A novel compact DPP dye with enhanced light harvesting and charge transfer properties for highly efficient DSCs

Article abstract of DOI:10.1039/c3ta10618g

A novel compact diketopyrrolopyrrole (DPP) dye was designed and synthesized using DPP core as a bridge to connect bis(4-tert-butylphenyl)phenylamine and cyanoacetic acid units with the D-π-A configuration, and employed as a sensitizer in dye-sensitized so

Full text of DOI:10.1039/c3ta10618g

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Materials Chemistry A
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PAPER
A novel compact DPP dye with enhanced light
harvesting and charge transfer properties for highly
Cite this: J. Mater. Chem. A, 2013, 1,
4858
efficient DSCs†
Fang Zhang,ab Ke-Jian Jiang, Jin-Hua Huang,a Chun-Chun Yu,a Shao-Gang Li,a
a
*
b
a
a
*
*
Ming-Gong Chen, Lian-Ming Yang and Yan-Lin Song
Received 8th February 2013
Accepted 14th February 2013
A novel compact diketopyrrolopyrrole (DPP) dye was designed and synthesized using DPP core as a bridge
to connect bis(4-tert-butylphenyl)phenylamine and cyanoacetic acid units with the D–p–A configuration,
and employed as a sensitizer in dye-sensitized solar cells, giving a high power conversion efficiency of
8.61% under AM 1.5 conditions.
DOI: 10.1039/c3ta10618g
www.rsc.org/MaterialsA
improved since the report in 2010.7a Recently, Han's group
introduced a strong electron donor indoline into a DPP dye,
Introduction
giving an efficiency of 7.4%.7d Very recently, an asymmetric DPP
sensitizer with intense absorption in the red/near-IR region was
prepared with an efficiency of 7.7%, and a higher efficiency of
8.6% was achieved by its co-sensitization with another organic
sensitizer with complementary spectral absorption in the
visible region.7f Thus, DPP is a desirable building block for
constructing highly efficient sensitizers in DSCs.
Dye-sensitized solar cells (DSCs) are one of the most promising
alternatives to the conventional inorganic devices due to their
potentially low material/fabrication cost and relatively high
conversion efficiencies.1 In DSCs, the sensitizer is one of the key
components, exerting a signicant inuence on the device
power conversion and stability. With ruthenium complexes2a–d
and zinc porphyrin dyes,2e,f high power conversion efficiencies
of up to 8–12% have been achieved. On the other hand, metal-
free organic dyes, usually with the electron donor–p bridge–
electron acceptor (D–p–A) conguration, have been actively
pursued due to their molecular tailoring exibility and raw
material abundance.3 In the D–p–A organic dyes, triarylamine
and cyanoacrylic acid were widely used as a donor and an
acceptor, respectively, and various p-conjugated bridges were
employed to bridge the donor and acceptor units to create a
large number of D–p–A dyes for DSCs. It is well known that the
bridge is of paramount importance in tuning the molecular
energy gap, and the electronic and steric structures, strongly
affecting device performances.4
The diketopyrrolopyrrole (DPP) chromophore has a uniquely
planar conjugated bicyclic structure with electron-withdrawing
property, and its derivatives are extensively used as high-
performance pigments due to its exceptional photochemical,
mechanical and thermal stability.5 Following its successful
investigations in a series of optical electronic devices,6 it was
tested as a bridge to construct D–p–A organic dyes for DSCs
recently,7 and the device performances have been greatly
In this article, we demonstrate a successful design of a new
metal-free organic dye, utilizing the DPP core as the p-conju-
gated bridge, bis(4-tert-butylphenyl)phenylamine as the elec-
tron donor and cyanoacrylic acid as the electron acceptor/
anchoring group (denoted as ICD-1). For comparison, a refer-
ence sensitizer was synthesized with an additional phenyl unit
inserted between the triphenylamine unit and DPP core
(denoted as ICD-2), whose structure was extensively studied in
DPP-based sensitizers recently.7 Our experiment showed that,
without the phenyl unit, the dye absorption band in ICD-1 is
dramatically red-shied with highly enhanced molar coefficient
efficiency, and the power conversion efficiency was signicantly
improved from 7.20% (ICD-2) to 8.61% (ICD-1) in DSCs with an
ꢀ
Iꢀ/I3 based electrolyte under standard AM 1.5 conditions.
