Synthesis, photophysical and photovoltaic properties of
a new class of two-dimensional conjugated polymers
containing donor
–acceptor chromophores as pendant
groups
†
Yu-Ying Lai, Yen-Ju Cheng,* Chiu-Hsiang Chen, Sheng-Wen Cheng, Fong-Yi Cao and Chain-Shu Hsu*
A new design for constructing two-dimensional conjugated copolymers with the D1–A(D2) repeating pattern (A: acceptor, D1: donor 1, D2: donor 2; molecules enclosed in parentheses are pendant groups.), is proposed. D1and A are employed to construct the linear main-conjugated polymer chain and D2is situated at the conjugated side chains connected to A. We designed and synthesized a series of D1– A(D2)-type copolymers, in which D1¼ diindeno[1,2-b:20,10-d]-thiophene (DIDT) or fluorene (F), A(D2)¼ bis-[4-(dioctylamino)-phenyl] quinoxaline (DOAQX) or bis-[4-(dioctylamino)-phenyl] thieno[3,4-b]-pyrazine (DOATP), and D2¼ N,N-dioctylanilines. The resultant copolymers, PDIDTDOAQX, PFDOAQX, PDIDTDOATP and PFDOATP, possess at least two strong ICT absorptions, thus resulting in better light harvesting. Their optical and electronic properties were thoroughly investigated experimentally and computationally. Bulk heterojunction photovoltaic cells on the basis of ITO/PEDOT:PSS/ polymer:PC71BM/Ca/Al configuration were fabricated and characterized. The photovoltaic performances of the devices incorporating these polymers follow the sequence: PDIDTDOAQX > PFDOAQX > PDIDTDOATP > PFDOATP, which is in good agreement with the magnitude of their hole-mobilities.
Introduction
Harvesting energy directly from sunlight using photovoltaic technology is considered one of the most promising approaches to address the growing global energy needs. Over the past few years, tremendous scientic effort has been made on organic photovoltaics (OPVs) to accomplish low-cost, lightweight, large-area, and exible photovoltaic devices.1 The development of novel conjugated polymers has been demonstrated to be effec-tive in improving the efficiency of OPV cells. To date, there are several design strategies for preparing the conjugated polymers which are classied into (a) D-based polymers,1e,2,3d (b) D–A alternating copolymers,3 (c) D–A random copolymers,4 (d) D1(D2) copolymers,5and (e) D1–D2(A) copolymers6(D: donor, A: acceptor, D1: donor 1, D2: donor 2). Parentheses are used to indicate that the enclosed molecules are the pendant groups in the polymer chains. As illustrated in Fig. 1, D-based, D–A alternating, and D–A random polymers belong to one-dimen-sional conjugated polymers and D1(D2) and D1–D2(A) polymers are categorized into two-dimensional conjugated polymers. The
concept of utilizing two-dimensional architecture can in prin-ciple broaden or intensify the absorption of the corresponding polymers. On the other hand, the donor–acceptor strategy has been established to be one of the most useful ways in preparing low band gap (LBG) polymers because of the efficient photo-induced intramolecular charge transfer (ICT) from the donor to the acceptor. Their optical properties can thus bene-tuned by adjusting the donating ability of the donor and/or the with-drawing strength of the acceptor. Consequently, two-dimen-sional-polymer construction in association with the D–A tactic, such as D1–D2(A), continues to receive attention.
Herein, we propose a new type of two-dimensional conjugated copolymer with the D1–A(D2) repeating pattern, where D1and A are used to construct the linear main-conjugated system and D2 is situated at the conjugated side chains connected to A (Fig. 2). Unlike the D1–D2(A) system, exchanging the position of D2 with A can not only preserve the ICT from D2 to A, but can further enhance the ICT from D1to A, resulting in better light harvesting. Based on this viewpoint, a series of D1–A(D2)-type copolymers were designed and synthesized, where D1 ¼ diin-deno[1,2-b:20,10-d]-thiophene (DIDT)3c,7oruorene (F), A(D
2)¼ (dioctylamino)-phenyl] quinoxaline (DOAQX) or bis-[4-(dioctylamino)-phenyl] thieno[3,4-b]pyrazine (DOATP), and D2 ¼ N,N-dioctylanilines (Fig. 2). Their thermal, optical, and electrochemical properties have been characterized and
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hseuh Road, Hsin-Chu, 30010 Taiwan. E-mail: [email protected]; cshsu@mail. nctu.edu.tw
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3py00168g
Cite this: Polym. Chem., 2013, 4, 3333
Received 31st January 2013 Accepted 13th March 2013 DOI: 10.1039/c3py00168g www.rsc.org/polymers
Polymer
Chemistry
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theoretical calculations were performed. Preliminary tests of the photovoltaic performance based on these polymers were carried out and investigated.
