Design and synthesis of solution processable small molecules towards high photovoltaic performance

Article abstract of DOI:10.1039/c0jm02510k

We successfully synthesized a series of novel symmetrical solution processable small molecules (APPM, AAPM and ATPM) consisting of the electron-accepting moiety (2-pyran-4-ylidenemalononitrile) (PM) and the electron-donating moiety (triphenylamine) linked by different electron-donating moieties (phenothiazine, triphenylamine and thiophene) through a Suzuki coupling reaction. Differential scanning calorimetry (DSC) measurement indicates that APPM and AAPM shows relatively high glass-transition temperature of ca. 137 °C and 163 °C, while the melting point of ATPM is at ca. 164 °C. UV-vis absorption spectra show that the combination of the PM moiety with moieties with a gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition, which leads to an extension of the absorption spectral range and a reduction of the band gap of the molecules. Both cyclic voltammetry measurement and theoretical calculations displayed that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be fine-tuned by changing the electron-donating ability of the electron-donating moieties. The bulk heterojunction (BHJ) photovoltaic devices with a structure of ITO/PEDOT/PSS/small molecules/PCBM/LiF/Al were fabricated by using the small molecules as donors and (6,6)-phenyl C ₆₁-butyric acid methyl ester (PCBM) as acceptor. Power conversion efficiencies (PCE) of 0.65%, 0.94% and 1.31% were achieved for the photovoltaic devices based on APPM/PCBM, AAPM/PCBM and ATPM/PCBM under simulated AM 1.5 illumination (100 mW cm^-2), respectively. The open circuit voltage of 1.0 V obtained from the device based on ATPM/PCBM is one of the highest values for organic solar cells based on solution processable small molecules. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0jm02510k

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PAPER
Design and synthesis of solution processable small molecules towards high
photovoltaic performance
Zaifang Li, Qingfeng Dong, Yaowen Li, Bin Xu, Meng Deng, Jianing Pei, Jibo Zhang, Feipeng Chen,
*
Shanpeng Wen, Yajun Gao and Wenjing Tian
Received 3rd August 2010, Accepted 26th October 2010
DOI: 10.1039/c0jm02510k
We successfully synthesized a series of novel symmetrical solution processable small molecules (APPM,
AAPM and ATPM) consisting of the electron-accepting moiety (2-pyran-4-ylidenemalononitrile) (PM)
and the electron-donating moiety (triphenylamine) linked by different electron-donating moieties
(phenothiazine, triphenylamine and thiophene) through a Suzuki coupling reaction. Differential
scanning calorimetry (DSC) measurement indicates that APPM and AAPM shows relatively high
glass-transition temperature of ca. 137 ꢀC and 163 ꢀC, while the melting point of ATPM is at ca. 164 ꢀC.
UV-vis absorption spectra show that the combination of the PM moiety with moieties with a gradually
increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT)
transition, which leads to an extension of the absorption spectral range and a reduction of the band gap
of the molecules. Both cyclic voltammetry measurement and theoretical calculations displayed that the
highest occupied molecular orbital (HOMO) energy levels of the molecules could be fine-tuned by
changing the electron-donating ability of the electron-donating moieties. The bulk heterojunction
(BHJ) photovoltaic devices with a structure of ITO/PEDOT/PSS/small molecules/PCBM/LiF/Al were
fabricated by using the small molecules as donors and (6,6)-phenyl C61-butyric acid methyl ester
(PCBM) as acceptor. Power conversion efficiencies (PCE) of 0.65%, 0.94% and 1.31% were achieved for
the photovoltaic devices based on APPM/PCBM, AAPM/PCBM and ATPM/PCBM under simulated
AM 1.5 illumination (100 mW cmꢁ2), respectively. The open circuit voltage of 1.0 V obtained from the
device based on ATPM/PCBM is one of the highest values for organic solar cells based on solution
processable small molecules.
to a relatively low fill factor (FF) and PCE of the photovoltaic
devices.4,5
Introduction
Photovoltaic devices based on organic semiconductors are
evolving into a promising cost-effective alternative to silicon-
based solar cells due to their low-cost fabrication through
solution processing, lightweight nature, as well as excellent
compatibility with flexible substrates.1,2 To date, the highest
power conversion efficiency (PCE) of the bulk heterojunction
(BHJ) polymer photovoltaic devices has been up to 7.73%.3
Generally, polymers have many advantages: such as strong
absorption ability, admirable solution processability, good film-
forming ability and tunable energy levels. However, the purifi-
cation of polymers is still one of the most difficult problems
troubling people. And as usual, a polymer is a mixture of
molecules with different molecular weight. The impurity and
relatively high molecular weight dispersity would significantly
decrease the charge carrier mobility of polymers and further lead
In contrast with polymers, small molecules have attracted
more and more attention for photovoltaic applications due to
their high purity, high charge carrier mobility (15 cm2 Vꢁ1 s),6
solution processability, well-defined molecular structures and
definite molecular weights. Until now, profound progress has
been achieved in the synthesis of new solution processable small
molecules and the corresponding photovoltaic applications.7–16
Although the highest PCE of a solution processable bulk-
heterojunction photovoltaic device based on small molecules has
reached 4.4%,16 the mismatch between the absorption spectrum
of small molecules and the solar spectrum is still one of the
primary problems for improving the PCE of photovoltaic
devices.
