Triphenylamine-containing D-A-D molecules with (dicyanomethylene)pyran as an acceptor unit for bulk-heterojunction organic solar cells

Article abstract of DOI:10.1039/c0jm03425h

Two new triphenylamine (TPA)-containing D-A-D molecules with TPA as the donor unit (D), (dicyanomethylene)pyran (PM) as the acceptor unit (A) and thienylenevinylene (TV) or 4-hexyl-thienylenevinylene (HTV) as the conjugated pi-bridge, TPA-TV-PM and TPA-HTV-PM, have been designed and synthesized for solution-processable organic solar cells (OSCs). The optical and electrochemical properties of these linear molecules were studied. The compounds exhibit broad absorption in the visible region with lower HOMO energy levels, which are desirable for application as a donor in organic solar cells. The OSC devices were fabricated by spin-coating the blend solution of the molecules as the donor and PC₇₀BM as the acceptor (1:3, w/w). The power conversion efficiency of the OSCs based on TPA-TV-PM and TPA-HTV-PM reached 2.06% and 2.10%, respectively, under the illumination of AM.1.5, 100 mW cm^-2. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03425h

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PAPER
Triphenylamine-containing D–A–D molecules with (dicyanomethylene)pyran
as an acceptor unit for bulk-heterojunction organic solar cells
a
a
a
Jing Zhang,ab Guanglong Wu, Chang He, Dan Dengab and Yongfang Li
*
*
Received 12th October 2010, Accepted 22nd December 2010
DOI: 10.1039/c0jm03425h
Two new triphenylamine (TPA)-containing D–A–D molecules with TPA as the donor unit (D),
(dicyanomethylene)pyran (PM) as the acceptor unit (A) and thienylenevinylene (TV) or 4-hexyl-
thienylenevinylene (HTV) as the conjugated pi-bridge, TPA–TV–PM and TPA–HTV–PM, have been
designed and synthesized for solution-processable organic solar cells (OSCs). The optical and
electrochemical properties of these linear molecules were studied. The compounds exhibit broad
absorption in the visible region with lower HOMO energy levels, which are desirable for application as
a donor in organic solar cells. The OSC devices were fabricated by spin-coating the blend solution of the
molecules as the donor and PC70BM as the acceptor (1 : 3, w/w). The power conversion efficiency of the
OSCs based on TPA–TV–PM and TPA–HTV–PM reached 2.06% and 2.10%, respectively, under the
illumination of AM.1.5, 100 mW cmꢀ2
.
molecules have attracted much interest recently as photovoltaic
donor materials in organic solar cells13–26 because they combine
the advantages of high purity, definite molecular weight of the
organic small molecules and the low-cost solution processing of
the polymers. The power conversion efficiencies (PCEs) of the
OSCs based on the solution-processable organic molecules have
reached 2–4%.18–20,25,26 However, the efficiencies are still lower
than that of either the vacuum-evaporated OSCs or the solution-
processable PSCs. So there is a lot of room for the further design
and synthesis of high efficiency solution-processable organic
photovoltaic materials.
Introduction
In recent years, there has been a great deal of research on poly-
mer solar cells (PSCs) composed of a polymer donor and
a fullerene acceptor, for their advantages of low cost fabrication
by a solution-processing method, flexibility of the photoactive
layer and the abundant availability of the conjugated donor
materials. Many efforts have been devoted to the design and
synthesis of new conjugated polymer donor and fullerene-
derivative acceptor materials.1–9 To date, the power conversion
efficiency of the PSCs has reached over 7%,3,5 but the polymer
materials always suffer from broad molecular weight distribu-
tions, difficult purification and batch-to-batch variations. There
is still a long way to go before approaching commercial appli-
cation.