Experimental
General remarks
All chemicals were purchased from commercial sources and used as received unless otherwise specied. NMR measure-ments are reported for Varian Unity-300 spectrometers (1H, 300 MHz;13C, 75 MHZ). Chemical shis (d values) are reported in ppm with respect to Me4Si (d ¼ 0 ppm) for13C and1H NMR. Coupling constants ( J) are given in Hz.13C NMR was proton
broad-band-decoupled. Multiplicities of peaks are denoted by the following abbreviations: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad. The molecular weight of polymers was determined by gel permeation chromatography on a Viscotek VE2001GPC instrument. Polystyrene was used as the internal standard and THF is the eluent. Thermogravimetric analyses (TGA) were recorded on a PerkinElmer Pyris analyzer under nitrogen atmosphere at a heating rate of 10C min1. Absorp-tion spectra were collected on a HP8453 UV-Vis spectropho-tometer. Electrochemical cyclic voltammetry (CV) was conducted on a Bioanalytical Systems Inc. analyzer. A carbon glass coated with a thin polymerlm was used as the working electrode and Ag/AgCl as the reference electrode, while 0.1 M tetrabutylammonium hexauorophosphate in acetonitrile was the electrolyte. CV curves were calibrated using ferrocene as the standard, whose oxidation potential is set at 4.8 eV with respect to zero vacuum level. The HOMO energy levels were obtained from the equation HOMO¼ (Eonsetox Eonset(ferrocene)+ 4.8) eV. The LUMO levels were obtained from the equation LUMO¼ (Eonset
red Eonset(ferrocene)+ 4.8) eV.
Fabrication and characterization of BHJ devices
An indium tin oxide (ITO)-coated glass substrate was ultrasoni-cally cleaned sequentially by detergent, water, acetone and iso-propyl alcohol. It was then covered by a 30 nm thick layer of PEDOT:PSS (Clevios P provided by H. C. Stark) by spin-coating, thermally annealed in air at 200C for 10 min, and cooled down to room temperature. PDIDTDOAQX, PFDOAQX, PDIDTDOATP and PFDOATP were dissolved in o-dichlorobenzene (ODCB) (0.5– 1.0 wt%), respectively and mixed with PC71BM (purchased from Nano-C). The individual solutions were heated at 70 C for 1 hour, stirred overnight at room temperature, and ltered through a 0.45mm lter. The ltrates were then spin-coated on top of the PEDOT:PSS layer at various spin-coating speeds in a glove box in order to tune theirlm thickness. Subsequent to drying, they were thermally annealed for 15 min. The spin-coating speeds for polymer/PC71BM are described as follows:
Fig. 1 Molecular architecture for constructing conjugated polymers.
Fig. 2 D1–A(D2)-type two-dimensional copolymers where D1¼
diindeno[1,2-b:20,10-d]-thiophene (DIDT) or fluorene (F), A(D2)¼ bis-[4-(dioctylamino)-phenyl]
quinoxaline (DOAQX) or bis-[4-(dioctylamino)-phenyl] thieno[3,4-b]pyrazine (DOATP), and D2¼ N,N-dioctylanilines.
PDIDTDOAQX/PC71BM (1000 rpm), PFDOAQX/PC71BM (1400 rpm), PDIDTDOATP/PC71BM (1400 rpm), and PFDOATP/PC71BM (1400 rpm). The top electrode was then prepared by sequential evaporation of calcium (35 nm thick) and aluminum (100 nm thick) through a shadow mask under high vacuum (<106torr). All devices contained an active area of 0.04 cm2and the photo-voltaic parameters were measured under air atmosphere. Electrical characterization under illumination
The devices were characterized under 100 mW cm2 AM1.5 simulated light (Yamashita Denso solar simulator). Current– voltage ( J–V) characteristics of the devices were obtained using a Keithley 2400 SMU. Solar illumination conforming to the JIS Class AAA was provided by a SAN-EI 300W solar simulator equipped with an AM1.5G lter. The light intensity was cali-brated with a Hamamatsu S1336-5BK silicon photodiode. The performances are the average of the 4 pixels of each device. Hole-only devices
In order to investigate the respective hole mobilities of the different polymer lms, unipolar devices have been prepared following the same procedure except that the active layer is made of pure polymer and the Ca/Al cathode is replaced by evaporated gold (40 nm). The hole mobilities were calculated according to the space charge limited current theory (SCLC). The J–V curves were tted according to the following equation:
J ¼983mV 2 L3
where 3 is the dielectric permittivity of the polymer, m is the hole mobility and L is thelm thickness (distance between the two electrodes).
