; 1H NMR (400 MHz, CDCl3) δ 13.94 (s,
1H), 8.31 (s, 1H), 8.06 (d, J = 8.4 Hz, 3H), 7.90-7.84 (m, 3H),
7.77- 7.72 (m, 2H), 7.40-7.37 (m, 1H), 7.31-7.23 (m, 2H), 6.70 (d,
J = 16.8 Hz, 1H), 3.30 (bs, 4H), 1.86 (bs, 4H). 13C NMR (100
MHz, CDCl3) δ 163.34, 153.49, 145.83, 142.46, 132.90, 131.22
65 (2C), 130.17, 130.08, 128.29, 127.91, 127.37, 126.62, 126.31,
122.82, 122.01, 116.97, 116.78, 102.300, 51.83, 25.30. HRMS
(ESI positive): [M+H]+ calcd for C26H22N2O2H, 395.17540;
found, 395.17546.
70
Dye YF02. Yield 75%; 1H NMR (400 MHz, CDCl3) δ 8.29 (s,
1H), 8.06 (d, J = 8.8 Hz, 2H), 7.96 (s, 1H), 7.82-7.77 (m, 4H),
7.68 (d, J = 8.8 Hz, 1H), 7.60 (d, J = 16.4 Hz, 1H), 7.42 (d, J =
16.4 Hz, 1H), 7.34-7.30 (m, 5H), 7.18 (dd, J = 8.8 Hz, 2.4 Hz,
1H), 7.10-7.06 (m, 6H). 13C NMR (100 MHz, CDCl3) δ 163.30,
75 153.33, 146.85, 145.39, 141.95, 133.89, 132.58, 131.88, 131.21,
130.21, 129.53, 129.32, 129.27, 127.24, 127.01, 126.87, 126.71,
124.21, 124.04, 123.79, 123.36, 118.56, 116.38, 102.27. HRMS
(ESI positive): [M+Na]+ calcd for C34H24N2O2Na, 515.17300;
found, 515.17291.
80
Dye YF03. Yield 70%; 1H NMR (400 MHz, CDCl3) δ 13.94 (s,
1H), 8.26 (s, 1H), 8.19-8.16 (m, 1H), 7.96- 7.92 (m, 4H), 7.54-
7.49 (m, 4H), 7.39 (d, J = 16.4 Hz, 1H), 7.28 (d, J = 8.8 Hz, 1H),
7.20-7.16 (m, 4H), 6.92-6.86 (m, 7H). 13C NMR (100 MHz,
85 CDCl3) δ 163.22, 153.44, 146.92, 141.90, 141.56, 132.86, 132.33,
131.53, 130.93, 130.64, 130.42, 129.54, 129.13, 128.28, 127.47,
126.81, 126.76, 125.64, 124.70, 121.77, 121.68, 116.22, 102.58.
HRMS (ESI positive): [M+Na]+ calcd for C34H24N2O2Na,
515.17300; found, 515.17256.
Figure 2. Molecular structure of YF dyes.
90
Experimental Section
Dye YF04. Yield 76%; 1H NMR (400 MHz, CDCl3) δ 8.20 (s,

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Journal of Materials Chemistry
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Scheme 1. Synthesis of new organic dyes YF01-04.
(DCA) (20 mM) as a co-adsorbent was added into the dye
solution to prevent aggregation of the dye molecules. The TiO2
films were immersed into the dye solution and then kept at 25 C
5
1H), 8.01 (d, J = 8.4 Hz, 2H), 7.83 (s, 1H), 7.77-7.69 (m, 4H),
7.62 (d, J = 8.8 Hz, 1H), 7.53 (d, J = 16.4 Hz, 1H), 7.29 (d, J =
16.4 Hz, 1H), 7.02 (d, J = 8.4 Hz, 1H), 6.75 (s, 1H), 3.35 (bs, 4H),
o
for 30 h. Photovoltaic measurements were performed in a
1.99 (bs, 4H). 13C NMR (100 MHz, THF-d8) δ 163.00, 152.94, 40 sandwich type solar cell in conjunction with an electrolyte
146.42, 143.08, 135.66, 132.80, 131.40, 130.34, 129.65, 128.96,
consisting of a solution of 0.6 M dimethylpropyl-imidazolium
iodide (DMPII), 0.05M I2, 0.1M LiI and 0.5M tert-butylpyridine
(TBP) in acetonitrile (AN). The dye-deposited TiO2 film and a
platinum-coated conducting glass were separated by a Surlyn
45 spacer (40 µm thick) and sealed by heating the polymer frame.
