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Oct 23, 2016 - Wenqin Li, Wene Shi, Zihua Wu, Jinmin Wang, Min Wu, Wei-Hong Zhu ...... P. Chen, B. Zhao, S. Tan, J. Power Sources 246 (2014) 831-839.
Accepted Manuscript Unsymmetrical Donor-Acceptor-Donor-Acceptor Type Indoline Based Organic Semiconductors with Benzothiadiazole Cores for Solution-Processed Bulk Heterojunction Solar Cells Wenqin Li, Wene Shi, Zihua Wu, Jinmin Wang, Min Wu, Wei-Hong Zhu PII:

S2468-0257(16)30040-1

DOI:

10.1016/j.gee.2016.10.004

Reference:

GEE 29

To appear in:

Green Energy and Environment

Received Date: 27 August 2016 Revised Date:

23 October 2016

Accepted Date: 24 October 2016

Please cite this article as: W. Li, W. Shi, Z. Wu, J. Wang, M. Wu, W.-H. Zhu, Unsymmetrical DonorAcceptor-Donor-Acceptor Type Indoline Based Organic Semiconductors with Benzothiadiazole Cores for Solution-Processed Bulk Heterojunction Solar Cells, Green Energy & Environment (2016), doi: 10.1016/ j.gee.2016.10.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Article

Unsymmetrical Donor-Acceptor-Donor-Acceptor Type Indoline Based Organic Semiconductors with Benzothiadiazole Cores for Solution-Processed Bulk Heterojunction Solar Cells

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Wenqin Li a, Wene Shi b, Zihua Wu a, Jinmin Wang a,*, Min Wu c and Wei-Hong Zhu b,* a

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School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China. Email: [email protected] b Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and Institute of Fine Chemicals, Collaborative Innovation Center for Coal Based Energy (i-CCE), School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China. E-mail: [email protected] c Department of Chemistry and Molecular Biology, University of Gothenburg, 40530, Göteborg, Sweden

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Abstract

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1. Introduction

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Bulk heterojunction (BHJ) solar cells based on small molecules have attracted potential attention due to their promise of conveniently defined structures, high absorption coefficients, solution process-ability and easy fabrication. Three DA-D-A type organic semiconductors (WS-31, WS-32 and WS-52) are synthesized, based on the indoline donor and benzotriazole auxiliary acceptor core, along with either bare thiophene or rigid cyclopentadithiophene as π bridge, rhodanine or carbonocyanidate as end-group. Their HOMO orbitals are delocalized throughout the whole molecules. Whereas the LUMOs are mainly localized on the acceptor part of structure, which reach up to benzothiadiazole, but no distribution on indoline donor. The first excitations for WS-31 and WS-32 are mainly originated by electron transition from HOMO to LUMO level, while for WS-52, partly related to transition between HOMO and LUMO+1 level. The small organic molecules are applied as donor components in bulk heterojunction (BHJ) organic solar cells, using PC61BM as acceptor material to check their photovoltaic performances. The BHJ solar cells based on blended layer of WS-31:PC61BM and WS-32:PC61BM processed with chloroform show overall photoelectric conversion efficiency (PCE) of 0.56% and 1.02%, respectively. WS-32 based BHJ solar cells show a higher current density originated by its relatively larger driving force of photo-induced carrier in photo-active layer to LUMO of PC61BM. Keywords: Indoline donor; Unsymmetrical organic semiconductors; BHJ solar cells; Photovoltaic performances

Bulk heterojunction (BHJ) solar cells, especially those based on small molecules, have attracted world-wide attention due to their promise of conveniently defined structures, high absorption coefficients, solution process-ability and easy fabrication.[1-5] For an efficient BHJ solar cell, the electron donor and acceptor parts always interpenetrate and entangle each other to deduce diffusion length, which can be realized through solution processing procedure during device fabrication.[6] Generally, P-type fullerene derivatives, such as PC61BM or PC71BM are ideal acceptors in BHJ organic solar cell due to their good solubility and lower LUMO energy level.[7-10] Comparably, small molecular semiconductor, acted as electron donor in BHJ solar cells, is a main factor influencing photovoltaic performance. An ideal small molecule should possess a broad absorption profile, large absorption coefficient, low band gap, suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels as well as good solubility in organic solvent.[6] Molecules containing D-A or D-π-A segment have been proved desirable architecture. Up to now, the