Synthesis of 1,2-bis(4-dioctylaminophenyl)ethane-1,2-dione (4) To a solution of 2 (31.4 g, 79.2 mmol) in dry THF (125 mL) under nitrogen was added a hexane solution of n-BuLi (2.5 M, 65.0 mmol) dropwise at 78 C. The mixture was stirred at this temperature for 30 min and 3 (5.0 g, 35.2 mmol) was then introduced in one portion. Aer stirring at 78C for 2 h, it was
gradually warmed up to room temperature, quenched with 10% HCl solution (100 mL), and extracted with ethyl acetate (400 mL 3). The collected organic layer was washed with water (200 mL), dried over MgSO4, and evaporated under reduced pressure. The residue was then puried by column chromatography on silica gel (hexane–ethyl acetate, v/v, 50/1) to give a deep yellow oil 4 (17.2 g, 71%):1H NMR (CDCl3, 300 MHz): 0.88 (t, J¼ 6.5 Hz, 12H), 1.16– 1.42 (m, 40H), 1.50–1.70 (m, 8H), 3.31 (t, J ¼ 7.7 Hz, 8H), 6.57 (d, J ¼ 9.0 Hz, 4H), 7.82 (d, J ¼ 9.0 Hz, 4H);13C NMR (CDCl 3, 75 MHz): 14.1, 22.6, 27.0, 27.1, 29.2, 29.4, 31.8, 51.1, 110.5, 120.9, 132.4, 152.4, 193.7; MS (EI, M+_, C 46H76N2O2): calcd, 688.59; found, 689. Synthesis of 3,6-bis(5-bromothiophen-2-yl)benzene-1,2-diamine (8)
To a solution of 7 (2.0 g, 4.36 mmol) in acetic acid (100 mL) was added zinc dust (5.82 g, 89.0 mmol) in one portion. The reaction
mixture was reuxed for 30 min, cooled to room temperature, andltered. The resulting solid was collected, dissolved in ether (70 mL), and washed with 5% NaOH(aq). Aer removal of the solvent under reduced pressure, the residue was puried by column chromatography on silica gel (hexane–ethyl acetate, v/v, 10/1) and recrystallized from methanol to give a white powder 8 (0.77 g, 41%):1H NMR (CDCl
3, 300 MHz): 3.83 (br, 4H), 6.79 (s, 2H), 6.92 (d, J¼ 3.8 Hz, 2H), 7.09 (d, J ¼ 3.8 Hz, 2H);13C NMR (CDCl3, 75 MHz): 112.1, 120.6, 121.2, 126.6, 130.4, 133.1, 142.5; MS (EI, M+_, C14H10Br2N2S2): calcd, 429.86; found, 430.
Synthesis of 2,3-bis(4-dioctylaminophenyl)-5,8-bis(5-bromothio-phen-2-yl)quinoxaline (9)
8 (0.70 g, 1.62 mmol), 4 (1.68 g, 2.44 mmol), and acetic acid (85 mL) were mixed together and heated at reux for 3 h. Aer removal of acetic acid under reduced pressure, the residue was puried by column chromatography on silica gel (hexane–ethyl acetate, v/v, 250/1) to give a red oil 9 (1.19 g, 68%): 1H NMR (CDCl3, 300 MHz): 0.89 (t, J¼ 7.5 Hz, 12H), 1.20–1.40 (m, 40H), 1.56–1.65 (m, 8H), 3.31 (t, J ¼ 7.5 Hz, 8H), 6.64 (d, J ¼ 8.4 Hz, 4H), 7.10 (d, J¼ 4.2 Hz, 2H), 7.52 (d, J ¼ 4.2 Hz, 2H), 7.74 (d, J ¼ 8.4 Hz, 4H), 7.91 (s, 2H);13C NMR (CDCl3, 75 MHz): 14.1, 22.7, 27.2, 27.3, 29.4, 29.5, 31.8, 51.0, 110.8, 116.4, 124.4, 125.0, 125.4, 128.9, 130.0, 131.8, 136.1, 140.3, 148.9, 152.3; MS (FAB, (M + H)+, C60H83Br2N4S2): calcd, 1083.44; found, 1084. Anal. calcd for C60H82Br2N4S2: C, 66.53; H, 7.63; N, 5.17; found: C, 66.94; H, 7.83; N, 4.97%.
Synthesis of 2,3-bis(4-dioctylaminophenyl)-5,7-bis(5-bromothio-phen-2-yl)thieno[3,4-b]pyrazine (14)
13 (0.60 g, 1.38 mmol), 4 (1.04 g, 1.51 mmol), and acetic acid (32 mL) were mixed together and heated at 60C for 5 h. Aer removal of acetic acid under reduced pressure, the residue was puried by column chromatography on silica gel (hexane–ethyl acetate, v/v, 250/1) to give a red solid 14 (0.82 g, 55%):1H NMR (CDCl3, 300 MHz): 0.89 (t, J¼ 6.9 Hz, 12H), 1.20–1.40 (m, 40H), 1.50–1.70 (m, 8H), 3.30 (t, J ¼ 7.5 Hz, 8H), 6.60 (d, J ¼ 8.9 Hz, 4H), 7.02 (d, J¼ 4.1 Hz, 2H), 7.24 (d, J ¼ 4.1 Hz, 2H), 7.60 (d, J ¼ 8.9 Hz, 4H);13C NMR (CDCl3, 75 MHz): 14.1, 22.7, 27.2, 27.3, 29.3, 29.5, 31.9, 51.1, 110.7, 113.6, 122.1, 123.0, 125.8, 129.6, 131.5, 136.7, 137.7, 149.1, 153.6; MS (FAB, (M + H)+, C58H81Br2N4S3): calcd, 1089.40; found, 1090. Anal. calcd for C58H80Br2N4S3: C, 63.95; H, 7.40; N, 5.14; found: C, 64.04; H, 7.63; N, 4.93%.