Potocurrent density-voltage (I-V) of sealed solar cells was
measured under AM 1.5G simulated solar light at a light intensity
of 100 mW cm-2 with a metal mask of 0.25 cm2. The photovoltaic
parameters, i.e. short circuit current (Jsc), open circuit voltage
50 (Voc), fill factor (FF), and power conversion efficiency (η) were
estimated from I-V characteristics under illumination.
10 127.83, 126.49, 126.32, 126.30, 126.08, 124.57, 123.42, 115.79,
104.70, 101.98, 47.55, 25.45. HRMS (ESI positive): [M+H]+
calcd for C26H22N2O2H, 395.17540; found, 395.17547.
Molecular modeling
Ground state equilibrium molecular geometries of YF01-04 were
15 calculated using the hybrid DFT energy functional B3LYP10,11
and the basis set DGDZVP (density Gauss double-ζ with
polarization functions).12,13 Subsequently, a single point energy
calculation was performed on the optimized geometry using
different long-range corrected time-dependent DFT energy
20 functional methods including CAM-B3LYP14 or WB97XD15 with
the basis set DGDZVP, and the electronic absorption spectra
were extracted. The TD-DFT calculations were performed in
EIS and IMVS measurements
The intensity-modulated photovoltage spectra (IMVS) were
measured with a potentiostat (Solartron1287) equipped with a
ethanol using the polarizable conductor calculation model (PCM). 55 frequency response analyzer (Solartron1255B) at an open-circuit
The DFT/TD-DFT calculations were performed on the East
25 Carolina University's Super Computer Jasta using 8 processors
and 4GB of RAM. The solvent effect was accounted for using the
polarizable conductor calculation model (PCM), implemented in
Gaussian 09. All calculations were performed using Gaussian 09.
condition based on a monochromatic illumination (420 nm)
controlled by Labview system to obtain the photovoltaic response
induced by the modulated light. The modulated light was driven
with a 10% AC perturbation current super imposed on a DC
60 current in a frequency range from 0.1 to 106 Hz. The charge
extraction method (CEM) was performed with the same
monochromatic light source. The solar cell was illuminated at an
open-circuit condition for 5 s to attain a steady state and then the
light source was switched off when the device simultaneously
65 switched to a short-circuit condition to extract the charges
generated at that light intensity. The electrochemical impedance
spectra were measured with an impedance analyzer (Solartron
Fabrication of dye sensitized solar cells
30 A double-layer TiO2 photoelectrode (10 + 5) μm in thickness
with a 10 μm thick nanoporous layer and a 5 μm thick scattering
layer (area: 0.25 cm2) was prepared by screen printing on
conducting glass substrate. A dye solution of YF04-01 with 3 x
10-4 M concentration in acetonitrile/tert-butyl alcohol (1/1, v/v)
35 was used to up take the dye on to the TiO2 film. Deoxycholic acid

Journal of Materials Chemistry
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Analytical, 1255B) connected with a potentiostat (Solartron
Analytical, 1287) under illumination using a solar simulator
(WXS-155S-10: Wacom Denso Co. Japan). EIS spectra were 50 thermodynamic ground states for efficient regeneration of the dye
sensitizers YF01-04, which were sufficiently lower than the
redox potential of I-/I3- (-5.20 eV),19 ensuring the favourable
recorded over a frequency range of 10–2–106 Hz at 298 K. The
applied bias voltage and AC amplitude were set at Voc of the
DSCs. The electrical impedance spectra were characterized using
Z-View software (Solartron Analytical).