Recently, people have paid more and more attention to the
design and synthesis of small molecules with donor–acceptor
(D–A) structure.7,8,14–17 The intramolecular charge transfer (ICT)
from the donor moiety to the acceptor moiety inside a D–A
molecule can efficiently extend the absorption spectrum of the
molecule for better matching the solar spectrum. Moreover, the
Jilin University - State Key Laboratory of Supramolecular Structure and
Materials, Changchun, 130012, China
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incorporation of functional moieties with varied electron-
donating ability will bring different degrees of ICT to the
conjugated system and thus provide a means to tune the optical
band gap, electrochemical properties and energy levels for
achieving low band gap, good air stability and high open circuit
voltage (Voc) small molecules.18 There are two requests for the
energy level of the D–A small molecules for achieving high
performance. One is the LUMO energy level of the donor must
be higher than that of the electron acceptor (at least 0.3 eV) so as
to drive the electron transfer from the donor to the acceptor
sufficiently. The other is the HOMO energy level of the donor
molecule must be low enough in order to get relatively high Voc
which is determined by the difference between the HOMO energy
level of the donor and the LUMO energy level of the acceptor.19
In recent years, a series of D–A small molecules based on
triphenylamine (TPA) had been synthesized and applied to
photovoltaic devices.20–22 These small molecules possessed good
film-forming ability, excellent solubility and the relatively high
Voc. However, these small molecules exhibited relatively narrow
absorption range from 300 to 630 nm, and the total photon
energy among this range is rather small considering the whole
solar spectra. For example, the D–A small molecules TPA-BT
(with benzothiadiazole as the acceptor, TPA as the donor) and
B(TPA-BT-HT) (with benzothiadiazole as the acceptor, TPA
and thiophene as the donor) showed relatively narrow absorp-
tion bands from 300 to 600 nm for TPA-BT and 300 to 630 nm
for B(TPA-BT-HT)20,22 which could be caused by the relatively
weak electron-withdrawing abilities of benzothiadiazole
resulting in the relatively weak ICT intensity from the electron-
donating moiety (TPA, TPA and thiophene) to electron-
accepting moiety (benzothiadiazole) and thus the narrow
absorption range. We previously reported three D–A small
molecules21 with sulfonyldibenzene as the acceptor and different
numbers of triphenylamines as donors. The molecules exhibited
good film-forming abilities, but the absorption range only
covered 300 to 550 nm due to the relatively weak electron-
withdrawing abilities of sulfonyldibenzene.
that the combination of PM moiety and TPA moiety linked by
phenothiazine, triphenylamine and thiophene moieties results in
an enhanced intramolecular charge transfer (ICT) transition,
which lead to an extension of the absorption spectral range (ca.
350 nm to 700 nm). Cyclic voltammetry measurement displayed
that the HOMO energy levels were fine-tuned (5.04 eV, 5.12 eV
and 5.25 eV) by introducing different electron-donating moieties
(phenothiazine, triphenylamine and thiophene) into these small
molecules. The bulk heterojunction photovoltaic devices were
fabricated by using small molecules (APPM, AAPM and ATPM)
as the donor and PCBM as the acceptor. The optimized photo-
voltaic devices exhibited that the gradually increased PCE
(0.65% for APPM, 0.94% for AAPM and 1.31% for ATPM) was
achieved because of the gradual increase of Voc (0.8 V for APPM,
0.9 V for AAPM and 1.0 V for ATPM) and short-circuit current
(2.33 mA cmꢁ2 for APPM, 2.89 mA cmꢁ2 for AAPM and
3.85 mA cmꢁ2 for ATPM) under simulated AM 1.5 illumination
(100 mW cmꢁ2).
Experimental
All reagents and chemicals were purchased from commercial
sources (Aldrich, Across, Fluka) and used without further
purification unless stated otherwise. All solvents were distilled
over appropriate drying agent(s) prior to use and were purged
with nitrogen. Compounds 1, 4, 7 and 11 were synthesized
Herein, a series of novel symmetrical solution processable
small molecules (APPM, AAPM and ATPM) consisting of the
electron-donating moiety (triphenylamine) and the electron-
accepting moiety (2-pyran-4-ylidenemalononitrile) (PM) linked
by different electron-donating moieties (phenothiazine, triphe-
nylamine and thiophene) were designed and synthesized towards
wider absorption spectra. In these small molecules, TPA was
introduced as the end group for its typical electron-donating
ability, excellent hydrotropy and good film-forming ability.23–25
PM was introduced because of its stronger electron-withdrawing
abilities than reported for the electron-accepting moiety (such as
benzenamine and sulfonyldibenzene), which could enhance the
ICT intensity and thus broad the absorption spectra when
combined with strong electron-donating moieties.7,8,18 The
purpose of incorporating different electron-donating ability
moieties (phenothiazine, triphenylamine and thiophene) is to
further strengthen the conjugated degrees and thus the ICT
intensity. Furthermore, the incorporation of different electron-
donating ability moieties (phenothiazine, triphenylamine and
thiophene) could regulate the oxidation potential of these small
molecules and thus provide a method for tuning the energy level
of the conjugated system. UV-vis absorption spectra illustrated
Scheme 1 Synthetic routes for compounds.
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pH ¼ 7 with 2 M NaOH aqueous solution and extracted with
chloroform. The extract was washed with plenty of water and
NaCl successively. The organic extracts were dried over anhy-
drous MgSO4, evaporated and purified with column chroma-
tography on silica gel with ethyl acetate : petroleum ether (1 : 10)
as the eluant and yielded a yellow solid of 4.59 g (13.07 mmol,
yield 85%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 9.826 (s,
1H, –CHO), 7.694 (d, 2H, –Ph), 7.435 (m, 2H, –Ph), 7.347 (t, 2H,
–Ph), 7.172 (m, 3H, –Ph), 7.035 (m, 4H, –Ph). 13C NMR (75
MHz, DMSO, TMS): d (ppm) 190.388, 132.718, 132.063,
131.280, 129.827, 129.281, 127.130, 126.211, 125.173, 124.408,
123.944, 123.271, 119.869.
Synthesis of compound 5
To a solution of 1 (5.5 g, 20 mmol) in THF (30 mL) at room
temperature was added N-bromosuccinimide (7.2 g, 40 mmol)
over a period of 5 min. The solution was stirred at room
temperature for 4 h. The solvent was then removed in vacuo and
hexane (200 mL) was added (to precipitate all the succinimide).
The mixture was filtered through a silica plug (to remove the
succinimide) and the solvent was removed in vacuo. Distillation
under vacuum yielded a colorless oil of 6.73 g (16.4 mmol, yield
82%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 2.560 (t, 4H,
J ¼ 8.0 Hz, –CH2), 1.517 (m, 4H, –CH2). 1.442–1.353 (m, 12H,
–CH2), 0.948 (t, 6H, J ¼ 8.0 Hz, –CH3).
