Solution-processable two-dimensional conjugated organic small molecules containing triphenylamine cores for photovoltaic application

Article abstract of DOI:10.1039/c4nj00814f

Two solution-processable two-dimensional conjugated organic small molecules based on triphenylamine (TPA) cores, TPA-BT-C8 and TPA-3Th, were designed and synthesized. As to TPA-BT-C8, two arms of the TPA core are symmetrically connected with a thiophene donor group and a benzothiadiazole acceptor group, while the third arm consists of a strong acceptor group of 2-(5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (DCM) connected through a trans double bond with the TPA core. For TPA-3Th, two arms of its TPA core are composed of only donor group, terthiophene, whereas the third arm consists of an acceptor group of cyano-n-octyl acetate connected through a trans double bond with the TPA core. The investigation indicated that TPA-BT-C8 has a lower energy band gap and wider absorption than TPA-3Th due to the strong intramolecular charge transfer effect in TPA-BT-C8. The two molecules have a deep highest occupied molecular orbital (HOMO) energy level. Bulk heterojunction photovoltaic devices were fabricated using TPA-BT-C8 or TPA-3Th as the donor and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) as the acceptor. All the devices have a high open-circuit voltage (V_oc) of about 0.9 eV. Devices based on TPA-BT-C8 have a much higher short circuit current (J_sc) (8.47 mA cm^-2) and power conversion efficiency (PCE) (2.26%) than devices based on TPA-3Th (4.32 mA cm^-2, 1.21%), resulting from wider solar light absorption of TPA-BT-C8 and better compatibility and film-formation ability of TPA-BT-C8 with PCBM than TPA-3Th. Incident photon-to-electron conversion efficiency (IPCE) spectra also confirmed that TPA-BT-C8 based devices have a wider and red-shifted response range than TPA-3Th based devices, which leads to a higher performance of the former devices.

Full text of DOI:10.1039/c4nj00814f

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Solution-processable two-dimensional conjugated
organic small molecules containing triphenylamine
cores for photovoltaic application
Cite this: New J. Chem., 2014,
38, 5009
Leijing Liu,a Hui Li,a Ji Bian,a Jingyu Qian,a Yingjin Wei,b Jiyang Lic and
Wenjing Tian*a
Two solution-processable two-dimensional conjugated organic small molecules based on triphenylamine
(TPA) cores, TPA-BT-C8 and TPA-3Th, were designed and synthesized. As to TPA-BT-C8, two arms of the
TPA core are symmetrically connected with a thiophene donor group and a benzothiadiazole acceptor
group, while the third arm consists of a strong acceptor group of 2-(5,5-dimethylcyclohex-2-en-1-
ylidene)malononitrile (DCM) connected through a trans double bond with the TPA core. For TPA-3Th, two
arms of its TPA core are composed of only donor group, terthiophene, whereas the third arm consists of
an acceptor group of cyano-n-octyl acetate connected through a trans double bond with the TPA core.
The investigation indicated that TPA-BT-C8 has a lower energy band gap and wider absorption than
TPA-3Th due to the strong intramolecular charge transfer effect in TPA-BT-C8. The two molecules have a
deep highest occupied molecular orbital (HOMO) energy level. Bulk heterojunction photovoltaic devices
were fabricated using TPA-BT-C8 or TPA-3Th as the donor and (6,6)-phenyl C61-butyric acid methyl ester
(PCBM) as the acceptor. All the devices have a high open-circuit voltage (Voc) of about 0.9 eV. Devices
based on TPA-BT-C8 have a much higher short circuit current (Jsc) (8.47 mA cmꢀ2) and power conversion
efficiency (PCE) (2.26%) than devices based on TPA-3Th (4.32 mA cmꢀ2, 1.21%), resulting from wider solar
light absorption of TPA-BT-C8 and better compatibility and film-formation ability of TPA-BT-C8 with
PCBM than TPA-3Th. Incident photon-to-electron conversion efficiency (IPCE) spectra also confirmed
that TPA-BT-C8 based devices have a wider and red-shifted response range than TPA-3Th based devices,
which leads to a higher performance of the former devices.
Received (in Montpellier, France)
19th May 2014,
Accepted 1st August 2014
DOI: 10.1039/c4nj00814f
www.rsc.org/njc
The TPA unit possesses a three-dimensional propeller struc-
ture, which makes the TPA containing molecules exhibit good
Introduction
Bulk-heterojunction organic solar cells (OSCs) have been receiving solution processability. Moreover, TPA is a strong electron-
a great deal of attention in recent years due to their advantages of donor unit and has a good charge transport ability, and
low cost, light weight and capability to be fabricated into flexible accordingly the TPA derivatives have high hole mobility and
devices.1 In comparison with conjugated polymers, solution- are widely used as hole-transporting materials in organic light-
processable organic small molecules possess the advantages of emitting diodes (OLEDs),3 sensitizers in dye-sensitized solar
high purity, definite molecular weight, simple purification and cells (DSSCs)4 and photovoltaic cells.3b,5,6 The Yongfang Li
better batch-to-batch reproducibility for the application in bulk- group synthesized many star-shaped D–p–A structural mole-
heterojunction OSCs. Therefore, solution-processable organic cules containing TPA as the core and different acceptor units or
small molecules have drawn considerable interest and achieved a combination of acceptor units and donor units as arms, and
great progress in recent years in OSCs.2
they applied these molecules in organic photovoltaic devices as
donor materials.6,7 The Jean Roncali group developed some
star-shaped TPA p-conjugated systems with internal charge-
transfer (ICT) and used them as donor materials for hetero-
junction solar cells.8 In addition, other research groups also
reported some star-shaped molecules based on TPA cores for
photovoltaic applications.5d,9 However, it is worth noting that
for all of these reported TPA derivatives, their three arms are
constructed by the same molecular structures or at least part of
a State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, P. R. China. E-mail: wjtian@jlu.edu.cn;
Fax: +86-431-85193421; Tel: +86-431-85166368
b Key Laboratory of Physics and Technology for Advanced Batteries,
Ministry of Education, Jilin University, Changchun 130012, P. R. China
c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, P. R. China
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their three arms are the same. This more or less brings about TPA-BT-C8 or TPA-3Th as the donor and PCBM as the acceptor.
