Novel dyes based on naphthalimide moiety as electron acceptor for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.01.010

Four novel naphthalimide-based dyes (D-1, D-2, D-3 and D-4) were synthesized and utilized as sensitizers in dye-sensitized solar cells (DSCs), in which the triphenylamine (TPA) or indoline groups, naphthalimide unit and carboxylic group were functionalized as electron donor, acceptor and anchoring group, respectively. The naphthalimide unit was employed as the π-conjugation ring and electron acceptor for effectively realizing intramolecular charge separation in the oxidized states. In the series of dyes, the LUMO orbital is delocalized mainly on naphthalimide moieties, especially on the carbonyl group. Consequently, the LUMO electrons are isolated from the carboxyl anchoring group (-CH₂CO₂H) due to the presence of the methylene group, which could suppress the electron injection efficiency from the excited dyes to the TiO₂ conduction band, thus leading to the inferior efficiencies of 1.10, 1.18, 2.27 and 2.70%, respectively, even though they exhibit broad spectral response and high extinction coefficients.

Full text of DOI:10.1016/j.dyepig.2011.01.010

Dyes and Pigments 90 (2011) 297e303
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel dyes based on naphthalimide moiety as electron acceptor
for efficient dye-sensitized solar cells
,
a
a
a,
*
Xiaomei Huang a, Yi Fang b , Xin Li , Yongshu Xie , Weihong Zhu
**
a Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials, and Institute of Fine Chemicals,
East China University of Science and Technology, Shanghai 200237, PR China
b Department of Physics, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel naphthalimide-based dyes (D-1, D-2, D-3 and D-4) were synthesized and utilized as sensitizers
in dye-sensitized solar cells (DSCs), in which the triphenylamine (TPA) or indoline groups, naphthalimide
unit and carboxylic group were functionalized as electron donor, acceptor and anchoring group,
Received 29 December 2010
Received in revised form
21 January 2011
Accepted 22 January 2011
Available online 31 January 2011
respectively. The naphthalimide unit was employed as the p-conjugation ring and electron acceptor for
effectively realizing intramolecular charge separation in the oxidized states. In the series of dyes, the
LUMO orbital is delocalized mainly on naphthalimide moieties, especially on the carbonyl group.
Consequently, the LUMO electrons are isolated from the carboxyl anchoring group (-CH2CO2H) due to the
presence of the methylene group, which could suppress the electron injection efficiency from the excited
dyes to the TiO2 conduction band, thus leading to the inferior efficiencies of 1.10, 1.18, 2.27 and 2.70%,
respectively, even though they exhibit broad spectral response and high extinction coefficients.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Naphthalimide
Electron acceptor
Indoline
Synthesis
Sensitizers
Solar cells
1. Introduction
supramolecular moieties for the study of photo-induced electron
transfer, fluorescence switcher, liquid crystal displays, and electro-
Dye-sensitized solar cells (DSCs) have attracted great attention
due to their relative high power conversion efficiency and low cost
with respect to conventional inorganic photovoltaic devices [1e4].
DSCs based on Ru dyes have been intensively investigated and
shown efficiencies up to 11% at simulated AM 1.5G irradiation
[5e10]. However, the utilization of rare and expensive metals with
substantial purification brings the major problem in cost. There-
fore, an increasing attention has been directed to the application of
metal-free organic dyes [11,12]. Several organic sensitizers based on
coumarin [13e16], indoline [17,18], perylene [19e21], cyanine
[22,23] and triphenylamine [24e26] derivatives have been devel-
oped as promising candidates with the high power conversion
efficiency, and even realizing efficiencies higher than 9% [27e29].