Synthesis of PDIDTDOAQX
A mixture of 9 (410 mg, 0.38 mmol), 2,8-bis(4,4,5,5-tetramethyl- [1,3,2]dioxaborolan-2-yl)-10,10,11,11-tetraoctyl-diindeno[1,2-b:2010-d]thiophene (364 mg, 0.38 mmol), Pd(PPh3)4 (8.7 mg, 0.0076 mmol), K2CO3 (395 mg, 2.86 mmol), Aliquant 336 (65 mg, 0.16 mmol), degassed toluene (20 mL), and degassed H2O (4 mL) was heated to 90C under nitrogen atmosphere for 72 h. It was then added to methanol dropwise. The precipitate was collected by ltration and washed by Soxhlet extraction with methanol and acetone sequentially for one week. The crude polymer was dissolved in hot THF and the residual Pd catalyst
in the THF solution was removed by Pd-thiol gel (Silicycle Inc.). Aer ltration and removal of the solvent, the polymer was re-dissolved in THF again and reprecipitated by methanol. The resultant polymer was collected byltration and dried under vacuum for 1 day to give a black solid (490 mg, 79%, Mn ¼ 39 000, PDI¼ 2.00):1H NMR (CDCl
3, 300 MHz): 0.60–1.80 (m, 120H), 2.00–2.25 (m, 8H), 3.20–3.60 (m, 8H), 6.60–6.80 (m, 4H), 7.30–8.10 (m, 16H).
Synthesis of PFDOAQX
A mixture of 9 (280 mg, 0.26 mmol), 2,7-bis(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-9,9-dioctyluorene (166 mg, 0.26 mmol), Pd(PPh3)4(6.0 mg, 0.005 mmol), K2CO3(270 mg, 1.95 mmol), Aliquant 336 (37 mg, 0.09 mmol), degassed toluene (15 mL), and degassed H2O (3 mL) was heated to 90 C under nitrogen atmosphere for 72 h. It was then added to methanol dropwise. The precipitate was collected byltration and washed
by Soxhlet extraction with methanol and acetone sequentially for one week. The crude polymer was dissolved in hot THF and the residual Pd catalyst in the THF solution was removed by Pd-thiol gel (Silicycle Inc.). Aer ltration and removal of the solvent, the polymer was re-dissolved in THF again and repre-cipitated by methanol. The resultant polymer was collected by ltration and dried under vacuum for 1 day to give a dark red solid (250 mg, 73%, Mn¼ 22 000, PDI ¼ 1.23):1H NMR (CDCl3, 300 MHz): 0.60–1.80 (m, 90H), 2.00–2.25 (m, 4H), 3.30–3.50 (m, 8H), 6.60–6.80 (m, 4H), 7.40–7.60 (m, 2H), 7.70–8.10 (m, 14H). Synthesis of PDIDTDOATP
A mixture of 14 (400 mg, 0.37 mmol), 2,8-bis(4,4,5,5-tetra- methyl-[1,3,2]dioxaborolan-2-yl)-10,10,11,11-tetraoctyl-diindeno-[1,2-b:2010-d]thiophene (353 mg, 0.37 mmol), Pd(PPh3)4(8.5 mg, 0.007 mmol), K2CO3(383 mg, 2.77 mmol), Aliquant 336 (63 mg, 0.16 mmol), degassed toluene (20 mL), and degassed H2O
Scheme 1 Synthesis of DOAQX and DOATP.
(4 mL) was heated to 90C under nitrogen atmosphere for 72 h. It was then added to methanol dropwise. The precipitate was collected by ltration and washed by Soxhlet extraction with methanol and acetone sequentially for one week. The crude polymer was dissolved in hot THF and the residual Pd catalyst in the THF solution was removed by Pd-thiol gel (Silicycle Inc.). Aer ltration and removal of the solvent, the polymer was re-dissolved in THF again and reprecipitated by methanol. The resultant polymer was collected byltration and dried under vacuum for 1 day to give a black solid (213 mg, 35%, Mn ¼ 12 000, PDI¼ 1.17):1H NMR (CDCl3, 300 MHz): 0.60–1.50 (m, 112H), 1.50–1.75 (m, 8H), 1.90–2.20 (m, 8H), 3.20–3.40 (m, 8H), 6.55–6.70 (m, 4H), 7.30–7.45 (m, 4H), 7.50–7.60 (m, 2H), 7.60– 7.80 (m, 8H).