Scheme 2 Synthetic routes and structures of small molecules.
according to the literature procedure.18,27–29 The synthesis routes
are shown in Scheme 1 while the molecular structures (APPM,
AAPM and ATPM) are shown in Scheme 2.
Synthesis of compound 6
The synthetic procedure for compound 6 was similar to that for
compound 2 and yielded a red brown oil of 0.46 g (1.29 mmol,
yield 53%). 1H NMR (500 MHz, CDCl3, TMS): d (ppm) 9.912 (s,
1H, –CHO), 2.872 (t, 2H, –CH2), 2.542 (t, 2H, –CH2), 1.573 (m,
2H, –CH2), 1.495 (m, 2H, –CH2), 1.397 (m, 4H, –CH2), 1.322 (m,
8H, –CH2), 0.899 (m, 6H, –CH3). 13C NMR (75 MHz, CDCl3,
TMS): d (ppm) 181.231, 151.359, 143.693, 138.249, 122.477,
32.227, 31.459, 31.429, 29.364, 29.214, 27.908, 27.714, 22.528,
22.483, 13.989.
Synthesis of compound 2
N-BuLi (6.86 mL of 2.5 M solution in hexane, 17.08 mmol) was
added dropwise to a solution of compound 1 (7.56 g, 17.08
mmol) in THF (150 mL) at ꢁ78 ꢀC under a dry argon atmo-
ꢀ
sphere. The solution was stirred at ꢁ78 C for 2 h, then dried
DMF (1.45 mL, 18.90 mmol) was added quickly and it was kept
at room temperature and stirred for 24 h before being poured
into water. The product was extracted with ether. The organic
layer was subsequently washed with water and brine and dried
over anhydrous magnesium sulfate (MgSO4), and the solvent
was removed by rotary evaporation. The crude product was
purified by column chromatography with CH2Cl2 : petroleum
ether (1 : 2) and yielded a yellow solid of 3.50 g (1.29 mmol, yield
Synthesis of compound 8
A mixture of compound 2 (3.41 g, 8.73 mmol), compound 7 (0.69
g, 3.97 mmol), piperidine (10 drops), and acetonitrile (30 mL)
were refluxed under N2 for 24 h. The reaction mixture was cooled
to room temperature and poured into water and extracted with
chloroform. The combined organic extractions were washed
three times with water, dried over anhydrous MgSO4, evaporated
under vacuum and purified with column chromatography on
silica gel with dichloromethane : petroleum ether (3 : 1) as the
eluant to yield a red solid of 2.92 g (3.19 mmol, yield 73.5%). 1H
NMR (500 MHz, CDCl3, TMS): d (ppm) 7.351 (m, 4H, –Ph,
–vinylic), 7.302 (m, 2H, –Ph), 7.255 (m, 4H, –Ph), 6.862 (4d, H,
–Ph), 6.721 (d, 2H, –Ph), 6.623 (s, 2H, –PM), 6.591 (d, J ¼ 16 Hz,
2H, –vinylic), 3.844 (t, 4H, –CH2), 1.801 (m, 4H, –CH2), 1.442
(m, 4H, -CH2), 1.319 (m, 8H, –CH2), 0.892 (t, 6H, –CH3). 13C
NMR (75 MHz, CDCl3, TMS): d (ppm) 158.221, 155.588,
146.731, 143.133, 136.352, 130.204, 129.682, 129.101, 127.722,
126.404, 125.962, 124.571, 116.845, 116.503, 115.501, 115.426,
115.118, 106.704, 59.032, 47.901, 31.384, 26.662, 26.501, 22.575,
13.982.
1
53%). H NMR (500 MHz, CDCl3, TMS): d (ppm) 9.79 (s, 1H,
–CHO), 7.64 (m, 1H, –Ph), 7.55 (m, 1H, –Ph), 7.23 (m, 2H, –Ph),
6.88 (d, 1H, –Ph), 6.70 (d, 1H, –Ph), 3.83 (t, 2H, –CH2), 1.77 (m,
2H, –CH2), 1.42 (m, 2H, –CH2), 1.30 (m, 4H, –CH2), 0.87 (m,
3H, –CH3). 13C NMR (75 MHz, DMSO, TMS): d (ppm) 189.79,
150.22, 142.57, 131.22, 130.18, 130.17, 129.70, 128.34, 126.06,
124.30, 117.01, 115.72, 114.92, 48.03, 31.28, 26.55, 26.38, 22.49,
13.91.
Synthesis of compound 3
DMF (10.56 mL) was put into a 50 mL flask, kept in ice-water,
and then phosphorous oxychloride (4.23 mL) was added drop-
wise to the stirred DMF. After 20 min, 4-bromo-N,N-diphe-
nylbenzenamine (5.00 g, 15.38 mmol) was added to the mixture
with stirring at 60 ꢀC for 6 h. After cooling, the solution was
poured into cold water. The resulting mixture was neutralized to
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Synthesis of compound 9
468 mg (0.40 mmol, 80.0%). 1H NMR (500 MHz, CDCl3, TMS):
d (ppm) 7.522 (d, 4H, –Ph), 7.455 (m, 10H, –Ph), 7.345 (t, 4H,
–Ph), 7.283 (t, 4H, –Ph), 7.200 (m, 8H, –Ph), 7.153 (t, 13H, –Ph),
7.102 (d, 4H, –Ph), 7.049 (t, 4H, –Ph), 6.651 (s, 2H, –PM), 6.623
(d, 2H, –vinylic). 13C NMR (75 MHz, CDCl3, TMS): d (ppm)
158.731, 155.830, 149.894, 147.636, 147.096, 146.627, 145.486,
137.431, 136.425, 134.205, 129.607, 129.274, 129.037, 127.582,
127.382, 125.610, 125.529, 124.422, 124.388, 124.378, 123.882,
122.988, 121.745, 115.780, 115.854, 106.340, 58.270. MALDI-
TOF MS: calcd for C84H60N6O 1169.41; found 1169.50.
The synthetic procedure for compound 9 was similar to that for
compound 8 and yielded a red solid of 2.02 g (2.40 mmol, 83.1%).
