Feasible energy level tuning in polymer solar cells based on broad band-gap polytriphenylamine derivatives

Article abstract of DOI:10.1039/c5nj02124c

A series of versatile broad band-gap alternating copolymers (P1, P2, P3 and P4) based on triphenylamine (TPA) and benzofurazan derivatives, differing in the substituted groups [-OC₈H₁₇, -C₈H₁₇, -CF₃, -(CF₃)₂] in their triphenylamine units, were designed and synthesized by Suzuki polycondensation. The relationships between the substituted groups in TPA and the highest occupied molecular orbital (HOMO) energy levels, as well as the open circuit voltages (V_ocs), were investigated in detail. The HOMO levels of these four polymers decreased sequentially when the substituted groups shifted from electron-donating groups [-OC₈H₁₇, -C₈H₁₇] to electron-withdrawing groups [-CF₃, -(CF₃)₂], which led to the successive increase in V_ocs of the polymer solar cells (PSCs) based on these polymers. Through the characterization of photovoltaic performance, the highest V_oc, which reached up to 1.00 V, was achieved by the polymer with bis(trifluoromenthyl) substituted group (P4), which is one of the highest V_oc values based on polytriphenylamine derived polymers for PSCs. Among these polymers, the one with octyl side chain (P2) showed the best photovoltaic performance with the highest short circuit current density (J_sc) and fill factor (FF), giving a J_sc of 4.84 mA cm^-2, FF of 50%, V_oc of 0.80 V and power conversion efficiency (PCE) of 2.22%.

Full text of DOI:10.1039/c5nj02124c

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Feasible energy level tuning in polymer solar cells
based on broad band-gap polytriphenylamine
derivatives
Cite this: New J. Chem., 2016,
40, 402
Bin Zhang,*a Guiting Chen,b Jin Xu,b Liwen Hub and Wei Yangb
A series of versatile broad band-gap alternating copolymers (P1, P2, P3 and P4) based on triphenylamine
(TPA) and benzofurazan derivatives, differing in the substituted groups [–OC8H17, –C8H17, –CF3, –(CF3)2]
in their triphenylamine units, were designed and synthesized by Suzuki polycondensation. The relation-
ships between the substituted groups in TPA and the highest occupied molecular orbital (HOMO) energy
levels, as well as the open circuit voltages (Vocs), were investigated in detail. The HOMO levels of these
four polymers decreased sequentially when the substituted groups shifted from electron-donating
groups [–OC8H17, –C8H17] to electron-withdrawing groups [–CF3, –(CF3)2], which led to the successive
increase in Vocs of the polymer solar cells (PSCs) based on these polymers. Through the characterization of
photovoltaic performance, the highest Voc, which reached up to 1.00 V, was achieved by the polymer with
bis(trifluoromenthyl) substituted group (P4), which is one of the highest Voc values based on polytriphenyl-
amine derived polymers for PSCs. Among these polymers, the one with octyl side chain (P2) showed the
best photovoltaic performance with the highest short circuit current density (Jsc) and fill factor (FF), giving a
Jsc of 4.84 mA cmꢀ2, FF of 50%, Voc of 0.80 V and power conversion efficiency (PCE) of 2.22%.
Received (in Montpellier, France)
10th August 2015,
Accepted 23rd October 2015
DOI: 10.1039/c5nj02124c
www.rsc.org/njc
(3) narrow band gap, (4) high molar extinction coefficient,
(5) deep highest occupied molecular orbital (HOMO), and (6) good
Introduction
Bulk-heterojunction polymer solar cells (BHJ-PSCs), due to their miscibility with acceptors.7
versatile merits of low cost, large-scale fabrication, flexibility and
Among these requirements for polymer donors, a deep
diversity of polymer donors, have attracted considerable interest, HOMO energy level is an indispensable parameter for evaluating
both academically and industrially. To date, some novel PSCs with polymers, because the open circuit voltage (Voc) is proportional
high photovoltaic performance and power conversion efficiencies to the energy level difference between the HOMO energy level of
(PCEs) exceeding 10% have been developed.1–3 Potentially, this donor and lowest unoccupied molecular orbital (LUMO) energy
success can lead to the application of PSCs in generating level of acceptor.8 If the HOMOs of polymer donors are much
electricity in the future.
deeper, higher Vocs in PSCs will be facilely accessible. Therefore,
Generally, two components that contain narrow bandgap researchers should select molecular units with deep HOMOs,
polymers and work as electron donators and electron-accepting such as fluorene,9 indenofluorene,10 benzene,11 carbazole,12–15
units (acceptors) are utilized widely in BHJ-PSCs. For acceptors, indolocarbazole,16 diindenothieno[2,3-b]thiophene ladder-type
fullerene derivatives are used comprehensively due to their hexacyclic arene17 and pyrrolo[3,4-g]quinoxaline-6,8-dione, to
advanced properties of high electron-accepting capability and construct polymers.18 However, some of these units are not
electron mobility.4–6 On the other hand, narrow bandgap poly- suitable for the construction of highly efficient narrow bandgap
meric donors are another significant part of BHJ-PSCs. To polymers due to their disadvantages of large band gaps and low
obtain high photovoltaic performance, these donors should hole mobility, for example, fluorene and benzene. Alternatively,
meet specific requirements: (1) high stability, (2) good solubility, another efficient strategy to achieve deep HOMO energy levels
in polymer donors is the introduction of electron-withdrawing
groups on the side chains of aromatic moieties. Liang et al.
obtained high-efficiency PSCs with a higher Voc of 0.76 V by
introducing the electron-withdrawing fluorine group on thieno-
[3,4-b]thiophene as compared to that of 0.68 V based on the
non-fluorine substituted polymer.19,20 You’s group, through the
introduction of fluorinated benzothiadiazole21 and benzotriazole22
a Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education
and Guangdong Province, College of Optoelectronic Engineering,
Shenzhen University, Shenzhen 518060, China. E-mail: msbzhang@scut.edu.cn
b Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of
Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, China
402 | New J. Chem., 2016, 40, 402--412
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into the backbones of polymers, acquired a series of versatile a waters gel permeation chromatography (GPC) system using
polymers with enhanced Voc values. In addition, Huang et al. linear polystyrene as standard and tetrahydrofuran (THF) as
developed a new series of novel two-dimensional (2D) narrow eluent. Cyclic voltammetry (CV) was carried out on a CHI600D
bandgap polymers containing strong electron-withdrawing electrochemical workstation with a standard three electrodes
groups in the side chains of the polymers, such as malononitrile cell using a platinum (Pt) working electrode and Pt wire counter
and 1,3-diethyl-2-thiobarbituric acid. Through the introduction electrode, against a saturated calomel electrode (SCE) as the
of these electron-withdrawing groups, Voc values can not only be reference electrode at a scan rate of 50 mV sꢀ1 in a nitrogen-
readily increased, but the band gaps of polymers are also saturated anhydrous solution of 0.1 mol Lꢀ1 tetrabutylammonium
decreased simultaneously, which is ascribed to the strong inter- hexafluorophosphate (Bu4NPF6) in acetonitrile versus ferrocene/
nal charge-transfer (ICT) effect between electron-rich polymer ferrocenium (Fc/Fc+) as the internal reference. UV-vis absorp-
main chains and side groups. On the basis of the strategy for tion spectra were obtained on an HP 8453 spectrophotometer.