Synthesis of PFDOATP
A mixture of 14 (270 mg, 0.25 mmol), 2,7-bis(4,4,5,5-tetra-methyl-[1,3,2]dioxaborolan-2-yl)-9,9-dioctyluorene (159 mg, 0.25 mmol), Pd(PPh3)4(5.7 mg, 0.005 mmol), K2CO3(259 mg, 1.87 mmol), Aliquant 336 (43 mg, 0.11 mmol), degassed toluene (14 mL), and degassed H2O (3 mL) was heated to 90C under nitrogen atmosphere for 72 h. It was then added to methanol dropwise. The precipitate was collected byltration and washed by Soxhlet extraction with methanol and acetone sequentially for one week. The crude polymer was dissolved in hot THF and the residual Pd catalyst in the THF solution was removed by Pd-thiol gel (Silicycle Inc.). Aer ltration and removal of the solvent, the polymer was re-dissolved in THF again and repre-cipitated by methanol. The resultant polymer was collected by ltration and dried under vacuum for 1 day to give a dark red solid (132 mg, 40%, Mn¼ 12 000, PDI ¼ 1.17):1H NMR (CDCl3, 300 MHz): 0.60–1.70 (m, 90H), 1.90–2.20 (m, 4H), 3.20–3.40 (m, 8H), 6.55–6.70 (m, 4H), 7.40–7.45 (m, 2H), 7.50–7.80 (m, 12H).
Results and discussion
Synthesis
Synthesis of DOAQX and DOATP moieties is depicted in Scheme 1. Aniline was treated with an excess amount of n-octylbromine in the presence of potassium carbonate and potassium iodide in ethanol to give 1 in 73% yield. Bromination of 1 occurred selectively at the para position by N-bromosuccinimide (NBS) to furnish compound 2. Amidation of diethyl oxalate in anhydrous diethylether yielded compound 3. The key intermediate dike-tone 4 with two N,N-dioctylaniline groups was obtained in 71% yield by treatment of the lithiumated compound 2 with compound 3 in THF. Bromination of benzothiadiazole with Br2/ HBr regio-selectively occurred at the 4,7 positions to form compound 5 in 68% yield. Compound 5 was then reacted with tributyl(thiophen-2-yl)stannane in the presence of catalytic amounts of Pd(PPh3)2Cl2to give compound 6. Compound 7 was synthesized by bromination of compound 6 with NBS, followed by zinc-promoted reduction to form diamine 8. The formation of quinoxaline dithiophene 9 (DOAQX) was achieved by double condensation of 8 with 4. The other monomer thieno[3,4-b]-pyrazine dithiophene 14 (DOATP) was synthesized through
analogous steps to 9, which are illustrated in Scheme 1. The A(D2) units, DOAQX and DOATP were then polymerized respectively with D1 units, 2,8-diboronic ester-DIDT3cand 2,7-diboronic ester-uorene via Suzuki cross-coupling to give alternating copolymers, poly(diindenothiophene-alt-bis-[4-(dioctylamino)-phenyl]quinoxaline) PDIDTDOAQX, poly-(uorene-alt-bis-[4-(dioctylamino)-phenyl]quinoxaline) PFDOAQX, poly(diindenothiophene-alt-bis-[4-(dioctylamino)phenyl]thieno-[3,4-b]pyrazine) PDIDTDOATP and poly(uorene-alt-bis-[4-(dio-ctylamino)-phenyl]thieno[3,4-b]pyrazine) PFDOATP, respectively (Scheme 2). All the intermediates, monomers and correspond-ing copolymers were fully characterized by NMR spectroscopy. The copolymers puried by successive reprecipitation and Soxhlet extraction have narrow molecular weight distributions with a polydispersity index (PDI) below two (Table 1). All of the resulting copolymers show excellent solubility in common organic solvents, such as THF, chloroform, toluene and 1,2-dichlorobenzene.
Thermal properties
PDIDTDOAQX, PFDOAQX, PDIDTDOATP and PFDOATP were analyzed by thermal gravimetric analysis (TGA) and the results are summarized in Table 1. The decomposition temperatures (Td) of the polymers are situated between 402 and 433 C
Scheme 2 Synthesis of PDIDTDOAQX, PFDOAQX, PDIDIDOATP, and PFDOATP.
(Fig. 3), indicating their sufficient thermal stability for polymer-solar-cell (PSC) applications.
Optical properties
Initially, the absorption spectra of the monomer DOATP and an analogue without additional N,N-dialkylaniline groups on thieno[3,4-b]pyrazine, DEHTP (Chart 1) are compared in Fig. 4 in order to investigate the inuence of the extra donating groups. DOATP possesses much stronger absorption in the region from 375 to 475 nm than DEHTP. This extra absorption band facilitates photon harvesting in the visible-light region, which is in resonance with our supposition that anking donating groups can induce additional ICT while photo-excited. The absorption spectra of all studied polymers are illustrated in
Fig. 5 (in toluene solution) and Fig. 6 (in thin lm), and the corresponding optical parameters are summarized in Table 2. The thin-lm spectra do not show signicant difference to those in toluene solution except that bathochromic shis and band-broadening behaviors of the absorption bands were observed in the solid state, suggesting that all polymers in the solid state have stronger intermolecular interactions. All the polymers exhibit two distinct absorption bands. Detailed assignment of the absorption bands with the help of DFT calculations is described in the ESI.† The optical band gaps (Eopt
g ) estimated from the absorption edges of thinlm spectra follow the order: PFDOAQX (2.03 eV) > PDIDTDOAQX (1.97 eV) > PFDOATP (1.62 eV) > PDIDTDOATP (1.57 eV). By comparison of these data, it could be deduced that the accepting ability of DOATP is stronger than that of DOAQX.