through the reaction of the oxidized dye with iodide. The IP value
of YF04 was shifted positively compared to the other YF dyes
due to the stronger electron donation of pyrrolidine moiety at the
6-position of naphthalene, which furnished more charge transfer
55 from D to A. Moreover, the positive shift increased the negative
free energy of electron injection, which furnishs more favorable
thermodynamic excited state for electron injection, which can
lead to more photocurrent (see Table 1, Jsc). The onset of the
optical energy gap (E0-0) of YF dyes was in the range of 1.91-
60 2.39 eV (Table 1).20 The excited-state oxidation potential (S+/*) of
sensitizers YF01-04 was in the range of -3.45 to -3.63 eV, which
lay above the conduction band edge (-4.2 eV)1c of the
nanocrystalline TiO2 (Table 1), ensuring the efficient electron
injection into the conduction band edge of TiO2 from the
65 adsorbed YF sensitizers.
Incident photon-to-current conversion efficiencies (IPCEs) for
DSCs based on YF dyes are plotted in Figures 4a. The IPCEs of
YF02 and YF03 are relatively blue-shifted compared to the dyes
YF04 and YF01, consistent with the UV-Vis absorption spectra
70 of dye adsorbed on TiO2 film (Figure 3b). Dyes YF02 and YF04
comprised of a donor moiety at the 6-position of naphthalene
showed higher maximum IPCEs than that of dyes with the donor
moiety at the 2-position of naphthalene (YF01 and YF03). The
maximum quantum efficiency of YF02 and YF04 reached up to
75 86% in the plateau region. These results implied that the rod-like
D-π-A structure should be favorable for efficient electron
injection quantum yield.
5
Results and discussion
The synthesis routes of compounds YF01-04 are shown in
10 Scheme 1. The 2-amino-substituted 1-naphthaldehydes were
prepared according to the reported methods (Scheme S1, ESI).16-
18
The substituted (E)-4-(2-(naphthalenyl)vinyl)benzaldehydes
were synthesized in high yield by using the Vilsmeier-Haack,
Wittig-Horner and Buchwald-Hartwig reactions which were
15 described in ESI. The target compounds YF01-04 were obtained
via Knoevenagel condensation from the corresponding aldehydes
with cyanoacetic acid in the presence of ammonium acetate in
high yields as described in the experimental section.
The photophysical properties of dyes YF01-04 in ethanol
20 solution and on TiO2 film are summarized in Table 1. The
wavelengths of absorption maxima (λmax) are in the range of 404-
423 nm due to the similar π-π* electron transition of YF dyes as
shown in Figure 3a. The absorption maxima of pyrrolidine based
sensitizers YF01 and YF04 were observed at a slightly longer
25 wavelength region compared to diphenylamine based sensitizers
due to the stronger donor ability of pyrrolidine than
diphenylamine. The molar extinction coefficients (ε) at the
maximum absorption wavelength for these dyes are in the range
of 1.1-3.2 x 104 M-1 cm-1, where YF02 exhibits the highest ε
30 value. When absorbed on transparent thin TiO2 film, YF01-04
showed broad absorption spectra similar to that in solution, while
the absorption peaks are slightly blue-shifted due to the
interaction between the carboxylic group and TiO2 (Figure 3b).
Upon dye adsorption on TiO2 film, the tails of the absorption
35 spectra of all sensitizers are red-shifted compared to that of the
absorption spectra in solution. It is worth mentioning that the tail
of the absorption spectrum of dye YF04 extended up to 650 nm,
which is highly desirable for harvesting more of the solar
spectrum and leads to a large photocurrent.
Figure 4. (a) Photocurrent action spectra (IPCE) of nanocrystalline TiO2
80 film sensitized by YF 01-04. (b) Photocurrent voltage (I-V) characteristics
of dyes YF01-04. The redox electrolyte solution was a mixture of 0.6 M
DMPII, 0.05 M I2, 0.1 M LiI and 0.3 M TBP in acetonitrile.