1,8-Naphthalimide and its derivatives with a strong electron-
withdrawing imide group have been extensively investigated due to
their quite good chemical stability, a large Stokes shift, and high
fluorescent quantum yield, known to act as solar energy collectors,
luminescent (EL) materials [30e36]. Naphthalimide dyes as a kind
of metal-free dyes for DSCs have also been reported by our group,
however, the unfavorable electron flow makes it a lower efficiency
[37]. Meanwhile, the indoline unit has been demonstrated to
possess a stronger electron-donating ability than triphenylamine
(TPA) unit, giving remarkable power conversion efficiency as high as
9.52% under AM 1.5 [14]. Bearing these considerations in mind, we
designed and synthesized a novel series of organic sensitizers (D-1,
D-2, D-3 and D-4 shown in Fig. 1) with carboxylic acid as anchoring
group. Essentially, these dyes were designed from the following
considerations: i) substituting the TPA group with stronger elec-
tron-donating indoline unit to achieve higher Jsc; ii) improving the
open-circuit voltages (Voc) by the introduction of bis-substituted
indoline units instead of the mono-substituted; and iii) employing
the strong electron-withdrawing properties of imide group
in naphthalimide unit as electron acceptor for effectively realizing
intramolecular charge separation in the oxidized dye and
decreasing the dark current. As demonstrated, we improved the
open-circuit voltages (Voc) by the introduction of bis-substituted
indoline units instead of the mono-substituted. Moreover, the cell
performance is greatly dependent upon the methylene group in
the nature of the anchoring group (-CH2CO2H), resulting in the
* Corresponding author. Fax: þ86 21 6425 2758.
** Corresponding author.
E-mail addresses: fangyi@ecust.edu.cn (Y. Fang), whzhu@ecust.edu.cn (W. Zhu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.01.010

298
X. Huang et al. / Dyes and Pigments 90 (2011) 297e303
(reagent grade) and used as received. The starting material of
4-bromo-1,8-naphthalic anhydride is commercially available.
2.3. Preparation of dye-sensitized nanocrystalline electrode TiO2
The dye-sensitized TiO2 electrode was prepared via a modified
procedure reported in the literature [38]. A screen-printed double
layer of TiO2 particles was used as photoelectrode. A 3
of 13 nm sized TiO2 particles (Ti-Nanoxide T/SP) was first printed on
the FTO conducting glass and further coated by a 4 m thick second
mm thick film
m
layer of 400 nm light-scattering anatase particles (Ti-Nanoxide 300).
Sintering was carried out at 275, 325 and 375 ꢁC for 5 min, 450 and
500 ꢁC for 15 min, respectively. Before immersed in the dye solution,
these films were soaked in 0.2 mol Lꢀ1 aqueous TiCl4 solution
overnight. After washed with deionized water and fully rinsed with
ethanol, the films were heated again at 450 ꢁC for 30 min, followed
by cooling to 80 ꢁC and dipping into a 3 ꢂ 10ꢀ4 mol Lꢀ1 solution of
dyes in methanol-chloroform (4:1, v/v) for 12 h at room temperature.
To prepare the counter electrode, the Pt catalyst was deposited on
the cleaned FTO glass by coating with a drop of H2PtCl6 solution
(0.02 mol Lꢀ1 in 2-propanol solution) with the heat treatment at
400 ꢁC for 15 min. A hole (0.8 mm diameter) was drilled on the
counter electrode by a Drill-press. The perforated sheet was cleaned
by ultrasound in an ethanol bath for 10 min.
In the assemblage of DSCs, the dye-covered TiO2 electrode and
Pt-counter electrode were assembled into a sandwich-type cell and
sealed with a hot-melt gasket of 25 mm thickness made of the
ionomer Surlyn 1702 (Dupont). The redox electrolyte was injected
through a pre-drilled hole in the counter electrode by capillary
force and driven into the cell by means of vacuum backfilling, then
the hole in the counter electrode was sealed by a film of Surlyn 1702
and a cover glass (0.1 mm thickness) using a hot iron bar. The
electrolyte employed was a solution of 0.6 mol Lꢀ1 BMII (1-butyl-3-
methyl imidazolium iodide), 0.1 mol Lꢀ1 LiI, 0.05 mol Lꢀ1 I2,
0.5 mol Lꢀ1 TBP (4-tertbutylpyridine) in acetonitrile.
Fig. 1. Chemical structures of novel dyes (D-1, D-2, D-3 and D-4).
electronic uncoupling of naphthalimide core to the TiO2 electrode
with inferior performance. The naphthalimide dye D-4, which
employed two indoline units as donor, exhibited a photoelectron
performance with Jsc of 5.42 mA cmꢀ2, Voc of 664 mV, FF of 0.75, and
an overall conversion efficiency
h of 2.70%.