Our group has synthesized a series of solution-processable
triphenylamine (TPA)-containing organic molecules for appli-
cation as donor materials in OSCs.21–26 The donor materials
include the star-shaped D–A molecules with TPA as the core and
donor unit and benzothiadiazole (BT) as the acceptor unit,21,24–26
and the D–A–D molecules with TPA as the donor unit, (dicya-
nomethylene)pyran (or 2-pyran-4-ylidenemalononitrile) (PM) as
the acceptor unit and styrene as the conjugated pi-bridge.22,23 The
TPA- and PM-containing molecules (such as TPA–DCM–
TPA,22 see Scheme 1) exhibit broad absorption and suitable
HOMO (the highest occupied molecular orbital) and LUMO
(the lowest unoccupied molecular orbital) energy levels. The
OSCs based on them as the donor showed a higher open circuit
voltage (Voc) but unsatisfactory power conversion efficiencies
(PCE) due to a lower short circuit current density (Jsc) and
smaller fill factor (FF). In fact, the styrene linkage in the mole-
cules negatively influenced the charge transfer properties and
solubility of the molecules,23,27 which could result in the lower
PCE of the molecules. Roncali and co-workers used a thienyle-
nevinylene (TV) unit in the TPA-containing organic molecules to
improve their photovoltaic performance.13 In order to improve
In a parallel effort, organic solar cells (OSCs) based on small
molecules with vacuum-evaporating multilayer structures have
reached a power conversion efficiency of 5–6%.10–12 It is
remarkable that these small molecules have well-defined molec-
ular structures, definite molecular weights without any distribu-
tion and high purity with easy purification methods like column
chromatography or sublimation. However, fabrication of OSCs
by vacuum evaporation is much more difficult and expensive
than that of the polymer solar cells by a solution-processed
method.
Compared with the polymers and the vacuum-deposited
organic small molecules, solution-processable organic small
aBeijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190, China.
E-mail: liyf@iccas.ac.cn; Fax: +86-10-62559373
bGraduate University of Chinese Academy of Sciences, Beijing, 100049,
China
3768 | J. Mater. Chem., 2011, 21, 3768–3774
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taken on a Hitachi U-3010 UV-vis spectrophotometer. The
organic molecule films on quartz used for absorption spectral
measurements were prepared by spin-coating their chloroform
solutions. The electrochemical cyclic voltammogram was
obtained using a Zahner IM6e electrochemical workstation in
a
0.1 mol Lꢀ1 tetrabutylammonium hexafluorophosphate
(Bu4NPF6) acetonitrile solution. A Pt electrode coated with the
sample film was used as the working electrode; a Pt wire and
Ag/Ag+ (0.01 M AgNO3 in acetonitrile) were used as the counter
and reference electrodes, respectively.
Fabrication and characterization of organic solar cells
Organic solar cells (OSCs) were fabricated in the traditional
sandwich structure with an ITO positive electrode and a metal
negative electrode. Patterned ITO glass with a sheet resistance of
ca. 10 U sqꢀ1 was purchased from CSG Holding Co., Ltd (China).
The ITO glass was cleaned in an ultrasonic bath of acetone and
isopropanol, and treated by UVO (ultraviolet ozone cleaner,
Jelight Company, USA). Then a thin layer (30 nm thickness) of
PEDOT : PSS (poly(3,4-ethylenedioxythiophene)–poly(styrene
sulfonate)) (Baytron PVP A1 4083, Germany) was spin-coated on
the ITO glass and baked at 150 ꢁC for 30 min. Subsequently, the
photosensitive layer was prepared by spin-coating the blend
chlorobenzene solution of TPA–TV–PM : PC70BM (1 : 3 w/w) or
TPA–HTV–PM : PC70BM (1 : 3 w/w) on the top of the
PEDOT : PSS layer and baked at 80 ꢁC for 30 min. Thickness of
the active layer is ca. 60 nm measured by an Ambios Tech. XP-2
profilometer. Finally, a metal electrode layer of Al (ca. 120 nm)
was vacuum evaporated on the photoactive layer under a shadow
mask in a vacuum of ca. 10ꢀ4 Pa. The effective area of one cell is 4
mm2. The current–voltage (I–V) measurement of the devices was
conducted on a computer-controlled Keithley 236 Source
Measure Unit. A xenon lamp coupled with A.M.1.5 solar spec-
trum filters was used as the light source, and the optical power at
Scheme 1 The chemical structures of TPA–DCM–TPA, TPA–TV–PM
and TPA–HTV–PM.
the photovoltaic properties of the TPA- and PM-containing
molecules, we designed and synthesized two new solution-
processable TPA- and PM-containing organic molecules
TPA–TV–PM and TPA–HTV–PM (Scheme 1). We changed the
styrene to thienylenevinylene (TV) and 4-hexyl-thienyleneviny-
lene (HTV) linkages in TPA–TV–PM and TPA–HTV–PM,
respectively, to obtain better solubility, charge transporting and
film-forming properties. The hexyl groups in TPA–HTV–PM are
for further improving the solubility and film-forming properties
of the molecule. The two compounds show good solubility in
common organic solvents, broad absorption from 350 nm to 650
nm and lower HOMO energy levels of ca. ꢀ5.15 eV. The OSC
devices based on the compounds as the donor and PC70BM as
the acceptor (1 : 3, w/w) demonstrate a PCE higher than 2%,
under the illumination of AM.1.5, 100 mW cmꢀ2, which is
significantly improved in comparison with a PCE of 1.40% for
the OSCs based on TPA–DCM–TPA : PC70BM under the same
experimental conditions.
the sample was ca. 100 mW cmꢀ2
.
