High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating π-linkage

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Contents lists available at ScienceDirect

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High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating p-linkage

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Q1

Lihui wang a,b, Lunxiang Yin a, Changyan Ji a, Yu Zhang a, Hang Gao a, Yanqin Li a,⇑ a

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b

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School of Chemistry, Dalian University of Technology, Dalian 116024, China College of Chemistry and Chemical Engineering, Inner Mongolia University for Nationalities, Tongliao 028023, China

a r t i c l e

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i n f o

Article history: Received 12 December 2013 Received in revised form 7 March 2014 Accepted 17 March 2014 Available online xxxx

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Keywords: Triphenylamine (TPA) Benzothiadiazole (BT) p-Linkage Small molecular solar cells Bulk heterojunction (BHJ) Solution-processed

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a b s t r a c t Two novel small molecular photovoltaic (PV) materials, BDPTBT and BDATBT were designed and synthesized, consisting of 5,6-bis-(octyloxy)benzo[c][1,2,5]thiadiazole (DOBT) as electron-withdrawing core (A), and triphenylamine (TPA) as electron-donating side group (D). Moreover, the benzene and ethynylbenzene as p-linkage were introduced to form donor–p-acceptor–p-donor (D–p-A–p-D) typed molecular structures, respectively. To fully investigate the linkage effect of a series of small molecules, two reference compounds BDCTBT and BDETBT were also studied systematically, consisting of 2-phenylacrylonitrile and styrene as p-linkage, respectively. As a result, the p-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving PV performance. Bulk heterojunction (BHJ) solar cells based on BDPTBT/PC61BM and BDATBT/PC61BM yielded the power conversion efficiencies (PCEs) of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high open-circuit voltage (Voc) of 1.03 V. Compared to the results we have reported previously, the reference devices based on BDCTBT/PC61BM and BDETBT/PC61BM with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for devices prepared from solution-processed small-molecule-donor materials, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed benzothiadiazole (BT)–TPA based solution-processed small molecules PV devices. Ó 2014 Published by Elsevier B.V.

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1. Introduction

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Organic photovoltaics (OPV) have attracted great interest during the past decade because of their potential for achieving large-area, flexible PV devices through low-cost solution processing techniques [1,2]. To date, a PCE of 9.2% has been achieved for solution-processed

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Q2

⇑ Corresponding author. Tel./fax: +86 411 84986040. E-mail address: [email protected] (Y. Li).

polymer-based solar cells (PSCs) [3]. Meanwhile, much work has been done on solution-processed small molecule organic solar cells (SMOSCs) in recent years, owing to their promising advantages of definite structure, easy purification and well PV performance reproduction in comparison with PSCs [4,5]. Currently, encouraging PCEs with 6–9% of SMOSCs have been achieved [6–11]. In particular, an impressive PCE of 9.02% for BHJ SMOSCs has been reported Q3 by Gupta et al. [12], which demonstrated that SMOSCs were comparable with polymeric counterparts. However, the novel solution-processed organic small molecules

http://dx.doi.org/10.1016/j.orgel.2014.03.013 1566-1199/Ó 2014 Published by Elsevier B.V.

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(OSMs) with high PCEs are inadequate. Toward high PCEs, exploring novel OSMs and deeply understanding the relationship between molecular structure and property are urgently needed [13]. Obviously, developing new D–A conjugated OSMs has become an efficient strategy to obtain high PV performance [14–16], because this design can not only reduce the band gap through intermolecular charge transfer (ICT) transition, but also efficiently control the energy levels through introducing appropriate D and A moieties into the molecules [17]. Therefore, researchers devoted so much effort to develop novel D and A building blocks, in order to improve PV performance of SMOSCs [18]. Relatively, little attention was paid to the investigation of p-linkage effect between D and A units [19]. In fact, modification of p-linkage in the OSMs is an effective approach to improve the PV performance as the following reasons [20,21]: (1) the narrow band gap was realized not only by selecting of appropriate D–A combinations, but also by tuning the energy levels through p-linkage. (2) The deep-lying high occupied molecular orbital (HOMO) level could be achieved by inserting an appropriate p-linkage into the D–A backbone, which is directly correlated to the Voc [22]. Voc is one of the key parameters of PCEs and is mainly governed by the energy level difference between the lowest unoccupied molecular orbital (LUMO) of A and the HOMO of D [23]. However, the solution-processable PV devices with a Voc over 1.0 V were seldom reported by using a p-type small molecule as a donor [24–26]. Therefore, systematic investigation of the p-linkage effect will be critical for deeply understanding the relationship between structure and property, thus improving the Voc and PCEs of BHJ OSCs. DOBT was designed with two octyloxy chains on the BT unit and has been widely investigated to construct D–A conjugated polymers due to the strongly electron-withdrawing ability and good planar conformation, however, there are few reports on DOBT-based BHJ SMOSCs [27–29]. On the other hand, TPA group was widely used as end-capping donors due to its excellent electron-donating property and good charge transportation [30,31]. Furthermore, the covalently linked BT–TPA backbone is a common combination in SMOSCs, however, the BT–TPA based linear D–A–D typed OSM donors with high PV performance over 2% were seldom reported [32–34]. In this work, two novel small molecules, BDPTBT and BDATBT were designed and synthesized successfully, consisting of DOBT as electron-withdrawing cores and TPA as electron-donating moieties. Moreover, the benzene and ethynylbenzene as p-linkage units were introduced to form D–p–A–p–D typed structure, respectively. To fully investigate the linkage effect of a series of small molecules, compounds BDCTBT and BDETBT were also studied as reference materials, consisting of 2-phenylacrylonitrile and styrene as p-linkage, respectively. The chemical structures of these four molecules were shown in Scheme 1. Previously, we have reported the synthesis and a preliminary PV performance of two molecules BDCTBT and BDETBT [34]. In this paper, to fully investigate the p-linkage effect on PV property, the optimized devices based on BDCTBT/ PC61BM and BDETBT/PC61BM were further studied and

