Carbazole-based small molecules for vacuum-deposited organic photovoltaic devices with open-circuit voltage exceeding 1?V

Article abstract of DOI:10.1016/j.orgel.2017.05.007

Fabrication of vacuum deposited small molecules organic solar cell with open-circuit voltage (V_oc) exceeding 1?V is crucial in advancing the applications of organic photovoltaics (OPVs). Here, a novel carbazole-based donor-π bridge-acceptor (D-

Full text of DOI:10.1016/j.orgel.2017.05.007

Accepted Manuscript
Carbazole-based small molecules for vacuum-deposited organic photovoltaic devices
with open-circuit voltage exceeding 1 V
Yuan Jay Chang, Jung-Lien Hsu, Yi-Hua Li, Sajal Biring, Tzu-Hung Yeh, Jhih-Yan
Guo, Shun-Wei Liu
PII:
S1566-1199(17)30198-2
10.1016/j.orgel.2017.05.007
ORGELE 4078
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 14 February 2017
Revised Date: 12 April 2017
Accepted Date: 4 May 2017
Please cite this article as: Y.J. Chang, J.-L. Hsu, Y.-H. Li, S. Biring, T.-H. Yeh, J.-Y. Guo, S.-W. Liu,
Carbazole-based small molecules for vacuum-deposited organic photovoltaic devices with open-circuit
voltage exceeding 1 V, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.05.007.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Carbazole-based p-type donor molecules for vacuum-deposited organic photovoltaic
devices exhibited the open-circuit voltage exceeding 1 V

ACCEPTED MANUSCRIPT
Carbazole-based small molecules for vacuum-deposited organic
photovoltaic devices with open-circuit voltage exceeding 1 V
Yuan Jay Changa,*, Jung-Lien Hsua, Yi-Hua Lib, Sajal Biringb, Tzu-Hung Yehb,c,
Jhih-Yan Guob, and Shun-Wei Liub,*
a Department of Chemistry, Tunghai University, Taichung 40704, Taiwan
b Department of Electronic Engineering and Organic Electronics Research Center, Ming Chi
University of Technology, New Taipei City 24301, Taiwan
cDepartment of Electronic Engineering, National Taiwan University of Science and Technology,
Taipei 10607, Taiwan
*Corresponding author.
E-mail address: jaychang@thu.edu.tw (Y. J. Chang) and swliu@mail.mcut.edu.tw (S. -W. Liu)
Abstract
Fabrication of vacuum deposited small molecules organic solar cell with
open-circuit voltage (Voc) exceeding 1V is crucial in advancing the applications of
organic photovoltaics (OPVs). Here,
a
novel carbazole-based donor-π
bridge-acceptor (D-π-A) of p-type material (F-series) in combination with fullerene
derivative C60 or C70 as n-type material for bulk-heterojunction OPVs with the
structure of ITO/ MoO3 (15 nm)/ F-series donor: C60 or C70 (40 or 80 nm)/BCP (7
nm)/ Ag (120 nm) have been proposed. The vacuum deposited small molecules OPV
with the donor layer consisting of F1 combined with the electron acceptor C70
exhibits a high power conversion efficiency (PCE) of 4.93%. The higher PCE of the
OPV is attributed to the large VOC value of 1.02 V. The analysis of photophysical
properties using a time-dependent density functional theory model and the B3LYP
functional corroborates the experimental results and provides the evidence on
increasing the Voc of OPVs.
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Keywords: Organic photovoltaics, small-molecule organic solar cells, planar mixed
heterojunction, highly open-circuit voltage
1. Introduction
The global energy demand is raised rapidly in current year [1-2]. Combustion of fossil
fuels produces large quantities of greenhouse gases, which may have catastrophic
consequences resulting in global warming. Therefore, there is an urge to investigate and
develop renewable and environmental friendly energy source. Especially solar energy
has tremendous potential as alternative energy resource. In the past two decades,
compared with silicon-based photovoltaic devices, the organic photovoltaics (OPVs)
have received considerable attention due to its ease of fabrication, extended absorption
range by tuning structure, colorful transparency, compatibility with flexible substrates,
large area processing, and low fabrication cost of device [3-5]. A general notion in
OPVs research field regarding the successful improvement of power conversion
efficiency (PCE) is to use a p-type conjugated polymer as a donor and the fullerene
derivative PC61BM as an acceptor. Currently, PCE of polymer-based solar cells have
been reached over 10% using solution processed bulk-heterojunction (BHJ) with
tandem cell structure [6-8].
The small molecules can receive the predominant advantages over the polymeric
materials having well-defined molecular structures, easier purification, higher reliability
and better reproducibility. Therefore, enormous research works have been carried out
with small-molecule OPVs, which exhibit appreciable PCEs of > 6% using
vacuum-deposition fabrication techniques [9-11]. Although, in general, the solution
processing is considered to be more cost-effective compared to vacuum deposition, but
solution processing creates leakage paths between anode and cathode due to either
p-type or n-type phase continuity arising from spin-coating of the solvent to form the
film [12-13]. On the other hand, vacuum deposition technique of OPVs shows higher
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potential availing the advantage of easy fabrication of multilayered tandem architectures.
Of late, a tandem cell of OPV device with a PCE of up to 11% has been reported [14].
Recently, researches on new small molecule-based p-type material in OPVs have
developed a large variety of promising organic materials with different donor-acceptor
moiety combinations. For example, symmetrical acceptor-donor-acceptor (A-D-A),
[15-19] donor-acceptor-donor (D-A-D), [20-22] unsymmetrical D-A, [23-25] and
D-A-A´ [9-10, 26-27] systems have been synthesized with PCE approaching those of
polymeric system. A wide variety of prominent chemical structures of photovoltaic
materials with vacuum processed deposition have been reported in the literatures
including the oligothiophene, [15, 28-29] phathalocyanines (Pc), [30]
subphathalocyanines (SubPc), [31-34] squaraine, [35-37] and triarylamine [38-40].
Although the device performance is improved continuously by using unique property
of materials and fabrication architectures of device, but it is still inadequate in meeting
commercial requirements [41-42]. The previous reports have focused on the
panchromatic absorption range and photocurrent output using various active materials
[38, 43-45] and device structures [46-48]. However, one of the crucial limitations for
device performance is the low value of open-circuit voltage (Voc). Carbazole structure
possesses weak electron donating ability which is one of curial factors for designing
higher HOMO energy level of p-type material in order to achieve high Voc value in
organic photovoltaics devices.