The solar cell performance for the DSCs based on the YF dyes
is listed in Table 1, and the corresponding photocurrent-voltage
85 curves are shown in Figure 4b. Dye YF02 showed the highest
solar to power conversion efficiency () (5.29%) among the four
dyes with the short-circuit current (Jsc), the open-circuit voltage
(Voc) and the fillfactor (FF) of 9.19 mA cm-2, 0.799 V and 0.721,
respectively. Under the same conditions, the overall power
90 conversion efficiencies () of dyes YF01, YF03, and YF04 were
3.57%, 3.67%, and 4.03%, respectively. DSCs based on the rod-
like dyes, YF02 and YF04, produced higher Jsc than YF01 and
YF03, which may be attributed to the more efficient electron
transfer from donor to acceptor moiety of the formers.
95 Remarkably, the Voc of the diphenyl(naphthyl)amine based dyes,
YF02 and YF03, reach about 0.8 V, which is much higher than
40
Figure 3. Absorption spectra of YF01-04 in ethanol (a) in EtOH and (b)
on a transparent TiO2 film.
The Ionization potential (IP) of YF dyes bound to
nanocrystalline TiO2 film was measured using the photoemission
45 yield spectrometer (Riken Keiki, AC-3E) (Fig. S1, ESI†). The IP
values which correspond to ground-state oxidation potentials
(S+/0) were found to be -5.71, -5.70, -5.91 and -5.54 eV for the

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Table 1. Photophysical properties and DSCs performance parameters of dyes YF01-04.
Dye
λmax/nma (ε×104M-1cm-1) λonset /nmb on TiO2 film
(IP)c (eV)
-5.71
-5.70
-5.91
E0-0 (eV)d
2.16
2.25
2.39
S+/*c (eV)e
-3.55
-3.45
-3.52
Jsc [mA cm-2] Voc [V]
[ff]
 [%]
3.57
5.29
3.67
4.03
YF01
YF02
YF03
YF04
309, 407 (1.1)
298, 406 (3.2)
324 (2.7), 404
316, 422 (2.1)
575
550
520
650
8.33
9.19
0.574
0.799
0.807
0.552
0.747
0.721
0.700
0.712
6.494
10.24
-5.54
1.91
-3.63
c
a Absorption maxima, measured in ethanol at room temperature; b Absorption measured on a transparent 4 µm TiO2 film. Ionization potential (IP) of
absorbed dyes on the nanocrystalline TiO2 film was determined by using the photoemission yield spectrometer (Riken Keiki, AC-3E). d E0-0 was estimated
from the absorption onset of dye loaded on TiO2 film; e The excited-state oxidation potential, S+/* levels were calculated from the expression of S+/*
= IP - E0-0. Measurements were performed under AM 1.5 irradiation on the DSC devices with 0.25 cm2 active surface area defined by a metal mask. Jsc:
short circuit current; Voc: open circuit voltage; ff: fill factor; η: conversion efficiency.
the naphthalene-substituted pyrrolidine based dyes, YF01 and
YF04. This can be rationalized by the increased intermolecular π-
π interaction of the pyrrolidine based dyes (YF04 and YF01)
which have less twisted structures compared to the
intensity modulated photovoltage spectroscopy (IMVS) as shown
in Figure 5b. The τe values for the DSCs based on YF02 and
YF03 dyes are much larger than that of YF01 and YF04 dyes,
5
which suggests that the YF02 and YF03 dyes having a low
-
diphenylamine based dyes (YF02 and YF03), which are more 30 planarity are favourable for the blocking I3 ions approaching the
twisted due to large torsion-angels. The strong intermolecular π-π
TiO2 surface for charge recombination, and thus show an
10 interactions on the TiO2 surface may cause self-quenching
process, resulting in a low Voc, and hence, poor device
performance.
improvement in Voc.
Figure 6. The EIS (a) Nyquist plots and (b) Bode plots for DSCs
35 sensitized with YF dyes.
Figure 5. (a) Voc as a function of electron density and (b) electron lifetime
15 (τe) as a function of Voc for DSCs sensitized with YF dyes.