2.4. Photoelectrochemical measurements
The photocurrent action spectra were measured with a Model
SR830 DSP Lock-In Amplifier and a Model SR540 Optical Chopper
(Stanford Research Corporation, USA), a 7IL/PX150 xenon lamp
with power supply, and a 7ISW301 Spectrometer. The irradiation
source for the photocurrent densityevoltage (JeV) measurement is
an AM 1.5 solar simulator (91160, Newport Co., USA). The incident
light intensity was 100 mW cmꢀ2 calibrated with a standard Si solar
cell. The tested solar cells were masked to a working area of
0.159 cm2. Voltecurrent characteristics were performed on Model
2400 Sourcemeter (Keithley Instruments, Inc., USA).
2. Experimental section
2.1. General procedure
1H NMR and 13C NMR spectra were recorded on a Bruker AV400
spectrometer operating at 400 and 100 MHz, respectively. HRMS
spectra were obtained using a Waters LCT Premier XE spectrometer.
UVevis spectra were determined with a Varain Cary 500 spec-
trometer. The cyclic voltammograms of dyes were estimated with
a Versastat II electrochemical workstation using a normal three-
electrode cell with dye-coated TiO2 electrode as working electrode,
a Pt wire auxiliary electrode, and Ag/AgCl reference electrode in
saturated KCl solution. Tetrabutylammonium hexaflourophosphate
(TBAPF6) 0.1 mol Lꢀ1 was used as the supporting electrolyte in
CH2Cl2.
2.5. Synthesis of D-1, D-2, D-3 and D-4
Synthesis of D-1.1-benzhydryl-4-bromobenzene (1.0 g, 3.1 mmol)
was dissolved in dry THF (20 mL) under nitrogen and cooled
to ꢀ78 ꢁC n-Butyllithium (2.5 M in hexanes,1.85 mL, 4.63 mmol) was
added dropwise and the mixture was stirred at ꢀ78 ꢁC for 45 min,
then B(OCH3)3 (2.1 mL, 9.30 mmol) was added dropwise and the
mixture was allowed to warm up slowly to room temperature and
used for the next Suzuki coupling reaction without purification.
The unpurified mixture was reacted with compound 2 (0.22 g,
0.63 mmol) under Suzuki coupling reaction using Pd(PPh3)4 (0.18 g,
0.15 mmol) and K2CO3 aqueous solution (3 mL, 2 mol Lꢀ1) as catalysts
in THF (15 mL) for 12 h. After cooling, the solvent was removed in
vacuo and the residual material was extracted with CH2Cl2. The
organic phase was washed with water and brine, and dried over
2.2. Materials
Optically transparent FTO conducting glass (fluorine doped SnO2,
sheet resistance < 15
U/square, transmission > 90% in the visible)
was obtained from the Geao Science andEducationalCo. Ltd. of China
and cleaned by a standard procedure. Titanium (IV) isopropoxide
and 3-methyl-2-oxazolidinone were purchased from Aldrich.
Lithium iodide was from Fluka. All other solvents and chemicals used
were produced by Sinopharm Chemical Reagent Co., Ltd., China

X. Huang et al. / Dyes and Pigments 90 (2011) 297e303
299
Na2SO4. After evaporation of the solvent, the crude product was used
for the next hydrolysis reaction without further purification.
is more powerful, which is even stronger than that of the cyano-
group [39]. Accordingly, the incorporated naphthalimide moiety
can be further expected to reinforce the intramolecular charge
transfer (ICT) process, which is effective for realizing intramolecular
charge separation in the oxidized dye and distinctly decreasing the
To a THF solution (15 mL) of crude product 5 was added
a 1 mol Lꢀ1 aqueous lithium hydroxide solution (10 mL). The mixture
was reacted at room temperature for 24 h. After the removal of most
THF, the reaction mixture was neutralized with 1 mol Lꢀ1 HCl, and
then filtered to give yellow solid product. The crude product was
purified by silica gel column chromatography (CH2Cl2/MeOH ¼ 15:1
v/v) to give D-1 as a yellow solid (0.13 g, 48% for two steps). 1H NMR
dark current. Moreover, extending
would lower the stability of dye molecules [40]. The introduction of
the naphthalimide -conjugation ring instead of methine unit will
simultaneously expand the -conjugation system and improve the
p-conjugated bonding bridges
p
p
d
(400 MHz, DMSO-d6): 8.54 (d, J ¼ 7.4 Hz, 2H), 8.41 (d, J ¼ 8.2 Hz,1H),
stability of dye molecules. By incorporating the naphthalimide unit
directly to the anchoring carboxylic group, the excited electrons will
flow directly from the donor moieties to the carboxylic groups
which are anchored on nanoporous TiO2 film, avoiding the unbe-
neficial transferring direction of photo-generated electrons [37].