Experimental section
Measurement and characterization
NMR spectra were recorded on a BRUKER AVANCE 400 MHz
instrument. The residual solvent protons (1H) or the solvent
carbons (13C) were used as internal standards. 1H-NMR data are
presented as follows: the chemical shi in ppm (d) downeld
from tetramethylsilane (multiplicity, coupling constant (Hz),
integration). The following abbreviations are used in reporting
NMR data: s, singlet; br.s, broad singlet; d, doublet; t, triplet; q,
quartet; dd, doublet of doublets; and m, multiplet. UV-vis
aLaboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, P. R. China, 100190. E-mail: kjjiang@iccas.ac.cn; ylsong@iccas.ac.cn
bSchool of Chemical Engineering of Anhui University of Science and Technology,
Huainan, P. R. China, 232001. E-mail: mgchen@aust.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta10618g
absorption spectra were recorded on
a
HP 8453
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spectrophotometer. Mass spectra were taken on a Bruker Dal- yield ¼ 42%). 1H-NMR (CDCl3; 400 MHz), d (ppm): 0.83 (m, 6H),
tonics Inc. APEXII FT-ICR spectrometer. The photocurrent– 1.22 (s, 12H), 1.55–1.60 (m, 4H), 3.71–3.80 (m, 4H), 7.50 (d, J ¼ 4
voltage (I–V) characteristics were recorded at room temperature Hz, 1H), 7.65–7.71 (m, 4H), 7.77 (d, J ¼ 4 Hz, 1H), 7.80 (d, J ¼ 8.4
using a computer-controlled Keithley 2400 source meter under Hz, 2H), 7.89 (d, J ¼ 8.4 Hz, 2H), 9.92 (s, 1H).
air mass (AM) 1.5 simulated illumination (100 mW cmꢀ2, Oriel,
Synthesis of 4. An oven-dried 100 mL three-necked ask was
67005). The action spectra of the monochromatic incident charged with compound 3 (0.64 g, 1.0 mmol), Pd(OAc)2 (22 mg,
photo-to-current conversion efficiency (IPCE) for solar cells were 0.1 mmol), P(t-Bu)3 (20 mg, 0.1 mmol), bis(4-(tert-butyl)phenyl)
collected using a commercial setup (PV-25 DYE, JASCO). A 300 amine (0.37 g, 1.30 mmol) and tBuONa (0.48 g, 5.0 mmol). The
W Xenon lamp was employed as light source for generation of a ask was evacuated and backlled with nitrogen, with the
monochromatic beam. Calibrations were performed with a operation being repeated twice. Then 20 mL of toluene were
standard silicon photodiode. IPCE is dened as IPCE(l) ¼ added and the mixture was heated at 90 ꢂC for 16 h. Aer
hcJsc/e4l, where h is Planck's constant, c is the speed of light in a cooling to room temperature, dichloromethane was added and
vacuum, e is the electronic charge, l is the wavelength in meters the organic phase was ltered through a silica-gel pad. The
(m), Jsc is the short-circuit photocurrent density (A mꢀ2), and 4 ltrate was washed with water (3 ꢁ 20 mL) and brine (3 ꢁ
is the incident radiation ux (W mꢀ2).