To date, most of the previous publications have reported Voc value in solution
processed devices using the symmetry structure based on carbazole core as p-type or
n-type materials [49-54]. In this work, we have used carbazole as electron donating
moiety with four acceptors applying D-π-A dipolar system as p-type material in
photovoltaic solar cells. We have fabricated a highly efficient OPVs device with
ITO/MoO3/Active layer/ BCP/Ag by vacuum deposition technique. The active layer
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consisting of F1 combined with the electron acceptor C70 exhibits a high PCE of 4.93%,
which is attributed to the higher Voc value of 1.02 V.
2. Experimental Section
2.1 General Information
All reactions and manipulations were performed in a nitrogen atmosphere. Solvents
1
13
were distilled freshly according to standard procedures. H and C NMR spectra were
recorded on a Bruker (AV 400/AVIII HD 400/AV 500 MHz) spectrometer, and CDCl3
was used as solvents. Chemical shifts were reported on the
δ scale downfield from the
tetramethylsilane peak. Absorption spectra were recorded on a SHIMADZU UV-1800
and Jasco V-630 spectrophotometer, and emission spectra were obtained using a Hitachi
F-4500 spectrofluorimeter. Emission spectra in solutions were measured using a spectral
grade solvent and right-angle detection. For material analysis, the photoelectron
spectrometer (AC-2, Riken Keiki) was used to measure the ITO work function and the
HOMO level of the organic materials. Mass spectra were recorded on a JEOL JMS-700
double-focusing mass spectrometer. The surface morphology of the materials was
characterized by the non-contact mode atomic force microscopy (AFM).
Thermogravimetric analysis (TGA; Pyris 1 TGA Lab system) measurements were
carried out with heating rate of 10 °C/min under a nitrogen gas atmosphere.
The chemicals N-Bromosuccinimide (NBS), trans-Dichlorobis(triphenylphosphine)
palladium(II) (PdCl2(PPh3)2), 2-(Tributyl-stannyl)thiophene, n-Butyllithium (1.6 M in
hexane), N,N’-Dimethylformamide, 4-Bromobenzaldehyde, tri-n-Butyltin chloride,
Malononitrile, 1,3-Indandione, Ammonium acetate, and Piperidine were purchased from
Acros, Alfa, Merck, Lancaster, TCI, Sigma-Aldrich, and Showa, separately, and purified
when necessary. Chromatographic separations were performed using silica gel from
Merck (Kieselgel Si 60; 40–63 ꢀm).
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2.2 Fabrication and Characterization of Bulk-Heterojunction OPVs
Devices were fabricated as described using the basic method described in our
pervious work [34, 55]. The bulk-heterojunction OPVs devices was consisted of
F-series organic dyes and fullerene derivative C60 or C70 as the p-type and n-type
materials, MoO3 as hole transporting material, and BCP as the buffer between the n-type
material and the cathode. All materials were purified by sublimation before use. The
patterned indium-tin-oxide (ITO) substrates as the anode with a sheet resistance of 10
ꢁ/sq were purchased from Lumtec. The ITO substrates were cleaned by detergent,
deionized water, acetone, and isopropanol in sequence, and subsequently the substrates
were transferred to a vacuum chamber of the thermal evaporator, and the organic active
materials were deposited in the sequence of ITO/ MoO3 (15 nm)/ F-series donor:C60 or
C70 (40 or 80 nm)/BCP (7 nm)/ Ag (120 nm) under the pressure of 6*10-6 Torr. The
constant deposition rates of MoO3 was 0.03 nm/s, and the other organic materials were
controlled within 0.1~0.2 nm/s. A cathode material of Ag was deposited in the chamber
through a shadow mask to define active regions of the devices (4 mm2) by the
overlapping area between ITO/organic layer/Ag. After the evaporation, the devices were
transferred to a grove box and go encapsulation process under the N2 environment to
prevent damage from the oxygen and moisture.
The J-V characteristics of the OPVs devices were measured with a current source
meter (Keithley 2400) under dark conditions and under the illumination of a 1-sun AM
1.5G solar simulator (91160, Newport), which was calibrated through a Si reference cell.
For measuring the external quantum efficiency (EQE) spectra, the AM 1.5G solar
simulator was used to generate the bias light. A monochromator, which was calibrated
with a photodiode and chopped at 250 Hz, was used to select wavelengths between 350
and 800 nm to illuminate the OPVs. The photocurrent of the OPVs was measured
through the lock-in amplifier, which was referenced to the chopper frequency. After
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measuring the devices, the results of the EQE spectra were integrated to obtain the Jsc of
the devices and were subsequently compared with the Jsc, which were measured under
illumination of the solar simulator.
2.3 Synthesis of dyes
The structures of carbazole-based organic dyes and their synthetic sequences are
presented in Fig. 1 and Scheme 1.
2.3.1 9-(4-tert-Butylphenyl)-3-bromo-9H-carbazole (2a).
A mixture of 1a (4.9 g, 16.38 mmol) in dry CH2Cl2 was placed in a three-necked
flask under a nitrogen atmosphere. Following this, added dropwise NBS (2.91 g, 16.38
mmol) in CH2Cl2 to this reaction and stirred 6 hours room temperature. The mixture
was quenched by adding DI water, and the mixture extracted with CH2Cl2. The organic
layer was dried over anhydrous MgSO4 and evaporated under vacuum. The product was
purified by silica gel column chromatograph with hexane as eluent. White solid of 2a
was obtained in 78% yield (4.81 g, 12.77 mmol). 1H NMR (400 MHz, CDCl3): δ 8.28 (s,
1H), 8.11 (d, 1H, J = 8.0 Hz), 7.63 (d, 2H, J = 8.4 Hz), 7.51-7.44 (m, 5H), 7.33-7.30 (m,
13
2H), 1.46 (s, 9H). C NMR (100 MHz, CDCl3): δ 150.8, 141.3, 139.6, 134.4, 128.4,
126.8, 126.6, 126.5, 124.9, 122.9, 122.1, 120.4, 120.1, 112.4, 111.4, 110.1, 34.8, 31.4.
MS (FAB, 70 eV): m/z (relative intensity) 377 (M+, 100); HRMS calcd for C22H20N79Br:
377.0779, found 377.0773.
2.3.2 3-Bromo-9-(4-(dodecyloxy)phenyl)-9H-carbazole (2b).