achieving 2D narrow bandgap polymers, the Voc value can reach Tapping-mode atomic force microscopy (AFM) was performed
0.99 V, which is an excellent result in BHJ-PSCs.23,24 However, on a Veeco Nanoscope V scanning probe microscope for the
due to the process of post-linking reaction between polymers identification of topography and to obtain blend film phase
and formyl groups and strong electron-withdrawing groups, images.
it is not a certain guarantee that every formyl group is reacted
completely, and ultimately this possibly affects the batch-to-
Device fabrication
batch repeatable production of polymers.
All the devices were fabricated on ITO-coated glass substrates.
Triphenylamine (TPA), which is a versatile unit, is utilized A thin layer (40 nm) of PEDOT:PSS (Clevios 4083) was spin-casted
widely in organic light-emitting diodes (OLEDs),25–27 small onto UV-ozone treated ITO substrates at 4000 rpm for 40 s, and
molecular organic solar cells (OSCs)28–30 and organic field- then baked at 140 1C for 15 min in air. The solutions containing
effect transistors (OFETs)31,32 due to its excellent merits of low the polymer and PC61BM with different weight ratios in o-dichloro-
cost, feasible chemical functionalization, thermal stability, and benzene (ODCB) were spin-casted inside a glove-box with nitrogen
high hole mobility. Normally, TPA, which is a strong electron- at 1200 rpm for 40 s to form active layers (B80 nm). Under the
donating moiety, acts as a donor in the preparation of narrow device structure of ITO/PEDOT-PSS/active layer/poly[(9,9-dioctyl-
bandgap polymers. However, very reactive points exist in the TPA 2,7-fluorene)-alt-(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)]
backbone, which can be used to decorate it, and different (PFN)/Al, the PFN layer (5 nm) was prepared by spin-coating a
substituting groups can be readily introduced to modify the mixed solution (0.2 mg mlꢀ1) in methanol with a small amount
chemical and physical properties of TPA. When electron- of acetic acid (volume ratio: 100 : 1) before cathode evaporation.
donating groups are introduced, the HOMOs of polymers The aluminum cathode (B80 nm) was then thermally evapo-
are promoted; in contrast, the HOMOs decrease by the intro- rated under vacuum (B10ꢀ6 Torr) through a shadow mask
duction of electron-withdrawing groups into the polymer side defining an active area of 0.16 cm2. Current density–voltage
chains. Herein, we developed a series of TPA based broad ( J–V) curves were obtained using a Keithley 2400 multimeter
bandgap polymers with different substituting groups ranging under AM 1.5 G solar illumination at 87 mW cmꢀ2 and a
from the electron-donating groups of octyloxy [–OC8H17] and Thermal-Oriel 150 W solar simulator. Incident photon-to-current
octyl [–C8H17] to the electron-withdrawing groups of trifluoro- conversion efficiency (IPCE) values were acquired with a mono-
methyl [–CF3] and bis(trifluoromethyl) [–(CF3)2], and four poly- chromator and calibrated with a standard silicon photodiode. The
mers P1, P2, P3 and P4, were prepared accordingly. Through hole mobility data of neat polymer films were characterized using
the introduction of these groups, the HOMO energy levels of the space charge limited current (SCLC) method with the device
the corresponding polymers can be tuned readily from ꢀ5.15, structure of ITO/PEDOT:PSS (40 nm)/active layer/MoO3 (10 nm)/Al
ꢀ5.23, ꢀ5.41, to ꢀ5.46 eV, respectively, and ultimately the Voc (80 nm). Preliminarily, PEDOT:PSS was spin coated on an ITO
values in PSCs based on these polymers were simultaneously covered glass substrate; then, active layers were prepared from an
adjusted from 0.70, 0.80, 0.95 to 1.00 V. Therefore, this is a ODCB solution of 1% concentration by spin casting. Finally,
facile strategy to tune the energy levels in polymer donors by MoO3 and Al layers were deposited via thermal evaporation under
substituting different groups (electron-donating or electron- high vacuum.
withdrawing) into the polymer side chains.
Synthesis of monomers and polymers
All the starting materials utilized were purchased commercially
and used without further purification. [6,6]-Phenyl-C61-butyric
Experimental section
Characterization and instrumentation
acid methyl ester (PC61BM) was purchased from Lumtec Corp.
4-Bromo-N-(4-bromophenyl)-N-(4-methoxyphenyl) aniline, 1,33
1H nuclear magnetic resonance (NMR) and 13C NMR character- 4-(bis(4-bromophenyl)amino)phenol, 2,34 4-bromo-N,N-diphenyl-
ization were carried out on a Bruker 300 MHz or 600 MHz DRX aniline, 5,35 and 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)-
spectrometer with tetramethylsilane (TMS) as the internal benzofurazan, 15,15 were synthesized according to the published
reference. The weight-averaged molecular weight (Mw) and literature. Other monomers and polymers were synthesized via
number-averaged molecular weight (Mn) were determined on the following synthetic methodologies.
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4-Bromo-N-(4-bromophenyl)-N-(4-(octyloxy)phenyl)aniline, 3. 2.55 (t, 2H), 1.62–1.55 (m, 2H), 1.31–1.26 (m, 10H), 0.88 (t, 3H).