20 mL). The combined organic phase was dried on MgSO4,
ltered and evaporated under reduced pressure and the residue
was puried by silica-gel column chromatography to give a
purple solid (0.53 g, 62%). 1H-NMR (CDCl3, 400 MHz) d (ppm):
Materials
All reagents were obtained from Alfa Aesar Chemical Co., 0.81–0.88 (m, 6H), 1.23–1.28 (m, 12H), 1.33 (s, 18H), 1.61–1.74
Aladdin Chemical Co., and J&K Chemical Co. and they were (m, 4H), 3.79 (t, J ¼ 7.8 Hz, 4H), 7.03 (d, J ¼ 8.8 Hz, 2H), 7.13 (d,
used as received unless otherwise specied. All manipulations J ¼ 8.8 Hz, 4H), 7.34 (d, J ¼ 8.8 Hz, 4H), 7.49 (d, J ¼ 4 Hz, 1H),
involving air-sensitive reagents were performed in an atmo- 7.77–7.82 (m, 5H), 7.90 (d, J ¼ 8.8 Hz, 2H), 9.92 (s, 1H).
sphere of dry argon. The solvents (tetrahydrofuran (THF) and
Synthesis of ICD-1. An oven-dried 100 mL one-necked ask
toluene) were puried by routine procedures and were distilled was charged with compound 4 (3.5 ꢁ 10ꢀ4 mol), cyanoacrylic
under dry argon before being used. 1,4-Diketo-3,6-bis(4-bro- acid (7 ꢁ 10ꢀ3 mol), 10 mL of dry THF and 0.4 mL piperidine.
mophenyl)pyrrolo[3,4-c]-pyrrole (DPP) was prepared according The solution was heated to reux for 15 h and the colour turned
to the reference procedures.8
deep red. Aer cooling to room temperature, dichloromethane
Synthesis of 2. An oven-dried 250 mL one-necked ask was was added and the organic phase was washed with water (3 ꢁ
charged with DPP (1) (3.6 g, 8 mmol), NaH (0.48 g, 20 mmol) (60% 20 mL) and brine (3 ꢁ 20 mL). The combined organic phase was
dispersion in mineral oil, in soluble bags in resealable cans) and dried on MgSO4, ltered and evaporated under reduced pres-
150 mL of DMF. The mixture was stirred for 1 h at room sure and the residue was puried by silica-gel column chro-
1
temperature, followed by dropping 1-bromohexane (13.3 g, 80 matography to give a purple solid (86%). H-NMR (DMSO, 400
mmol) in 30 mL of DMF. The mixture was kept for an additional MHz) d (ppm): 0.75–0.85 (m, 6H), 1.12–1.16 (m, 12H), 1.29 (s,
30 h at room temperature and ltered. The solid was washed with 18H), 1.47 (s, 4H), 3.71 (s, 4H), 6.84 (d, J ¼ 8 Hz, 2H), 7.10 (d, J ¼
60 mL of chloroform, and the organic phase was washed with 8.4 Hz, 4H), 7.41 (d, J ¼ 8.4 Hz, 4H), 7.77–7.80 (m, 4H), 7.88 (s,
water (3 ꢁ 50 mL) and brine (3 ꢁ 50 mL). The combined organic 4H), 8.18 (s, 1H); HRMS (ESI): m/z found (M + 1): 913.4696.
phase was evaporated under reduced pressure and the residue
Synthesis of 5. An oven-dried 100 mL three-necked ask was
was puried by silica-gel column chromatography to give an charged with compound 3 (0.64 g, 1 mmol), Pd(PPh3)4 (81 mg,
orange-red polycrystalline powder (2.2 g, yield ¼ 45%). 1H-NMR 0.07 mmol), and Na2CO3 (2.3 g, 0.02 mol). The ask was evac-
(CDCl3; 400 MHz), d (ppm): 0.83 (t, J ¼ 6.8 Hz, 6H), 1.20–1.30 (m, uated and backlled with nitrogen, with the operation being
12H), 1.52–1.58 (m, 4H), 3.72 (t, J ¼ 7.6 Hz, 4H), 7.67 (s, 8H).