some limitations to tune the photophysical properties, energy Both of the two devices have a high Voc. The device based on
level and photovoltaic properties of the molecules.
TPA-BT-C8 has a higher Jsc and PCE than the device based on
The Alex K.-Y. Jen group reported two conjugated two- TPA-3Th.
dimensional copolymers PFDCN and PFPDT with donor–p-bridge–
acceptor side chains,10 which take advantage of nonlinear optical
chromophores to optimize the absorption spectra and energy levels
Experimental section
Materials
of the resultant polymers,11 and the two-dimensional conjugated
structure of these polymers may improve their isotropic charge
transport.12 Resultantly, both of the two copolymers exhibit excellent
photovoltaic properties.
Potassium acetate, [1,10-bis(diphenylphosphino)ferrocene]dichloro-
palladium(II) (PdCl2(dppf)), bis(triphenylphosphine)palladium(II)
chloride (PdCl2(PPh3)2), tetrakis(triphenylphosphine)palladium
(Pd(PPh3)4) and n-octyl cyanoacetate were purchased from J&K
Company. Butyl lithium was obtained from Acros. Piperidine was
purchased from Shanghai Chemical Reagent Company. Dichloro-
methane, dioxane, chloroform and acetonitrile were obtained from
Beijing Chemical Reagent Company. The other reagents and
solvents were purchased from Tianjin Tiantai Chemical Reagent
Company. All reagents and chemicals were used without further
purification unless stated otherwise.
Herein, we designed and synthesized two novel solution-
processable two-dimensional conjugated organic small molecules,
(E)-2-(3-(4-(bis(4-(7-(5-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-
yl)phenyl)amino)styryl)-5,5-dimethylcyclohex-2-en-1-ylidene)malono-
nitrile (TPA-BT-C8) and (E)-octyl-3-(4-(bis(4-([2,20:50,200-terthiophen]-5-
yl)phenyl)amino)phenyl)-2-cyanoacrylate (TPA-3Th) (Scheme 1) by
using a TPA based structure. Different from the common linear or
star-shaped TPA derivatives, in TPA-BT-C8 and TPA-3Th, two arms
of the TPA core are symmetrically connected with the donor and
acceptor or just donor groups, while the third arm is connected
with different electron-withdrawing groups through a trans
Instruments
double bond. The results indicated that both of the two mole- 1H NMR and 13C NMR spectra were measured using a Bruker
cules have a deep HOMO energy level, and TPA-BT-C8 has a lower AVANCE-500 NMR spectrometer and a Varian Mercury-300
energy band gap and wider solar light absorption than TPA-3Th. NMR, respectively. The mass spectra were recorded on a Kratos
Bulk heterojunction photovoltaic devices were fabricated using MALDI-TOF mass system. Thermogravimetric analysis (TGA)
Scheme 1 Synthetic route of TPA-BT-C8 and TPA-3Th.
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was performed on a Perkin-Elmer Pyris 1 analyzer under a nitrogen and then the solvent was removed using a rotary evaporator. The
atmosphere (100 mL minꢀ1) at a heating rate of 10 1C minꢀ1
crude product was purified by column chromatography using a
.
UV-visible absorption spectra were measured using a Shimadzu cosolvent (petroleum ether/ethyl acetate, 10 : 1 V/V) as the eluent
UV-3000 spectrophotometer. Electrochemical measurements were to give 0.48 g of red solid with a yield of 90%. 1H NMR (300 MHz,
performed using a Bioanalytical Systems BAS 100 B/W electro- CDCl3, TMS): d (ppm) 9.88 (s, 1H), 8.02 (t, J = 6.3 Hz, 6H), 7.90
chemical workstation. The atomic force microscopy (AFM) (d, J = 7.5 Hz, 2H), 7.79 (d, J = 8.8 Hz, 2H), 7.75 (d, J = 7.5 Hz, 2H),
morphology was scanned using a Nanoscope IIIa Dimension 7.41 (d, J = 8.7 Hz, 4H), 7.29 (d, J = 8.7 Hz, 2H), 6.91 (d, J = 3.7 Hz,
3100 atomic force microscope.