1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.405 (m, 10H,
–Ph), 7.323 (m, 4H, –Ph, –vinylic), 7.140 (t, 6H, –Ph), 7.026 (m,
8H, –Ph), 6.643 (s, 2H, –PM), 6.614 (d, 2H, J ¼ 16.2 Hz,
–vinylic), 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 158.589,
156.741, 149.432, 146.296, 145.866, 137.243, 132.529, 129.666,
129.048, 128.025, 126.538, 125.554, 124.633, 121.976, 116.632,
116.099, 115.518, 106.433, 58.540.
Synthesis of compound ATPM
Synthesis of compound 10
After evaporation of the solvent, the residue was purified with
column chromatography on silica gel with dichloro-
methane : petroleum ether (2 : 3) as the eluant to dark red solid
426 mg (0.36 mmol, 72.0%). 1H NMR (300 MHz, CDCl3, TMS):
d (ppm) 7.033 (d, 2H, J ¼ 15.6 Hz, –vinylic), 7.297 (d, 8H, –PH),
7.248 (s, 4H, –PH), 7.146 (d, 8H, –PH), 7.068 (t, 8H, –PH), 6.581
(s, 2H, –PM), 6.447 (d, 2H, J ¼ 15.6 Hz, –vinylic), 2.719
(t, 4H, –CH2), 2.586 (t, 4H, –CH2), 1.421 (m, 32H, –CH2), 0.853
(t, 12H, –CH3). 13C NMR (75 MHz, CDCl3, TMS): d (ppm)
158.328, 155.438, 147.887, 147.319, 146.953, 141.858, 139.406,
132.873, 129.780, 129.374, 128.816, 127.538, 124.929, 123.474,
122.529, 115.672, 115.618, 106.249, 58.353, 31.840, 31.615,
31.388, 30.721, 29.489, 29.348, 27.830, 27.314, 22.614, 22.522,
14.002, 13.945. MALDI-TOF MS: calcd for C80H86N4OS2
1245.64; found 1245.60.
The synthetic procedure for compound 10 was similar to that for
compound 8 and yielded a dark green solid of 2.1 g (2.46 mmol,
1
53.2%). H NMR (500 MHz, CDCl3, TMS): d (ppm) 7.509 (d,
2H, J ¼ 15.5 Hz, –vinylic), 6.613 (s, 2H, –PM), 6.368 (d, 2H, J ¼
15.5 Hz, –vinylic), 2.678 (t, 4H, –CH2), 2,529 (t, 4H, –CH2), 1.500
(m, 8H, –CH2), 1.332 (m, 16H, –CH2), 1.285 (m, 8H, –CH3),
0.908 (t, 6H, –CH3), 0.843 (t, 6H, –CH3). 13C NMR (75 MHz,
CDCl3, TMS): d (ppm) 157.792, 157.693, 155.164, 144.954,
143.322, 134.634, 127.844, 116.501, 115.133, 113.724, 106.635,
59.598, 31.445, 31.420, 29.343, 29.191, 28.312, 27.960, 22.486,
13.951, 13.868.
General procedures of small molecules
Suzuki coupling reaction was used to synthesize small molecules
shown in Scheme 1 and Scheme 2. compound 8, (or 9 or 10),
compound 11, and (PPh3)4Pd(0) (2 mol% with respect to
compound 8 or 9 or 10) were dissolved in a mixture of toluene (6
mL) and aqueous 2 M K2CO3 (4 mL, 3/2 volume ratio). The
solution was stirred under the Ar atmosphere and refluxed with
vigorous stirring for 48 h. The resulting solution was then poured
into water and was extracted with dichloromethane. The extract
was washed with brine and dried over anhydrous MgSO4.
Instruments and measurements
Differential scanning calorimetry (DSC) was performed under
nitrogen flushing at a heating rate of 20 ꢀC minꢁ1 with
1
a NETZSCH (DSC-204) instrument. H NMR and 13C NMR
spectra were measured using
a
Bruker AVANCE-500
NMR spectrometer spectrometer and a Varian Mercury-300
NMR, respectively. The time-of-flight mass spectra were recor-
ded with a Kratos MALDI-TOF mass system. UV-visible
absorption spectra were measured using a Shimadzu UV-3100
spectrophotometer. The photoluminescence spectra of spin-cast
films and solution were measured with a RF-5301PC spectro-
fluorophotometer. Electrochemical measurements of these
derivatives were performed with a Bioanalytical Systems BAS
100 B/W electrochemical workstation. Atomic force microscopy
(AFM) images of blend films were carried out using a Nanoscope
IIIa Dimension 3100.
Synthesis of compound APPM
After evaporation of the solvent, the residue was purified with
column chromatography on silica gel with dichlor-
omethane : petroleum ether (2 : 1) as the eluant to dark red solid
473 mg (0.38 mmol, 76.0%). 1H NMR (500 MHz, CDCl3, TMS):
d (ppm) 7.354 (m, 14H, –Ph), 7.273 (d, 2H, –Ph), 7.250 (s, 2H,
–Ph), 7.116 (t, 12H, –Ph), 7.031 (t, 4H, –Ph), 6.875 (d, 4H, –Ph),
6.591 (d, 4H, –PM, –vinylic), 3.887(m, 4H, –CH2), 1.844
(m, 4H, –CH2), 1.471 (m, 4H, –CH2), 1.339 (m, 8H, –CH2), 0.894
(m, 6H, –CH3). 13C NMR (75 MHz, CDCl3, TMS): d (ppm)
158.293, 155.554, 147.611, 147.083, 146.830, 142.467, 136.455,
135.004, 133.497, 129.250, 128.883, 128.739, 127.852, 127.086,
126.238, 125.547, 125.225, 124.743, 124.382, 123.887, 122.945,
116.088, 115.784, 115.484, 115.140, 106.480, 58.731, 47.881,
31.387, 26.743, 26.541, 22.580, 13.951. MALDI-TOF MS: calcd
for C84H72N6OS2 1245.64; found 1245.60.