Electrochemical impedance spectroscopy (EIS) was employed
to study the influence of electron recombination and interfacial
charge transfer process on Voc.21 Figure 6 shows the EIS Nyquist
and Bode plots for the DSCs based on YF dyes. The values of the
40 first semicircles in Nyquist plots corresponding to the series
resistance and electrolyte resistance in all YF-DSCs are almost
same. The reciprocal of the peak frequency for the left-frequency
peak in the Bode plot is regarded as the electron lifetime which
represents the charge transfer process at the dye/TiO2/electrolyte
45 interface. The YF02 and YF03 dyes afford the higher lifetimes
and charge transfer resistances at the dye/TiO2/electrolyte
interface (Rrec) than that of YF01 and YF04 dyes. This trend is in
The generation of Voc is related to the energy level of the TiO2
conduction band (ECB) and the charge recombination rate in
DSCs.21 To clarify the effect of molecular planarity on Voc, the
charge extraction and lifetime (τe) of YF dyes based DSCs were
20 measured. As shown in Figure 5a, the measurement of the ECB
with the charge extraction method (CEM) shows that all of the
dyes have almost same effect on the conduction band of TiO2.
Therefore, the improvement of Voc of YF-dyes based DSCs
should be attributed to the suppression of charge recombination,
25 which is related to the τe. The τe was measured by means of

Journal of Materials Chemistry
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accordance with the τe vs Voc results, implying that the decrease
of molecular aggregation of dyes by introduction of naphthalene
linked diphenylamine donor moiety may suppress the electron 30 YF01. On the other hand, in the case of dye YF02, the
moiety out of the plane by 44.04 (2) (Table 2). This explains
why dye YF04 is bathochromically shifted compared to dye
recombination rate between the TiO2 film and the electrolyte.
In order to gain more insights and better understanding into the
electronic and molecular geometry of YF dyes, equilibrium
molecular geometry, vertical electronic excitation values,
diphenylamine is at position 6 and it is twisted by 33.33 out of the
plane. While in the case of dye YF03, the diphenylamine is at
position 2, therefore it is more twisted out of the plane by 62.16°
(4) and also forces the stilbene moiety out of the plane by 46.66°
5
delocalization of HOMO and LUMO, and torsion angels θ1-θ6 35 (5). Considering the strength of pyrrolidine compared to
were calculated.
biphenyl amine as an electron-donor auxochorme and the more
twist in the torsion angles generated by the introduction of the
latter at position 2, one should expect that in the case of dye
YF03, the π-π overlap and π-π* charge transfer should be weaker
10 The calculated HOMOs and LUMOs isodensity superimposed
on the molecular structures of dyes are shown in Figure 7. The
HOMOs are delocalized considerably on the electron donor
synthon where the auxochrome (pyrrolidine or diphenylamine) 40 than that of dye YF01 because of the more twist in the case of the
resides, which facilitates the reduction with I-, making the dye
15 regeneration more efficient. On the other hand, the LUMO was
mainly delocalized on the π* cyanuric unit and has significant
contributions from the carboxylic groups, which facilitates the
former, which would significantly enhance the Voc but decrease
the intermolecular charge transfer within the molecule. Therefore,
it should be expected that the electron injection and hence in
theory the Jsc of dye YF03 should be weaker than that of dye
electron injection from the photoexcited sensitizer into the TiO2 45 YF01, and the Voc of dye YF03 should be higher than YF01,
semiconductor. This confirms that the D-π-A structural motif is
20 an excellent design for efficient charge transfer and separation in
DSCs.
which is consistent with the experimental results. Moreover, by
the same analogy, the torsion angle 6 (4.01) in dye YF04 is
significantly less twisted compared to 3 (33.33) in dye YF02,
which produces stronger π-π overlap and π-π* charge transfer in
Results of the equilibrium molecular geometry demonstrated
that the pyrrolidine ring at position 6 in dye YF04 is coplanar 50 the case of the former, and that explains why dye YF04 produced
with the rest of the dye molecule furnishing strong intermolecular
25 π-π overlap in the D-π-A motif. In the case of dye YF01, however,
the location of the pyrrolidine ring is at position 2, which is
twisted out of the plane by 33.99 (1) and also forces the stilbene
the higher Jsc and lower Voc compared to dye YF02. Because of
the introduction of diphenylamine at position 2 in dye YF03, the
twist in the torsion angles 4-5 were the largest among all dyes,
55
Figure 7. The calculated HOMO and LUMO isodensity of YF01-04 showing the regions of delocalization.