2H), 2.93 (t, J = 7.6 Hz, 4H), 1.77 (m, 4H), 1.29 (m, 20H), 0.90
(t, 6H), 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 190.35, 152.59,
148.03, 146.47, 145.88, 136.54, 133.92, 132.22, 131.26, 130.37,
Synthesis
Compound 1 was synthesized following the procedures in 127.83, 127.64, 125.89, 125.68, 125.19, 124.99, 121.06, 31.79,
ref. 13, and tributyl(5-octylthiophen-2-yl)stannane and 2,20:50,200- 31.56, 30.24, 29.27, 29.15, 29.10, 22.59, 14.04.
terthiophene were synthesized following the procedures in ref. 14.
2-([2,20:50,200-Terthiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-
4-(Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)- dioxaborolane (5). In a three-necked round bottom flask under
amino)benzaldehyde (2). In a two-necked round bottom flask, nitrogen, 2,20:50,200-terthiophene (1 g, 4 mmol) was dissolved in
compound 1 (2.15 g, 5 mmol), bis(pinacolato)diboron (3.81 g, 50 mL of THF. The mixture was cooled to ꢀ78 1C and stirred.
15 mmol), potassium acetate (3.93 g, 40 mmol) and PdCl2(dppf) Then 1.8 mL of a solution of butyl lithium in hexane (2.4 M) was
(0.12 g, 0.15 mmol) were dissolved in 50 mL of dioxane, and the slowly dropped into the flask, and the reactant was warmed to
mixture was heated to 80 1C, stirred for 8 h and the reaction ꢀ40 1C, stirred at this temperature for 2 h. The mixture was
was finished. Then the reaction mixture was cooled to room cooled to ꢀ78 1C again, and isopropoxyboronic acid pinacol
temperature and poured into 300 mL of ice water, and extracted ester (0.9 mL) was added. The reaction mixture was warmed
using dichloromethane (3 ꢁ 300 mL). The organic phase was to room temperature, stirred overnight, and then poured into
dried over anhydrous magnesium sulfate (MgSO4), and then 150 mL of water, and extracted using dichloromethane (3 ꢁ 130 mL).
the solvent was removed using a rotary evaporator. The crude The organic phase was dried over anhydrous MgSO4, and then the
product was purified by column chromatography using a solvent was removed using a rotary evaporator. The crude product
cosolvent (petroleum ether/ethyl acetate, 5 : 1 V/V) as the eluent was purified by column chromatography using a cosolvent (petro-
to give 2 g of white solid with a yield of 76%. 1H NMR (300 MHz, leum ether/ethyl acetate, 10 : 1 V/V) as the eluent to give 0.96 g of
1
CDCl3, TMS): d (ppm) 9.84 (s, 1H), 7.76 (d, J = 8.7 Hz, 4H), 7.71 green solid with a yield of 64%. H NMR (300 MHz, CDCl3, TMS):
(d, J = 8.7 Hz, 2H), 7.14 (d, J = 8.7 Hz, 4H), 7.11 (d, J = 8.7 Hz, d (ppm) 7.54 (d, J = 3.6 Hz, 1H), 7.24 (d, J = 3.6 Hz, 1H), 7.22 (d, J =
2H), 1.35 (s, 24H).
3.6 Hz, 1H), 7.19 (dd, J = 3.6 Hz, J = 1.2 Hz, 1H), 7.15 (d, J = 3.8 Hz,
4-Bromo-7-(5-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (3). 1H), 7.09 (d, J = 3.8 Hz, 1H), 7.03 (dd, J = 5.1 Hz, J = 3.6 Hz, 1H), 1.36
In a one-necked round bottom flask under nitrogen, tributyl- (s, 12H). 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 137.89, 127.81,
(5-octylthiophen-2-yl)stannane (0.5 g, 1.03 mmol), 4,7-dibromo- 124.92, 124.79, 124.56, 124.40, 123.81, 84.15, 24.72.
benzo[c][1,2,5]thiadiazole (0.5 g, 1.7 mmol), and PdCl2(PPh3)2
4-(Bis(4-([2,20:50,200-terthiophen]-5-yl)phenyl)amino)benzalde-
(10 mg) were dissolved in 70 mL of tetrahydrofuran (THF). The hyde (6). In a one-necked round bottom flask under nitrogen, a
mixture was heated to 60 1C, and stirred overnight at this mixture of compound 2 (107 mg, 0.25 mmol), compound 5
temperature. Then the reactant was cooled to room temperature, (253 mg, 0.68 mmol), Pd(PPh3)4 (15 mg), 2 mL of solution of
and the solvent was removed using a rotary evaporator. The crude Na2CO3 in water (2 M) and 3 mL of toluene was heated to reflux
product was purified by column chromatography using a cosol- for 24 h under stirring. Then the reactant was cooled to room
vent (petroleum ether/dichloromethane, 10: 1 V/V) as the eluent temperature, and poured into 50 mL of water, and extracted
to give 0.3 g of orange solid with a yield of 71%. 1H NMR using dichloromethane (3 ꢁ 60 mL). The organic phase was
(300 MHz, CDCl3, TMS): d (ppm) 7.91 (d, J = 3.7 Hz, 1H), 7.80 dried over anhydrous MgSO4, and then the solvent was removed
(d, J = 7.7 Hz, 1H), 7.62 (d, J = 7.7 Hz, 1H), 6.87 (d, J = 3.7 Hz, 1H), using a rotary evaporator. The crude product was purified by
2.89 (t, J = 7.6 Hz, 2H), 1.76 (m, 2H), 1.28 (m, 10H), 0.88 (t, 3H). column chromatography using dichloromethane as the eluent
13C NMR (75 MHz, CDCl3, TMS): d (ppm) 153.73, 151.73, 148.52, to give 0.13 g of yellow solid with a yield of 68%. 1H NMR
135.77, 132.24, 128.01, 127.39, 125.33, 125.02, 111.47, 31.83, (300 MHz, CDCl3, TMS): d (ppm) 9.86 (s, 1H), 7.76 (d, J = 8.9 Hz,
31.56, 30.27, 29.31, 29.20, 29.12, 22.64, 14.10.