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PCE of BHJ based on small molecular heterojunction organic solar cells has reached more than 9%, but there remains much upside potential with respect to polymer counterparts through molecular modification.[11-14] Triphenylamine has been widely used in organic semiconductors due to its good hole-transporting and electronic donating capacity.[15-18] With respect to triphenylamine, indoline shows better electron donating ability and widely used in dye sensitized solar cells with gratifying results.[19-22] Accordingly, a series of indoline bridged benzothiadiazole-based D-A-D-A small molecules were reported based on the following consideration: (i) Electron-rich indoline moiety was incorporated as donor unit, taking advantage of its three dimensional propeller structure to depress molecular aggregation as well as its excellent electron-donating ability; (ii) Benzothiadiazole, which has a strong coplanar, high oxidation potential, high electron affinity and super stability, is introduced to modify HOMO-LUMO gap;[23,24] and (iii) the solubility and film-formation ability can be optimized through introducing long alkyl chains to cyclopentadithiophene. Herein, unsymmetrical small organic molecules WS-31, WS-32, and WS-52 (Fig. 1) using indoline as electron donor, benzotriazole as auxiliary acceptor, bare thiophene or cyclopentadithiophene as π bridge, rhodanine or carbonocyanidate as electron acceptor were designed. BHJ solar cells were fabricated using the small molecules as donor materials, along with PC61BM as acceptor, with overall PCE of 0.56, 1.02 and 0.62% for WS-31, WS-32, and WS-52, respectively.

2. Experimental 2.1. Characterization

H and 13C NMR spectra were recorded on a Bruker AVANCE III HD 400 spectrometer using tetramethylsilane (TMS) as the internal standard. HRMS were recorded on a Waters LCT Premier XE instrument. The UV-Vis spectra were measured with a Varian Cary 100 spectrophotometer, while the fluorescence spectra were obtained with a Varian Cary Eclipse spectrometer. The cyclic voltammograms were obtained with a Versastat II electrochemical workstation (Princeton Applied Research). Three-electrode cell was applied using a Pt working electrode, a Pt wire auxiliary electrode, and a saturated calomel electrode (SCE) reference electrode, 0.1 mol·L-1 tetrabutylammoniumhexaflourophosphate (TBAPF6) in CH3Cl was used as the supporting electrolyte. The scan rate was 100 mV·s-1. Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a 300 W xenon lamp (model NO. 91160, Oriel). J-V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with Keithley2400 source meter. The voltage step and delay time of the photocurrent were 10 mV and 40 ms, respectively.

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Fig. 1. Molecular structures of WS-31, WS-32 and WS-52

2.2. General methods 2

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All reagents and materials were commercial obtained unless otherwise specially stated. THF applied in Suzuki reaction was thoroughly dried with sodium and benzophenone. Dimethylformamide, dichloromethane and chloroform was pretreated with calcium hydride.