Fabrication and characterization of photovoltaic devices
For device fabrication, the ITO glass was precleaned and
modified by a thin layer of PEDOT/PSS, which was spin-cast
from a PEDOT:PSS aqueous solution (H. C. Starck) on the ITO
substrate, and the thickness of the PEDOT/PSS layer was about
50 nm. The active layer contained a blend of small molecules as
electron donor and PCBM as electron acceptor, which was
prepared from chloroform and chlorobenzene solution with the
different weight ratios (small molecule : PCBM ¼ 2 : 1, 1 : 1,
1 : 2, and 1 : 3, respectively). After spin-coating the blend from
solution at 900 rpm, The devices were completed by evaporating
a 0.6 nm LiF layer and protected by 100 nm of Al at a base
Synthesis of compound AAPM
After evaporation of the solvent, the residue was purified with
column chromatography on silica gel with dichloro-
methane : petroleum ether (1 : 1) as the eluant to dark red solid
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pressure of 5 ꢂ 10ꢁ4 Pa. The effective photovoltaic area as
defined by the geometrical overlap between the bottom ITO
electrode and the top cathode was 5 mm2. The current–voltage
(J–V) characteristics were recorded using Keithley 2400 Source
Meter in the dark and under simulated AM 1.5 illumination (100
mW cmꢁ2) by Solar Simulators (SCIENCETECH SS-0.5 K). The
spectral response was recorded by SR830 lock-in amplifier under
short circuit condition when devices were illuminated with
a monochromatic light from a Xeon lamp. Films thickness was
measured by Veeco DEKTAK 150 surface profilometer. AFM
images were measured by atomic force microscopy (S II Nano-
navi probe station 300hv). All fabrication and characterizations
were performed in an ambient environment.
Material synthesis and structural characterization
Fig. 1 Differential scanning calorimetry (DSC) measurement of APPM,
AAPM and ATPM, scan rate 10 ꢀC minꢁ1
The general synthetic routes toward the compounds and the final
small molecules are outlined in Scheme 1. The compounds 8, 9
and 10 were prepared through Knoevenagel condensation of
compound 7 with 2, 3 and 6 respectively. The structures of
compounds were confirmed by 1H NMR, 13C NMR spectra, and
the data were included in the Experimental section. In 1H-NMR
spectroscopy of compound 8, 9 and 10, the coupling constant
(J ꢃ 15.5 Hz) of olefinic protons indicates that the Knoevenagel
reaction afforded the pure all-trans isomers. The final products
(shown in Scheme 2) were prepared by the well-known palla-
dium-catalyzed Suzuki coupling reaction between varied
compounds (8, 9, 10) and compound 11. 1H NMR and 13C NMR
spectras were used to characterize the structure of these mole-
cules, which clearly indicate that well-defined APPM, AAPM
and ATPM have been obtained. The two legible double peaks
that appear at ꢃ6.400 ppm and 7.600 ppm with the coupling
constant (J ꢃ 16 Hz) are due to all-trans double bond, which
further confirms the regular structure. Molecular weights were
determined by Kratos MALDI-TOF. These results are consis-
tent with the proposed structure and molecule weights. All the
molecules exhibited excellent solubility in common organic
solvents such as chloroform, tetrahydrofuran, dichloromethane,
and chlorobenzene.
.
Thermal properties
Differential scanning calorimetry (DSC) was performed to
investigate the thermal properties of APPM, AAPM and ATPM.
Fig. 1 shows the DSC curves of these molecules after they are
purified through recrystallization. When APPM and AAPM
were heated, the endothermic peak due to glass transition
ꢀ
temperatures were observed at ca. 137 and 163 C respectively.
Fig. 2 The absorption spectra of the small molecules (a) in chloroform
solutions with the concentration of 10ꢁ5 mol Lꢁ1; (b) films spin-coated
from a 10 mg mLꢁ1 chloroform solution.
When ATPM was heated, the exothermic peak due to crystalli-
zation was observed at ca. 131 ꢀC. On further heating above the
crystallization temperature, the apparently endothermic peak
ꢀ
due to melting point was observed at ca. 164 C.
497 nm in dilute solution (Fig. 2a), which can be assigned to the
intrinsic absorption of triphenylamine moiety, p–p* transition
of the conjugated small molecule backbone and ICT interaction
between the electron-donating moieties (thiophene and triphe-
nylamine) and electron-accepting moiety (PM). The absorption
spectra of APPM in dilute solutions also exhibited three
absorption peaks at 328 nm, 400 nm and 503 nm corresponding
to the intrinsic absorption of phenothiazine moiety, p–p*
Optical properties
The normalized UV-vis absorption spectra of APPM, AAPM
and ATPM in dilute chloroform solution (concentration 10ꢁ5 M)
are shown in Fig. 2a, and the main optical properties are listed in
Table 1. ATPM with the weak electron-donating moiety (thio-
phene) showed three absorption peaks at 312 nm, 400 nm and
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Table 1 Optical and electrochemical data of ATPM, AAPM and APPM
In solutiona
In film b
abs
max
abs
onset
Ox
onset
E /V/LUMO/
Red
eV
l
/nm
ledge
nm
/
l
max
nm
/
ledge
nm
/
E
eV
/V/HOMO/
Electrochem.
Eg,ec/eV
Small molecules
(3max/Mꢁ1 cmꢁ1
)
Opticalc Eg,opt/eVc
APPM
AAPM
ATPM
503 (52501)
502 (58496)
497 (56844)
612
589
603
512
504
499
665
616
638
0.35/ꢁ5.04
0.43/ꢁ5.12
0.56/ꢁ5.25
ꢁ1.21/ꢁ3.48
ꢁ1.25/ꢁ3.44
ꢁ1.22/ꢁ3.47
1.56
1.68
1.78
1.86
2.01
1.94
a
b
c
1 ꢂ 10ꢁ5 M in anhydrous chloroform. Spin-coated from a 10 mg mLꢁ1 chloroform solution. The optical band gap (Eg,opt ) was obtained from
absorption edge.
Table 2 Characteristic current–voltage parameters from device spin-
coated from chlorobenzene solution and testing at standard AM 1.5 G
conditions
transition of the conjugated small molecule backbone and ICT
interaction between the electron-donating moieties (phenothia-
zine and triphenylamine) and electron-accepting moiety (PM),
respectively. But the absorption peak from p–p* transition of
APPM becomes much weaker than that of ATPM, which could
be explained by its worse conjugation as the molecular dimen-
sions increased. In the absorption spectra of AAPM, only two
absorption peaks at 350 nm and 502 nm were observed, which
could be ascribed to the intrinsic absorption of triphenylamine
moiety and ICT interaction between the electron-donating
moieties (triphenylamine) and electron-accepting moiety (PM).