2H), 7.58 (d, J = 8.8 Hz, 4H), 7.20 (m, 18H), 7.05 (dd, J = 5.1 Hz,
4-(Bis(4-(7-(5-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)- J = 3.6 Hz, 2H), 13C NMR (75 MHz, CDCl3, TMS): d (ppm) 190.30,
phenyl)amino) benzaldehyde (4). In a one-necked round bottom 145.36, 131.30, 130.81, 130.09, 128.81, 127.85, 126.83, 126.15,
flask under nitrogen, a mixture of compound 2 (0.3 g, 0.57 mmol), 124.60, 124.54, 124.38, 124.22, 123.75, 120.67.
compound 3 (0.3 g, 0.57 mmol), Pd(PPh3)4 (0.033 g), 4 mL of
(E)-2-(3-(4-(Bis(4-(7-(5-octylthiophen-2-yl)benzo[c][1,2,5]thia-
solution of sodium carbonate in water (Na2CO3) (2 M) and 6 mL diazol-4-yl)phenyl)amino)styryl)-5,5-dimethylcyclohex-2-en-1-
of toluene was heated to reflux for 24 h under stirring. Then the ylidene)malononitrile (TPA-BT-C8). In a two-necked round
reactant was cooled to room temperature, and poured into bottom flask under nitrogen, a mixture of compound 4 (130 mg,
100 mL of water, and extracted using dichloromethane (3 ꢁ 0.14 mmol), acetonitrile (6 mL), piperidine (10 drops) and 2-(5,5-
110 mL). The organic phase was dried over anhydrous MgSO4, dimethylcyclohex-2-en-1-ylidene)malononitrile (39 mg) was heated
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to reflux for 24 h. Then the reactant was cooled to room tempera- (I–V) characteristics were measured using
a computer-
ture, and the solvent was removed using a rotary evaporator. The controlled Keithley 2400 Source Meter Measurement System
crude product was purified by column chromatography using a in the dark or under 100 mW cmꢀ2 illumination from a
cosolvent (petroleum ether/dichloromethane, 1 : 1 V/V) as the Sciencetech 500 W solar simulator (AM 1.5G). The class of
1
eluent to give 110 mg of red solid with a yield of 72%. H NMR Sciencetech 500 W solar simulator (AM 1.5G) is A.
(300 MHz, CDCl3, TMS): d (ppm) 7.98 (m, 6H), 7.88 (d, J = 7.5 Hz,
2H), 7.73 (d, J = 7.5 Hz, 2H), 7.46 (d, J = 8.7 Hz, 2H), 7.37 (d, J =
8.7 Hz, 4H), 7.25 (d, J = 8.7 Hz, 2H), 7.07 (d, J = 16.2 Hz, 1H), 6.93
Results and discussion
(d, J = 16 Hz, 1H), 6.90 (d, J = 3.9 Hz, 2H), 6.82 (s, 1H), 2.92
(t, J = 7.6 Hz, 4H), 2.60 (s, 2H), 2.47 (s, 2H), 1.76 (m, 4H), 1.29
(m, 20H), 1.06 (s, 6H), 0.89 (t, 6H). 13C NMR (75 MHz, CDCl3, TMS):
d (ppm) 168.97, 154.10, 153.89, 152.85, 148.77, 147.95, 146.65,
136.71, 136.59, 132.88, 131.27, 130.19, 129.93, 128.82, 127.61,
127.35, 126.65, 125.19, 125.11, 124.91, 123.37, 122.81, 113.65,
112.90, 77.76, 43.01, 39.31, 31.97, 31.82, 31.59, 30.28, 29.64,
29.30, 29.17, 27.99, 22.60, 14.01. MALDI-TOF MS calcd for
C67H67N7S4 1098.44; found 1099.44.
Synthesis and characterization
The synthesis procedures of the two-dimensional conjugated
organic small molecules, TPA-BT-C8 and TPA-3Th, are depicted
in Scheme 1. Compound 3 was synthesized via Stille reaction,
and compounds 4 and 6 were synthesized via Suzuki reaction.
Compound 5 was synthesized from isopropoxyboronic acid
pinacol ester and lithiated 2,20:50,200-terthiophene at ꢀ78 1C.