Compound 2b was synthesized according to the same procedure as that of 2a. Yellow
1
solid was obtained in 50% yield. H NMR (400 MHz, CDCl3): δ 8.26 (s, 1H), 8.10 (d,
1H, J = 7.72 Hz), 7.44-7.50 (m, 4 H), 7.41 (d, 2H, J = 8.68 Hz), 7.33 (d, 1H, J = 8.44
Hz), 7.29 (d, 1H, J = 7.76 Hz), 7.21 (d, 1H, J = 8.64 Hz), 7.12 (d, 2H, J = 8.76 Hz),
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4.07 (t, 2H, J = 6.48 Hz), 1.84-1.90 (m, 2 H), 1.50-1.56 (m, 2 H), 1.32-1.44 (m, 16 H),
13
0.92 (t, 3H, J = 6.96 Hz); C NMR (100 MHz, CDCl3): δ 158.7, 141.7, 140.0, 129.5,
128.5, 128.4, 126.5, 124.8, 122.9, 122.0, 120.4, 120.0, 115.6, 112.3, 111.2, 109.9, 68.4,
31.9, 29.7, 29.6, 29.4, 29.3, 29.2, 26.0, 22.7, 14.1. MS (FAB, 70 eV): m/z (relative
intensity) 505 (M+, 100); HRMS calcd for C30H36BrNO: 505.1975, found 505.1958.
2.3.3 9-(4-tert-Butylphenyl)-3-(thiophen-2-yl)-9H-carbazole (3a).
To a three-necked flask containing a mixture of 2a (4.5 g, 11.93 mmol), PdCl2(PPh3)2
(0.26 g, 0.36 mmol), 2-tributylstannylthiophene (10.2 mL, 27.44 mmol) was added
DMF (30 mL). The reaction mixture was stirred at 90 °C for 24 h. After cooling, the
reaction was quenched by adding methanol and KF(aq) (saturated 45 mL). The mixture
was extracted with CH2Cl2 and the organic layer dried over anhydrous MgSO4.
Evaporation of the solvent gave the crude, which was purified by silica gel with
CH2Cl2/hexane (1/3) as eluent. Yellow solid of 3a was obtained in 82% (3.73 g, 9.78
1
mmol). H NMR (400 MHz, CDCl3): δ 8.38 (s, 1H), 8.20 (d, 1H, J = 7.6 Hz), 7.69 (d,
1H, J = 8.8 Hz), 7.64 (d, 2H, J = 8.4 Hz), 7.51 (d, 2H, J = 8.4 Hz), 7.45-7.42 (m, 3H),
13
7.41 (d, 1H, J = 3.6 Hz), 7.34-7.28 (m, 2H), 7.14 (t, 1H, J = 4.0 Hz), 1.46 (s, 9H). C
NMR (100 MHz, CDCl3): δ 150.6, 145.6, 141.5, 140.5, 134.7, 127.9, 126.7, 126.6,
126.5, 126.1, 124.5, 123.7, 123.7, 123.1, 122.2, 120.4, 119.9, 117.7, 110.2, 110.1, 34.8,
31.4. MS (FAB, 70 eV): m/z (relative intensity) 381 (M+, 100); HRMS calcd for
C26H23NS: 381.1551, found 381.1554.
2.3.4 9-(4-(Dodecyloxy)phenyl)-3-(thiophen-2-yl)-9H-carbazole (3b)
Compound 3b was synthesized according to the same procedure as that of 3a. Green
1
solid of 3b was obtained in 80% yield. H NMR (400 MHz, CDCl3): δ 8.38 (s, 1H),
7.69 (dd, 1H, J = 8.52, 1.52 Hz), 7.42-7.47 (m, 3 H), 7.31-7.38 (m, 4 H), 7.28 (d, 1H, J
= 6.24 Hz), 7.12-7.15 (m, 3 H), 4.08 (t, 2H, J = 6.48 Hz), 1.87-1.90 (m, 2 H), 1.50-1.55
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(m, 2 H), 1.32-1.43 (m, 16 H), 0.92 (t, 3H, J = 6.92 Hz); 13C NMR (100 MHz, CDCl3):
δ 158.5, 145.7, 141.9, 140.9, 129.8, 128.4, 127.9, 126.5, 126.1, 124.5, 123.7, 123.5,
123.0, 122.1, 120.4, 119.8, 117.7, 115.6, 110.0, 109.9, 68.4, 31.9, 29.6, 29.4, 29.3, 29.2,
26.1, 22.7, 14.1. MS (FAB, 70 eV): m/z (relative intensity) 509 (M+, 100); HRMS calcd
for C34H39NOS: 509.2747, found 509.2751.
2.3.5 4-(5-(9-(4-tert-Butylphenyl)-9H-carbazol-3-yl)thiophen-2-yl)benzaldehyde (4a).
A mixture of 3a (2.5 g, 6.56 mmol) was added dropwise BuLi (6.2 mL, 9.84 mmol,
1.6 M in hexane) in dry THF at -78 °C in a three-necked flask under a nitrogen
atmosphere. After then the solution was cooled to -40 °C and was stirred by a magnetic
bar for 30 minutes. The solution was cooled again to -78 °C and to it was added
tri-n-butyltin chloride (1.7 mL, 9.84 mmol). The reaction was warmed up to room
temperature and stirred overnight. The reaction mixture was quenched by the addition
of water, and was extracted with CH2Cl2. The combined organic solution was dried over
anhydrous MgSO4, and dried under vacuum. The crude product was dissolved in dry
DMF, then to it was added 4-bromobenzaldehyde (1.21 g, 6.56 mmol) and PdCl2(PPh3)2
(148 mg, 0.2 mmol). The solution was heated to 90 °C for 24 h and then cooled. The
reaction was quenched by the addition of methanol and saturated KF(aq) (15 mL). The
mixture was extracted with CH2Cl2, while the organic layer was dried over anhydrous
MgSO4. Evaporation of the solvent gave a crude product, which was purified by silica
gel column chromatograph eluted by CH2Cl2/hexane (1/1). Yellow solid of 4a was
obtained in 75% yield. 1H NMR (400 MHz, CDCl3): δ 10.02 (s, 1H), 8.42 (s, 1H), 8.21
(d, 1H, J=7.6 Hz), 7.92 (d, 2H, J=8.4 Hz), 7.83 (d, 2H, J=8.4 Hz), 7.71 (d, 1H, J=8.4
Hz), 7.65 (d, 2H, J=8.4 Hz), 7.52-7.44 (m, 6H), 7.41 (d, 1H, J=3.6 Hz), 7.36-7.32 (m,
13
1H), 1.46 (s, 9H). C NMR (100 MHz, CDCl3): δ 191.4, 150.7, 147.4, 141.6, 140.8,
140.5, 140.3, 134.8, 134.5, 130.5, 126.8, 126.5, 126.3, 126.1, 125.9, 125.4, 124.1, 123.8,
123.4, 123.0, 120.4, 120.1, 117.6, 110.4, 110.2, 34.8, 31.4. MS (FAB, 70 eV): m/z
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(relative intensity) 485 (M+, 100); HRMS calcd for C33H27NOS: 485.1813, found
485.1814.