the detailed optoelectronic properties of the series of materials were reported.

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2. Experimental

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2.1. Reagent and materials

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All reagents were obtained commercially and used as received unless specified. Tetrahydrofuran (THF) and toluene were distilled over Na/benzophenone under nitrogen prior to use. Compound A, BDETBT and BDCTBT were synthesized according to the reported methods and the detailed synthetic procedures were reported in our previous work [34,35]. All reactions and manipulations were carried out under nitrogen atmosphere with the use of standard Schlenk techniques.

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2.2. Device fabrication

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The solution-processed organic PV devices were fabricated with a conventional device configuration of ITO/ PEDOT:PSS/OSMs:PC61BM/Al. The ITO substrate (10 X/h) Q4 was pre-cleaned in water, acetone, toluene and isopropyl alcohol for 10 min, respectively. Then a thin layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS from Baytron P VP AI 4083) was spin-coated (4000 rpm, 60 s) onto the ITO glass. After being baked at 140 °C for 15 min, the substrate was transferred into a nitrogen-filled glove box. Subsequently, the active layer was spin-coated at 1000 rpm for 60 s with the blend solution of OSMs and PC61BM (total concentration of 21 mg mL1 in dichlorobenzene) on the top of ITO/PEDOT:PSS substrate. Finally, a 100 nm aluminium electrode was deposited by thermal evaporation under vacuum (ca. 104 Pa) through a shadow mask, yielding six individual devices with 5 mm2 nominal area.

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2.3. Measurements and characterization

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H NMR and 13C NMR spectra were measured on a Bruker AVANCE II 400-MHz spectrometer with CDCl3 as solvent and TMS as the internal standard. High-resolution mass spectra (HRMS) were obtained using MALDI Micro MX spectrometer. The UV–visible absorption spectra of the synthesized molecules were recorded on Agilent Cary 5000 spectrophotometer at room temperature in dilute chloroform solutions and in thin films. Fluorescence quenching experiments were performed on a Shimadzu RF-5301PC spectrofluorometer. Cyclic voltammetry (CV) measurements were performed in Bu4NBF4/CH2Cl2 solutions using a CHI610D electrochemical workstation from CH Instrument, Inc. The glass-carbon electrode served as the working electrode, and Ag/Ag+ electrode (Ag in 0.1 M AgNO3 solution of MeCN) and platinum wire was chosen as the reference electrode and the count electrode, respectively. Ferrocene–ferrocenium (Fc/Fc+) couple was selected as internal standard. TGA analysis were performed using a TGAQ501163 thermal analysis system under a nitrogen atmosphere at a heating rate of 10 °C min1. The current density–voltage (J–V) curves were obtained using a

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Scheme 1. The chemical structures of BDPTBT, BDETBT, BDATBT and BDCTBT.

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computer-controlled Keithley 2400 Source Measure Unit under AM 1.5 G illumination with an intensity of 100 mW cm2. The monochromatic incident photon-toelectron conversion efficiency (IPCE) spectra were recorded using a SM-25 photoelectric conversion analyzer system. The hole mobility measurement of the device with a structure of ITO/PEDOT:PSS/OSMs:PC61BM/Au were carried out in the dark using a computer-controlled Keithley 2400 Sourcemeter.

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2.4. Synthesis

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The synthetic routes of the compounds BDPTBT and BDATBT were shown in Scheme 2.