What’s more, the incorporation of indoline units can be expected to
broaden the spectral region of absorption and improve the molar
extinction coefficients.
The brief synthetic route of naphthalimide dyes (D-1, D-2, D-3
and D-4) is shown in Fig. 2. The important intermediate of 4,5-
dibromo-l,8-naphthalic anhydride (10) was prepared by the estab-
lished literature procedure [41]. The corresponding 1,8-naphthalic
anhydrides (1 and 10) were imidated with glycine methyl ester
hydrochloride [42]. Mono- and bis-substituted 1,8-naphthalimide
derivatives were synthesized through Suzuki cross-coupling with
the corresponding boronic acids (3 and 4) utilizing Pd(PPh3)4 as
catalyst in a biphasic mixture of aqueous K2CO3 and THF. Finally, the
target dyes were obtained via hydrolysis under basic conditions
using a standard protocol [18], which were fully characterized by 1H
NMR, 13C NMR and HRMS.
7.89 (t, J ¼ 7.2 Hz, 1H), 7.82 (d, J ¼ 7.6 Hz,1H), 7.53 (d, J ¼ 8.3 Hz, 2H),
7.41 (t, J ¼ 7.2 Hz, 4H), 7.15 (m, 8H), 4.44 (s, 2H). 13C NMR
d (100 MHz,
DMSO-d6): 168.44, 168.21, 152.92, 151.99, 150.91, 137.51, 136.58,
136.22, 135.89, 135.61, 134.98, 134.43, 133.31, 133.00, 132.59, 129.99,
129.04,127.74,127.12,126.09, 48.54. HRMS (ESI) calcd for C32H21N2O4
[M ꢀ H]þ 497.1501, found 497.1499.
The Synthesis of D-2, D-3 and D-4 were followed according to
the procedure as described above for the synthesis of D-1.
D-2. (0.15 g, 45% for two steps), 1H NMR
d (400 MHz, DMSO-d6):
8.53 (m, 2H), 8.47 (d, J ¼ 8.4 Hz, 1H), 7.87 (t, J ¼ 7.2 Hz, 1H), 7.79 (d,
J ¼ 7.6 Hz, 1H), 7.37 (s, 1H), 7.25 (m, 5H), 6.99 (d, J ¼ 8.4 Hz, 1H), 4.95
(m, 1H), 4.64 (s, 2H), 3.92 (m, 1H), 2.3 (s, 3H), 2.07 (m, 1H), 1.82 (m,
3H), 1.65 (m, 1H), 1.46 (m, 1H). 13C NMR
d (100 MHz, DMSO-d6):
163.38, 163.14, 147.81, 146.79, 139.39, 135.44, 132.62, 131.13, 130.60,
130.50, 129.83, 129.66, 129.23, 128.92, 128.23, 127.71, 127.41, 127.08,
126.29, 122.42, 120.05, 119.87, 106.76, 68.36, 44.50, 43.44, 34.89,
33.07, 24.00, 20.35. HRMS (ESI) calcd for C32H25N2O4 [M ꢀ H]þ
501.1814, found 501.1833.
D-3. (0.10 g, 35% for two steps), 1H NMR
d (400 MHz, DMSO-d6):
8.58 (d, J ¼ 7.6 Hz, 2H), 7.78 (d, J ¼ 7.6 Hz, 2H), 7.29 (t, J ¼ 8.0 Hz, 8H),
7.09 (t, J ¼ 8.0 Hz, 4H), 7.08 (d, J ¼ 8.0 Hz, 8H), 6.98 (d, J ¼ 8.4 Hz,
4H), 6.75 (d, J ¼ 8.4 Hz, 4H), 4.63 (s, 2H). 13C NMR
d
(100 MHz,
3.2. Optical properties of sensitizers
DMSO-d6): 169.82, 163.08, 146.74, 146.64, 146.19, 134.50, 130.74,
130.47, 130.26, 129.91, 129.53, 127.21, 124.68, 123.66, 120.70, 120.15,
42.62. HRMS (ESI) calcd for C50H34N3O4 [M ꢀ H]þ 740.2549, found
740.2554.