2.3.6
4-(5-(9-(4-(Dodecyloxy)phenyl)-9H-carbazol-3-yl)thiophen-2-yl)benzaldehyde
(4b).
Compound 4b was synthesized according to the same procedure as that of 4a. Yellow
1
solid of 4b was obtained in 71% yield. H NMR (400 MHz, CDCl3): δ 9.99 (s, 1H),
8.38 (s, 1H) , 8.17 (d, 1H, J = 7.68 Hz), 7.89 (d, 2H, J = 8.12 Hz), 7.79 (d, 2H, J = 8.2
Hz), 7.58 (dd, 1H, J = 8.56, 1.48 Hz), 7.47 (d, 1H, J = 3.8 Hz), 7.41-7.44 (m, 3H), 7.36
(d, 1H, J = 3.8 Hz), 7.28-7.34 (m, 3H), 7.10 (d, 2H, J = 8.72 Hz), 4.06 (d, 2H, J = 6.52
Hz), 1.84-1.87 (m, 2H), 1.48-1.53 (m, 2H), 1.28-1.39 (m, 16H), 0.88 (d, 3H, J = 6.08
13
Hz); C NMR (100 MHz, CDCl3): δ 191.3, 158.7, 147.5, 142.0, 141.2, 140.5, 140.3,
134.8, 130.4, 129.6, 128.4, 126.3, 126.1, 125.8, 125.4, 124.2, 123.6, 123.4, 122.9, 120.3,
120.0, 117.6, 115.6, 110.2, 110.0, 68.4, 31.9, 29.6, 29.4, 29.3, 29.2, 26.0, 22.6, 14.0; MS
(FAB, 70 eV): m/z (relative intensity) 613 (M+, 100); HRMS calcd for
C41H43NO2S:613.3009, found 613.2998.
2.3.7 2-((4-(5-(9-(4-tert-Butylphenyl)-9H-carbazol-3-yl)thiophen-2-yl)phenyl)-
methylene)malononitrile (F1).
A mixture of 4a (1.8 g, 3.71 mmol), and malononitrile (0.28 mL, 4.45 mmol) were
placed in a three-necked flask containing dry THF (20 mL) under N2. The resulted
mixture was heated to reflux for 24 hours. After cooling, the reaction was quenched by
pouring into water and it was extracted with CH2Cl2. The combined organic layer was
dried over anhydrous MgSO4 and evaporated under vacuum. The mixture was purified
by silica gel column chromatograph with CH2Cl2 as eluent. The red solid of F1 was
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1
collected in 80% yield (1.58 g, 2.96 mmol). MP: 145-147 °C. H NMR (400 MHz,
CDCl3): δ 8.34 (d, 1H, J = 1.48 Hz), 8.16 (d, 1H, J = 7.72 Hz), 7.85 (d, 2H, J = 8.52 Hz),
7.72 (d, 2H, J = 8.52 Hz), 7.65 (dd, 1H, J = 8.52, 1.84 Hz), 7.59-7.61 (m, 3H),
7.38-7.48 (m, 6H), 7.35 (d, 1H, J = 3.84 Hz ), 7.28-7.32 (m, 1H), 1.42 (s, 9H); 13C
NMR (100 MHz, CDCl3): δ 158.4, 150.8, 148.8, 141.6, 140.9, 140.5, 139.8, 134.4,
131.6, 129.2, 126.9, 126.8, 126.4, 125.5, 124.1, 123.8, 123.6, 123.0, 120.3, 120.2, 117.6,
114.1, 113.0, 110.4, 110.2, 80.6, 34.8, 31.4; MS (FAB, 70 eV): m/z (relative intensity)
533 (M+, 100); HRMS calcd for C36H27N3S: 533.1920, found 533.1911.
2.3.8 2-((4-(5-(9-(4-tert-Butylphenyl)-9H-carbazol-3-yl)thiophen-2-yl)phenyl)-
methylene)-2H-indene-1,3-dione (F2).
Compound F2 was synthesized according to the same procedure as that of F1. Red
solid of F2 was obtained in 76% yield. MP: 176-178 °C. 1H NMR (400 MHz, CDCl3): δ
8.53 (d, 2H, J = 8.36 Hz), 8.38 (d, 1H, J = 1.44 Hz), 8.20 (d, 1H, J = 7.88 Hz),
7.98-8.03 (m, 2H), 7.86 (s, 1H), 7.77-7.80 (m, 4H), 7.64 (d, 2H, J = 8.52 Hz),
7.49-8.7.52 (m, 3H), 7.42-7.45 (m, 3H), 7.38 (d, 1H, J = 3.92 Hz), 7.31-7.35 (m, 1H),
13
1.46 (s, 9H); C NMR (100 MHz, CDCl3): δ 190.4, 189.1, 150.7, 147.4, 146.0, 142.5,
141.6, 141.0, 140.8, 140.0, 139.1, 135.2, 135.1, 134.9, 134.6, 131.9, 128.3, 126.8, 126.5,
126.3, 126.2, 126.0, 125.1, 124.1, 123.9, 123.8, 123.5, 123.2, 123.1, 120.4, 120.1, 117.6,
110.3, 110.1, 34.8, 31.4. MS (FAB, 70 eV): m/z (relative intensity) 613 (M+, 100);
HRMS calcd for C42H31NO2S: 613.2076, found 613.2083.
2.3.9 2-(2-tert-Butyl-6-((E)-2-(4-(5-(9-(4-tert-butylphenyl)-9H-carbazol-3-yl)-
thiophen-2-yl)phenyl)vinyl)-4H-pyran-4-ylidene)malononitrile (F3).