With the purpose of further examining the optical effect of the N,N-dioctylaniline groups on the polymer chains, the interaction between PDIDTDOATP and B(C6F5)3 was investi-gated. Given that B(C6F5)3 binds to the Lewis basic nitrogen atoms on the N,N-dioctylaniline moieties, it will result in a
Table 1 Molecular weights, polydispersity and thermal properties of polymers
Copolymer Mwa Mna PDIa Tdb(C)
PDIDTDOAQX 78 000 39 000 2.00 433
PFDOAQX 27 000 22 000 1.23 432
PDIDTDOATP 14 000 12 000 1.17 409
PFDOATP 14 000 12 000 1.17 402
aMolecular weights were determined by gel permeation chromatography (GPC) in THF using polystyrene as the standard. bOnset decomposition temperature (5% weight loss) measured by TGA.
Fig. 3 Thermal gravimetric analysis (TGA) measurements of PDIDTDOAQX, PFDOAQX, PDIDTDOATP and PFDOATP with a ramping rate of 10C min1.
Chart 1 Structure of DEHTP.
Fig. 4 Normalized absorption spectra of DOATP and DEHTP in the toluene solution.
Fig. 5 Normalized absorption spectra of PDIDTDOAQX, PFDOAQX, PDIDT-DOATP and PFPDIDT-DOATP in toluene solution (0.1 mg per 4 mL).
diminution in the ICT band induced by the N,N-dioctylaniline units and bathochromic shis of certain absorption bands.8 Based on the calculation results illustrated in Fig. S1,† the ICT absorptions resulted from the N,N-dioctylaniline groups are primarily located in the valley from 500–600 nm. As illustrated in Fig. 7, subsequent to the addition of B(C6F5)3to the toluene solution of PDIDTDOATP, the absorption intensity of this valley decreased signicantly, indicating that the nitrogen atoms on the N,N-dioctylaniline moieties coordinate to B(C6F5)3. The thienopyrazine unit is considered to be less nucleophilic and sterically shielded in PDIDTDOATP. Therefore, complexation of the thienopyrazine unit with B(C6F5)3is less likely. Coordina-tion to B(C6F5)3 should also make the thienopyrazine moiety more electron-decient, resulting in the enhancement of the ICT intensity, which is not the case in our observation.
Furthermore, an obvious bathochromic shi was observed for the absorption peak at 632 nm which is mainly the ICT from DIDT to thienopyrazine. Since the complexation of B(C6F5)3 with the N,N-dioctylaniline units would make the thienopyr-azine group more electron-decient, the energy band gap of the ICT band from DIDT to thienopyrazine is thus lowered, result-ing in the evident bathochromic shi.
Electrochemical properties
Cyclic voltammetry (CV) was employed to examine the electro-chemical properties and evaluate the HOMO and LUMO energy levels of the polymers. The spectra are illustrated in Fig. 8 and the results are summarized in Table 3. All polymers reveal stable and reversible p-doping and n-doping processes that are important prerequisites for p-type semiconductor materials. The LUMO energy levels of PDIDTDOAQX, PFDOAQX, PDIDT-DOATP, and PFDOATP are determined to be 3.29, 3.27, 3.61, 3.59 eV, respectively. It is evident that with the same
Fig. 6 Normalized absorption spectra of PDIDTDOAQX, PFDOAQX, PDIDT-DOATP and PFPDIDT-DOATP in thinfilm.
Table 2 Optical properties and band gaps of the model compounds and copolymers in toluene solution and in thinfilma
Polymer
Toluene solution Thinlm
lmax (nm) lonset (nm) Eopt g (eV) lmax (nm) lonset (nm) Eopt g (eV) DOATP 507 638 1.94 364 320 DEHTP 525 619 2.00 354 322 PDIDTDOAQX 425, 533 614 2.02 434, 580 629 1.97 PFDOAQX 389, 494 580 2.14 392, 514 611 2.03 PDIDTDOATP 452, 632 747 1.66 464, 656 790 1.57 PFDOATP 433, 632 726 1.71 441, 640 765 1.62 aEopt
g was estimated from the onset of UV absorption.
Fig. 7 Normalized absorption spectra of PDIDTDOATP (0.064 mg) in toluene (4 mL) solution with and without excess amounts of B(C6F5)3(4 mg).