Synthesis
The synthesis routes to TPA–TV–PM and TPA–HTV–PM are
shown in Scheme 2. The detailed synthetic processes are as
follows.
Experimental section
2-Bromo-5-formyl-3-hexylthiophene (1). Lithium diisopropy-
lamide (22 ml, 2.0 M in hexane, 44 mmol) was added dropwise to
a stirred solution of 2-bromo-3-hexyl-thiophene (10 g, 40 mmol)
in dry THF (100 ml), under argon at ꢀ78 ꢁC. The reaction
mixture was maintained under this condition for a further 2 h
before DMF (3.39 ml, 44 mmol) was added dropwise. The
mixture was stirred at room temperature for 12 h and quenched
with hydrochloric acid (3%, 10 ml). The product was extracted
into dichloromethane (3 ꢂ 150 ml) and the combined organic
extracts were washed with brine and dried by anhydrous
magnesium sulfate. The crude product was purified by column
chromatography (petroleum ether (bp: 60–90 ꢁC)/dichloro-
methane, 10 : 1) to afford 2-bromo-5-formyl-3-hexylthiophene
Materials
2,6-Dimethyl-4-pyrone, 2-bromo-3-hexyl-thiophene, 5-bromo-2-
thiophenecarboxaldehyde, n-butyllithium (2.50 mol Lꢀ1 in
hexane), pyridine, tetrabutylammonium bromide, sodium
acetate, palladium acetate, acetic anhydride, acetonitrile and
DMF were obtained from Acros Organics. Tetrahydrofuran
(THF) was dried over Na/benzophenoneketyl and freshly
distilled prior to use. Other chemicals were common commercial
grade and were used as received.
Measurements
1
Nuclear magnetic resonance (NMR) spectra were taken on
a Bruker DMX-400 spectrometer. MALDI-TOF spectra were
recorded on a Bruker BIFLEXIII. Absorption spectra were
(1) as yellow liquid. Yield 7.8 g (70%). MS: m/z ¼ 274 (M+). H
NMR (CDCl3, 400 MHz) d/ppm: 9.75 (s, 1H), 7.46 (s, 1H), 2.62
(t, 2H), 1.61 (m, 2H), 1.36 (m, 6H), 0.85 (t, 3H).
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N,N-diphenyl-N-(4-vinylphenyl)amine (5). To a solution of
methyltriphenylphosphonium bromide (7.19 g, 20 mmol) in dry
ꢁ
THF (250 ml) under argon at ꢀ78 C was added n-butyllithium
(8 ml, 2.5 M, 20 mmol) by syringe. The mixture was warmed to
room temperature slowly. Compound 4 (5 g, 18 mmol) in dry THF
(50ml)wasaddeddropwisetothemixtureandtheresultantmixture
was kept at room temperature for 12 h. The mixture was poured
into water and extracted into dichloromethane (3 ꢂ 150 ml). The
combined organic extracts were washed with brine and dried over
anhydrousmagnesiumsulfate. After evaporation of the solvent, the
crude product was purified by column chromatography (hexane) to
get compound 5 as a white solid. Yield 4.33 g (87%). MS: m/z ¼ 271
(M+). 1H NMR (CDCl3, 400 MHz) d/ppm: 7.25 (m, 6H), 7.08 (d,
4H), 6.99 (d, 4H), 6.67 (q, 1H), 5.62 (d, 1H), 5.14 (d, 1H).