repeated twice. 30 mL of THF and 8 mL of H2O were added and
Synthesis of 3. An oven-dried 100 mL three-necked ask was the blend was heated at 45 ꢂC for 0.5 h, then a solution of
charged with compound 2 (1.84 g, 3 mmol), Pd(PPh3)4 (0.24 g, bis(4-(tert-butylphenyl)amino)phenyl boronic acid (0.61 g,
0.21 mmol), and sodium carbonate (5.4 g, 51 mmol). The ask 1.50 mmol) in 10 mL of THF was added using a syringe at this
ꢂ
was evacuated and backlled with nitrogen, with the operation time. The temperature was increased to 80 C and maintained
being repeated twice. 40 mL of THF and 15 mL of H2O were for 16 h. Aer cooling to room temperature, dichloromethane
added and the blend was heated at 45 ꢂC for 0.5 h, then a was added and the organic phase was ltered through a silica-
solution of 5-formyl-2-thiophenylboronic acid (0.47 g, 3.0 gel pad. The ltrate was washed with water (3 ꢁ 20 mL) and
mmol) in 10 mL of THF was added using a syringe at this time. brine (3 ꢁ 20 mL). The combined organic phase was dried on
The temperature was increased to 80 ꢂC and maintained for MgSO4, ltered and evaporated under reduced pressure and the
16 h. Aer cooling to room temperature, dichloromethane was residue was puried by silica-gel column chromatography to
added and the organic phase was ltered through a silica-gel give a red solid (0.31 g). Yield: 36%. 1H-NMR (CDCl3, 400 MHz) d
pad. The ltrate was washed with water (3 ꢁ 20 mL) and brine (ppm): 0.83 (t, J ¼ 6.4 Hz, 6H), 1.24 (s, 12H), 1.33 (s, 18H), 1.64–
(3 ꢁ 20 mL). The combined organic phase was dried on MgSO4, 1.65 (m, 4H), 3.78–3.84 (m, 4H), 7.08 (d, J ¼ 8.8 Hz, 4H), 7.12 (d,
ltered and evaporated under reduced pressure and the residue J ¼ 8.8 Hz, 2H), 7.31 (d, J ¼ 8.8 Hz, 4H), 7.50–7.52 (m, 3H), 7.72
was puried by silica-gel column chromatography to give an (d, J ¼ 8.4 Hz, 2H), 7.78 (d, J ¼ 3.6 Hz, 1H), 7.83 (d, J ¼ 8.8 Hz,
orange-red fraction corresponding to pure compound 3 (0.81 g, 2H), 7.92 (t, J ¼ 7.8 Hz, 4H), 9.28 (s, 1H).
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mixture–tetrahydrofuran (1 : 1 : 0.2, v/v) and kept for 24 h at
room temperature. The dyed electrodes were then rinsed with
the mixed solvent to remove the excess dye. A platinum-coated
counter electrode was prepared according to a previous report,10
and two holes were drilled on its opposite sides. The two elec-
trodes were sealed together with a 25 mm thick thermoplastic
Surlyn frame. An electrolyte solution was then introduced
through one of the two holes in the counter electrode, and the
holes were sealed with the thermoplastic Surlyn. The electrolyte
contains 0.68 M dimethyl imidazolium iodide, 0.05 M iodine,
0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40 M tert-
butylpyridine in the mixture of acetonitrile and valeronitrile
(85 : 15, v/v). All the devices were prepared with a photoactive
area of about 0.3 cm2, and a metal mask of 0.165 cm2 was used
to cover the device for photovoltaic property measurements.
Synthesis of ICD-2. An oven-dried 100 mL one-necked ask
was charged with compound 5 (3.5 ꢁ 10ꢀ4 mol), cyanoacrylic
acid (7 ꢁ 10ꢀ3 mol), 10 mL of dry THF and 0.4 mL piperidine.