The absorption spectra of dyes (D-1, D-2, D-3 and D-4) in
methanol-chloroform (4:1, v/v) solutions are shown in Fig. 3, and
the data are listed in Table 1. All dyes show broad absorption
spectra in the visible region. Generally, the absorption bands at
D-4. (0.11 g, 30% for two steps), 1H NMR
d (400 MHz, CDCl3): 8.63
(d, J ¼ 7.6 Hz, 2H), 7.69 (d, J ¼ 7.4 Hz, 2H), 7.25e6.54 (m, 14H), 4.95
around 300 nm can be ascribed to the pep
* electron transition, and
(m, 2H), 4.52 (s, 2H), 3.63 (m, 2H), 2.31 (s, 6H), 2.07e1.45 (m, 12H).
bands at around 450 nm to the ICT band between TPA or indoline
based donor and the naphthalimide-carboxylic acid, resulting in an
efficient charge separation excited state [43]. Mono-TPA based D-1
shows the absorption band (lmax) at 419 nm with a relatively low
molar extinction coefficient (8200 Mꢀ1 cmꢀ1). In contrast, mono-
indoline based D-2 shows red shift absorption at 458 nm, the bis-
TPA based D-3 red shifts to 438 nm with higher molar extinction
coefficient (14700 Mꢀ1 cmꢀ1). Moreover, the bis-indoline based dye
D-4 shows the largest absorption band red shifted to 473 nm.
Apparently, the indonline unit has stronger electron-donating
capability than that of TPA, and the incorporation of bis-donor unit
instead of the mono-donor is further beneficial to the red shift in
the absorption band.
13C NMR
d (100 MHz, CDCl3): 164.95, 149.09, 147.06, 133.88, 132.24,
131.44, 131.02, 130.62, 130.30, 129.61, 129.11, 127.59, 126.86, 126.25,
125.35, 120.95, 120.04, 106.16, 68.61, 44.97, 43.83, 34.79, 33.11,
24.15, 20.60. HRMS (ESI) calcd for C50H42N3O4 [M ꢀ H]þ 748.3175,
found 748.3179.
3. Results and discussion
3.1. Design and synthesis of sensitizers
The sensitizers in DSCs always play a great important role in
realizing high power conversion efficiency. Generally, the efficiency
of metal-free organic DSCs can be improved by two channels: i)
improving Jsc via increasing spectral response and molar extinction
coefficients; ii) increasing Voc via tuning orbital levels and inhibit-
ing dark current. Accordingly, it is very flexible in molecular designs
for metal-free organic sensitizers to improve the photovoltaic
properties of DSCs.
In order to monitor the light response, a monolayer of theses
dyes was loaded on a thin TiO2 film (3 mm in thickness). As shown in
Fig. 4, the absorption spectra of D-2 and D-4 became broadened
significantly with respect to those in methanol-chloroform solu-
tion. Notably, the spectral onsets of D-2 and D-4 were extended by
100 and 50 nm, respectively. Consequently, the indoline unit can
efficiently decrease the band-gap and optimize energy levels, thus
resulting in a longer responsive wavelength to NIR region with
several favorable characteristics of the beneficial light harvesting
and photocurrent generation in DSCs. Compared to that in solu-
tions, the absorption spectra of mono-donor based D-1 and D-2 on
thin TiO2 film have slightly blue-shifted by 2 and 21 nm, respec-
tively. In contrast, the absorption peaks of bis-donor based dyes D-3
and D-4 on TiO2 film exhibit a red shift by 4 and 33 nm, respectively.
In traditional Donor-p-Acceptor (D-p-A) system, herein we
introduce the naphthalimide and indoline or TPA unit to sensitizers
as electron acceptor and donor, respectively. The incorporation of
naphthalimide unit shows several favorable characteristics in the
beneficial of light harvesting and efficiency. The imide group with
strong electron-withdrawing nature has serious effect on the whole
molecular polarity. Compared with traditional acceptors, the elec-
tron-withdrawing ability of imide group at the naphthalimide unit

300
X. Huang et al. / Dyes and Pigments 90 (2011) 297e303
Fig. 2. Synthetic route of dyes D-1, D-2, D-3 and D-4.