The final molecules TPA-BT-C8 and TPA-3Th were synthesized
via Knoevenagel condensation reaction. The chemical struc-
tures of the compounds were confirmed by 1H NMR and 13C
NMR spectra. The double peaks of coupling constant J B 16 in
1H NMR spectra indicate that the double bond presents a trans
configuration in TPA-BT-C8. The characteristic peak of the
aldehyde group at a chemical shift of 10.0 ppm in the
1H NMR spectrum of compound 6 disappears in that of
TPA-3Th, while a single peak at a chemical shift of 8.114 ppm
appears, which indicates that the trans double bond forms in
TPA-3Th. Both TPA-BT-C8 and TPA-3Th exhibit good solubility
in common organic solvents such as chloroform, dichloro-
methane and chlorobenzene etc. at room temperature.
(E)-Octyl-3-(4-(bis(4-([2,20:50,200-terthiophen]-5-yl)phenyl)amino)-
phenyl)-2-cyanoacrylate (TPA-3Th). In a two-necked round bottom
flask under nitrogen, a mixture of compound 6 (70 mg,
0.09 mmol), n-octyl cyanoacetate (0.18 mL, 0.09 mmol), triethyla-
mine (0.15 mL) and chloroform (5 mL) was heated to reflux for
24 h. Then the reactant was cooled to room temperature, and the
solvent was removed using a rotary evaporator. The crude product
was purified by column chromatography using a cosolvent (petro-
leum ether/dichloromethane, 2 : 1 V/V) as the eluent to give 60 mg
1
of orange solid with a yield of 71%. H NMR (300 MHz, CDCl3,
TMS): d (ppm) 8.11 (s, 1H), 7.91 (d, J = 8.7 Hz, 2H), 7.59 (d, J =
8.7 Hz, 4H), 7.21 (m, 18H), 7.05 (m, 2H), 4.31 (t, J = 6.6 Hz, 2H),
2.06 (m, 2H), 1.75 (m, 2H), 1.26 (m, 8H), 0.89 (t, 3H). 13C NMR
(75 MHz, CDCl3, TMS): d (ppm) 163.50, 162.88, 153.64, 151.61,
144.96, 142.29, 137.10, 136.55, 136.41, 136.11, 133.10, 131.11,
127.89, 126.90, 126.34, 124.66, 124.58, 124.41, 124.28, 123.89,
123.79, 120.47, 67.12, 31.71, 29.07, 28.34, 25.68, 24.67, 22.58,
14.00. MALDI-TOF MS calcd for C54H44N2O2S6 944.17; found 945.18.
Thermal properties
Fig. 1 shows the TGA curves of TPA-BT-C8 and TPA-3Th
scanned at a heating rate of 10 1C minꢀ1 under the protection
of nitrogen. The 5% weight-loss temperature (Td) of TPA-BT-C8
is 227 1C, much higher than that of TPA-3Th which is 153 1C. As
for low molecular weight materials, the Td of TPA-BT-C8 and
TPA-3Th is high enough for photovoltaic device application.
Photovoltaic device fabrication and characterization
Mixed solutions for the active film and morphology investiga-
tion were prepared by dissolving TPA-BT-C8:PCBM and
TPA-3Th:PCBM in the solvent of chlorobenzene with blend
ratios of 1 : 1, 1 : 2.5 and 1 : 3, respectively, then continuously
stirred overnight at 40 1C. The bulk heterojunction photovoltaic
devices were prepared as follows: the ITO-covered glass sub-
strate was used as the anode in the devices. It was cleaned in
an ultrasonic bath with detergent, chloroform, acetone and
isopropyl alcohol in turn. Then the substrates were treated in
an oxygen plasma cleaner for about 5 min. Subsequently,
PEDOT:PSS was spin-coated from an aqueous dispersion onto
the cleaned ITO substrate to form a thin film with a thickness
of about 50 nm. The active layer was prepared by spin-coating
on top of the PEDOT:PSS layer, and the thickness was around
73 nm. Finally, the cathode of LiF/Al about 100 nm was
deposited by thermal evaporation in a vacuum chamber at a
pressure of 5 ꢁ 10ꢀ4 Pa. The effective photovoltaic area as
defined by the geometrical overlap between the bottom ITO
electrode and the top cathode was 4 mm2. The current–voltage
Fig. 1 TGA curves of TPA-BT-C8 and TPA-3Th at a heating rate of 10 1C minꢀ1
under the protection of nitrogen.
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Table 1 Summary of optical and electrochemical properties of TPA-BT-C8
and TPA-3Th
Optical properties
The normalized UV-vis absorption spectra of TPA-BT-C8 and
TPA-3Th in a dilute chloroform solution (10ꢀ5 M) and in a solid
thin film are shown in Fig. 2. The relevant spectroscopic data of
abs
a
abs
max
b
onset
ox
onset
l
l
Egopt c
E
HOMO E
red
LUMO Egel
(eV) (eV)
max
Molecule (nm)
(nm)
(eV) (V)
(eV)
(V)
the two small molecules are summarized in Table 1. In a dilute TPA-BT-C8 329, 500 330, 517 1.98 0.68 ꢀ5.38 ꢀ1.13 ꢀ3.57 1.81
TPA-3Th 420
438
2.21 0.65 ꢀ5.35 ꢀ1.20 ꢀ3.50 1.85
chloroform solution, TPA-BT-C8 exhibits two parts of absorp-
tion, a relative narrow absorption band with a peak at 329 nm
and a wide absorption band with a maximum at 500 nm, which
can be assigned to the intramolecular p–p* transition and the
ICT interaction between the electron-donor and acceptor
groups, respectively. As to TPA-3Th, it has no electron-acceptor
group in the main chain except a weak cyano acrylate group in
the third arm, so there is only a weak ICT band resulting from a
somewhat smaller charge transfer from TPA to the cyano acrylate
group. Moreover, the absorption band with a peak at 420 nm and
a weakly discernible shoulder observed for TPA-3Th should
correspond to the addition of a p–p* transition and the weak
ICT band. Both of the two compounds have a relatively high
molar absorption coefficient, which is 6.0 ꢁ 104 Mꢀ1 cmꢀ1 for
TPA-BT-C8 and 6.7 ꢁ 104 Mꢀ1 cmꢀ1 for TPA-3Th calculated using
Lambert–Beer’s law.15
a
b
c
film
In chloroform (1 ꢁ 10ꢀ5 M). In thin solid film. Egopt = 1240/l
.