The solution was heated to reux for 15 h and the colour turned
deep red. Aer cooling to room temperature, dichloromethane
was added and the organic phase was washed with water (3 ꢁ
20 mL) and brine (3 ꢁ 20 mL). The combined organic phase was
dried on MgSO4, ltered and evaporated under reduced pres-
sure and the residue was puried by silica-gel column chro-
matography to give a purple solid (86%). 1H-NMR (CDCl3,
400 MHz) d (ppm): 0.82 (t, J ¼ 7.2 Hz, 6H), 1.20–1.25 (m, 12H),
1.33 (s, 18H), 1.58 (s, 4H), 3.72–3.83 (m, 4H), 7.07–7.11 (m, 6H),
7.29 (d, J ¼ 8.8 Hz, 4H), 7.47–7.50 (m, 3H), 7.68–7.70 (m, 3H),
7.76–7.80 (m, 4H), 7.86 (d, J ¼ 8.4 Hz, 2H), 8.05 (s, 1H); HRMS
(ESI): m/z found (M + 1): 989.5036.
DSC fabrication
Results and discussion
Optical and electrochemical properties
The nanocrystalline TiO2 pastes (particle size, 20 nm) were
prepared using a previously reported procedure.9 Fluorine
doped thin oxide (FTO, 4 mm thickness, 10 ohm sqꢀ1, Nippon The synthetic routes to ICD-1 and ICD-2 are described in
Sheet Glass, Japan) conducting electrodes were washed with Scheme 1. Their UV-Vis absorption spectra examined in
soap and water, followed by sonication for 10 min in acetone dichloromethane solutions and on a transparent TiO2 lm
and isopropanol, respectively. Following a drying period, the (4.5 mm) are displayed in Fig. 1, and the relevant parameters are
electrodes were submerged in a 40 mM aqueous solution of collected in Table 1. Both the dyes showed broad absorption
TiCl4 for 30 min at 75 ꢂC, and then washed by water and spectra with two absorption bands covering a wide range in the
ethanol. On the electrodes, an 11 mm thick nanocrystalline TiO2 visible region. Interestingly, the longer wavelength band of ICD-
layer and a 6 mm thick TiO2 light scattering layer (particle size, 1 (lmax ¼ 543 nm), assigned to intramolecular charge transfer
400 nm, PST-400C) were prepared by the screen-printing (ICT), is signicantly red-shied compared to that of ICD-2
method. The TiO2 electrodes were heated at 500 ꢂC for 30 min, (lmax ¼ 519 nm), although ICD-2 has one additional phenyl
followed by treating with a 40 mM aqueous solution of TiCl4 for ring. The UV-Vis spectra of dyes ICD-1 and ICD-2 adsorbed on
30 min at 75 ꢂC and subsequent sintering at 500 ꢂC for 30 min. transparent TiO2 lms were recorded and are shown in Fig. 2,
The thickness of TiO2 lms was measured using a proler, where the CT bands of ICD-1 and ICD-2 were slightly blue-
Sloan, Dektak3.
shied with lmax at 528 nm and 514 nm, respectively, compared
The electrodes were immersed in a dye bath containing to the solution spectra. Moreover, the molar extinction coeffi-
0.2 mM ICD-1 or ICD-2 and 20 mM 3a,7a-dihydroxy-5b-cholic cient of ICD-1 (3 ¼ 48 100 L molꢀ1 cmꢀ1) is higher than that of
acid (chenodeoxycholic acid) in 4-tert-butanol–acetonitrile ICD-2 (3 ¼ 43 200 L molꢀ1 cmꢀ1). The results imply that the
Scheme 1 The synthetic routes to ICD-1 and ICD-2.
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that in ICD-2. In this case, the additional benzene ring between
the donor and DPP units is not an effective conjugation bridge
for electron transfer in ICD-2. The reason could be explained by
the presence of a non-planar biphenyl structure, which would
hinder charge transfer from the donor to acceptor in the
molecule.11
To investigate the molecular energy levels, cyclic voltammetry
measurements were performed in a 0.1 M dichloromethane
solution of tetra-n-butylammonium hexauorophosphate with
ferrocene as an internal standard at 0.63 V vs. NHE. The rst
oxidation potentials (E+/0) of ICD-1 and ICD-2 were observed to
be 0.97 and 0.99 V vs. NHE, respectively, which are assigned to
the oxidation of triphenylamine units. The positive shi of the
oxidation potential in ICD-2 means the weaker donor–acceptor
interaction due to poor charge transfer from triphenylamine to
cyanoacetic acid, in keeping with the analysis presented above.