TPA–TV–PM. A mixture of compound 2 (1 g, 1.9 mmol), 5
(1.13 g, 4.18 mmol), palladium (II) acetate (15 mg), tetra-n-
butylammonium bromide (198 mg, 0.6 mmol), and anhydrous
sodium acetate (3.2 g, 38 mmol) were dissolved and kept in
degassed anhydrous DMF (30 ml) under argon at 100 ꢁC for 24
h. The mixture was poured into water (30 ml). The precipitate
was filtered, washed with water, dissolved in dichloromethane
and dried over anhydrous sodium sulfate. After evaporation of
the solvent, the residue was purified by column chromatography
(petroleum/dichloromethane, 1 : 1) to get TPA–TV–PM. Yield
500 mg (29%). MALDI-TOF MS: 898.5, Calcd for C60H42N4OS2
Scheme 2 Synthesis routes to the compounds: (I) CNCH2CN, acetic
anhydride, N2, reflux at 140 ꢁC for 24 h; (II) and (III) CH3CH2CN,
piperidine, N2, reflux at 80 ꢁC for 24 h; (IV) POCl3, DMF, N2, 45 ꢁC for 2
h; (V) MePh3PBr, n-butyllithium, THF, N2, ꢀ78 ꢁC; (VI) Pd(OAc)2,
NaOAc, n-Bu4NBr, DMF, N2, 100 ꢁC for 24 h.
2-(2,6-Bis(2-(4-bromothienyl)vinyl)pyran-4-ylidene)-malononi-
trile (2). A mixture of 2-(2,6-dimethylpyran-4-ylidene)-malono-
nitrile (2.44 g, 14 mmol), 5-bromo-2-thiophenecarboxaldehyde
(5.8 g, 30.5 mmol), piperidine (2 ml) and freshly distilled aceto-
nitrile (60 ml) were refluxed at 80 ꢁC under argon for 24 h. After
cooling the reaction mixture to room temperature, the dark red
precipitate was purified by recrystallization from methanol to get
compound 2 as a brown solid. Yield 6.7 g (84.8%). MS: m/z ¼
516 (M+). 1H NMR (CDCl3, 400 MHz) d/ppm: 7.44 (q, 2H), 7.09
(d, 2H), 7.06 (s, 2H), 6.92 (d, 2H), 6.41 (q, 2H).
1
899.13. H NMR (CDCl3, 400 MHz) d/ppm: 7.36 (d, 4H), 7.29
(m, 18H), 7.21 (s, 2H), 7.13 (d, 10H), 7.06 (d, 4H), 7.00 (d, 2H),
6.79 (d, 2H).13C NMR (CDCl3, 400 MHz): d/ppm 158.04, 148.18,
147.28, 140.14, 138.20, 132.14, 130.78, 130.52, 130.00, 129.39,
129.04, 128.48, 128.23, 127.63, 126.81, 125.30, 124.88, 123.48,
122.92, 119.18, 117.54, 116.68, 115.43, 106.74, 58.49.
TPA–HTV–PM. A mixture of compound 3 (1 g, 1.45 mmol), 5
(0.869 g, 3.2 mmol), palladium (II) acetate (10 mg), tetra-n-
butylammonium bromide (150 mg, 0.46 mmol), and anhydrous
sodium acetate (2.39 g, 29 mmol) was dissolved and kept in
degassed anhydrous N,N-dimethyl formamide (30 ml) under
argon at 100 ꢁC for 24 h. The mixture was poured into water
(30 ml). The precipitate was filtered, washed with water,
dissolved in dichloromethane and dried over anhydrous sodium
sulfate. After evaporation of the solvent, the residue was purified
by column chromatography (petroleum/dichloromethane, 2 : 1)
to get TPA–HTV–PM. Yield 650 mg (42%). MALDI-TOF MS:
2-(2,6-Bis(2-(4-bromo-3-hexylthienyl)vinyl)pyran-4-ylidene)-
malononitrile (3). 2-(2,6-dimethylpyran-4-ylidene)-malononitrile
(2.44 g, 14 mmol), compound 1 (7.8 g, 28.4 mmol), piperidine
(2 ml) and freshly distilled acetonitrile (28.4 ml) were mixed
under argon and refluxed at 80 ꢁC under argon for 24 h. The
reaction mixture was cooled to room temperature, dissolved in
dichloromethane, washed with brine, and dried by anhydrous
magnesium sulfate. After evaporation of the solvent, the residue
was purified by column chromatography (petroleum/dichloro-
methane, 5 : 1) to produce compound 3 as a red solid. Yield 5 g
(55.56%). MALDI-TOF MS: 686.6, calcd for C32H34Br2N2OS2
1
1066.7, calcd for C72H66N4OS2 1067.45. H NMR (CDCl3, 400
1
MHz) d/ppm: 7.37 (d, 4H), 7.29 (s, 2H), 7.27 (d, 8H), 7.13
686.56. H NMR (CDCl3, 400 MHz) d/ppm: 7.41 (d, 2H), 7.00
(d, 8H), 7.09 (d, 4H), 7.05 (d, 8H), 6.93 (d, 2H), 6.58 (s, 2H), 6.45
(d, 2H), 2.67 (t, 4H), 1.64 (m, 4H), 1.45 (m, 12H), 0.91 (t, 6H). 13
(s, 2H), 6.61 (s, 2H), 6.40 (d, 2H), 2.56 (t, 4H), 1.6 (t, 4H), 1.33
(m, 12H), 0.90 (t, 6H).