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2.4.1. N,N-diphenyl-4-((trimethylsilyl)ethynyl)aniline (1) 4-Iodo-N,N-diphenylaniline (3.71 g, 10 mmol), Pd (PPh3)2Cl2 (176 mg, 0.25 mmol) and CuI (96 mg, 0.5 mmol) were dissolved in a mixture of THF (40 mL) and Et3N (40 mL) under a nitrogen atmosphere, and then trimethylsilyl acetylene (1.55 mL, 11 mmol) was added. The mixture was refluxed at 70 °C for 15 h, concentrated under reduced pressure and purified by chromatography on silica gel with petroleum ether:dichloromethane (v/v, 8: 1) to give (1) as a white solid (3.07 g, yield: 90%). 1H NMR (400 MHz, CDCl3, ppm): d 7.30 (d, J = 8.8 Hz, 2H), 7.25 (t, J = 8.0 Hz, 4H), 7.02–7.09 (m, 6H), 6.95 (d, J = 8.4 Hz, 2H), 0.23 (s, 9H). 2.4.2. 4-Ethynyl-N,N-diphenylaniline (2) To a stirred mixed solution of CH3OH (30 mL) and THF (40 mL), compound (1) (683 mg, 2.0 mmol) and K2CO3 (607 mg, 4.4 mmol) were added. The reaction mixture was stirred at room temperature for 2 h and then poured into aqueous 1 M HCl (50 mL). The precipitate was collected and the crude product was purified by chromatography on silica gel with petroleum ether to give compound

(2) as a white solid (511 mg, yield: 95%), M.p.: 101– 102 °C. 1H NMR (400 MHz, CDCl3, ppm): d 7.32 (d, J = 8.8 Hz, 2H), 7.24–7.29 (m, 4H), 7.04–7.11 (m, 6H), 6.96 (d, J = 8.4 Hz, 2H), 3.01 (s, 1H).

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2.4.3. N-{4-[2-(4-Bromophenyl)ethynyl]phenyl}diphenylamine (3) Compound (2) (538 mg, 2.0 mmol), Pd(PPh3)4 (59 mg, 0.085 mmol) and CuI (10 mg, 0.024 mmol) were dissolved in distilled toluene (8 mL) under a nitrogen atmosphere, and then i-Pr2NH (5.2 mL, 29 mmol) and 1-bromo-4-iodobenzene (672 mg, 2.2 mmol) were added. The mixture was refluxed at 110 °C for 15 h, concentrated under reduced pressure and purified by chromatography on silica gel with petroleum ether: dichloromethane (v/v, 10:1) to give compound (3) as a yellow solid (787 mg, yield: 93%), M.p.: 159–160 °C. 1H NMR (400 MHz, CDCl3, ppm): d 7.46 (d, J = 8.4 Hz, 2H), 7.35 (d, J = 8.8 Hz, 4H), 7.27 (t, J = 8.0 Hz, 4H), 7.04–7.12 (m, 6H), 7.00 (d, J = 8.4 Hz, 2H).

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2.4.4. N-{4-{2-[4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan2-yl)phenyl}.ethynylphenyl}-diphenylamine (D1) A solution of compound (3) (2.12 g, 5 mmol), bis(pinacolato)diborane (1.52 g, 6 mmol), CH3COOK (1.47 g, 15 mmol), Pd(PPh3)2Cl2 (175 mg, 0.25 mmol) and PPh3 (131 mg, 0.5 mmol) in toluene (50 mL) was refluxed at 110 oC under a nitrogen atmosphere for 24 h. After being cooled to room temperature, the mixture was poured into water (100 mL) and extracted with dichloromethane (3  50 mL). The combined organic layers were dried over anhydrous Na2SO4 and evaporated under reduced pressure. The crude product was purified by silica column chromatography eluting with petroleum ether:ethyl-acetate (v/ v, 20:1) to afford compound D1 as a yellow solid (1.719 g, yield: 73%), M.p.: 160–163 °C. 1H NMR (400 MHz, CDCl3, ppm): d 7.76 (d, J = 8.0 Hz, 2H), 7.49 (d, J = 8.4 Hz, 2H),

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Scheme 2. Synthetic route of the compounds BDPTBT and BDATBT.