3.3. Electrochemical properties
and regenerate from the oxidized state [44]. Thus, the electro-
chemical properties of dyes were studied to clarify their HOMO
and LUMO levels, using SCE as a reference electrode, 0.1 M tetra-
butylammonium hexafluorophosphate as a supporting electrolyte
(Table 1). The HOMO levels corresponding to the first oxidation
potentials (Eox vs NHE) are 1.08, 0.79, 0.93 and 0.61 V, respectively,
which are sufficiently more positive than the Iꢀ3 /Iꢀ redox potential
value. And the LUMO levels are ꢀ1.42, ꢀ1.45, ꢀ1.43 and ꢀ1.46 V,
For highly efficient DSCs, suitable LUMO and HOMO levels of
sensitizer are generally required to match the conduction band
(Ecb) of TiO2 electrode and the HOMO level of Iꢀ3 /Iꢀ, respectively.
That is, the LUMO level must be more negative than the Ecb of TiO2
electrode for the injection of electrons, and the HOMO level must
be more positive than the HOMO level of Iꢀ3 /Iꢀ to accept electrons,
Table 1
Optical and electrochemical properties of dyes D-1, D-2, D-3 and D-4.
HOMOc
E0e0
LUMO
a
b
a
d
Dyes
lmax
lmax
3
at lmax
nm
nm
Mꢀ1 cmꢀ1
V vs NHE
V vs NHE
V vs NHE
D-1
D-2
D-3
D-4
419
458
438
473
417
437
442
506
8200
5100
14700
19000
1.08
0.79
0.93
0.61
2.50
2.24
2.36
2.07
ꢀ1.42
ꢀ1.45
ꢀ1.43
ꢀ1.46
a
Maximum absorption in methanol-chloroform (4:1, v/v) solutions.
b
c
Maximum absorption on TiO2 film (3
mm thick).
Cyclic voltammetry of the oxidation behavior of the dyes adsorbed on TiO2 film
was measured in dry acetonitrile containing 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as supporting electrolyte (working electrode, dye-coated
TiO2 electrode; reference electrode, a saturated calomel electrode in saturated KCl
solution calibrated with ferrocene/ferrocenium (Fcþ/Fc) as reference; counter
electrode, Pt).
d
Fig. 3. Absorption spectra of dyes D-1, D-2, D-3 and D-4 in methanol-chloroform (4:1,
The zerothezeroth transition E0e0 values were estimated from the energy at the
v/v).
absorption threshold wavelength of dyes adsorbed on TiO2.

X. Huang et al. / Dyes and Pigments 90 (2011) 297e303
301
Table 2
Performance parameters of dye-sensitized solar cells.a
Dye
Jsc (mA cmꢀ2
)
Voc (mV)
FF
h (%)
D-1
D-2
D-3
D-4
N719
2.51
2.90
4.62
5.42
11.95
597
594
662
664
727
0.73
0.68
0.74
0.75
0.68
1.10
1.18
2.27
2.70
5.91
a
The listed data are results from the measurement which has the closest values to
the average of 6 times.
the corresponding photocurrent-voltage curves are shown in Fig. 6.
We obtained a solar energy to electricity conversion efficiency of
1.10% (Jsc ¼ 2.51 mA cmꢀ2, Voc ¼ 597 mV, FF ¼ 0.73), 1.18% (Jsc
¼
2.90 mA cmꢀ2, Voc ¼ 594 mV, FF ¼ 0.68), 2.27% (Jsc ¼ 4.62 mA cmꢀ2
Voc ¼ 662 mV, FF ¼ 0.74) and 2.70% (Jsc ¼ 5.42 mA cmꢀ2
,
,
Fig. 4. Normalized absorption spectra of Dyes D-1, D-2, D-3 and D-4 adsorbed on TiO2
film (3 m in thickness).
m
Voc ¼ 664 mV, FF ¼ 0.74) with DSCs based on D-1, D-2, D-3 and D-4,
respectively. In contrast with D-1 and D-2, the improved cell
performance of D-3 and D-4 can be mainly attributed to the
bathochromic shift in the absorption band, resulting in a Jsc increase
in the high IPCE values.
respectively, which are also more negative than Ecb of TiO2 elec-
trode (ꢀ0.5 V vs. NHE), indicating that the electron injection
process from the excited dye molecule to TiO2 conduction band is
thermodynamically favorable. Notably, the introduction of stronger
electron-donating indoline unit can make the HOMO levels in the
series of dyes more negative (D-2 < D-1 and D-4 < D-3). Among
them, the bis-indoline based D-4 exhibits the highest LUMO level,
indicative of high injection driving force to photo-induced
electrons.