A mixture of 4a (1.8 g, 3.71 mmol), 2-(2-tert-Butyl-6-methyl-4H-pyran-4-ylidene)-
malononitrile (794 mg, 3.71 mmol), and piperidine (0.41 mL, 4.08 mmol) were placed
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in a three-necked flask containing dry acetonitrile (20 mL) under N2. The resulted
mixture was heated to reflux for 24 hours. After cooling, the reaction was quenched by
pouring into water and it was extracted with CH2Cl2. The combined organic layer was
dried over anhydrous MgSO4 and evaporated under vacuum. The mixture was purified
by silica gel column chromatograph with CH2Cl2 as eluent. The red solid of F3 was
1
collected in 60% yield (1.51 g, 2.23 mmol). MP: 145-147 °C. H NMR (500 MHz,
CDCl3): δ 8.37 (d, 1H, J = 1.35 Hz), 8.16 (d, 1H, J = 7.70 Hz), 7.69 (d, 2H, J = 8.30 Hz),
7.66 (dd, 1H, J = 8.55, 1.55 Hz), 7.60 (d, 2H, J = 8.40 Hz), 7.55 (d, 2H, J = 8.30 Hz),
7.47 (d, 2H, J = 8.35 Hz), 7.40-7.41 (m, 4H), 7.28-7.37 (m, 3H), 6.72 (d, 1H, J = 15.90
Hz), 6.69 (d, 1H, J = 1.80 Hz), 6.55 (d, dH, J = 1.90 Hz), 1.42 (s, 9H), 1.38 (s, 9H); 13C
NMR (125 MHz, CDCl3): δ 172.1, 158.8, 156.5, 150.7, 146.3, 141.6, 141.1, 140.7,
137.1, 136.5, 134.6, 133.1, 128.5, 126.8, 126.5, 126.3, 126.1, 125.7, 125.1, 124.1, 123.8,
123.3, 123.0, 120.4, 120.1, 117.9, 117.5, 115.2, 110.3, 110.2, 107.2, 102.6, 36.7, 34.8,
31.4, 28.1; MS (FAB, 70 eV): m/z (relative intensity) 681 (M+, 100); HRMS calcd for
C46H39N3OS: 681.2814, found 681.2819.
2.3.10 2-(2-tert-Butyl-6-((E)-2-(4-(5-(9-(4-tert-butylphenyl)-9H-carbazol-3-yl)-
thiophen-2-yl)phenyl)vinyl)-4H-pyran-4-ylidene)-2H-indene-1,3-dione (F4).
Compound F4 was synthesized according to the same procedure as that of F3.
1
Orange solid of F4 was obtained in 66% yield. MP: 176-178 °C. H NMR (500 MHz,
CDCl3): δ 8.44 (s, 1H), 8.40 (s, 1H), 8.35 (d, 1H), 8.14 (d, 1H, J = 7.75 Hz), 7.73 (dt,
2H, J = 18.7, 3.3 Hz), 7.64-7.67 (m, 3H), 7.53-7.60 (m, 6H), 7.45 (d, 2H, J = 8.35 Hz),
7.39-7.39 (m, 3H), 7.36 (d, 2H, J = 8.60 Hz), 7.32 (d, 1H, J = 3.70 Hz), 7.25-7.28 (m,
1H), 6.85 (d, 1H, J = 15.95 Hz), 1.41 (s, 9H), 1.40 (s, 9H); 13C NMR (125 MHz,
CDCl3): δ 192.7, 173.8, 160.8, 150.6, 149.2, 146.0, 141.5, 141.3, 140.7, 140.6, 136.0,
135.5, 134.6, 133.6, 133.2, 133.1, 128.3, 126.7, 126.4, 126.2, 126.1, 125.6, 124.8, 124.1,
11

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123.7, 123.2, 123.0, 121.3, 121.2, 120.3, 120.0, 119.6, 117.4, 110.3, 110.1, 109.0, 108.7,
103.5, 36.9, 34.8, 31.4, 28.3. MS (FAB, 70 eV): m/z (relative intensity) 761 (M+, 100);
HRMS calcd for C52H43NO3S: 761.2964, found 761.2964.
2.3.11 2-((4-(5-(9-(4-(Dodecyloxy)phenyl)-9H-carbazol-3-yl)thiophen-2-yl)-
phenyl)methylene)malononitrile (F5).
Compound F5 was synthesized according to the same procedure as that of F1. Red
solid of F5 was collected in 80% yield. MP: 145-147 °C. 1H NMR (500 MHz, CDCl3):
δ 8.35 (s, 1H), 8.14 (d, 1H, J = 7.70 Hz), 7.88 (d, 2H, J = 8.45 Hz), 7.73 (d, 2H, J = 8.40
Hz), 7.64-7.66 (m, 3H), 7.48 (d, 1H, J = 3.85 Hz), 7.39-7.41 (m, 3H), 7.35 (d, 1H, J =
3.85 Hz), 7.27-7.31 (m, 3H), 7.08 (d, 2H, J = 8.70 Hz), 4.03 (t, 2H, J = 6.50 Hz),
1.81-1.84 (m, 2H), 1.47-1.51 (m, 2H), 1.22-1.38 (m, 16H), 0.86 (t, 3H, J = 7.05 Hz );
13C NMR (125 MHz, CDCl3): δ 158.7, 158.5, 148.5, 141.9, 141.3, 140.6, 139.8, 131.7,
129.5, 129.2, 128.4, 127.0, 126.4, 125.6, 124.1, 123.6, 122.8, 120.4, 120.1, 117.7, 115.6,
114.1, 113.0, 110.2, 110.1, 80.6, 68.4, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 26.1, 22.7,
14.1; MS (FAB, 70 eV): m/z (relative intensity) 661 (M+, 100); HRMS calcd for
C44H43N3OS: 661.3121, found 663.3119.
2.3.12 2-((4-(5-(9-(4-(Dodecyloxy)phenyl)-9H-carbazol-3-yl)thiophen-2-yl)phenyl)-
methylene)-2H-indene-1,3-dione (F6).
Compound F6 was synthesized according to the same procedure as that of F1. Red
solid of F6 was obtained in 76% yield. MP: 176-178 °C. 1H NMR (500 MHz, CDCl3): δ
8.48 (d, 2H, J = 8.35 Hz), 8.33 (d, 1H, J = 1.04 Hz), 8.14 (d, 1H, J = 7.65 Hz),
7.93-7.98 (m, 2H), 7.81 (s, 1H), 7.72-7.75 (m, 4H), 7.64 (dd, 2H, J = 8.50, 1.45 Hz),
7.46 (d, 1H, J = 3.75 Hz), 7.37-7.41 (m, 3H), 7.33 (d, 1H, J = 3.80 Hz), 7.25-7.30 (m,
3H), 7.07 (d, 2H, J = 8.75 Hz), 4.03 (t, 2H, J = 6.50 Hz), 1.81-1.84 (m, 2H), 1.45-1.50
12

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(m, 2H), 1.22-1.38 (m, 16H), 0.86 (t, 3H, J = 6.55 Hz ); 13C NMR (125 MHz, CDCl3): δ
190.5, 189.2, 158.6, 147.5, 146.1, 142.5, 141.9, 141.2, 141.0, 139.1, 135.3, 135.2, 135.0,
131.8, 129.6, 128.4, 128.3, 126.3, 126.2, 125.9, 125.1, 124.1, 123.6, 123.4, 123.2, 123.1,
122.9, 120.4, 120.0, 117.6, 115.6, 110.1, 110.0, 68.4, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3,
29.2, 26.1, 22.7, 14.1. MS (FAB, 70 eV): m/z (relative intensity) 742 ((M+H)+, 100);
HRMS calcd for C50H47NO3S: 741.3272, found 742.3377.