To a solution of 4-(bis(4-bromophenyl)amino)phenol 2 (16.5 g, 13C NMR (CDCl3, 75 MHz, d) 148.07, 145.35, 137.84, 129.19,
39.4 mmol) in dry methanol (170 ml) under nitrogen, sodium 124.72,124.31, 124.18, 123.70, 123.10, 122.68, 122.24, 35.41,
methoxide (3.2 g, 59.1 mmol) was added in one portion. Then, 31.93, 31.55, 29.73, 29.69, 29.52, 22.71, 14.16.
the mixture was heated to reflux for one hour and 1-bromo-
4-Bromo-N-(4-bromophenyl)-N-(4-octylphenyl)aniline 7. 4-Octyl-
octane (14 ml, 78.8 mmol) was added dropwise. The reaction N,N-diphenylaniline 6 (5.19 g, 14.5 mmol) was dissolved in
mixture was refluxed overnight. Then, it was cooled to room N,N-dimethylformamide (DMF) (50 ml) at 0 1C. Then, N-bromo-
temperature, poured into water, extracted with ethyl acetate succinimide (NBS) (5.16 g, 29 mmol) was slowly added in
three times and washed with an aqueous solution of sodium portions in the absence of light. When NBS was added com-
chloride and water. The organic extract was dried using MgSO4, pletely, the reaction mixture was warmed to room temperature
and the solvent was removed by evaporation. The resulting spontaneously, and allowed to react overnight. The mixture was
product was purified by column chromatography using petro- then poured into water, and extracted with DCM three times,
leum as the eluent to obtain 18.0 g of a viscous liquid, yield then washed with sodium chloride aqueous solution and water,
1
86%. H NMR (CDCl3, 300 MHz, d) 7.31 (d, 4H), 7.03 (d, 2H), and dried over MgSO4. The solvent was removed by evapora-
6.92–6.85 (m, 6H), 3.96 (t, 2H), 1.88–1.77 (m, 2H), 1.50–1.33 tion, and the crude product was purified by column chromato-
(m, 10H), 0.93 (t, 3H). 13C NMR (CDCl3, 75 MHz, d) 152.37, graphy in petroleum as the eluent to obtain 6.3 g of a viscous
146.76, 136.12, 130.87, 124.34, 123.86, 117.71, 112.03, 31.81, liquid, yield 84%. 1H NMR (CDCl3, 300 MHz, d) 7.25–7.19
29.43, 29.32, 29.47, 26.13, 24.75, 22.57, 14.14.
(m, 4H), 7.11–7.05 (m, 2H), 6.98–6.89 (m, 6H), 2.58 (t, 2H),
4-(Octyloxy)-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan- 1.65–1.55 (m, 2H), 1.33–1.28 (m, 10H), 0.93 (t, 3H). 13C NMR
2-yl)phenyl)aniline 4. 4-Bromo-N-(4-bromophenyl)-N-(4-(octyloxy)- (CDCl3, 75 MHz, d) 146.70, 146.06, 144.37, 138.92, 132.52,
phenyl)aniline 3 (17.0 g, 32 mmol) was dissolved in 300 ml of 132.32, 129.47, 125.60, 124.96, 124.52, 116.10, 114.98, 35.37,
dry tetrahydrofuran (THF) under the protection of dry nitrogen 31.87, 31.42, 29.68, 29.63, 29.45, 22.64, 14.09.
gas. The mixture was cooled to ꢀ78 1C, then BuLi (2.5 M, 51 ml,
4-Octyl-N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
128 mmol) was slowly added dropwise. When BuLi was added phenyl)aniline 8. 4-Bromo-N-(4-bromophenyl)-N-(4-octylphenyl)-
completely, it was left to react for another 1 hour at this aniline 7 (23.4 g, 45.4 mmol) was dissolved in 300 ml dry THF
temperature. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane under the protection of dry nitrogen gas. The mixture was
(53 ml, 256 mmol) was added slowly, and the reaction was allowed cooled to ꢀ78 1C, then BuLi (2.5 M, 73 ml, 181.4 mmol) was
to continue for 1 hour at ꢀ78 1C. Then, the reaction mixture was slowly added dropwise. When the BuLi was added completely,
warmed to room temperature spontaneously, and allowed to react it was allowed to react for another hour at this temperature.
overnight. It was poured into water, and extracted with dichloro- 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (75.5 ml,
methane (DCM) three times, then washed with sodium chloride 362.8 mmol) was added slowly, and the reaction was continued
aqueous solution and water, and dried over MgSO4. The solvent for 1 hour at ꢀ78 1C. Then, the reaction mixture was warmed to
was removed by evaporation, and the crude product was purified room temperature spontaneously, and allowed to react over-
by recrystallization in THF and methanol to obtain 14.3 g of night. The mixture was then poured into water, and extracted
target compound as a white crystal, yield 72%. 1H NMR (CDCl3, with DCM three times, then washed with sodium chloride
300 MHz, d) 7.65 (d, 4H), 7.05–7.01 (m, 6H), 6.83 (d, 2H), 3.94 aqueous solution and water, and dried over MgSO4. The solvent
(t, 2H), 1.80–1.73 (m, 2H), 1.46–1.29 (m, 34H), 0.89 (t, 3H). was removed by evaporation, and the crude product was purified
13C NMR (CDCl3, 75 MHz, d) 156.41, 150.34, 139.62, 135.81, by recrystallization in THF and methanol to obtain 18.7 g of a
128.06, 127.75, 121.73, 115.40, 31.82, 29.38, 29.35, 29.25, 26.09, white solid, yield 68%.1H NMR (CDCl3, 300 MHz, d) 7.67–7.64
24.87, 24.60, 22.66, 14.11.
(m, 4H), 7.09–7.00 (m, 8H), 2.57 (t, 2H), 1.64–1.53 (m, 2H), 1.33–
4-Octyl-N,N-diphenylaniline 6. 4-Bromo-N,N-diphenylaniline 1.27 (m, 34H), 0.89 (t, 3H). 13C NMR (CDCl3, 75 MHz, d) 150.26,
5 (6.48 g, 20 mmol) was dissolved in 100 ml of dry THF under 144.48, 138.98, 135.88, 129.32, 125.83, 123.88, 122.39, 35.44,
the protection of dry nitrogen gas. The mixture was cooled to 31.89, 31.42, 29.48, 29.41, 29.27, 24.86, 22.67, 14.12.
ꢀ78 1C, and then BuLi (2.5 M, 12 ml, 30 mmol) was slowly
N,N-Diphenyl-4-(trifluoromethyl)aniline 9. Diphenylamine
added dropwise. When BuLi was added completely, it was allowed (6.1 g, 36 mmol), 4-iodobenzotrifluoride (5 g, 18 mmol), palladium
to react for another 1 hour at this temperature. 1-Bromooctane acetate (202 mg, 0.9 mmol), tris(2-methylphenyl)phosphine
(14 ml, 78.8 mmol) was added dropwise and 1-iodooctane (12 g, (548 mg, 1.8 mmol) and sodium tert-butoxide (3.57 g, 36 mmol)
50 mmol) was added in one portion. Then, the reaction mixture were dissolved in dry toluene (60 ml). The mixture was refluxed
was warmed to room temperature spontaneously, and allowed overnight. Then, it was cooled to room temperature and filtered
to react overnight. The mixture was then poured into water, and to remove any insoluble solids. The filtrate was concentrated
extracted with DCM three times, then washed with sodium to remove the solvent, and the crude product was purified by
chloride aqueous solution and water, and dried over MgSO4. column chromatography using petroleum as the eluent to obtain
The solvent was removed by evaporation, and the crude product 7.8 g of a viscous liquid, yield 69%. 1H NMR (CDCl3, 300 MHz, d)
was purified by column chromatography using petroleum 7.55 (d, 2H), 7.37 (t, 4H), 7.19–7.11 (m, 6H), 6.96 (d, 2H).
as the eluent to obtain 5.3 g of a viscous liquid, yield 74%. 13C NMR (CDCl3, 75 MHz, d) 151.30, 146.51, 130.38, 126.94,
1H NMR (CDCl3, 300 MH, d) 7.31–7.27 (m, 4H), 7.24–6.94 (m, 10H), 126.89, 125.24, 123.28, 120.20, 40.01.