edge
solid films have a red shift of B20 nm. Because the three-
dimensional structure of TPA restrains the p–p stacking effect
of TPA-BT-C8 and TPA-3Th, so the red shift in the solid state is
not so high as other molecules. The maximum absorption peak
of TPA-BT-C8 is at 330 nm, indicating that in thin solid films,
the p–p* transition absorption is dominant, not like in dilute
solution, the p–p* transition absorption and ICT interaction
absorption are comparable. According to the onset of the
optical absorption in thin films, the optical band gap (Eogpt) of
TPA-BT-C8 and TPA-3Th is estimated to be 1.98 eV and 2.21 eV,
respectively.
Compared with the absorption spectra in dilute chloroform
solutions, the absorption spectra of TPA-BT-C8 and TPA-3Th in
Electrochemical properties
The electrochemical properties of TPA-BT-C8 and TPA-3Th were
investigated by cyclic voltammetry (CV). The measurements were
carried out in anhydrous acetonitrile under the protection of
nitrogen, using platinum button as the working electrode,
Ag/AgNO3 as the reference electrode, platinum wire as the
counter electrode, and tetrabutylammonium hexafluoropho-
sphate (TBAPF6, 0.1 mol Lꢀ1) as the supporting electrolyte.
Ferrocene was used as the internal standard and showed a peak
at +0.1 V versus Ag/Ag+. The CV curves are shown in Fig. 3, and
the electrochemical parameters are summarized in Table 1. It
can be found that TPA-BT-C8 and TPA-3Th exhibit semi-
onset
reversible redox behavior. The onset oxidation potential (E
)
ox
and onset reduction potential (Eorendset) of TPA-BT-C8 are ca. 0.68
Fig. 3 Cyclic voltammetry curves of TPA-BT-C8 and TPA-3Th films on a
Fig. 2 Absorption spectra of TPA-BT-C8 and TPA-3Th in chloroform
solution with a concentration of 10ꢀ5 mol Lꢀ1 (a) and in a thin solid film (b).
platinum electrode in 0.1 mol Lꢀ1 acetonitrile solution of TBAPF6 at a scan
rate of 100 mV sꢀ1
.
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and ꢀ1.13 V respectively, and for TPA-3Th, they are 0.65 and
ꢀ1.20 V. From Eooxnset and Eroendset of TPA-BT-C8 and TPA-3Th, their
HOMO and lowest unoccupied molecular orbital (LUMO) energy
levels as well as electrochemical energy band gap (Eegl) can be
calculated according to the following equations:16
onset
HOMO (eV) = ꢀe(E
+ 4.7 V)
+ 4.7 V)
ox
onset
red
LUMO (eV) = ꢀe(E
Eegl = ꢀ(HOMO ꢀ LUMO) (eV)
The HOMO and LUMO energy levels as well as the electro-
chemical band gap of TPA-BT-C8 are ꢀ5.38, ꢀ3.57 and 1.81 eV, and
as to TPA-3Th, they are ꢀ5.35, ꢀ3.50 and 1.85 eV. It can be found
that the two molecules have a deep HOMO energy level, far below
the air oxidation threshold (ca. ꢀ5.2 eV),17 which ensures a good air
stability of the molecules and allows a high Voc of the photovoltaic
device based on TPA-BT-C8 and TPA-3Th. Both TPA-BT-C8 and
TPA-3Th have a TPA group which mainly determines their HOMO
energy level, so they have nearly equal HOMO energy level. In
addition, since the electron-donor ability of the trithiophene group
in TPA-3Th is stronger than that of the single thiophene group in
TPA-BT-C8, and they have some contribution to the HOMO energy
level, the HOMO energy level of TPA-3Th is somewhat higher than
that of TPA-BT-C8. The electron-acceptor ability of benzothiadiazole
and DCM groups in TPA-BT-C8 is a little stronger than that of
cyano-n-octyl acetate in TPA-3Th, resulting in that TPA-BT-C8 has a
somewhat lower LUMO energy level than TPA-3Th.
Fig. 4 HOMO and LUMO pictograms and corresponding energy levels of
TPA-BT-C8 and TPA-3Th obtained at the B3LYP/6-31G* level.
electrochemical and optical values are a well-known consequence
of performing calculations that lack medium-related factors.