Fig. 8 Cyclic voltammograms of PDIDTDOAQX, PFDOAQX, PDIDTDOATP and PFDOATP in thinfilms at a scan rate of 80 mV s1.
acceptor, either DOAQX or DOATP, the LUMO energy levels of the conjugated polymers do not vary appreciably with variation of the donor unit, indicating that the LUMO energy level is primarily decided by the acceptor. Furthermore, all LUMO energy levels are higher than that of the n-type material, PC71BM (3.8 eV), ensuring energetically favorable electron transfer. The HOMO energy levels of PDIDTDOAQX, PDIDT-DOATP, PFDOAQX, and PFDOATP are estimated to be 5.26, 5.18, 5.30, and 5.21 eV. With the same donor (DIDT or F), the energy level of the HOMO of the polymers rises considerably when the acceptor unit changes from DOAQX to DOATP, sug-gesting that the electron density of the HOMO for the studied polymers is not only localized on the donor unit but also distributed elsewhere. An energy diagram of HOMO and LUMO levels of PDIDTDOAQX, PFDOAQX, PDIDTDOATP and PFDOATP relative to PC71BM is shown in Fig. 9.
Theoretical calculations
The absorption optical characteristics of the conjugated mole-cules were investigated at the
B3LYP/6-311G(d,P)//TD-B3LYP/6-311G(d,p) level of theory by applying the polarized continuum model (PCM). The computational details are described in the ESI.† Considering an insignicant effect on electronic proper-ties, all the aliphatic substituents were replaced with methyl groups for simplicity. Repeating units, denoted as DIDTDOAQX, FDOAQX, DIDTDOATP, and FDOATP, in their most stable conformations were used as simplied model compounds for PDIDTDOAQX, PFDOAQX, PDIDTDOATP, and PFDOATP, respectively. The calculated HOMO/LUMO energy, excitation energy, oscillator strength, and congurations of the excited states are summarized in Table 4 and the frontier orbitals, HOMO (H) and LUMO (L) are illustrated in Fig. 10. Although there are variations in the absolute values, the calculated absorptions are still in good agreement with the experimental values. First of all, by comparison of the electronic transitions of DOATP and DEHTP, it shows that DOATP possesses noticeable absorptions between 629 and 378 nm, which are yet absent in DEHTP. Analogous phenomena are observed as well in the experimental absorption spectra, albeit the variations in the absolute values exist. In order to have further understanding of these electronic transitions, electron density difference maps (EDDMs) were conducted.9 The electronic transitions can therefore be visualized through EDDMs. Red indicates a decrease in charge density, while green indicates an increase. For DEHTP, the two major transitions (lcalc¼ 597 and 362 nm) are illustrated in Fig. 11, which reveals that both absorptions mainly derive from the ICT from the thiophene moieties to the pyrazine unit. Furthermore, the electron redistributions of DEHTP at 597 and 362 nm are similar to those of DOATP at 629 and 378 nm, where the contribution of theanking N,N-dime-thylaniline groups is insignicant (Fig. 11). In contrast, the additional absorption bands at 584 and 500 nm in DOATP
Table 3 Electrochemical onset potentials and electronic energy levels of the polymers Copolymer Eonsetox (V) Eonsetred (V) HOMO (eV) LUMOel (eV) LUMOopta (eV) Eelg (eV) PDIDTDOAQX 0.46 1.85 5.26 2.95 3.29 2.31 PFDOAQX 0.50 1.90 5.30 2.90 3.27 2.40 PDIDTDOATP 0.38 1.65 5.18 3.15 3.61 2.03 PFDOATP 0.41 1.73 5.21 3.07 3.59 2.14
aLUMOopt¼ HOMO + Eopt
g .
Fig. 9 Energy diagram of HOMO–LUMO levels for PDIDTDOAQX, PFDOAQX, PDIDTDOATP, PFDOATP and PC71BM.
primarily result from the intramolecular charge transfer from the N,N-dimethylaniline groups to the TP unit. The calculation results strongly suggest that anking donating groups can induce additional ICT absorptions when photo-excited and validate that our strategy of employing theanking donating groups to build the D–A(D) two-dimensional conjugated poly-mer is an effective approach to enhance the absorption ability of the PSC devices.