404 | New J. Chem., 2016, 40, 402--412
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4-Bromo-N-(4-bromophenyl)-N-(4-(trifluoromethyl) phenyl)- sodium chloride aqueous solution and water, and dried over
aniline 10. To a solution of N,N-diphenyl-4-(trifluoromethyl)- MgSO4. The solvent was removed by evaporation, and the crude
aniline 9 (21.2 g, 67.7 mmol) dissolved in DMF (210 ml) at room product was purified by recrystallization in ethanol to obtain
temperature, NBS (24.7 g, 138.8 mmol) dissolved in DMF 5.6 g of a white crystal, yield 82%. 1H NMR (CDCl3, 300 MHz, d)
(210 ml) was added dropwise slowly in the absence of light. 7.48–7.37 (m, 7H), 6.96 (d, 4H). 13C NMR (CDCl3, 75 MHz, d)
When NBS was added completely, the reaction was continued at 148.46, 144.92, 133.53, 126.62, 121.22, 118.08, 115.35, 115.25.
room temperature for one hour. The mixture was then poured
N,N-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
into water and extracted with DCM three times, then washed 3,5-bis(trifluoromethyl)aniline 14. N,N-Bis(4-bromophenyl)-3,5-
with sodium chloride aqueous solution and water, and dried bis(trifluoromethyl)aniline 13 (15.0 g, 27.8 mmol) was dissolved
over MgSO4. The solvent was removed by evaporation, and the in 350 ml of dry THF under the protection of dry nitrogen gas.
crude product was purified by recrystallization in ethanol The mixture was cooled to ꢀ78 1C, and then BuLi (2.5 M, 33 ml,
1
to obtain 31.5 g of white crystals, yield 99%. H NMR (CDCl3, 83.5 mmol) was slowly added dropwise. When the BuLi was
300 MHz, d) 7.47–7.32 (m, 6H), 7.06 (d, 2H), 6.97 (d, 4H). added completely, it was left to react for another hour at this
13C NMR (CDCl3, 75 MHz, d) 150.00, 145.61, 132.77, 126.61, temperature. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
126.54, 126.49, 121.92, 117.17.
(35 ml, 167 mmol) was added slowly, and the reaction was
N,N-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)- continued for 1 hour at ꢀ78 1C. Then, the reaction mixture was
(4-(trifluoromethyl)phenyl)aniline 11. 4-Bromo-N-(4-bromo- warmed to room temperature spontaneously, and allowed to
phenyl)-N-(4-(trifluoromethyl)phenyl)aniline 10 (15.0 g, 31.8 mmol) react overnight. It was then poured into water and extracted
was dissolved in 300 ml of dry THF under the protection of dry with DCM three times, then washed with sodium chloride aqueous
nitrogen gas. The mixture was cooled to ꢀ78 1C, and then BuLi solution and water, and dried over MgSO4. The solvent was
(2.5 M, 45 ml, 111.4 mmol) was slowly added dropwise. When removed by evaporation, and the crude product was purified by
the BuLi was added completely, it was left to react for another recrystallization in THF and methanol to obtain 13.2 g of a
hour at this temperature. 2-Isopropoxy-4,4,5,5-tetramethyl- white crystal, yield 75%. 1H NMR (CDCl3, 300 MHz, d) 7.75
1,3,2-dioxaborolane (40 ml, 191 mmol) was added slowly, and (d, 4H), 7.44–7.41 (m, 3H), 7.08 (d, 4H), 1.35 (s, 24H). 13C NMR
the reaction was continued for 1 hour at ꢀ78 1C. Then, the (CDCl3, 75 MHz, d) 148.69, 148.61, 136.43, 132.84, 132.40,
reaction mixture was warmed to room temperature sponta- 123.89, 122.35, 115.40, 83.88, 24.87.
neously, and allowed to react overnight. It was then poured into
Poly(4-(octyloxy)-N,N-diphenylaniline-alt-4,7di(thiophen-2-yl)-
water, extracted with DCM three times, washed with sodium 5,6-bis(octyloxy) benzofurazan) P1. 4-(Octyloxy)-N,N-bis(4-(4,4,5,5-
chloride aqueous solution and water, and dried over MgSO4. The tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline 4 (313 mg,
solvent was removed by evaporation, and the crude product was 0.5 mmol), 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzo-
purified by recrystallization in THF and methanol to obtain furazan 5 (349 mg, 0.5 mmol) and Pd(PPh3)4 (11.6 mg, 0.01 mmol)
12.4 g of a white crystal, yield 69%. 1H NMR (CDCl3, 300 MHz, d) were dissolved in 15 ml toluene under nitrogen protection.
7.72 (d, 4H), 7.44 (d, 2H), 7.14–7.07 (m, 6H), 1.35 (s, 24H). 13C NMR Then, a tetraethyl ammonium hydroxide aqueous solution (2 ml,
(CDCl3, 75 MHz, d) 150.24, 149.31, 136.14, 126.39, 126.33, 126.11, 20 wt/wt%) was added in one portion and the mixture was heated
124.40, 122.99, 83.77, 25.60.