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Figure 8. The calculated torsion angles of YF02 and TA-St-CA.
which significantly reduced the π-π* charge transfer. This
torsion angles are 33.33 and 40.81, while TA-ST-CA(a) and (b)
5
explains why YF03 produced the most inferior Jsc and highest Voc 30 torsion angles are 34.33 and 35.44, respectively. The overall twist
(0.807 V) among all dyes. In comparison to YF03, YF02 shows a
much higher Jsc and a comparably high Voc (0.799), which are the
important parameters for dye YF02 to produce the highest solar
to electric efficiency of 5.29%.
around diphenylamine in YF02 is greater than that of TA-ST-CA,
supporting our experiment results that DSCs based on the new
dye YF02 with a diphenyl(naphthyl)amine as a donor, showed a
higher Voc than the DSC based on TA-ST-CA which has a
10 The calculated and the experimental λmax of YF dyes are shown 35 triphenylamine as a donor.
in Table 3. Excellent correlation between the experimental and
calculated λmax values was founded by using the energy functional
B3LYP and the basis set DGDZVP for geometry optimization,
and the TD-DFT energy calculations utilizing the long-range
15 corrected energy functional WB97XD and basis set DGDZVP,
which was found superior to the long-range corrected energy
functional CAM-B3LYP.
Conclusion
We have synthesized a series of new organic dyes YF01-04 based
on naphthylamine moiety as a donor unit in the D-π-A system for
nanocrystalline TiO2 solar cells. The dyes YF02 and YF04
40 having amine donors substituted at 6-position of naphthalene
moiety furnished better charge separation and more photocurrent
than the dyes YF01 and YF03 which have amine donors at 2-
position of naphthalene moiety. Moreover, the dyes YF02 and
YF03 with diphenylamine as a donor showed a very high Voc
45 about 0.8 V due to their highly twisted geometry, which led to the
highest overall conversion efficiency of 5.29% for YF02. The
EIS and electron lifetime measurements indicated that the DSCs
sensitized with YF02 and YF03 dyes have a larger τe and Rrec
than that of YF01 and YF04 dyes, which resulted in higher Vocs.
50 DFT calculations indicated that the torsion angels around
naphthalene in the phenylamine based dyes (YF02 and YF03) are
more twisted than the pyrroline based dyes (YF01 and YF04),
which contributed to less dye aggregation on TiO2, resulting in a
high Voc. Further molecular geometry calculation of the
55 triphenylamine based dye TA-St-CA and the result that a higher
Voc of YF02 than that of TA-St-CA, indicated that the existence
of naphthalene moiety linked to diphenylamine is favourable for
retarding the charge recombination. The introduction of
naphthalene moiety in D-π-A dyes might be a promising way to
60 inhibit the molecular aggregation of dyes and hence enhance the
solar cell performance. With the aid of molecular modelling,
molecular engineering and synthesis in our laboratory is
Table 2. Calculated torsion angels of YF01-04.
Dyes
Dihedral
angles (°)
YF01
YF02
3
33.33
YF03
YF04
6
4.01
1
31.99
2
4
62.16
5
44.04
46.66
20 Table 3. Calculated versus experimental λmax of YF01-04.
λmax
Calculated Energy caculations
Expt.
Dyes
YF01
YF02
YF03
YF04
WB97XD/DGDZVP
CAM-B3LYP/DGDZVP
410
413
401
429
446
433
420
453
407
406
404
422
Although the dyes containing triarylamine unit as donors always
show high Voc compared to other organic dyes,22 the Voc of dyes
up to 0.8 V have been rarely reported.23 To better understand the
25 relationship between the high Voc and molecular geometry, the
equilibrium molecular geometries of dyes YF02 and TA-St-CA
were further calculated using the hybrid energy functional B3LP
and basis set DGDZVP. As shown in Figure 8, YF02(a) and (b)

Journal of Materials Chemistry
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21 (a) H. Tian, I. Bora, X. Jiang, E. Gabrielsson, K. M. Karlsson, A.
underway to develop a novel generation of more efficient
naphthalene-based dyes that have the potential to exhibit
significant increase in DSC performance across a wider range of
the solar spectrum.