2.3.13 (E)-2-(2-(tert-Butyl)-6-(4-(5-(9-(4-(dodecyloxy)phenyl)-9H-carbazol-3-yl)-
thiophen-2-yl)styryl)-4H-pyran-4-ylidene)malononitrile (F7).
Compound F7 was synthesized according to the same procedure as that of F3. The
1
red solid of F7 was collected in 60% yield (1.51 g, 2.23 mmol). MP: 145-147 °C. H
NMR (500 MHz, CDCl3): δ 8.35 (s, 1H), 8.15 (d, 1H, J = 7.70 Hz), 7.65-7.68 (m, 3H),
7.52 (d, 2H, J = 8.2 Hz), 7.38-7.42 (m, 4H), 7.27-7.35 (m, 5H), 7.09 (d, 2H, J = 8.7 Hz),
6.70 (d, 1H, J = 15.95 Hz), 6.66 (d, 1H, J = 1.55 Hz), 6.53 (d, 1H, J = 1.65 Hz), 4.04 (t,
2H, J = 6.5 Hz), 1.82-1.85 (m, 2H), 1.46-1.51 (m, 2H), 1.33 (s, 9H), 1.23-1.32 (m, 16H),
13
0.87 (t, 3H, J = 6.5 Hz); C NMR (125 MHz, CDCl3): δ 172.1, 158.8, 158.6, 156.5,
146.3, 141.9, 141.1, 141.0, 137.0, 136.4, 133.0, 129.6, 128.4, 128.3, 126.3, 125.9, 125.6,
125.1, 124.0, 123.6, 123.2, 122.9, 120.3, 120.0, 117.8, 117.4, 115.6, 115.2, 115.1, 110.1,
110.0, 107.1, 102.6, 68.4, 63.0, 59.6, 36.6, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 28.1,
26.0, 22.6, 14.1; MS (FAB, 70 eV): m/z (relative intensity) 809 (M+, 100); HRMS calcd
for C54H55N3O2S: 809.4015, found 809.4033.
2.3.14 (E)-2-(2-(tert-Butyl)-6-(4-(5-(9-(4-(dodecyloxy)phenyl)-9H-carbazol-3-yl)-
thiophen-2-yl)styryl)-4H-pyran-4-ylidene)-1H-indene-1,3(2H)-dione (F8).
Compound F8 was synthesized according to the same procedure as that of F3.
1
Orange solid of F8 was obtained in 66% yield. MP: 176-178 °C. H NMR (500 MHz,
13

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CDCl3): δ 8.44 (s, 1H), 8.41 (s, 1H), 8.35 (d, 1H), 8.16 (d, 1H, J = 7.65 Hz), 7.74 (dd,
2H, J = 19.45, 4.55 Hz), 7.65-7.67 (m, 3H), 7.52-7.58 (m, 4H), 7.27-7.43 (m, 9H), 7.10
(d, 2H, J = 8.40 Hz), 6.84 (dd, 1H, J = 16.0, 5.4 Hz), 4.05 (t, 1H, J = 6.3 Hz), 1.82-1.88
(m, 2H), 1.60-1.62 (m, 2H), 1.48-1.54 (m, 2H), 1.44 (s, 9H), 1.37-1.41 (m, 2H),
13
1.27-1.32 (m, 12H), 0.89 (t, 3H, J = 6.95 Hz); C NMR (125 MHz, CDCl3): δ 192.9,
173.9, 160.9, 158.7, 149.3, 146.2, 142.0, 141.4, 141.2, 140.9, 140.8, 136.1, 135.6, 133.8,
133.3, 129.8, 128.5, 128.4, 126.4, 126.2, 125.8, 125.0, 124.2, 123.7, 123.3, 123.1, 121.5,
121.3, 120.5, 120.1, 119.7, 117.6, 115.7, 110.2, 110.1, 109.2, 108.8, 103.7, 68.5, 37.1,
32.0, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 28.5, 26.2, 22.8. MS (FAB, 70 eV): m/z (relative
intensity) 890 ((M+H)+, 100); HRMS calcd for C60H59NO4S: 889.4165, found
890.4248.
2.4 Quantum Chemistry Computations
All organic dyes were optimized by using B3LYP/6-31G* hybrid functional. For
excited states, the time-dependent density functional theory (TDDFT) and the B3LYP
functional were used. All analyses were performed using the Q-Chem 3.0 software. The
frontier orbital plots of the highest and lowest occupied molecular orbitals (hereafter
abbreviated as HOMO and LUMO, respectively) were drawn using GaussView 04.
3. Results and Discussion
3.1 Synthesis
The synthesis was started with 1a or 1b, onto which a bromo-substituent was added
by using a bromination reaction to yield 2. Then a thiophene unit was added to 2 by a
Stille coupling reaction yielding 3 [56]. The π-bridge length was further increased by
inserting another phenyl unit through
a
Stille coupling reaction with
4-bromobenzaldehyde to yield 4. The eight final products were obtained through
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Knoevenagel condensation with malononitrile, 1,3-indanddione, and pyran derivatives,
which were used to build up the four kinds of acceptor unit [57]. The structures of all
new compounds were characterised through spectroscopic data.
3.2 Thermal stability
Thermal stability is essential for organic donor materials because of the vacuum
deposition process followed in the fabrication of the photovoltaic devices. The thermal
property of F-series was investigated through TGA and is shown in Fig. 2 and Table 1.
The F1~F4 exhibited a good thermal stability with a decomposition temperature (Td,
5% weight loss) at 384~406 under N2 condition. The F5~F8 with flexible long chain
showed slightly low Td at 317~405 .