Both the potential values are substantially more positive than
that of the iodide/triiodide couple redox (0.4 V vs. NHE), indi-
cating that the ground-state sensitizer regeneration is energeti-
cally favorable in DSCs.12 The optical transition energies (E0–0
)
were 2.04 eV for ICD-1 and 2.12 eV for ICD-2, estimated from the
intersection of their absorption and emission spectra. The
excited-state redox potentials, E+/*, determined by subtracting
E0–0 from E+/0, were ꢀ1.07 V for ICD-1 and ꢀ1.13 eV for ICD-2.
Both the values are negative enough to allow their excited state
electron transfer into the TiO2 conduction band (ꢀ0.5 V vs.
NHE).13
Fig. 2 shows the action spectra in the form of mono-
chromatic incident photon-to-current conversion efficiencies
(IPCEs). The ICD-1 based device clearly exhibited a stronger and
broader response in the entire visible spectral region, especially
in the red region, as compared to that of the ICD-2 based device.
The former showed plateaus of over 80% from 450 to 630 nm
with the highest value of 87% at 600 nm and an onset at 800 nm,
while ICD-2 presented a relatively narrow response spectrum
with the highest value of 82% at 543 nm. The difference
between IPCE spectra is in good agreement with their difference
in absorption spectra as shown in Fig. 1. The higher IPCE values
for ICD-1 in the whole spectrum imply efficient charge transfer
upon photo-excitation, partly due to the efficient electron
communication, as discussed above.
Fig. 1 Absorption spectra of ICD-1 and ICD-2 in dichloromethane (a) and on a
transparent TiO2 film (4.5 mm) (b).
device with ICD-1 would harvest more photons and give a
higher conversion efficiency as compared to the ICD-2 based
device. In general, the ICT band is strongly related to the nature
of electron donor, acceptor, and the bridge connecting them in
a D–p–A system. Thus, the ICT in ICD-1 is more efficient than
Table 1 Photoelectrochemical properties of ICD-1 and ICD-2 and their solar cell performances
3 (L molꢀ1 cmꢀ1
)
lemb (nm)
652
E0–0 (eV)
E+/0 (V)
E+/*e (V)
ꢀ1.07
Jsc (mA cmꢀ2
16.65 ꢃ 0.20
13.67 ꢃ 0.20
)
Voc (mV)
708 ꢃ 10
712 ꢃ 10
FF
hf (%)
8.61
c
d
Dye
lmaxa (nm)
ICD-1
ICD-2
543
389
519
393
48 100
41 900
43 200
41 000
2.04
2.12
0.97
0.99
0.73 ꢃ 0.02
0.74 ꢃ 0.02
664
ꢀ1.13
7.20
a
b
c
Absorption in CH2Cl2 solutions (1 ꢁ 10ꢀ5 M) at rt. Emission in CH2Cl2 solutions (1 ꢁ 10ꢀ5 M) at rt. E0–0 values were estimated from the
d
intersection of the absorption and emission spectra. The oxidation potentials of the dyes were measured in CH2Cl2 solutions with tetra-butyl-
ammoniumhexauorophosphate (TBAPF6, 0.1 M) as an electrolyte, Pt wires as working and counter electrodes, Ag/Ag+ as a reference electrode;
calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV. The estimation was
e
f
determined by subtracting E0–0 from Es+/0
.
The data were recorded under AM 1.5 G simulated solar light at a light intensity of 100 mW cmꢀ2
,
and represent the average of three devices, where TiO2 lms with a 11 mm thick nanocrystalline layer and a 6 mm thick scattering layer were
used with an electrolyte containing 0.68 M dimethyl imidazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40
M tert-butylpyridine in a mixture of acetonitrile and valeronitrile (85/15, v/v), and each value was averaged by three parallel samples.