to 85 1C for 48 h. Phenylboronic acid (0.609 g, 5 mmol) was added
N,N-Diphenyl-3,5-bis(trifluoromethyl)aniline 12. Diphenyl- and the mixture was left to react for 12 h, then bromobenzene
amine (3.7 g, 22.1 mmol), 1-iodo-3,5-bis(trifluoromethyl)benzene (1.57 g, 10 mmol) was added in one portion and allowed to react
(5 g, 14.7 mmol), palladium acetate (166 mg, 0.74 mmol), tris(2- for an additional 12 h. The reaction mixture was cooled to room
methylphenyl)phosphine (447 mg, 1.47 mmol) and sodium tert- temperature and precipitated in methanol. The crude polymer
butoxide (2.9 g, 29.4 mmol) were dissolved in dry toluene was purified by Soxhlet extraction with methanol, acetone,
(50 ml). The mixture was refluxed overnight. Then, it was hexane and chloroform. The final chloroform solution was
cooled to room temperature, and filtered to remove any inso- precipitated in methanol, filtered and dried at 80 1C in a
luble solid. The filtrate was concentrated to remove the solvent, vacuum oven for 10 hours to yield 247 mg of the target polymer
and the crude product was purified by column chromatography as a red solid. 1H NMR (CDCl3, 600 MHz, d): 7.71–7.31 (br, 8H,
using petroleum as the eluent to obtain 4.3 g of a viscous liquid, Ar-H), 7.22–7.00 (br, 6H, Ar-H), 6.91–6.86 (m, 2H, Ar-H),
yield 77%. 1H NMR (CDCl3, 300 MHz, d) 7.42–7.34 (m, 7H), 4.20–4.14 (m, 4H, CH2), 3.73–3.72 (m, 2H, CH2), 2.04–2.03 (m,
7.22–7.14 (m, 6H). 13C NMR (CDCl3, 75 MHz, d) 150.14, 146.31, 4H, CH2), 1.83–1.79 (m, 2H, CH2), 1.29–1.27 (br, 30H, CH2),
133.14, 125.39, 125.33, 125.11, 124.40, 113.99.
0.90–0.86 (m, 9H, CH3). GPC (THF): Mn = 12 400, Mw = 26 800,
N,N-Bis(4-bromophenyl)-3,5-bis(trifluoromethyl)aniline 13. PDI = 2.16.
To a solution of N,N-bis(4-bromophenyl)-3,5-bis(trifluoromethyl)-
Poly(4-octyl-N,N-diphenylaniline-alt-4,7di(thiophen-2-yl)-5,6-
aniline 13 (4.8 g, 12.6 mmol) dissolved in DMF (50 ml) at room bis(octyloxy)benzofurazan) P2. 4-Octyl-N,N-bis(4-(4,4,5,5-tetra-
temperature, NBS (4.5 g, 25.2 mmol) dissolved in DMF (50 ml) methyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline 8 (305 mg, 0.5 mmol),
was slowly added dropwise in the absence of light. When NBS 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzofurazan
5
was added completely, the reaction was continued at room (349 mg, 0.5 mmol) and Pd(PPh3)4 (11.6 mg, 0.01 mmol) were
temperature for one hour. Then, the mixture was poured into dissolved in 15 ml toluene under nitrogen protection. Then,
water and extracted with DCM three times, then washed with tetraethyl ammonium hydroxide aqueous solution (2 ml, 20 wt/wt%)
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was added in one portion and the mixture was heated to 85 1C 1.53–1.26 (br, 20H, CH2), 0.91–0.84 (m, 6H, CH3). GPC (THF):
for 48 h. Phenylboronic acid (0.609 g, 5 mmol) was added and Mn = 7600, Mw = 10 700, PDI = 1.41.
reacted for 12 h. Then, bromobenzene (1.57 g, 10 mmol) was
added in one portion and reacted for an additional 12 h. The
reaction mixture was cooled to room temperature and precipi-
tated in methanol. The crude polymer was purified by Soxhlet
extraction in methanol, acetone, hexane and chloroform. The
Results and discussion
Synthesis
final chloroform solution was precipitated in methanol, filtered The synthetic routes of the monomers are presented in
and dried at 80 1C in a vacuum oven for 10 hours to yield 189 mg Scheme 1. For the synthesis of 4-(octyloxy)-N,N-bis(4-(4,4,5,5-
of the target polymer as a red solid. 1H NMR (CDCl3, 600 MHz, d): tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline 4, 4-bromo-
7.70–7.58 (br, 4H, Ar-H), 7.49–7.30 (br, 4H, Ar-H), 7.18–7.06 N-(4-bromophenyl)-N-(4-methoxyphenyl) aniline 1 was prepared
(br, 8H, Ar-H), 4.20–4.15 (m, 4H, CH2), 2.62–2.59 (m, 2H, CH2), preliminarily from 1-bromo-4-iodobenzene and p-methoxyl-
2.05–1.99 (m, 4H, CH2), 1.38–1.28 (br, 32H, CH2), 0.90–0.85 aniline via the Ullmann reaction, and then compound 1 was
(m, 9H, CH3). GPC (THF): Mn = 11 700, Mw = 20 600, PDI = 1.76. demethylated to afford 4-(bis(4-bromophenyl)amino)phenol 2
Poly(N,N-diphenyl-4-(trifluoromethyl)aniline-alt-4,7di(thiophen- with the assistance of boron tribromide as the catalyst. By the
2-yl)-5,6-bis(octyl oxy)benzofurazan) P3. N,N-Bis(4-(4,4,5,5- alkylation of monomer 2 in the presence of 1-bromooctane,
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-(4-(trifluoromethyl)- 4-bromo-N-(4-bromophenyl)-N-(4-(octyloxy)phenyl)aniline 3 was
phenyl)aniline 11 (283 mg, 0.5 mmol), 4,7-bis(5-bromothiophen-2- obtained with good yield, and finally target compound 4 was
yl)-5,6-bis(octyloxy)benzofurazan 5 (349 mg, 0.5 mmol) and prepared from monomer 3 by a general low temperature reaction
Pd(PPh3)4 (11.6 mg, 0.01 mmol) were dissolved in 15 ml toluene (ꢀ78 1C) in the presence of n-butyllithium and 2-isopropoxy-
under nitrogen protection. Then, a tetraethyl ammonium hydroxide 4,4,5,5-tetramethyl-1,3,2-dioxaborolane. 4-Octyl-N,N-diphenyl-
aqueous solution (2 ml, 20 wt/wt%) was added in one portion aniline 6 was prepared feasibly from 4-bromo-N,N-diphenylaniline
and the mixture was heated to 85 1C for 48 h. Phenylboronic 5 via the reaction with n-butyllithium and 1-bromooctane at
acid (0.609 g, 5 mmol) was added and allowed to react for 12 h. ꢀ78 1C sequentially; then, through general bromination in
Then, bromobenzene (1.57 g, 10 mmol) was added in one DMF, 4-bromo-N-(4-bromophenyl)-N-(4-octylphenyl)aniline 7 was
portion and reacted for an additional 12 h. The reaction obtained in 84% yield, and ultimately 4-octyl-N,N-bis(4-(4,4,5,5-
mixture was cooled to room temperature and precipitated in tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline 8 was prepared
methanol. The crude polymer was purified by Soxhlet extrac- through the same procedure used for the synthesis of compound
tion in methanol, acetone, hexane and chloroform. The final 4. The synthetic procedures of N,N-bis(4-(4,4,5,5-tetramethyl-
chloroform solution was precipitated in methanol, filtered and 1,3,2-dioxaborolan-2-yl)phenyl)-(4-(trifluoromethyl)phenyl)aniline
dried at 80 1C in a vacuum oven for 10 hours to yield 207 mg of 11 and N,N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
the target polymer as a red solid. 1H NMR (CDCl3, 600 MHz, d): 3,5-bis (trifluoromethyl)aniline 14 are very similar, where
8.37–8.17 (m, 2H, Ar-H), 7.64–7.31 (br, 8H, Ar-H), 7.19–7.01 (br, N,N-diphenyl-4-(trifluoromethyl)aniline 9 and N,N-diphenyl-3,5-
6H, Ar-H), 4.14–4.09 (m, 4H, CH2), 1.98–1.91 (m, 4H, CH2), bis(trifluoromethyl)aniline 12 were synthesized first via the
1.40–1.16 (br, 20H, CH2), 0.87–0.78 (m, 6H, CH3). GPC (THF): Buchwald–Hartwig reaction; then, through general bromination
Mn = 8700, Mw = 12 400, PDI = 1.43.