3.3 Absorption spectra
The ultraviolet-visible absorption spectra of all p-type materials in THF solution and
the thin film are shown in Fig. 3 and Fig. S15~16. The photochemical and
electrochemical parameters are listed in Table 1. All p-type materials exhibit broad and
strong absorption in the range of 320~575 nm. Absorption in the short-wavelength
region, 325–350 nm, is attributed to localised π-π* and n-π* transitions, and that in the
long-wavelength region, 447~472 nm, results from intramolecular charge-transfer
transitions (ICT). Compared to F1, F3, F5, and F7, the ICT transitions of F2, F4, F6,
and F8 are more red-shifted with the 1,3-indandione or pyran-based 1,3-indandione
acceptor moiety assuming that 1,3-indandione group possess
a
stronger
electron-withdrawing ability than dicyanovinylene moiety. The solid state absorption of
ICT band on thin film shows a red shift of approximately 10~24 nm with respect to
those in THF solution, and it is supposed to be the result of intermolecular J-aggregation
15

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on the surface of substance. The compound F1 and F5 displays large stoke shift of 151
nm and 153 nm, respectively.
3.4 Electrochemical Properties
The ionic potentials (IP) corresponding to the highest occupied molecular orbital
(HOMO) level of the dyes were measured by photo-electron spectroscopy (AC-2 PESA)
and are presented in Fig. 4 and S17. Apparently, the dipolar π-conjugation system in the
F-series and the electron-withdrawing moiety can strongly influence electron
delocalization and the oxidation potential. The structures containing dicyanovinylene
moeity as electron-withdrawing group in F1, F3, F5, and F7 shows larger IP energy
level compared to that with 1,3-indanddione moiety. This observation also agrees well
with the theoretical result (Fig. S18). The IP of the dyes decreased in the order F1≈F5 >
F2≈F6, and F3≈F7 > F4≈F8. The EA levels of the dyes were estimated from the
HOMO levels and zero-zero band gaps at the intersection of absorption and emission
spectra (Table 1). The compound F2, and F6 shows a narrow HOMO-LUMO energy
gap, which is consistent with the wide absorption range and higher red-shift of ICT
absorption band in Figure 3. F1 compound shows the highest IP energy level and the
largest gap with the LUMO energy level of acceptor materials C60 and C70, indicating
the high Voc value in OPV devices.
3.5 Computational analysis
The electronic characteristics of F-series donor materials were investigated using
theoretical models. Full geometrical optimisations were performed by the
B3LYP/6-31G* hybrid functional, which was implemented in Q-Chem 3.0 [58]. The
optimised molecular geometry of F-series donor materials are shown in Fig. 5 and S19.
Clearly, the bridge conformation of the adjacent phenyl rings and anchoring group is
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nearly coplanar, but the orientation of the adjacent carbazole (or phenyl group) and
thiophene ring is slightly twisted to 13°-29° because of steric hindrance. The slight
distortion in the bridge does not seem to inhibit the resonance between the electron
donor and electron acceptor moieties. As depicted In Fig. 5 and S18, the electron
densities in the HOMOs are distributed mainly around the carbazole donor sites, and
those in the LUMOs are distributed around the acceptor moieties. As a consequence,
HOMO to LUMO transition contributes more than 98% of first singlet excited state (S1
state) in all F-series materials.
The bar chart in Fig. 4 shows that F1 exhibits the highest HOMO energy level than
the others. The trend F1≈F5 > F2≈F6, and F3≈F7 > F4≈F8 is similar with computed
result. This observation can also be verified by comparing the dipole moments. F1
(12.7477 D) and F5 (14.2723 D) shows a larger dipole moment than that of F2
(4.7393 D) and F6 (8.717 D) as shown in Table S1. This large dipole moment could
result from strong electron donating moiety of dicyanovinylene in F1, F3, F5, and F7.
The donor material F1 displays the highest HOMO energy level (-5.38 eV) and
supposed to have the largest gap with the LUMO energy level of acceptor materials C60
or C70, which is supported by the experimental data from AC-2. We expect that F1
should provide high Voc value in the OPVs device which further could be verified
straightforwardly through the photovoltaic performance of the device fabricated by
thermal deposition in vacuum.
3.6 Photovoltaic performance of bulk-heterojunction organic photovoltaic cells
To understand the relationship between molecular properties and device performance,
we have prepared the bulk-heterojunction OPVs based on F-series donor materials and
C60 or C70 acceptor for comparison. To address the effect of Voc, OPV devices fabricated
with our proposed donor materials were measured under the 1-sun illuminance for the
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electrical characterizations. The electrical characteristics of the organic photovoltaics
with F1~F4 are summarized in Table 2 and the current density versus voltage curves of
such devices are shown in Fig. 6. It has been reported vastly by many research groups
that several mechanisms could strongly influence the value of Voc. For example, the
energy difference between donor and acceptor [59], charge transfer state [60] or energy
coupling [61] of donor/acceptor, organic/electrode’s energy barrier [62], the built-in
potential in OPV [63], and interfacial energy at organic/organic [64] or
organic/electrode [65] have been studied thoroughly on the variation in Voc. Concerning
the intrinsic analysis of our proposed materials in previous section, we expect that F1
has a suitable energy level of HOMO/LUMO to achieve a high Voc in OPVs due to a big
energy-gap between the donor’s HOMO and acceptor’s LUMO. The OPV performance
of F1 exhibits a Voc of 1.01 V (in Table 2), which is obviously higher than that of F2-F4
ones. To date, only few researchers have presented Voc value exceeding 1 V for a
vacuum-deposited bulk-heterojunction small molecule OPVs [66]. It is obvious that a
donor material with high HOMO level could achieve the high Voc resulting in a high
device performance. On the other hand, F4 exhibits a lower value of Voc due to its low
HOMO level compared with F1-F3. In general, such result is consistent with previous
work in the field of variation of Voc [67]. On the other hand, in this work we still
observed that the F3 was not fully followed with such concept to obtian the high Voc.
The possible reason is because the chemical structure of F3 donor showed the the bulky
tert-butyl group in pyran moiety as compared to those F-series materials (Fig. S19).
Accordingly, it is possible that the Voc value of F3:C60 is much lower than that of
F1:C60 device although the F1 (5.59 eV) and F3 (5.52 eV) exhibited very similar
HOMO values measured by by low-energy photoelectron spectrometer. To confirm this
hypothesis, we have compared the surface morphologies of F1:C60 and F3:C60 thin
films in Fig. S20. Thin film of F3:C60 with few monolayers exhibits a rough surface
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morphology affected by their chemical structure, resulting in the OPV device with a
lower Voc.