Photovoltaic properties
Bulk heterojunction photovoltaic cells with a device structure of
glass/ITO/PEDOT:PSS/TPA-BT-C8 or TPA-3Th:PCBM/LiF/Al were
fabricated and their performances were measured under a
simulated AM1.5G illumination of 100 mW cmꢀ2. The current
Theoretical calculation
The ground-state geometry and electron-state-density distribu- density–voltage characteristics of the devices are illustrated in
tion of the HOMO and LUMO energy levels of TPA-BT-C8 and Fig. 5. The optimized conditions and photovoltaic parameters
TPA-3Th have been fully optimized using density functional for the devices are summarized in Table 2. Both of the two
theory (DFT) based on the B3LYP method with a 6-31G* basis in devices achieve the best performance at a weight blend ratio of
the Gaussian 03 program package. The HOMO and LUMO 1 : 2.5. The device based on TPA-BT-C8 shows an optimal
pictograms and corresponding energy levels for TPA-BT-C8 performance with a Voc of 0.91 V, a Jsc of 8.47 mA cmꢀ2, a fill
and TPA-3Th are presented in Fig. 4. The calculation results factor (FF) of 0.30 and a PCE of 2.26%. The device based on
demonstrate that the HOMO energy level of TPA-BT-C8 distri- TPA-3Th exhibits its best performance with a Voc of 0.86 V, a Jsc
butes mainly on the triphenylamine core with some extending of 4.32 mA cmꢀ2, a FF of 0.34 and a PCE of 1.21%. The much
to the symmetrical two arms, and the LUMO energy level higher PCE of the TPA-BT-C8 based device than that of the
distributes on the acceptor groups, DCM and benzothiadiazole TPA-3Th based device is resulted from the higher Jsc, 8.47 vs.
groups. However, as to TPA-3Th, the HOMO energy level 4.32 mA cmꢀ2, and this is mainly attributed to the much wider
distributes on the triphenylamine core and the six thiophene solar light absorption of TPA-BT-C8 than TPA-3Th (see Fig. 2).
groups, and the LUMO energy level distributes only on the The somewhat higher Voc of the device based on TPA-BT-C8
cyano-n-octyl acetate group. The DFT calculation indicated that (0.91 V) than that of the device based on TPA-3Th (0.86 V) is
ICT absorption can take place in both TPA-BT-C8 and TPA-3Th, consistent with their HOMO energy levels obtained from electro-
but the intensity of TPA-BT-C8 should be stronger than that of chemical measurement and DFT theoretical calculation, which
TPA-3Th, which accords with their optical properties. The is ꢀ5.38 eV vs. ꢀ5.35 eV by electrochemical measurement and
calculated HOMO, LUMO energy levels and the band gap of ꢀ5.06 eV vs. ꢀ4.99 eV by DFT theoretical calculation (vide ut
TPA-BT-C8 are ꢀ5.06, ꢀ2.73 and 2.33 eV, and as to TPA-3Th, supra), since Voc of the bulk heterojunction photovoltaic cells is
they are ꢀ4.99, ꢀ2.31 and 2.68 eV, respectively. In general, the mainly determined by the difference between the HOMO energy
theoretical HOMO energy levels of TPA-BT-C8 and TPA-3Th are level of the donor and the LUMO energy level of the acceptor.1d
consistent with those obtained from electrochemical measure-
The charge carrier mobility of materials is another
ment, and the trend of theoretical band gaps agrees with that of important factor which influences the performance of OSCs.
optical band gaps. The differences between the theoretical and Accordingly, we investigated the hole mobility of TPA-BT-C8
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Fig. 6 J–V curves in the dark of ITO/PEDOT:PSS/TPA-BT-C8 or TPA-3Th/
Au devices in the log axis for estimating the hole mobility of TPA-BT-C8
(a) and TPA-3Th (b). The thicknesses of TPA-BT-C8 or TPA-3Th films are
around 70 nm. The open triangle symbols are the experimental data. The
solid line from 0.02 to 0.6 V means log J is fitted linearly dependent on log V
with a slope of 1. The solid line from 0.6 to 1 V means log J is fitted linearly
dependent on log V with a slope of 2 (SCLC area).
Fig. 5 The current–voltage (J–V) curves of the solar cells based on
TPA-BT-C8 or TPA-3Th:PCBM with varying blend ratios under simulated
AM1.5 solar illumination (100 mW cmꢀ2).