in DMF, 4-bromo-N-(4-bromophenyl)-N-(4-(trifluoromethyl)phenyl)-
Poly(N,N-diphenyl-3,5-bis(trifluoromethyl)aniline-alt-4,7di(thio- aniline 10 and N,N-bis(4-bromophenyl)-3,5-bis(trifluoromethyl)-
phen-2-yl)-5,6-bis(octyloxy)benzofurazan) P4. N,N-Bis(4-(4,4,5,5- aniline 13 were obtained, respectively, and finally using the same
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-3,5-bis(trifluoromethyl)- synthetic procedure for the preparation of compound 4 and 8,
aniline 14 (317 mg, 0.5 mmol), 4,7-bis(5-bromothiophen-2-yl)- we acquired target compound 11 and 14.
5,6-bis(octyloxy) benzofurazan 5 (349 mg, 0.5 mmol) and
The synthetic routes of polymers are shown in Scheme 2. All
Pd(PPh3)4 (11.6 mg, 0.01 mmol) were dissolved in 15 ml toluene the polymers, P1, P2, P3 and P4, were synthesized by Suzuki
under nitrogen protection. Then, a tetraethyl ammonium polycondensation in toluene under catalysis of tetrakis(tri-
hydroxide aqueous solution (2 ml, 20 wt/wt%) was added in phenylphosphine)palladium, and then purified by Soxhlet
one portion and the mixture was heated to 85 1C for 48 h. extraction in methanol, acetone, hexane and chloroform, respec-
Phenylboronic acid (0.609 g, 5 mmol) was added and reacted tively. The final polymer solution in chloroform was precipitated
for 12 h. Then bromobenzene (1.57 g, 10 mmol) was added in in methanol, filtered and dried at 60 1C under vacuum for
one portion and reacted for an additional 12 h. The reaction 24 hours. These four polymers have excellent solubility in general
mixture was cooled to room temperature and precipitated in organic solvents such as toluene, tetrahydrofuran, chloroform,
methanol. The crude polymer was purified by Soxhlet extrac- and chlorobenzene. Molecular weights were characterized by
tion in methanol, acetone, hexane and chloroform. The final gel permission chromatography (GPC), which shows that the
chloroform solution was precipitated in methanol, filtered and number-averaged molecular weights (Mn) and weight-averaged
dried at 80 1C a the vacuum oven for 10 hours to yield 162 mg of molecular weights (Mw) are 12 400, 11 700, 8700, 7600, and
the target polymer as a red solid. 1H NMR (CDCl3, 600 MHz, d): 26 800, 20 600, 12 400 and 10 700 with the polydispersity index
8.64 (s, 1H, Ar-H), 7.68–7.33 (br, 10H, Ar-H), 7.20–7.14 (br, 4H, (PDI) of 2.16, 1.76, 1.43 and 1.41 for P1, P2, P3 and P4,
Ar-H), 4.22–4.18 (m, 4H, CH2), 2.05–2.04 (m, 4H, CH2), respectively (Table 1). In comparison with P3 and P4, P1 and
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Scheme 1 Synthetic routes for the monomers.
Scheme 2 Synthetic routes for the polymers.
P2 have higher molecular weights due to the longer octyl side are summarized in Table 1. The degradation temperatures at 5%
chains in P1 and octoxyl in P2, which may facilitate the good weight loss of P1, P2, P3 and P4 are 324, 321, 321 and 321 1C,
solubility of the polymers and realize the increment of mole- respectively. All of them are thermally stable up to 300 1C, which
cular weights when polymerization occurs.
indicates that they potentially possess high stability for the
construction of PSCs.
Thermal stability
UV-vis absorption
The thermal stability of polymers is an indispensable parameter
for evaluating the performance of PSCs. Therefore, thermal The UV-vis absorption spectra of P1, P2, P3 and P4 in chloro-
gravimetric analysis (TGA) was used to characterize the polymers form and in film are shown in Fig. 2, and the data in detail
and the resulting curves are displayed in Fig. 1, and detailed data are summarized in Table 2. In Fig. 2, we can see that all
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Table 1 Molecular weights and thermal properties
Polymers
Mn
Mw
PDI
Td (1C)
P1
P2
P3
P4
12 400
11 700
8700
26 800
20 600
12 400
10 700
2.16
1.76
1.43
1.41
324
321
321
323
7600
Fig. 1 Thermal gravimetric analysis curves of the polymers.
the polymers, both in chloroform and in film, show two distinct
absorption peaks. The peaks at short wavelengths, which range
from 360 to 380 nm, are attributed to the localized p–p* transi-
tion, and the other peaks at longer wavelengths, which range
from 500 to 560 nm, are due to the internal charge transfer
(ICT) interaction between the donor and acceptor units in the
polymer main chains.36 The onsets of absorption spectra of P1,
P2, P3 and P4 in films are 649, 656, 643 and 640 nm, which
correspond to the optical band gaps of 1.91, 1.89, 1.93 and 1.94
eV, respectively. The absorption spectra in film are clearly red-
shifted by about 20 nm in comparison to that in chloroform,
which indicates that strong intermolecular p–p stacking exists
between the polymer backbones. In comparison with the
absorption spectra of P3, which has trifluoromethyl, and P4,
which has bis(trifluoromethyl), P1, which has octyloxy, and P2,
which has octyl, display much more red-shifted absorptions
and much lower optical band gaps. This result is possibly due
to the fact that octyloxy and octyl belong to strong electron-
donating groups that enhance the ICT effect and decrease the
band gaps of polymers, whereas the electron-withdrawing
groups of trifluoromethyl or bis(trifluoromethyl) weaken the
ICT effect and cause the absorption peaks of P3 and P4 to be
blue-shifted.