Fig. 2. Synthetic procedure of WS-31, WS-32 and WS-52

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Synthesis of intermediate M1 2-Thenaldehyde was protected according to the following procedure. 2-Thenaldehyde (2.00 g, 17.8 mmol), anhydrous p-toluene sulfonic acid (614 mg, 3.57 mmol), toluene (60 mL) and ethylene glycol (8 mL) were added to a round-bottom flask to reflux for 10 h. The obtained mixture was poured into 40 mL of dichloromethane, followed by thoroughly washed with brine. The organic phase was separated, dried over anhydrous potassium carbonate and evaporated in vacuo to afford green solid 1.8 g. The aforementioned product and dry tetrahydrofuran (60 mL) was added to 250 mL of schlenk tube to cool to -78 ºC under N2 atmosphere, B(OCH3)3 (1.6 mL, 14.1 mmol) was added dropwise. After 2 h, the mixture was warmed to room temperature and stirred overnight to obtain thiophene boric acid ester. Compound 1 (4.72 g, 10.21 mmol), dry THF (35 mL), K2CO3 aqueous solution (2 M, 10 mL) and Pd(PPh3)4 (300 mg) was added to 100 mL of three-necked, round-bottomed flask to reflux at 110 ºC for 0.5 h, the thiophene boric acid ester was added dropwise and reacted for another 10 h. The mixture was poured to 100 mL H2O, and extracted with dichloromethane (3×50 mL). The organic phase was collected and removed through evaporation. A purple power was obtained through column chromatography in yield of 72.6%. 1H NMR (400 MHz, CDCl3, ppm): δ = 9.98 (s, 1H), 8.19 (d, J = 4.1 Hz, 1H), 8.05 (d, J = 7.6 Hz, 1H), 7.86 (d, J = 4.2 Hz, 1H), 7.79 (s, 1H), 7.73 - 7.78 (m, 1H), 7.69 (d, J = 7.5 Hz, 1H), 7.21 (d, J = 8.4 Hz, 2H), 7.16 (d, J = 8.4 Hz, 2H), 7.03 (d, J = 8.5 Hz, 1H), 4.86 (m, 1H), 3.92 (m, 1H), 2.31 (s, 3H), 2.07 (m, 1H), 1.95 (m, 2 H), 1.82 (m, 1H), 1.65 (m, 1H), 1.60 (m, 1H). 13C NMR (100 MHz, CDCl3, ppm): δ = 182.92, 153.91, 152.79, 149.38, 149.03, 142.98, 139.95, 136.85, 135.64, 135.50, 132.09, 129.81, 129.21, 127.90, 127.42, 126.60, 125.68, 125.60, 123.09, 120.65, 107.39, 69.44, 45.47, 35.28, 33.69, 24.47, 20.81. HRMS (ESI, m/z): [M + H]+ calcd for C29H24N3OS2, 494.1361; found, 494.1362. Synthesis of compound WS-31 Compound M1 (310 mg, 0.63 mmol) and octylcarbonocyanidate (1.25 mg, 6.34 mmol) in 60 mL CHCl3 and 0.1 mL triethylamine was collected in 100 mL three-necked flask to react for 70 h at room temperature. The obtained mixture was poured into 50 mL water and extracted with dichloromethane (3 × 50 mL). The organic phase was separated, dried with anhydrous sodium sulfate and evaporated in vacuo to remove the solvent. A purple solid was obtained after column chromatography (CH2Cl2: petroleum ether =1:1) in yield of 72.2%. 1H NMR (400 MHz, CDCl3, ppm): δ = 8.35 (s, 1H), 8.26 (d, J = 4.1 Hz, 1H), 8.02 (d, J = 7.6 Hz, 1H), 7.89 (d, J = 4.1 Hz, 1H), 7.81 (s, 1H), 7.77 (d, J = 8.4 Hz, 1H), 7.70