Table 2 Photovoltaic parameters of the solar cells based on TPA-BT-C8 or
TPA-3Th:PCBM under simulated AM1.5 solar illumination (100 mW cmꢀ2
)
permittivity of the vacuum (8.85 ꢁ 10ꢀ12 F mꢀ1), respectively. mh is
the hole mobility and L is the thickness of the organic layer. Vbi is
fitted to be about 0.1 V, which is determined by the difference
between the work functions of the cathode and anode. Then
Small molecule : PCBM Voc
(w : w ratio) (V)
Jsc
PCE
(%)
Molecule
(mA cmꢀ2
)
FF
TPA-BT-C8 1 : 2
TPA-BT-C8 1 : 2.5
TPA-BT-C8 1 : 3
0.86 2.75
0.91 8.47
0.93 5.04
0.87 3.26
0.86 4.32
0.86 2.29
0.37 0.88
0.30 2.26 the hole mobility of TPA-BT-C8 and TPA-3Th is estimated to be
0.36 1.68
7.1 ꢁ 10ꢀ5 and 1.6 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1, respectively. Both of the
TPA-3Th
TPA-3Th
TPA-3Th
1 : 2
1 : 2.5
1 : 3
0.35 0.98
two small molecules have a relatively high hole mobility, which
0.34 1.21
0.35 0.69 is favorable for performance of the photovoltaic cell. On the
other hand, the hole mobility of TPA-3Th is higher than that of
TPA-BT-C8, but the performance of the photovoltaic cell based
and TPA-3Th by the space charge limited current (SCLC)
method.18 Devices with a structure of ITO/TPA-BT-C8 or TPA-
3Th/LiF/Al were fabricated and their J–V curves in the dark are
shown in Fig. 6. The hole mobility of TPA-BT-C8 and TPA-3Th
was estimated according to the following equation:
on TPA-BT-C8 is better than that of the photovoltaic cell based
on TPA-3T. We believe this is because the hole mobility of the
donor materials is not the dominant factor influencing the
performance of these two devices. Just as we analyzed above,
the much wider solar light absorption of TPA-BT-C8 than
TPA-3Th is an important factor that cannot be ignored.
2
9
ðV ꢀ VbiÞ
To further investigate the reason why the photovoltaic
performance of the device based on TPA-BT-C8 is much better
J ¼ ere0mh
8
L3
where V is the applied voltage, and J is the current density. er and e0 than that of the device based on TPA-3Th, the morphology of
are the relative dielectric constant (here we fit it to be 3.) and the their active layers was investigated by AFM. As demonstrated in
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Fig. 7 AFM topography images (size 5 mm ꢁ 5 mm) of TPA-BT-C8 : PCBM (1 : 2.5) (a) and TPA-3Th : PCBM (1 : 2.5) (b) films.
Fig. 7, many large aggregates with several hundreds of nan- which is more close to the maximum photon flux of the solar
ometers can be found in the TPA-3Th:PCBM blend film, while irradiation at B700 nm,19 and so the TPA-BT-C8:PCBM based
the surface of the TPA-BT-C8:PCBM blend film appears to be device has a stronger ability of absorbing photons than the
much smooth. In addition, the root mean square (rms) rough- TPA-3Th:PCBM based device. Consequently, it is reasonable that
ness of the TPA-BT-C8:PCBM blend film is much lower than the device based on TPA-BT-C8 has a higher Jsc and PCE than the
that of the TPA-3Th:PCBM blend film, which is 0.96 and 4.61, device based on TPA-3Th.
respectively. Therefore, TPA-BT-C8 has a better compatibility
and film-formation property with PCBM than TPA-3Th, and this
Conclusions
partially leads to that the photovoltaic device based on TPA-BT-C8
has a higher Jsc and PCE than the device based on TPA-3Th.
In summary, two solution-processable two-dimensional conjugated
Fig. 8 shows the IPCE curves of photovoltaic devices based on
organic small molecules, TPA-BT-C8 and TPA-3Th, were synthesized
TPA-BT-C8:PCBM (w: w, 1 : 2.5) and TPA-3Th:PCBM (w: w,
successfully by Knoevenagel condensation reaction. For TPA-BT-C8,
1 : 2.5). It was found that IPCE spectra of the devices are basically
two arms of the TPA core are symmetrically connected with a
consistent with the UV-vis absorption spectra of TPA-BT-C8 and
thiophene donor group and a benzothiadiazole acceptor group,
TPA-3Th. The device based on TPA-BT-C8:PCBM has a wider
which brings about ICT and it ensures TPA-BT-C8 has a red-shifted
response range than the device based on TPA-3Th:PCBM. More
and wider absorption than TPA-3Th whose two arms of the TPA core
importantly, the response range of the TPA-3Th:PCBM based
are composed of only a donor group, terthiophene. Both of the two
device is mainly between 350 and 530 nm, while that of the
molecules have a deep HOMO energy level, which ensures that the
TPA-BT-C8:PCBM based device is extended from 350 to 630 nm
two molecules have a good air stability and the photovoltaic devices
based on TPA-BT-C8 or TPA-3Th have a high Voc of about 0.9 V. The
device based on TPA-BT-C8 has a higher Jsc (8.47 mA cmꢀ2) and
power conversion efficiency (PCE) (2.26%) than the device based on
TPA-3Th (4.32 mA cmꢀ2, 1.21%), resulting from the wider solar light
absorption of TPA-BT-C8 and good compatibility and film-
formation ability of TPA-BT-C8 with PCBM than TPA-3Th. The IPCE
spectra also confirmed that the device based on TPA-BT-C8 has a
wider and red-shifted response range than the device based on
TPA-3Th. The investigations indicate that the two-dimensional
conjugated organic small molecules are a type of promising organic
photovoltaic materials, and offer a new strategy for designing
organic small molecular solar cell materials.
Acknowledgements
This work was supported by 973 program (2014CB643506), the
Project Sponsored by the Scientific Research Foundation for the
Fig. 8 IPCE curves of the OSC devices based on TPA-BT-C8 or
TPA-3Th : PCBM (1 : 2.5).
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