Fig. 2 UV-vis absorption spectra of the polymers in chloroform (a) and in
film (b).
Table 2 UV-vis absorption properties of the polymers
lmax, in chloroform
(nm)
lmax, in film
(nm)
lonset
(nm)
Eg, optical
(eV)
Polymer
P1
P2
P3
P4
371, 533
377, 547
360, 505
361, 516
381, 550
381, 558
370, 525
372, 542
649
656
643
640
1.91
1.89
1.93
1.94
Electrochemical properties
To further investigate the energy levels of the polymers, the
highest occupied molecular orbit (HOMO) and the lowest
unoccupied molecular orbit (LUMO) energy levels of P1, P2,
P3 and P4 were acquired from the cyclic voltammogram (CV)
Fig. 3 Cyclic voltammogram curves of the polymers in films.
characterization. Fig. 3 gives the CV curves of P1, P2, P3 and P4 0.53, 0.71, 0.76 and ꢀ1.61, ꢀ1.62, ꢀ1.62, ꢀ1.61 V versus SCE,
in films and the detailed data are summarized in Table 3. The respectively. The redox potential of the Fc/Fc+ internal reference is
CV curves present reversibly oxidative and reductive curves for 0.1 V vs. SCE. The HOMO and LUMO energy levels of P1, P2, P3 and
all the polymers, in which the onsets of the oxidation and P4, which were determined using the empirical formula EHOMO
=
reduction potentials for P1, P2, P3 and P4 are located at 0.45, ꢀe(Eox + 4.8 ꢀ E1/2,(Fc/Fc+)) and ELUMO = ꢀe(Ere + 4.8 ꢀ E1/2,(Fc/Fc+)),
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Table 3 Electrochemical properties of the polymers
Polymer
Eox (V)
Ere (V)
EHOMO (eV)
ELUMO (eV)
Eg, CV (eV)
P1
P2
P3
P4
0.45
0.53
0.71
0.76
ꢀ1.61
ꢀ1.62
ꢀ1.62
ꢀ1.61
ꢀ5.15
ꢀ5.23
ꢀ5.41
ꢀ5.46
ꢀ3.09
ꢀ3.08
ꢀ3.08
ꢀ3.09
2.06
2.15
2.33
2.37
are ꢀ5.15, ꢀ5.23, ꢀ5.41, ꢀ5.46 and ꢀ3.09, ꢀ3.08, ꢀ3.08,
ꢀ3.09 eV, respectively. It is found that the HOMO energy levels
decreased obviously from P1, P2, P3 to P4, which is because of
the electron-withdrawing ability of the substituted groups of
octoxy, octyl, trifluoromethyl and bis(trifluoromenthyl) in P1, Fig. 5 J–V characteristics of neat films of the polymers from SCLC devices.
P2, P3 and P4, which increase sequentially. Octoxy and octyl are
electron-donating groups and they promote the HOMO energy
levels, while trifluoromethyl and bis(trifluoromenthyl) are attri-
Table 4 Hole mobility of neat polymers
Polymer
Thickness (nm)
m (ꢁ 10ꢀ4, cm2 Vꢀ1 sꢀ1
)
buted to strong electron-withdrawing groups and they reduce
the HOMO energy levels. Therefore, it is a feasible method to
tune the energy levels of semiconducting polymers by adjusting
the properties of substituted groups in the construction units
(Fig. 4). On the other hand, the reduction potentials and LUMO
energy levels of P1, P2, P3 and P4 are very similar, because they
have the same accepting unit of bis(octyloxy)benzofurazan in
their polymer main chains, thus they show similar reductive
properties.
P1
P2
P3
P4
79
73
77
78
0.27
1.60
0.17
0.27
Due to the higher hole mobility of P2, it is more beneficial for
the improvement of the photovoltaic performance of the P2
based PSC (Table 4).
Hole mobility
Photovoltaic properties
Hole mobility is one of the indispensable parameters to evaluate
donor polymers in BHJ PSCs. In this study, the hole mobility of
neat polymers were measured using the space charge limited
current (SCLC) method with the device configuration of ITO/
PEDOT:PSS/active layer/MoO3/Al. The thickness of the active
layers is very similar, in which for P1 it is 79 nm, P2 it is 73 nm,
P3 it is 77 nm and P4 it is 78 nm. Fig. 5 shows the current
density–voltage ( J–V) curves of the neat polymer films from
SCLC devices. The calculated hole mobilities of P1, P2, P3 and
PSCs were fabricated under the device structure of ITO/PEDOT:
PSS/polymer:PC61BM/PFN/Al. Herein, PFN was utilized as a
cathode interfacial layer due to its versatile functions as a pro-
tecting active layer during cathode evaporation and the possible
existence of built-in electric fields between the active layer and
the cathode.37,38 The current density–voltage ( J–V) curves are
shown in Fig. 6a, and the photovoltaic properties in detail are
summarized in Table 5. The P2 based PSC exhibits a higher
short circuit current density ( Jsc), which is ascribed to its better
light absorption in the visible region and higher hole mobility
than P1, P3 and P4. The open circuit voltages (Voc) of the P1, P2,
P3 and P4 based PSCs increase from 0.70, 0.80, 0.95 to 1.00 V,
respectively, which result from the fact that the HOMO energy
levels of P1, P2, P3 to P4 decrease successively from ꢀ5.15,
ꢀ5.23, ꢀ5.41 to ꢀ5.46 eV, where the value of Voc is proportional
to the energy level difference between the HOMO energy level of
the donor and LUMO energy level of the acceptor.8 Herein, the
P4 based PSC gives the highest Voc of 1.00 V which is, to our
best knowledge, one of the highest Voc values based on poly-
triphenylamine derived broad band-gap polymers for PSCs. On
the other hand, although the P2 based PSC gives the lower Voc
of 0.80 V, it also exhibits the highest Jsc and FF among all four
polymers, which is ascribed to its higher absorption and hole
mobility. Comprehensively, the P2 based PSC has the best
photovoltaic performance with the Jsc of 4.84 mA cmꢀ2, FF of
50%, Voc of 0.80 V and PCE of 2.22%.
P4 are 2.7 ꢁ 10ꢀ5, 1.6 ꢁ 10ꢀ4, 1.7 ꢁ 10ꢀ5 and 2.7 ꢁ 10ꢀ5
,
respectively. In comparison to P1, P3 and P4, the hole mobility
of P2 is an order of magnitude higher, which is possible
because the octyl substituted TPA unit in P2 is straighter than
the other group substituted TPA derivatives in P1, P3 and P4,
and this effect can improve the polymer main chain stacking
and ultimately strengthen the hole transporting ability in P2.