The peak corresponding to p–p* transition disappeared as
molecular dimensions increased further. The solution absorption
spectra of APPM, with an absorption maximum at 503 nm and
the absorption edge at 612 nm, is broadened and red-shifted
compared to those of ATPM (497 nm, 603 nm), and AAPM (502
nm, 589 nm), which can be explained by much stronger ICT
effect in APPM than that in ATPM and AAPM. Among the
three small molecules, there is a D–A–D structure, where D
represents the electron-donor (thiophene, triphenylamine and
phenothiazine) while A represents the electron-acceptor (2-
pyran-4-ylidenemalononitrile). The stronger electron-donating
ability D possesses, the higher electronic delocalization degree
and the stronger ICT the molecule has. Since the order of the
electron-donating abilities of the three D is phenothiazine >
triphenylamine > thiophene, the strongest electron-donating
ability of phenothiazine compared with triphenylamine and
thiophene improves the effective conjugation length along the
molecule backbone, resulting in an increase in the ICT strength
and thus electronic delocalization.30 Moreover, the relatively
high absorption coefficients could be calculated from the Beer’s
law equation with the same dilute concentration of the molecules
in chloroform (absorption coefficients are listed in Table 1),
which assures these small molecules can absorb enough photons.
Fig. 2b shows the optical absorption spectra of the small
molecules in thin films. The absorption spectra in thin films are
generally similar in shape to those in dilute solution. And we
could easily find that there is a little red shift of the maximum
absorption peak and a certain degree of broadening for
absorption edge between the absorption spectra in film and
solution for the three molecules (for APPM, 9 and 53 nm; for
AAPM, 2 and 27 nm; and for ATPM, 2 and 35 nm, respectively).
The red shifts and broadening of absorption edge may be caused
by the existence of some aggregation of the molecules, which is
beneficial for p–p* stacking in the solid state and thus could
broaden their absorption spectra.31 As for APPM, the red shift
Small
molecule :
PCBM
Small
molecule
Voc/v
Jsc/mA cmꢁ2
FF
PCE (%)
APPM
AAPM
ATPM
2 : 1
1 : 1
1 : 2
1 : 3
2 : 1
1 : 1
1 : 2
1 : 3
2 : 1
1 : 1
1 : 2
1 : 3
0.86
0.82
0.80
0.78
0.96
0.90
0.88
0.90
1.04
1.02
1.00
0.92
1.16
1.95
2.33
2.20
1.43
1.66
2.00
2.89
0.61
2.16
3.85
3.50
0.27
0.33
0.35
0.35
0.30
0.34
0.36
0.36
0.20
0.27
0.34
0.36
0.27
0.53
0.65
0.60
0.41
0.51
0.63
0.94
0.13
0.59
1.31
1.16
degree of the maximum absorption peak and the broadening of
absorption edge are larger than ATPM and AAPM, which
maybe attributed to its smaller steric hindrance in the molecule
structure than that of ATPM and AAPM. As regards AAPM,
the red shift degree of the maximum absorption peak and the
broadening of absorption edge is only 2 and 27 nm. This may
result from its bulky steric hindrance conformations due to the
existence of four triphenylamine moieties in AAPM, which is not
favorable for the p–p* stacking in the solid state. In the case of
ATPM, it also exhibits only 2 nm red shift of the aximum
absorption peak and 35 nm broadening of absorption edge
compared with that in dilute solutions. It could be explained by
the existence of triphenylamine moieties and long alkyl side
chains which can increase immensely the steric hindrance in the
molecule structure. As the absorption edge of the three small
molecules are tuned from 616 to 665 nm, the optical band gap
Eg,opt of the three small molecules derived from the absorption
edge of the thin film spectra is in the range of 2.01–1.86 eV
(Table 2). As expected, APPM with the strongest intramolecular
charge transfer interaction thus has the lowest optical band gap
of 1.86 eV.
Electrochemical properties
Fig. 3 shows the cyclic voltammetry (CV) diagrams of the small
molecules using TBAPF6 as supporting electrolyte in methylene
dichloride solution with platinum button working electrodes,
a platinum wire counter electrode and an Ag/AgNO3 reference
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Fig. 4 Band diagram for acceptor PCBM and donors APPM, AAPM
and ATPM. Dashed lines indicate the thresholds for air stability (ꢁ5.2
eV) and effective charge transfer from APPM, AAPM or ATPM to
PCBM (ꢁ3.5 eV).
Fig. 3 Cyclic volatammetry curves of APPM, AAPM and ATPM
solutions on platinum electrode in 0.1 mol Lꢁ1 n-Bu4NPF6 in CH2Cl2
PCBM used in our experiments by cyclic voltammetry in the
same condition as APPM, AAPM, and ATPM. The calculated
values of the LUMO and HOMO energy levels of PCBM were
ꢁ3.80 and ꢁ5.93 eV respectively. Therefore, the threshold value
for realizing the effective charge transfer should be ꢁ3.5 eV.
Furthermore, the calculated electrochemical band gaps (listed in
Table 1) indicate that the relatively low band gaps were achieved
(1.56 eV, 1.68 eV and 1.78 eV for APPM, AAPM and ATPM,
respectively) and it is an effective manners for reducing the
electrochemical band gap according to controlling the molecule
structure.
solution, at a scan rate of 100 mV sꢁ1
.
electrode under the N2 atmosphere. Ferrocene was used as the
internal standard. The redox potential of Fc/Fc+ which has an
absolute energy level of ꢁ4.8 eV relative to the vacuum level for
calibration is located at 0.11 V in 0.1 M TBAPF6/methylene
dichloride solution.26 The results of the electrochemical
measurement and calculated energy levels of the small molecules
are listed in Table 1.