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(d, J = 7.6 Hz, 1H), 7.25 (d, J = 8.4 Hz, 2H), 7.19 (d, J = 8.3 Hz, 2H), 7.02 (d, J = 8.4 Hz, 1H), 4.87-4.91 (t, J = 7.1 Hz, 1H), 4.29-4.33 (t, J = 6.7 Hz, 2H), 3.93-3.97 (t, J = 7.4 Hz, 1H), 2.36 (s, 3H), 2.04-2.18 (m, 1H), 1.92-2.01 (m, 2H), 1.72-1.86 (m, 3H), 1.18-1.54 (m, 12H), 0.87-0.92 (t, J = 6.9 Hz, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ = 163.13, 153.95, 152.76, 149.81, 149.04, 146.38, 139.84, 138.48, 135.71, 135.64, 135.54, 132.05, 129.87, 129.27, 128.05, 127.94, 126.55, 125.66, 125.62, 122.77, 120.57, 116.17, 107.37, 98.16, 69.35, 66.63, 45.36, 35.28, 33.64, 31.80, 29.21, 29.18, 28.58, 25.83, 24.44, 22.67, 20.86, 14.14. HRMS (ESI, m/z): [M + H]+ calcd for C40H41N4O2S2 , 673.2671; found, 673.2676. Synthesis of compound WS-32 M1 (260 mg, 0.53 mmol), 3-ethylrhodanine (171 mg, 1.06 mmol), ammonium acetate (0.49 g, 6.36 mmol) and 100 mL acetic acid were collected in a bottom flask under the nitrogen atomsphere to reflux for 10 h. The obtained mixture was pored into 50 mL dichloroformmethane, deeply washed with water to remove acetic acid. Organic phase was dried with anhydrous sodium sulfate and evaporated to remove the solvent. Viscous large purple solid was obtained after column chromatography (CH2Cl2:petroleum ether = 2:1). After recrystallization with methanol, a purple powder was obtained in yield of 60.2%. 1H NMR (400 MHz, CDCl3, ppm): δ = 8.16 (d, J = 4.0 Hz, 1H), 7.99 (d, J = 7.6 Hz, 1H), 7.92 (s, 1H), 7.81 (s, 1H), 7.74-7.78 (m, 1H), 7.69 (d, J = 7.6 Hz, 1H), 7.48 (d, J = 4.1 Hz, 1H), 7.17-7.25 (m, 4H), 7.02 (d, J = 8.4 Hz, 1H), 4.87-4.91 (m, 1H), 4.21 (m, 2H), 3.92-3.97 (m, 1H), 2.36 (s, 3H), 2.05-2.16 (m, 1H), 1.91-2.01 (m, 2H), 1.76-1.88 (m, 1H), 1.65-1.75 (m, 1H), 1.59-1.64 (m, 1H), 1.29-1.33 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ = 192.20, 167.32, 153.91, 152.60, 148.83, 147.82, 139.89, 138.16, 135.50, 131.97, 130.13, 129.86, 129.14, 127.96, 126.88, 125.66, 125.38, 124.72, 123.08, 120.76, 120.49, 107.41, 69.35, 45.39, 39.95, 35.28, 33.67, 24.46, 20.87, 12.34. HRMS (ESI, m/z): [M + H]+ calcd for C34H29N4OS4, 637.1224; found, 637.1219. Synthesis of intermediate 3 Compound 2 (800 mg, 1.98 mmol) in 60 mL dry THF was cooled to -78 ºC, n-BuLi (1.1 mL, 2.6 mmol) and 0.4 mL B(OCH3)3 was added drop-wise and stirred for 2 h. The mixture was warmed to room temperature and reacted overnight. The obtained borate ester was used without any further purification. A mixture of 4,7-dibromobenzothiadiazole (5.4 g, 18.4 mmol), K2CO3 (2 M, 20 mL) and Pd(PPh3)4 (50 mg) in THF (50 mL) was heated to reflux under argon atmosphere for 30 min. The obtained borate ester was added slowly, and refluxed for further 8 h. After cooling to room temperature, the mixture was extracted with CH2Cl2 (3 × 50 mL). The organic layer was collected and dried over anhydrous Na2SO4. After evaporation, the residue was purified by column chromatography on silica (CH2Cl2: petroleum ether=1:5) to give a red oil in yield of 81.7%. 1H NMR (400 MHz, CDCl3, ppm): δ = 8.03 (s, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 7.6 Hz, 1H), 7.26 (d, J = 4.8 Hz, 1H), 6.98 (d, J = 4.8 Hz, 1H), 1.88-1.92 (m, 4H), 1.15-1.22 (m, 20H), 1.00-1.02 (m, 4H), 0.82 (t, J = 7.2 Hz, 6H). Synthesis of intermediate 4 Under N2 atmosphere, 15 mL distillated DMF was added to 100 mL round bottom flask, 2.5 mL POCl3 was added slowly at ice-bath and stirred for further 30 min to obtain Vilsmeier reagent. Compound 3 (1 g, 1.6 mmol) in 20 mL DMF was added drop-wise under ice-bath within 30 min and warmed to 40~50 ºC to react for another 12 h. The mixture was poured into saturated CH3COOK aqueous solution (120 mL) and stirred for 0.5 h. The obtained solution was washed with water, extracted with dichloromethane and evaporated in vacuo to remove the solvent. The residue was purified by column chromatography on silica (CH2Cl2: petroleum ether = 3:1) to give a red oil in yield of 80%. 1H NMR (400 MHz, CDCl3, ppm): δ = 9.87 (s, 1H), 8.04 (s, 1H), 7.88 (d, J = 7.6 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.61 (s, 1H), 1.93-1.97 (m, 4H), 1.16-1.25 (m, 20 H), 0.97-1.01 (m, 4H), 0.82 (t, J = 6.8 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): δ = 182.67, 162.94, 158.62, 153.88, 151.47, 147.19, 144.10, 143.02, 137.59, 132.30, 127.24, 125.19, 122.37, 112.59, 54.30, 37.65, 31.77, 29.91, 29.70, 29.29, 29.21, 24.63, 22.59, 14.07. HRMS (ESI, m/z): [M + H]+ calcd for C32H40BrN2OS3, 643.1486; found, 643.1489. Synthesis of intermediate M2 The synthesis of intermediate M2 resembles the procedure of M1 in yield of 57%. 1H NMR (400 MHz, CDCl3, ppm): δ = 9.88 (s, 1H), 8.07 (s, 1H), 7.98 (d, J = 7.6 Hz, 1H), 7.83 (s, 1H), 7.76 -7.79 (m, 1H), 7.71 (d, J = 7.6 Hz, 1H), 7.63 (s, 1H), 7.28 (d, J = 8.2Hz, 2 H), 7.21 (d, J = 8.3 Hz, 2H), 7.06 (d, J = 8.4 Hz, 1H), 4.90 (m, 1H), 3.98 (m, 1H), 2.38 (s, 3H), 2.08-2.19 (m, 1H), 1.96-2.01 (m, 6H), 1.80-1.90 (m, 1H), 1.68-1.75 (m, 1H), 1.58-1.67 (m, 1H), 1.15-1.30 (m,