The incident photon-to-current conversion efficiency (IPCE)
curves of the PSCs are shown in Fig. 6b. All the polymers display
the corresponding photon-to-current conversion effect, which is
Fig. 4 Energy level diagram of the polymers.
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Fig. 6 Current density–voltage (J–V) characteristics of polymers:PC61BM
under AM 1.5 G, 87 mW cmꢀ2 (a); incident photon-to-current conversion
efficiency (IPCE) curves of polymers:PC61BM blend films (b).
Table 5 Photovoltaic performances with the device structure of ITO/
PEDOT:PSS/polymer:PC61BM/PFN/Al under AM 1.5 G, 87 mW cmꢀ2
Fig. 7 AFM images of blend films (1 : 3 weight ratio for P1 and 1 : 2 weight
ratio for P2, P3 and P4); topography of P1: PC61BM (a), P2: PC61BM (b), P3:
PC61BM (c), and P4: PC61BM (d); phase images of P1: PC61BM (e), P2:
PC61BM (f), P3: PC61BM (g), and P4: PC61BM (h).
Polymer
Ratio
Jsc (mA cmꢀ2
)
FF (%)
Voc (V)
PCE (%)
P1
P2
P3
P4
1 : 3
1 : 2
1 : 2
1 : 2
2.06
4.84
2.57
1.87
43
50
36
38
0.70
0.80
0.95
1.00
0.86
2.22
1.28
1.05
According to the photovoltaic performances displayed in Table 5,
P1 and P2 exhibit higher FF values than P3 and P4, which are
ascribed to the better nano-scale phase separation in P1:PC61BM
and P2:PC61BM blend films.39
in agreement with the results of the absorption spectra. Among
these polymers, the P2 based PSC gives the best result with the
broadened and intensified IPCE in the visible region, which is
ascribed to its natural characteristic of better light absorption
than P1, P3 and P4.
Conclusions
In summary, a series of novel broad band-gap D–A copolymers
(P1, P2, P3, and P4) based on triphenylamine and benzofurazan
AFM morphologies
To study the interaction of the blend films between the polymers derivatives were prepared. The HOMO energy levels of these
and PC61BM, AFM images of the blend films were obtained, polymers can be tuned feasibly by changing the side chains
which are shown in Fig. 7. In the topography of Fig. 7a–d, the [–OC8H17, –C8H17, –CF3, –(CF3)2] in the triphenylamine unit.
root-mean-square (RMS) roughness of the blend films are 0.476, Their HOMO energy levels are decreased successively from
0.23, 2.488 and 2.550 nm for P1, P2, P3 and P4, respectively, ꢀ5.15, ꢀ5.23, ꢀ5.41 to ꢀ5.46 eV, as the electron-withdrawing
which means that the P1 and P2 based blend films are much ability of the octoxy, octyl, trifluoromethyl and bis(trifluoromenthyl)
smoother than the P3 and P4 based films. Furthermore, the substituted groups are increased sequentially. Octoxy and octyl
phase images (Fig. 7e–h) display that P1 and P2 based blend are electron-donating groups that promote the HOMO energy
films give better nano-scale phase separation, while the P3 and levels, while the trifluoromethyl and bistrifluoromenthyl are
P4 based blend films show worse phase separation and bigger the strong electron-withdrawing groups that should reduce the
aggregates, which inhibit holes and electrons transport and HOMO energy levels. As a result, the Vocs of the PSCs with
ultimately decrease their FF values and photovoltaic properties. the device structure of ITO/PEDOT:PSS/polymer:PC61BM/PFN/Al
410 | New J. Chem., 2016, 40, 402--412
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´
13 S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon,
D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics,
2009, 3, 297–303.
can be adjusted from 0.70, 0.80, 0.95 to 1.00 V. To the best of
our knowledge, the Voc of 1.00 V (P4) is one of the highest Voc
values based on polytriphenylamine derived narrow band-gap
polymers for PSCs. Therefore, it is a feasible way to achieve high
Vocs and thus high PCE values by optimizing the substituted
groups of the polymers. Among these four polymers, the one
with the octyl side chain shows the best photovoltaic perfor-
mance with the highest Jsc (due to the best light absorption in
visible region and highest hole mobility) and FF (resulting from its
14 R. P. Qin, W. W. Li, C. H. Li, C. Du, C. Veit, H. F.
Schleiermacher, M. Andersson, Z. S. Bo, Z. P. Liu,
¨
O. Inganas, U. Wuerfel and F. L. Zhang, J. Am. Chem. Soc.,
2009, 131, 14612–14613.
15 B. Zhang, X. W. Hu, M. Q. Wang, H. P. Xiao,
X. Gong, W. Yang and Y. Cao, New J. Chem., 2012, 36,
2042–2047.
better nano-scale phase separation), giving a Jsc of 4.84 mA cmꢀ2
,
16 B. Zhang, L. Yu, L. Fan, N. Wang, L. W. Hu and W. Yang,
New J. Chem., 2014, 38, 4587–4593.
FF of 50%, Voc of 0.80 V and PCE of 2.22%.
17 Y. J. Cheng, S. W. Cheng, C. Y. Chang, W. S. Kao,
M. H. Liao and C. S. Hsu, Chem. Commun., 2012, 48,
3203–3205.
Acknowledgements
This study was financially supported by the National Nature
Science Foundation of China (No. 51273069, 91333206), the
Research Fund for the Doctoral Program of Higher Education
of China (No. 20130172110005) and the Fundamental Research
Funds for the Central Universities (No. 2014ZB0017).
¨
18 X. F. Xu, C. F. Wang, O. Backe, D. I. James, K. Bini,
E. Olsson, M. R. Andersson, M. Fahlman and E. G. Wang,
Polym. Chem., 2015, 6, 4624–4633.
19 Y. Y. Liang, D. Q. Feng, Y. Wu, S. T. Tsai, G. Li, C. Ray and
L. P. Yu, J. Am. Chem. Soc., 2009, 131, 7792–7799.
20 Y. Y. Liang, Z. Xu, J. B. Xia, S. T. Tsai, Y. Wu, G. Li, C. Ray
and L. P. Yu, Adv. Mater., 2010, 22, E135–E138.
21 H. X. Zhou, L. Q. Yang, A. C. Stuart, S. C. Price, S. B. Liu and
W. You, Angew. Chem., Int. Ed., 2011, 50, 2995–2998.
22 S. C. Price, A. C. Stuart, L. Q. Yang, H. X. Zhou and W. You,
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