Table 1 shows the calculated HOMO energy level (ꢁ5.04 eV)
and LUMO energy level (ꢁ3.48 eV) of APPM. The LUMO
energy levels of AAPM and ATPM are ꢁ3.44 and ꢁ3.47 eV,
which are very similar to those of APPM and the reported PM-
containing small molecules.7,8 Therefore, the substitution of
varied electron-donating moieties with different electron-
donating abilities have almost no effect on the reduction poten-
tial of the small molecules and the relatively low LUMO energy
levels of the three small molecules should result from the stronger
reduction of PM-based acceptor moiety.
Theoretical calculation
The geometry and electronic properties of APPM, AAPM and
ATPM have been investigated by means of theoretical calcula-
tion with the Gaussian 03 program package at a hybrid density
functional theory (DFT) level.24 Becke’s three-parameter
gradient corrected functional (B3LYP) with a 6-31G basis was
used for full geometry optimization. Fig. 5 presents the
geometry, the HOMO and LUMO. H atoms were used in place
of the n-hexyl groups to limit computation time. The calculation
results demonstrate that the periplanatic PM moiety with its two
adjacent phenyl vinyl moieties constitutes a large conjugated
planar structure, and the triphenylamine moieties located at the
end of the molecules result in an nonplanar structure for the
small molecules. Electron density of the HOMO energy level
It is clear that the HOMO levels of APPM (ꢁ5.04 eV), AAPM
(ꢁ5.12 eV) and ATPM (ꢁ5.25 eV) are gradually decreased with
the weakened electron-donating abilities of phenothiazine, tri-
phenylamine and thiophene. As we know that the HOMO energy
level of the small molecule is very important for high perfor-
mance photovoltaic device. Firstly, the small molecule should
have good air stability with HOMO energy level being below the
air oxidation threshold (ca. ꢁ5.2 eV).32 Secondly, the open circuit
potential (Voc) value for the photovoltaic device is determined by
the difference between the HOMO energy level of donor and
LUMO energy level of acceptor, and the relatively low HOMO
level of the small molecules may allow a high open circuit voltage
(Voc) for the photovoltaic device.31 Therefore, the gradually
decreased HOMO levels of the small molecules are benefit for
enhancing their air stability and raising their Voc values.
The energy band structure of the three small molecules and
PCBM is presented in Fig. 4. The first dashed line indicates the
threshold for air stability, and the second dashed line represents
the threshold value for an effective charge transfer from the small
molecules to PCBM.24,33 The LUMO values of PCBM were
reported to be ꢁ4.30 eV24 or ꢁ4.10 eV34 or around ꢁ3.70 eV.35
Therefore, we measured the LUMO and HOMO values of
Fig. 5 Molecular orbital surfaces of the HOMO and LUMO of ATPM,
AAPM, and APPM obtained at B3 LYP/6-31G* level.
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distributes mainly on the TPA etc.-donor moieties, while that of
the LUMO energy level mainly delocalizes on the PM-acceptor
moiety, indicating a charge-transfer nature of HOMO /
LUMO from the electron-donating moieties to the PM-acceptor
moiety. Furthermore, the calculated HOMO energy levels are
consistent with the electrochemical measurement while the
LUMO energy levels exhibit some bias.
chlorobenzene (CB) solutions. The photovoltaic parameters of
the photovoltaic devices are summarized in Table 2 and the
corresponding current–voltage characteristics with various blend
weight ratios are presented in Fig. 6, respectively. The photo-
voltaic parameters of the devices are summarized in Table 2. The
optimized photovoltaic devices with a structure of indium tin
oxide (ITO)/PEDOT/PSS/small molecules/PCBM/LiF/Al exhibit
a Voc of 0.80 V, a short-circuit current (Jsc) of 2.33 mA cmꢁ2, a fill
factor (FF) of 0.35, and a PCE of 0.65% for the device based on
APPM/PCBM; 0.90 V, 2.89 mA cmꢁ2, 0.36, 0.94% for the device
based on AAPM/PCBM; and 1.00 V, 3.85 mA cmꢁ2, 0.34, and
1.31% for the device based on ATPM/PCBM, respectively.
The optimized photovoltaic devices based on APPM, AAPM
and ATPM exhibited the gradually increased Voc of 0.8 V, 0.9 V
and 1.0 V respectively, which is consistent with the HOMO
energy level order of these small molecules (APPM > AAPM >
ATPM). The result indicates that it is an effective method for
achieving high Voc by adjusting the electron-donating abilities of
the bridge moieties (phenothiazine, triphenylamine and thio-
phene), and the value of 1.0 V is also one of the highest Voc for
photovoltaic devices.
Photovoltaic performance
To demonstrate the application potential of the small molecules
as an electron donor in organic solar cell, photovoltaic devices
were fabricated by spin-coating at a constant concentration of
20 mg mLꢁ1 comprising a mixture of small molecule and PCBM.
As we know that the weight ratio of donor material/PCBM have
a large effect on the photovoltaic performance because there is
a balance between the absorbance and the charge transporting
network of the active layer in the photovoltaic devices. Too low
a PCBM content will limit the electron transporting ability, while
too high a PCBM content will decrease the absorbance and hole
transporting ability of the active layer. Therefore, the photo-
voltaic devices with different weight ratios for small mole-
cule : PCBM (2 : 1, 1 : 1, 1 : 2 and 1 : 3) were achieved using
As for Jsc, the higher Jsc of the optimized photovoltaic devices
based on ATPM/PCBM (3.85 mA cmꢁ2) and AAPM/PCBM
(2.89 mA cmꢁ2) had been obtained than that of the device based
on APPM/PCBM (2.33 mA cmꢁ2). This could be explained by
the relatively high absorption coefficients of ATPM (56844) and
AAPM (58490) than that of APPM (52501) while these mole-
cules exhibited nearly the same absorption range. However, the
Jsc of the device based on AAPM/PCBM is lower (2.89 mA cmꢁ2
)
than that of ATPM (3.85 mA cmꢁ2), even though AAPM
possesses a higher absorption coefficient than that of ATPM.