Hagfeldt and L. Sun, J. Mater. Chem. 2011, 21, 12462; (b) S. Qu, C.
Qin, A. Islam, Y. Wu, W. Zhu, J. Hua, H. Tian and L. Han, Chem.
Commun. 2012, 48, 6972; (c) C. Qin, A. Islam and L. Han, J. Mater.
Chem. 2012, 22, 19236; (d) S. Haid, M. Marszalek, A. Mishra, M.
Wielopolski, J. Teuscher, J.-E. Moser, R. Humphry-Baker, S. M.
Zakeeruddin, M. Grätzel and P. Bäuerle, Adv. Funct. Mater. 2012, 22,
1291.
70
5
Notes and references
a
WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku 75 22 (a) D. P. Hagberg, J.-H. Yum, H. J. Lee, F. De Angelis, T. Marinado,
University, 2-1-1 Katahira, Aobaku, Sendai 980-8577, Japan. Fax: +81-
22-217-5979; Tel: +81-22-217-6177; E-mail: tjin@m.tohoku.ac.jp
Department of Chemistry, Faculty of Science, University of Malaya,
K. M. Karlsson, R. Humphry-Baker, L. Sun, A. Hagfeldt, M. Grätzel
and M. K. Nazeeruddin, J. Am. Chem. Soc., 2008, 130, 6259; (b) Z.
Ning, Q. Zhang, H. Pei, J. Luan, C. Lu, Y. Cui and H. Tian, J. Phys.
Chem. C, 2009, 113, 10307; (c) J. S. Song, F. Zhang, C. H. Li, W. L.
Liu, B. S. Li, Y. Huang and Z. S. Bo, J. Phys. Chem. C, 2009, 113,
13391; (d) R. Li, X. Jv, D. Shi, D. Zhou, Y. Cheng, G. Zhang and P.
Wang, J. Phys. Chem. C, 2009, 113, 7469; (e) S.-H. Lin, Y.-C. Hsu, J.
T. Lin, C.-K. Lin and J.-S. Yang, J. Org. Chem., 2010, 75, 7877; (f)
B.-S. Chen, D.-Y. Chen, C.-L. Chen, C.-W. Hsu, H.-C. Hsu, K.-L.
Wu, S.-H. Liu, P.-T. Chou and Y. Chi, J. Mater. Chem., 2011, 21,
1937; (g) S.-B. Ko, A.-N. Cho, M.-J. Kim, C.-R. Lee and N.-G. Park,
Dyes Pigm., 2012, 94, 88.
b
10 50603 Kuala Lumpur, Malaysia. Fax: 603-7967-4193; Tel: 603-7967-
4053; E-mail: akhtar@um.edu.my
80
85
90
c Photovoltaic Materials Unit, National Institute for Materials Science, 1-
2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
d Polymer and color Chemistry Program, North Carolina State University
15 1000 Main Campus Dr. Raleigh, NC 27695, USA. Fax: 1-919-515-6532;
Tel: 1-919-515-6548; E-mail: Ahmed_El-Shafei@ncsu.edu
e Chemistry Department, Faculty of Science, and Center of Excellence for
Advanced Materials Research, King Abdulaziz University, P.O. Box
80203, Jeddah, Saudi Arabia
23 (a) G. L. Zhang, H. Bala, Y. M. Cheng, D. Shi, X. J. Lv, Q. J. Yu and
P. Wang, Chem. Commun., 2009, 2198; (b) L. Y. Lin, C. H. Tsai, K.
T. Wong, T. W. Huang, L. Hsieh, S. H. Liu, H. W. Lin, C. C. Wu, S.
H. Chou, S. H. Chen and A. I. Tsai, J. Org. Chem., 2010, 75, 4778.
20 † Electronic Supplementary Information (ESI) available: [Experimental
procedures for synthesis, DFT calculations, Ionization potential and all
fundamental data of DSCs]. See DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
25 spectral data, and crystallographic data.
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65 20 E0-0 value was estimated from the 5% intensity level of the absorption
spectrum of YF-series adsorbed onto transparent TiO2 film.