The short circuit current Jsc is always corresponding to the molar
extinction coefficient of dye molecules. Compared with D-1, D-2
has a lower molar extinction coefficient, but a higher Jsc, which
might be probably due to the broader absorption of D-2 (up to
550 nm). However, compared with N719, all the dyes have lower Jsc,
which might be resulted from the fact that the methylene group in
the naphthalimide framework disrupts the p-conjugation between
naphthalimide and the carboxylic acid, hence decreasing the elec-
tron injection efficiency in a dynamic manner [45].
3.4. Performances of DSCs
A larger Voc is also a prerequisite for higher power conversion
efficiencies in DSCs. As listed in Table 2, the open-circuit voltages
(Voc) of bis-donor based dyes D-3 and D-4 are higher than that of
mono-donor based D-1 and D-2, which can be resulted from the
retardation of charge recombination at the nanocrystallite/dye/
redox electrolyte interface [46,47]. What’s more, the power
conversion efficiencies of bis-donor dyes are almost twice higher
than those of the corresponding mono-donor dyes due to a better
light harvesting ability from the bis-donor dyes. Interestingly, the
fill factors (FF) of four dyes are even better than that of N719.
Among them, D-4 with bis-indoline units has the highest power
conversion efficiency up to 2.70%, further confirming that the
indoline substituted dyes are superior to the corresponding TPA
substituted ones. And the introduction of multi-donor units is
beneficial to the improvement of power conversion efficiencies.
Fig. 5 shows the action spectra of monochromatic incident
photon-to-current conversion efficiency (IPCE) for these naph-
thalimide dyes, obtained from a sandwich-type cell using platinized
ITO glass as counter electrodes and 0.5 mol Lꢀ1 LiI, 0.05 mol Lꢀ1 I2,
0.6 mol Lꢀ1 methylbutylimidazoliumiodide, 0.5 mol Lꢀ1 tert-butyl-
pyridine in acetonitrile solution as electrolyte. D-4 gives broader
IPCE spectra than the other three dyes in the spectral range of
400e700 nm, which is in accordance with their absorption spectra
on TiO2 films (Fig. 4). With respect to the mono-donor based dyes
D-1 and D-2, both bis-donor based dyes D-3 and D-4 have higher
IPCE values which may benefit from their higher LHE.
The photovoltaic performances of solar cells constructed from
these organic dye-sensitized TiO2 electrodes were measured under
standard global AM 1.5 illumination (100 mW cmꢀ2). Their
photovoltaic data as well as N719 are summarized in Table 2, and
Fig. 5. IPCE action spectra of solar cells sensitized with dyes D-1, D-2, D-3 and D-4
Fig. 6. Photocurrentevoltage curves of solar cells sensitized with dyes D-1, D-2, D-3
(subtract the affection of TiO2).
and D-4.

302
X. Huang et al. / Dyes and Pigments 90 (2011) 297e303
Fig. 7. Calculated frontier molecular orbitals of HOMO and LUMO and experimental energy level diagrams of D-1, D-2, D-3 and D-4.
To get a deep insight into the geometrical and electronic prop-
Acknowledgements
erties of DSCs based on these naphthalimide dyes, density func-
tional theory (DFT) calculations [48] were performed by hybrid
density functional theory (B3LYP) with 6-31G* basis set as imple-
mented in the Gaussian 03 program for the geometry optimization.
The isodensity plots of the frontier molecular orbitals of dyes are
shown in Fig. 7. Obviously, the HOMO orbital is almost delocalized
over TPA and indoline moieties. But the LUMO is delocalized mainly
on the naphthalimide moieties, especially on the carbonyl, and as
a result, the position of the LUMO is isolated from the carboxyl
anchoring group (-CH2CO2H) due to the presence of the methylene
group, which could suppress the electron injection efficiency from
the excited dyes to the TiO2 conduction band via the carboxyl
group, and thus, leading to the inferior efficiencies of 1.10, 1.18, 2.27
and 2.70%, respectively, even though they exhibit broad spectral
response and high extinction coefficients. [49]
This work was financially supported by NSFC/China, the Oriental
Scholarship, SRFDP 200802510011 and 20100074110015, the Funda-
mental Research Funds for the Central Universities (WK1013002),
Innovation Program of Shanghai Municipal Education Commission,
and State Key Laboratory of Precision Spectroscopy (ECNU).
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