For the polyaromaticp-electron systems, consistent with the electrochemical experiments, the calculated HOMO energy levels of the four model compounds follow the order: FDOAQX (5.09 eV) < DIDTDOAQX (4.96 eV) < FDOATP (4.81 eV) < DIDTDOATP (4.75 eV) and the calculated LUMO energy levels are in the following sequence: DIDTDOATP (2.59 eV) z FDOATP (2.59 eV) < DIDTDOAQX (2.30 eV) < FDOAQX (2.29 eV). Furthermore, as demonstrated in Fig. 10, for all model compounds, the LUMO electron density is mainly located on the acceptors (DOAQX and DOATP), supporting the deduction
obtained from the electrochemical measurements that the LUMO energy level is primarily decided by the acceptor. The HOMO electron density is not only distributed on the donor but also elsewhere, which is also in resonance with the experi-mental results. The EDDMs of the polymers are depicted in the ESI.†
Hole-mobility and photovoltaic characteristics
Bulk heterojunction PSCs were fabricated on the basis of ITO/ PEDOT:PSS/polymer:PC71BM/Ca/Al conguration and their performances were measured under 100 mW cm2AM1.5 illu-mination. Hole-only devices (ITO/PEDOT:PSS/polymer/Au) were also fabricated in order to estimate the hole mobilities of these polymers by means of the space-charge limit current (SCLC) theory. The photovoltaic characteristics are summarized in Table 5 and the J–V curves of these polymers are shown in Fig. 12. The hole mobilities of the polymers are in the following
Table 4 CalculatedaHOMO/LUMO energy, excitation energy, oscillator strength, and configuration (with large CI coefficients) of the excited state
Compound HOMO [ev] LUMO [ev]
Excitation energy
Oscillator
strength Symmetry Congurationc
lmax,expb[nm] lcalc[nm] DEHTP 5.29 2.86 597 0.5155 Singlet-A H/L 362 0.6186 Singlet-A H/L+1 DOATP 5.02 2.71 629 0.2715 Singlet-A H/L 584 0.4007 Singlet-A H1/L 500 0.2212 Singlet-A H2/L 378 0.1884 Singlet-A H3/L H/L+1 H6/L 373 0.6874 Singlet-A H/L+2 DIDTDOAQX 4.96 2.30 533 551 0.875 Singlet-A H/L H1/L 514 0.3267 Singlet-A H/L H1/L 456 0.0877 Singlet-A H2/L H3/L 425 427 0.9777 Singlet-A H3/L H/L+1 FDOAQX 5.09 2.29 494 527 0.4946 Singlet-A H/L H1/L 508 0.406 Singlet-A H/L H1/L 448 0.1914 Singlet-A H2/L 389 386 0.5775 Singlet-A H/L+1 H3/L H1/L+1 DIDTDOATP 4.75 2.59 632 684 0.7262 Singlet-A H/L 570 0.3551 Singlet-A H1/L 508 0.1155 Singlet-A H3/L H2/L 452 466 0.9754 Singlet-A H/L+1 H2/L FDOATP 4.81 2.59 632 662 0.5282 Singlet-A H/L 569 0.3659 Singlet-A H1/L 493 0.1781 Singlet-A H2/L 433 433 0.6958 Singlet-A H/L+1 H3/L aTD-B3LYP/6-311G(d,p), PCM ¼ toluene. bExperimental values were measured for non-simplied compounds in the toluene solution. cCongurations with largest coefficients in the CI expansion of each state are highlighted in boldface.
order: PFDOATP (3 106cm2V1s1)z PDIDTDOATP (3 106 cm2 V1 s1) < PFDOAQX (1 105 cm2 V1 s1) < PDIDTDOAQX (5 105 cm2 V1 s1), suggesting that the DOAQX-based polymers have higher hole mobility than the
DOATP-based polymers. The device based on PDIDTDOAQX/ PC71BM and PFDOAQX/PC71BM showed PCEs of 1.94 and 1.65%, respectively, which are higher than the analogous devices based on PFDOATP/PC71BM (PCE ¼ 0.5%) and
Fig. 10 Plots (isovalue¼ 0.02 au) of frontier orbitals of DEHTP, DOATP, DIDTDOAQX, FDOAQX, DIDTDOATP, and FDOATP, calculated at the level of B3LYP/6-311G(d,P) in THF.
PDIDTDOATP/PC71BM (PCE ¼ 0.77%). The performances of these solar devices are in good agreement with their Jscvalues, which are highly associated with the hole mobilities of the polymers. Besides, the molecular weight of polymers may also affect the device efficiency. PDIDTDOAQX with the highest
molecular weight among the four polymers has the highest PCE. It is likely that longer polymer chains could enhance the hole mobility of polymers and provide more contact between polymers and fullerenes, leading to higher device performances.
Conclusions
We have successfully designed and synthesized a series of novel two-dimensional conjugated copolymers with the D1–A(D2) repeating pattern. This construction strategy can result in at least two strong ICT absorptions and therefore superior light harvesting, which was evaluated experimentally by absorption spectroscopy and computationally by DFT calculations. The photovoltaic performances of the devices incorporating these polymers follow the sequence: PDIDTDOAQX > PFDOAQX > PDIDTDOATP > PFDOATP, which is in good agreement with the trend of their hole-mobilities. The device based on PDIDT-DOAQX/PC71BM exhibited the highest Jscof 6.79 mA cm2and a PCE of 1.94%.
Notes and references
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Fig. 11 Electron density difference maps (EDDMs) of selected singlet electronic transitions of DEHTP (at 597 and 362 nm) and DOATP (at 629, 584, 500, and 378 nm). Red indicates a decrease in charge density, while green indicates an increase. All EDDMs are plotted with isovalue 0.0012 au.
Table 5 Photovoltaic characteristics
Copolymer Polymer:PC71BM (wt %) Mobility (cm2V1s1) V oc(V) Jsc (mA cm2) FF (%) PCE (%) PDIDTDOAQX 1 : 1 5 105 0.62 6.79 46 1.94 PFDOAQX 1 : 1 1 105 0.72 5.52 41 1.65 PDIDTDOATP 1 : 1 3 106 0.60 3.11 41 0.77 PFDOATP 1 : 1 3 106 0.68 2.25 32 0.50
Fig. 12 J–V characteristics of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al under
illumination of AM1.5, 100 mW cm2.
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