C
NMR (CDCl3, 400 MHz): d/ppm 158.14, 155.47, 148.11, 147.45,
142.11, 141.48, 136.70, 134.30, 130.65, 130.45, 130.25, 129.52,
127.68, 124.93, 123.55, 123.22, 117.67, 116.69, 115.71, 106.81,
58.88, 31.84, 30.91, 29.21, 28.52, 22.77, 14.27.
4-Formyltriphenylamine (4). Phosphorus oxychloride (14.7 ml)
was added dropwise into a stirred solution of triphenylamine
ꢁ
(5 g, 20 mmol) in DMF (4 ml), under argon at 0 C. Then, the
reaction mixture was stirred at 45 ꢁC for 2 h, poured into ice
water, and neutralized with sodium hydroxide (2 M). The crude
product was filtered and recrystallized from ethyl acetate to
afford 4-formyltriphenylamine (4) as the white solid. Yield 3 g
(54%). MS: m/z ¼ 273 (M+). 1H NMR (CDCl3, 400 MHz) d/ppm:
9.80 (s, 1H), 7.68 (d, 2H), 7.34 (m, 4H), 7.17 (m, 6H), 6.90
(d, 2H).
Results and discussion
Synthesis and thermal stability of the compounds
TPA–TV–PM and TPA–HTV–PM were synthesized by the
routes shown in Scheme 2. First, 2-(2,6-bis(2-(4-bromothienyl)-
vinyl)pyran-4-ylidene)-malononitrile (2) and 2-(2,6-bis(2-(4-
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Fig. 1 A TGA plot of TPA–TV–PM and TPA–HTV–PM.
bromo-3-hexylthienyl)vinyl)pyran-4-ylidene)-malononitrile (3)
were synthesized according to the method reported in the liter-
ature.28 Compound 5 was prepared by Wittig reaction. Then,
TPA–TV–PM and TPA–HTV–PM were synthesized from
compound 2 and 5, and 3 and 5, respectively by a palladium-
catalyzed Heck reaction.24 TPA–TV–PM and TPA–HTV–PM
are soluble in common organic solvents, such as CHCl3, THF,
chlorobenzene and toluene.
The thermal stability of TPA–TV–PM and TPA–HTV–PM
was investigated by thermogravimetric analysis (TGA). The
ꢁ
temperatures with 5% weight loss are 398 C for TPA–TV–PM
and 355 ꢁC for TPA–HTV–PM, as shown in Fig. 1. The stability
of TPA–TV–PM and TPA–HTV–PM is good enough for
applications in optoelectronic devices.
Fig. 2 (a) Absorption spectra of TPA–DCM–TPA, TPA–TV–PM and
TPA–HTV–PM in chloroform solution at a concentration of 10ꢀ5mol
Lꢀ1; (b) absorption spectra of TPA–TV–PM and TPA–HTV–PM films.
Absorption spectra
absorption peaks (see Fig. 2b) red shifted a little for both of the
molecules, indicating that some aggregation of the molecules in
the film state exists. The absorption edge of TPA–TV–PM and
TPA–HTV–PM films are at ca. 681 nm and ca. 707 nm, corre-
sponding to band gaps of 1.82 eV and 1.75 eV, respectively
(Table 1).