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Fig. 2 IPCE spectra of DSCs with ICD-1 and ICD-2, the inset shows their current–
voltage characteristics.
The inset in Fig. 2 shows the energy conversion efficiency for
DSCs with ICD-1 and ICD-2 under AM 1.5 G simulated solar
light at a light intensity of 100 mW cmꢀ2. The ICD-1
based-device gave a short circuit photocurrent density (Jsc) of
16.65 ꢃ 0.20 mA cmꢀ2, an open circuit voltage (Voc) of 708 ꢃ
10 mV, and a ll factor (FF) of 0.73 ꢃ 0.02, corresponding to an
overall conversion efficiency (h), derived from the equation h ¼
JscVocFF/light intensity, of 8.61%, while the ICD-2 based device
gave a Jsc of 13.67 ꢃ 0.20 mA cmꢀ2, Voc of 712 ꢃ 10 mV, FF of
0.74 ꢃ 0.02, and h of 7.20% under the same conditions. The
large difference in the efficiency comes mainly from the
differences in the Jsc between them. The higher Jsc for ICD-1 is
consistent with its higher and broader IPCE spectrum as
compared with that for ICD-2. The comparison between ICD-1
and ICD-2 clearly reects the importance of molecular cong-
uration in the performance.
Fig. 3 The geometry optimized ground state molecular structures of ICD-1 and
ICD-2 with the corresponding dihedral angles between each plain.
Density functional theory calculations
In order to gain insight into the geometrical conguration and
electron distribution of the frontier orbitals of the two dyes,
density functional theory (DFT) calculations were made on a
B3LYP/6-31G level. Their optimized ground state molecular
structures and the corresponding dihedral angles are presented
in Fig. 3. As expected, the additional phenyl unit in ICD-2
caused an additional torsion in the biphenyl unit with a dihe-
dral angle of z32ꢂ. Correspondingly, the electron distribution
of the HOMO is obstructed by the additional phenyl unit, and
located mainly on the separate triphenylamine and DPP units in
ICA-2 as shown in Fig. 4. By contrast, the HOMO of ICD-1 is
homogeneously delocalized over the whole triphenylamine and
DPP units. For both dyes, the LUMO orbitals show a similar
electron distribution over the DPP core, phenyl unit, thiophene
unit, and cyanoacetic acid unit with little contribution from the
former. According to the observation, electron communication
may be more favorable between the donor and acceptor in
ICD-1, allowing an efficient electron transfer from the dye to
TiO2 electrode under light irradiation, which may result in a
higher device performance as compared with that for ICD-2.
Fig. 4 HOMO–LUMO frontier molecular orbitals of ICD-1 and ICD-2, calculated
with DFT on a B3LYP/6-31+G(d) level (isodensity ¼ 0.03 au).
Conclusions
In summary, a novel compact DPP-based dye ICD-1 was
designed and employed as a sensitizer in DSCs, giving a high
power conversion efficiency of 8.61%, which is much higher
than that (7.20%) for the reference dye with an additional
phenyl unit inserted between the triphenylamine unit and DPP
core, whose structure was extensively studied in DPP-based
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sensitizers recently. Our results indicate that molecular engi-
neering is crucial for constructing highly efficient sensitizers in
DSCs, and the results would provide valuable, basic guidelines
for rational designs of D–p–A molecules for high-performance
DSCs and other optoelectronic devices. Further improvement of
the performance could be expected for DPP sensitizers with a
broader spectral response in the red/near-infrared region, and
the research is in progress along with the stability test.
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Acknowledgements
The authors greatly appreciate the nancial support from the
National Natural Science Foundation of China (Grant no.
21174149, 51173190, 21073203, 21076002 and 21121001), the
National 863 Program (no. 2011AA050521), the 973 Program
(2009CB930404, 2011CB932303, 2011CB808400), and the
Scientic Equipment Program, ACS (YZ201106).
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This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 4858–4863 | 4863