More recently, Wong group reported that an OPV device with novel
donor-acceptor-acceptor type organic dye extended to near-IR absorption region
achieved a high PCE of 5.81% [26]. This work also pointed out a key issue of common
organic donor with absorption range limited to visible light resulting in the poor device
performance. To address this issue, we have studied the effect of the mixture of
subphthalocyanine as donor and C70 as acceptor in OPV at different mixing ratio [68].
The experimental result demonstrated that the PCE of OPVs can be significantly
improved from 1.8% to 6.0% using a high concentration of C70 in active layer owing to
sufficient extended absorption range. Since, C70 shows a wide and strong spectral
absorption compared to C60 or other acceptors [69]. Therefore, the active layer (F1:C70)
of 40 or 80 nm with two mixing ratios of 1:1 and 1:4 could be employed to improve the
device performance. The EQE results are shown in Figure 6b and 6d. Using C70 as
acceptor into our proposed active layer, it is obvious that the F1 in OPV shows a wide
absorption spectrum in the range of 500-700 nm in comparison to their C60 counterparts.
We also observed that the 80 nm thick F1:C70 (1:4) in OPV exhibited the highest PCE
of 4.93%, which is much better than that of F2-F4. This result indicates that the active
layer with high concentration of C70 shows a wide absorption range, which, in turn, may
improve the device performance. Indeed, we also observed that the active layer of
F1:C70 with optimal ratio shows a good charge balance and improved efficiency of
carrier collection (see Fig. S21). Therefore, this phenomenon may contribute to improve
the fill factor (FF) and hence an increase in the short-circuit current density (Jsc). On the
other hand, in our previous work we have studied the optical interference effect on the
varied film thickness of OPVs, which may affect the device performance by optimizing
the optical-field intensity in the active layer [69]. In order to understand the effect of
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optical interference on our proposed OPV with various thicknesses, the details of EQE
and IQE of F1:C70 were measured for comparison as shown in Fig. S22. It is obvious
that various thicknesses of active layer influence the optical distribution in the OPV
structure, which strongly change the feature of IQE. For example, when the 80 nm
F1:C70 active layer is used, the IQE showed a wide absorption profile compared to 40
nm ones solely due to the optical interference effect. This study might explain why OPV
with the 80 nm active layer exhibited a good device performance. In addition, the film
morphology is also a key factor to determine the device performance. To confirm this
intrinsic property, we measured the rms roughness of the films and are presented in Fig.
7 which shows that rms roughness of the film increases from 0.913 to 2.52 nm as the
C70 content increases. In the EQE data (see Fig. 6d), our devices with active layer of 1:1
or 1:4 show the same value at wavelength of 450 nm nevertheless the different mixing
ratios of F1:C70. This result probably indicates that the effective phase separation might
be generated in our active layer to provide a better carrier transportation pathways
resulting in the improvement of Jsc in our proposed device.
In order to evaluate the effect of the energy level of carbazole-based donors on Voc,
we have prepared the F5-F8 OPVs for comparison as shown in Fig. 8 and summarized
all results in Table 3. Obviously, the trend of the values of Voc is inconsistent with their
HOMO/LUMO levels. For example, the F6 showed the higher Voc as compared with F5
and F7-F8. However, we believe that the Voc is strongly dependent on serveral factors
related to the molecular interaction of F6:C60. To comfirm this argument, the AFM
measurement was taken to investigate the morphology difference between the F5:C60
and F6:C60 thin films as shown in Fig. S23. As expected, the rms value of F6:C60 film is
0.48 nm and this film exhibits very smooth surface morphlogy compared to other
F-series donors. In previous work, we have demonstrated that the smooth active layer
could suppress leakage current or reverse saturation current of the device, resulting in
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ACCEPTED MANUSCRIPT
high Voc [70]. Indeed, the high efficiency of F6 OPV is possibly because the active layer
provides greater charge collection efficiency and extends the absorption range. In
constrast, we found that the F8 showed a very poor device performance in our study.
Such issue may be attributed to the effect of intrinsic structural character of longer
conjugation system, which results in the high LUMO level of 3.08 eV. Note that this
phenomenon is consistent with theoretical calculation in Fig. S19. To find the
relationship between the device performance and energy level, few research groups
already reported that the interfacial layers at donor/acceptor [71], i.e. HOMO-HOMO,
LUMO-LUMO or HOMO-LUMO [72], could highly affect the charge separation
efficiency in active layer, resulting in the change of the OPV’s Jsc and Voc. Thus, the
poor performance of F8 is due to an increase in the gap of LUMO-LUMO in this study.
Although the detail experiments are needed to study the interfacial mechanism of
donor/acceptor, in this work we have synthesized the F-series donors to understand the
reliationship between the moecular structure and device performance, implying that the
different intermolecular interaction or intrinsic structure of F-series donors:fullerene is
crucial for improving OPV device characteristics.
4. Conclusions
In summary, we have synthesized the F-series p-type materials adopting carbazole
moiety as the electron-donating (D) group combined with thiophenyl and phenyl as
π-bridge, and the dicyanovinyl substituted or indendionevinyl or pyran-based as
electron-withdrawing group (A) to form a D-π-A molecule in order to demonstrate its
application in high performance OPVs. The high Voc value in the OPVs device is
supported strongly by computational result. The donor material F1 displays the highest
HOMO energy level (-5.38 eV) and the largest gap with the LUMO energy level of
acceptor materials C60 or C70, which is consistent with the experimental data from AC-2.
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Further, we verified the photovoltaic performance of the device fabricated by thermal
deposition in vacuum. The optimal OPVs device fabricated with F1:C70 (1:4) as the
active layer under vacuum deposition technique shows Voc of 1.02 V and PCE of 4.93%,
respectively.
Acknowledgments
This work was financial supported by the Ministry of Science and Technology of
Taiwan
(MOST
103-2113-M-029-007-MY2,
105-2221-E-131-016,
105-2119-M-131-001, and 105-2218-E-131-003), Tunghai University and Academia
Sinica are gratefully acknowledged. Special thanks to Professor C.-P. Hsu at the
Institute of Chemistry Sinica of Taiwan for her assistance on quantum chemistry
computations.
Appendix. Supplementary data
1
The H and 13C NMR spectra of all compounds, absorption spectra and emission
spectra in THF solution, the absorption spectra on the film, TDDFT calculated orbitals,
low energy transitions, space-charge limited current, CV spectra, HOMO/LUMO level,
AFM images, internal quantum efficiency and absorption spectra. Supplementary data
associated with this article can be found in the online version, at doi: http://dx.doi.org.
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