Fig. 2 shows the UV-vis absorption spectra of TPA–TV–PM and
TPA–HTV–PM in chloroform solution and the film state. The
optical absorption maxima (l) are summarized in Table 1. Fig. 2a
shows the UV-vis absorption spectra of TPA–DCM–TPA, TPA–
TV–PM and TPA–HTV–PM in dilute chloroform solution with
a concentration of 10ꢀ5 mol Lꢀ1. Benefiting from their D–A
molecular structure connected with a conjugated pi-bridge, the
absorption spectra of TPA–TV–PM and TPA–HTV–PM solu-
tions (see Fig. 2a) exhibit two absorption peaks in the wavelength
range 350–650 nm. The absorption peak with a maximum at 420
nm for TPA–TV–PM and at 422 nm for TPA–HTV–PM corre-
sponds to the p–p* absorption of the molecules, and the
absorption band at longer wavelength with a peak at 530 nm for
TPA–TV–PM and at 538 nm for TPA–HTV–PM could be
assigned to the intramolecular charge transfer transition between
the TPA moiety and the PM unit.13,22 Compared with the
absorption peak at 466 nm of TPA–DCM–TPA solution for the
intramolecular charge transfer absorption, TPA–TV–PM (530
nm) and TPA–HTV–PM (538 nm) red shifted ca. 60–70 nm,
which implies that the thienylenevinylene and 4-hexyl-thienyle-
nevinylene groups are better conjugated pi-bridges than styrene
in the molecules, from the view point of broadening the
absorption. The molar absorptivity of TPA–DCM–TPA, TPA–
TV–PM and TPA–HTV–PM at their peak wavelength is 9.13 ꢂ
104, 8.19 ꢂ 104 and 8.09 ꢂ 104, respectively, which demonstrates
that all three of the molecules exhibit strong absorbance in the
visible region. In comparison with the solution, the film
Electrochemical properties and electronic energy levels
Electrochemical cyclic voltammetry was carried out for TPA–
TV–PM and TPA–HTV–PM films on Pt electrode, in order to
measure the HOMO and LUMO energy levels of the compounds.
Fig. 3 shows the cyclic voltammograms of TPA–TV–PM and
TPA–HTV–PM films in the positive potential range, showing the
p-doping/dedoping processes of the compounds. The onset
oxidation potentials (Eoxonset) of TPA–TV–PM and TPA–HTV–
PM are 0.45 V and 0.44 V vs. Ag/Ag+ respectively, obtained from
the cyclic voltammograms. Then the HOMO energy levels were
estimated to be ꢀ5.16 eV for TPA–TV–PM and ꢀ5.15 eV for
TPA–HTV–PM, according to the following equation: EHOMO
¼
ꢀe(Eox
+ 4.71) (eV).29 The LUMO energy levels of the
onset
compounds were estimated from LUMO ¼ HOMO + Eg. The
values of the onset oxidation potential, HOMO and LUMO
energy levels of the two compounds were also listed in Table 1. The
two compounds of TPA–TV–PM and TPA–HTV–PM exhibit
slightly lower-lying HOMO energy levels than that (ꢀ5.14 eV) of
TPA–DCM–TPA,22 although their band gap is lowered. The
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Table 1 Absorption and electrochemical properties and electronic energy levels of TPA–DCM–TPA, TPA–TV–PM and TPA–HTV–PM
Eoxonset (Va)/HOMO(eV)
LUMOb (eV)
opt
(Eg )film (eV)
Compounds
lsolution (nm)
3 (L molꢀ1 cmꢀ1
)
lfilm (nm)
TPA–DCM–TPAc
TPA–TV–PM
TPA–HTV–PM
393, 466
420, 530
422, 538
9.13 ꢂ 104
8.19 ꢂ 104
8.09 ꢂ 104
411, 498
432, 536
433, 540
1.88
1.79
1.71
0.75/ꢀ5.14
0.45/ꢀ5.16
0.44/ꢀ5.15
ꢀ2.76
ꢀ3.37
ꢀ3.44
a
b
c
opt
. The
The reference electrode is Ag/Ag+. LUMO energy level was calculated from the HOMO level and optical energy gap: LUMO ¼ HOMO + Eg
data were taken from Ref. 22.
lower HOMO energy level of the organic molecules is very
important for the higher Voc of the OSCs based on the molecules
as donor materials, because the Voc of the OSCs is proportional to
the difference between the LUMO level of acceptor and the
HOMO level of the donor.30
the OSCs in the dark and under the illumination of AM.1.5, 100
mW cmꢀ2. The photovoltaic performance data of the OSCs,
including open circuit voltage (Voc), short circuit current density
(Jsc), fill factor (FF) and power conversion efficiency (PCE)
values, are summarized in Table 2. For TPA–DCM–TPA, the
device with PC60BM as acceptor material, Ba/Al as cathode and
a donor : acceptor weight ratio of 1 : 3 showed a Voc of 0.9 V,
a Jsc of 2.14 mA cmꢀ2, a FF of 29%, corresponding to a PCE of
0.79%.22 In comparison, the OSC devices based on TPA–DCM–
TPA : PC70BM (1 : 3, w/w) exhibited a Voc of 0.71 V, Jsc of 5.07
mA cmꢀ2, a FF of 38% and a PCE of 1.4%, which implied that
PC70BM is a better acceptor than PC60BM in the OSCs for its
stronger absorption in the wavelength range 350–550 nm.