Considering that charge carrier mobility of photovoltaic
materials is another important factor which influences the Jsc of
photovoltaic devices,36 we measured the hole mobility of APPM,
AAPM and ATPM by space charge limited current (SCLC)
method.37 The hole only devices with a structure of ITO/PEDOT/
small molecule/Au were fabricated and their current–voltage
Fig. 7 J–V curve of an ITO/PEDOT/small molecule/Au device in the
dark with log axis for estimating the hole mobility of APPM, AAPM,
ATPM. The thicknesses of the small molecules are around 110 nm,
100 nm, 120 nm respectively. The solid line from 0.01 to 0.50 V means log
J is fitted linearly dependent on log V with a slope of 1. For the solid line
from 0.5 to 1.5 V log J is fitted linearly dependent on log V with a slope of
2 (SCLC area).
Fig. 6 Current–voltage characteristics of photovoltaic devices based on
APPM, AAPM and ATPM under illumination of AM 1.5, 100 mW cmꢁ2
.
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curves in the dark are shown in Fig. 7. The hole mobility values
were measured to be 7.91 ꢂ 10ꢁ6 cm2 vꢁ1 sꢁ1, 3.15 ꢂ 10–5 cm2 vꢁ1
sꢁ1 and 7.47 ꢂ 10ꢁ5 cm2 vꢁ1 sꢁ1 for APPM, AAPM and ATPM,
respectively, which are relatively high values for donors. The
gradually increased hole mobility from APPM to ATPM could
due to their enhanced conjugation.36 Obviously, the hole
mobility value of ATPM is higher than that of APPM and
AAPM, which is well consistent with its higher Jsc (3.85 mA
miscibilities with PCBM. As for AAPM/PCBM blend film, the
relatively low rms of 0.42 nm was also obtained, however, some
leakage sources were observed, which indicates that AAPM
possesses the worse film-forming abililties and miscibility with
PCBM than APPM and ATPM. Moreover, the investigations
also indicate that the lower Jsc (2.89 mA cmꢁ2) obtained from the
AAPM/PCBM device could attribute to the existence of some
leakage sources in AAPM/PCBM blend film, which is especially
disadvantageous for the transport of charge carrier and thus may
reduce the Jsc of AAPM/PCBM device.
cmꢁ2
) based on ATPM:PCBM than those of based on
APPM:PCBM (2.89 mA cmꢁ2) and AAPM:PCBM (2.33 mA
cmꢁ2). As for APPM, the lowest Jsc (2.33 mA cmꢁ2) might be
caused by its lowest hole mobility, which leads to the charge
mobility imbalance and thus could reduce the Jsc of the photo-
voltaic devices.
Fig. 9 shows the external quantum efficiency (EQE) spectra of
devices based on APPM/PCBM, AAPM/PCBM and ATPM/
PCBM blend films with the weight ratio of 1 : 2, 1 : 3 and 1 : 2,
respectively. It can be seen that all the devices show the efficient
photoconversion efficiency in the range 350–650 nm, with EQE
values of ca. 20–35%. Although the response range of these
photovoltaic devices is almost equal, the maximum values are
close to 18.8% at 445 nm based on APPM/PCBM, 25.4% at 506
nm based on AAPM/PCBM and 34.3% at 476 nm based on
ATPM/PCBM.
To gain further insight into what might affect the Jsc of the
photovoltaic device, we analyzed the morphology of APPM/
PCBM, AAPM/PCBM and ATPM/PCBM blend films. Fig. 8
shows the AFM height images of APPM/PCBM, AAPM/PCBM
and ATPM/PCBM blend films with the weight ratio 1 : 2, 1 : 3
and 1 : 2, respectively. It is clearly evidenced by AFM that the
APPM/PCBM and ATPM/PCBM blend films exhibit relatively
flat surfaces with the root-mean-square (rms) of 0.37 nm and
0.36 nm, respectively, which demonstrate that APPM and
ATPM possess the excellent film-forming abilities and
The photovoltaic devices based on APPM/PCBM exhibited
a PCE of 0.65%, while a PCE of 0.94% for the photovoltaic
device based on AAPM/PCBM and a PCE of 1.31% for the
photovoltaic device based on ATPM/PCBM under simulated
AM 1.5 illumination (100 mW cmꢁ2). Compared with APPM and
AAPM, ATPM based photovoltaic device exhibited higher Voc,
Jsc and thus the higher PCE while the FF stays nearly the same
value. The results indicate that it is an effect way to improve PCE
by adjusting the structure of small molecules and ATPM is
a promising donor material for photovoltaic devices.
Conclusions
We have designed and synthesized three novel D–A conjugated
small molecules consisting of electron-accepting moiety (PM)
and electron-donating moiety (triphenylamine) linked by
different electron-donating ability moieties (thiophene, triphe-
nylamine and phenothiazine). Optical property investigations
clearly indicated that these new small molecules exhibited long
wavelength ICT absorption bands. Both the electrochemical
properties and molecular orbital distribution calculations
revealed that the HOMO energy levels of the resulting small
molecules could be fine-tuned by changing the electron-donating
ability moieties. Meanwhile, the morphologies of the blend films
containing small molecules and PCBM demonstrated these small
molecules possess the excellent film-forming abilities. The
photovoltaic devices based on these small molecules showed the
PCE in the range of 0.65–1.31%, which indicate that it is an
effective way to improve the PCE of photovoltaic devices by
adjusting the electron-donating abilities for the type of D–A
molecules.
Fig. 8 Topography image (size 5 mm ꢂ 5 mm) obtained by tapping-mode
AFM showing the morphology of the blend films spin-coated from
chlorobenzene solutions for APPM, AAPM and ATPM. (a) APPM/
PCBM (1 : 2 w/w); (b) AAPM/PCBM (1 : 3 w/w); (c) ATPM/PCBM
(1 : 2 w/w).
Acknowledgements
This work was supported by the State Key Development
Program for Basic Research of China (Grant No.
2009CB623605), the National Natural Science Foundation of
China (Grant No. 20874035), the 111 Project (Grant No.
B06009), and the Project of Jilin Province (20080305).
Fig. 9 External quantum efficiency (EQE) curve for device using 1 : 2,
1 : 3, 1 : 2 blend of APPM/PCBM, AAPM/PCBM, ATPM/PCBM,
respectively.
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18 Y. W. Li, L. L. Xue, H. Li, Z. F. Li, B. Xu, S. P. Wen and W. J. Tian,
Macromolecules, 2009, 42, 4491.
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