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20H), 1.00-1.09 (m, 4H), 0.85 (t, J = 7.0 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): δ = 182.56, 163.11, 158.23, 154.12, 152.66, 148.58, 147.89, 144.78, 143.52, 140.04, 136.51, 135.48, 133.70, 131.90, 129.86, 128.94, 127.06, 126.05, 125.76, 125.54, 124.77, 121.18, 120.42, 107.54, 69.42, 54.22, 45.45, 37.74, 35.23, 33.68, 31.81, 29.98, 29.33, 29.25, 24.67, 24.48, 22.63, 20.86, 14.10. HRMS (ESI, m/z): [M + H]+calcd for C50H58N3OS3, 812.3742; found, 812.3746. Synthesis of compound WS-52 The synthesis of compound WS-52 resembles the procedure of compound WS-32 in yield of 49.1%. 1H NMR (400 MHz, CDCl3, ppm): δ = 8.04 (s, 1H), 7.93-7.96 (d, J = 7.6 Hz, 1H), 7.90 (s, 1H), 7.80 (s, 1H), 7.73-7.76 (m, 1H), 7.677.70 (d, J = 7.6 Hz, 1H), 7.23-7.25 (m, 3H), 7.17-7.21 (m, 2H), 7.01-7.04 (d, J = 8.4 Hz, 1H), 4.86-4.90 (m, 1H), 4.174.24 (m, 2H), 3.93-3.98 (m, 1H), 2.35 (s, 3H), 2.04-2.16 (m, 1H), 1.93-198 (m, 6H), 1.76-1.88 (m, 1H), 1.65-1.72 (m, 1H), 1.28-1.34 (m, 4H), 1.12-1.28 (m, 20H), 0.97-1.07 (m, 4H), 0.79-0.84 (t, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): δ = 191.72, 167.31, 162.07, 159.68, 154.12, 152.63, 148.61, 146.78, 144.34, 142.05, 138.71, 136.85, 135.44, 133.53, 131.81, 129.84, 128.91, 127.98, 126.99, 126.57, 126.04, 125.55, 125.52, 124.82, 121.28, 120.38, 117.38, 69.33, 54.30, 45.43, 39.92, 37.81, 35.24, 33.72, 31.81, 29.97, 29.33, 29.25, 24.65, 24.48, 22.63, 20.85, 14.10, 12.33. HRMS (ESI, m/z): [M + H]+ calcd for C55H63N4OS5 , 955.3605; found, 955.3604.

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2.3 Device fabrication and characterization

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The divices was fabricated with architecture of ITO/PEDOT:PSS(AI4083)/ DONOR:PC61BM/LiF/Al, amongst ITO glass acted as anode, Al as cathode, PEDOT:PSS as hole-transporting layer and a blended film of WS-31, WS-32 or WS-52: PC61BM as photoactive layer as follows. PEDOT: PSS (Clevios P VP AI4083, 0.45 µm) was spin-coated onto ITO glass at speed of 3000 rpm. After dried at 150 ºC for 20 min, the electrode was transferred to glove box under atmosphere of argon. A solution of organic semiconductors and PC61BM (1:0.8, mass ratio) in chloroform, with concentration of donor materials 8 mg/mL, was spin coated on the surface of ITO/PEDOT:PSS. The thickness of active layer was measured to be 100±10 nm. Cathode electrode of 0.8 nm LiF and 80 nm Al was attached on the surface of active layer through evaporation under depressed pressure (