The OSC device based on the TPA–TV–PM : PC70BM (1 : 3,
w/w) blend showed a Voc of 0.79 V, a Jsc of 5.94 mA cmꢀ2, a FF of
44% and a PCE of 2.06%, and the OSC device based on the
Morphology of the compound : PC70BM films
The morphology of the photoactive layer is very important for
the photovoltaic performance of OSCs.31,32 We used atomic force
microscopy (AFM) to investigate the morphology of the two
organic molecules : PC70BM (1 : 3, w/w) blend films. TPA–TV–
PM : PC70BM and TPA–HTV–PM : PC70BM blend films were
prepared by spin-coating their chlorobenzene solution on the top
of the PEDOT : PSS layer, which was spin-coated on the ITO
glass. Both of the AFM images of the blend films exhibit typical
amorphous morphology without any crystalline domains, as
shown in Fig. 4. We can see intuitively that the surfaces of the
blend films of TPA–TV–PM and TPA–HTV–PM are smooth
and uniform, which is beneficial for application in OSCs.
Photovoltaic properties
Bulk-heterojunction OSCs were fabricated by using the two
linear organic molecules as the donor materials, PC70BM as the
acceptor material33–35 and Al as the negative electrode. For
comparison, we also fabricated bulk-heterojunction OSCs based
on the TPA–DCM–TPA : PC70BM (1 : 3, w/w) blend. The
device structure is ITO/PEDOT : PSS/photoactive layer/Al.
Fig. 5 shows the typical current density–voltage (J–V) curves of
Fig. 4 AFM images of the molecule : PC70BM films spin-coated from
their solution at a weight ratio of 1 : 3: (a) TPA–TV–PM : PC70BM film,
(b) TPA–HTV–PM : PC70BM film, the scan size of the images is 5 mm ꢂ
5 mm.
Fig. 3 Cyclic voltammograms of TPA–TV–PM and TPA–HTV–PM
films on Pt electrode in an acetonitrile solution of 0.1 mol Lꢀ1 Bu4NPF6
(Bu ¼ butyl) with a scan rate of 100 mV sꢀ1
.
3772 | J. Mater. Chem., 2011, 21, 3768–3774
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ꢀ5.15 eV. The OSC devices were fabricated by spin-coating the
blend solution of the organic molecules as the donor and
PC70BM as the acceptor (1 : 3, w/w). The OSCs based on TPA–
TV–PM and TPA–HTV–PM showed power conversion effi-
ciencies (PCE) of 2.06% and 2.10%, respectively, under the illu-
mination of AM.1.5, 100 mW cmꢀ2, which are much higher than
the 1.40% PCE for the OSC based on
a TPA–DCM–
TPA : PC70BM (1 : 3, w/w) blend. The results indicate that in the
TPA- and PM-containing D–A molecules, thienylenevinylene
and 4-hexyl-thienylenevinylene as conjugated pi-bridges in TPA–
TV–PM and TPA–HTV–PM are superior to styrene in TPA–
DCM–TPA for the application as donor materials in the solu-
tion-processed OSCs. Moreover, the D–A structured molecules
could be promising organic photovoltaic materials by careful
molecular design.
Fig. 5 Current density-voltage characteristics of the OSC devices based
on the blend of TPA–DCM–TPA, TPA–TV–PM or TPA–HTV–PM/
PC70BM (1 : 3, w/w) with Al as the negative electrode at dark and under
the illumination of AM.1.5, 100 mW/cm2.
Acknowledgements
This work was supported by NSFC (Nos. 50633050, 50803071,
20874106 and 20721061).
Table 2 The photovoltaic performance of ITO/PEDOT : PSS/TPA–
DCM–TPA, TPA–TV–PM or TPA–HTV–PM : PC70BM (1 : 3, w/w)/Al
under the illumination of AM.1.5, 100 mW cmꢀ2
Voc
(V)
Jsc
(mA cmꢀ2
FF
(%)
PCE
(%)
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