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7.37 (d, J = 8.8 Hz, 2H), 7.27 (t, J = 8.0 Hz, 4H), 7.11 (d, J = 7.6 Hz, 4H), 7.06 (t, J = 7.2 Hz, 2H), 7.00 (d, J = 8.8 Hz, 2H), 1.35 (s, 12H). 2.4.5. N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaboro lan-2-yl)aniline (4) A solution of compound N,N-diphenyl-4-bromoaniline (1.62 g, 5.0 mmol), bis(pinacolato)diborane (1.52 g, 6 mmol), Pd(PPh3)2Cl2 (702 mg, 0.1 mmol), PPh3 (53 mg, 0.2 mmol) and CH3COOK (2.45 g, 25 mmol) in dry toluene (50 mL) was refluxed at 110 °C under a nitrogen atmosphere for 24 h. After being cooled to room temperature, the mixture was poured into water (100 mL) and extracted with dichloromethane (3  50 mL). The combined organic layers were dried over anhydrous Na2SO4 and organic solvent was evaporated under reduced pressure. The crude product was purified by silica column chromatography eluting with petroleum ether:ethyl acetate (v/v, 50:1) to afford compound (4) as a white solid (1.45 g, yield: 78%), M.p.: 92–100 °C. 1H NMR (400 MHz, CDCl3, ppm): d 7.66 (d, J = 8.4 Hz, 2H), 7.25 (m, 4H), 7.10 (d, J = 8.8 Hz, 4H), 7.03 (t, J = 8.0 Hz, 4H), 1.33 (s, 12H). 2.4.6. N-{4-[2-(4-Bromophenyl)]phenyl}-diphenylamine (5) A 100 mL three-neck round-bottom flask was charged with compound (4) (742 mg, 2.0 mmol), 1-bromo-4-iodobenzene (564 mg, 2.0 mmol), Pd(PPh3)4 (230 mg, 0.2 mmol), toluene (36 mL) and 2 M aqueous Na2CO3 (18 mL), under a nitrogen atmosphere. The reaction mixture was heated at 110 °C for 24 h and monitored via thin layer chromatography (TLC). After the reaction mixture

was cooled to room temperature, 50 mL water was added, and the mixture was extracted with dichloromethane (3  30 mL). The combined organic phases were dried over anhydrous Na2SO4 and then evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel using dichloromethane:petroleum ether (v/v, 1:10) as eluent to give the compound (5) as a dark purple solid (332 mg, yield: 65%), M.p.: 96–100 °C. 1H NMR (400 MHz, CDCl3, ppm): d 7.52 (d, J = 8.8 Hz, 2H), 7.40–7.42 (dd, J1 = 8.4 Hz, J2 = 1.0 Hz, 4H), 7.26 (t, J = 7.8 Hz, 4H), 7.11–7.13 (m, 6H), 7.04 (t, J = 7.4 Hz, 2H).

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2.4.7. N-{4-{2-[4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan2-yl)phenyl}phenyl}-diphenylamine (D2) A solution of compound (5) (798 mg, 2 mmol), bis(pinacolato)diborane (911 mg, 3.6 mmol), CH3COOK (588 mg, 6 mmol), Pd(PPh3)2Cl2 (29 mg, 0.04 mmol) and PPh3 (53 mg, 0.2 mmol) in dry toluene (15 mL) was refluxed at 110 °C under a nitrogen atmosphere for 24 h. After being cooled to room temperature, the mixture was poured into water (100 mL) and extracted with dichloromethane (3  50 mL). The combined organic layers were dried over anhydrous Na2SO4 and evaporated under reduced pressure. The crude product was purified by silica column chromatography eluting with petroleum ether:ethyl acetate (v/v, 50:1) to afford compound D2 as a white solid (536 mg, yield: 60%), M.p.: 173–176 °C. 1H NMR (400 MHz, CDCl3, ppm): d 7.86 (d, J = 8.4 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.50 (d, J = 8.4 Hz, 2H), 7.24–7.29 (m, 4H), 7.13 (d, J = 8.4 Hz, 6H), 7.03 (t, J = 7.2 Hz, 2H), 1.36 (s, 12H).

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2.4.8. Compound BDPTBT A mixture of compound D2 (224 mg, 0.50 mmol), compound A (179 mg, 0.25 mmol), Na2CO3 (848 mg, 8.00 mmol) and Pd(PPh3)4 (4.00 mg, 0.013 mmol) in the solution of toluene (8 mL), H2O (4 mL) and EtOH (2 mL) was heated at 110 °C under a nitrogen atmosphere for 24 h. After being cooled to room temperature, 50 mL water was poured into the solution, and the mixture was extracted with dichloromethane (3  20 mL). The combined organic phases were dried over anhydrous Na2SO4 and then evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel column eluting with petroleum ether:dichloromethane (v/v, 3:1) to give a red solid (288 mg, yield: 96%), M.p.: 86–87 °C. 1H NMR (400 MHz, CDCl3, ppm): d 8.53 (d, J = 2.4 Hz, 2H), 7.77 (d, J = 8.4 Hz, 4H), 7.62 (d, J = 8.0 Hz, 4H), 7.52 (d, J = 8.8 Hz, 4H), 7.47 (d, J = 7.2 Hz, 2H), 7.28 (t, J = 8.0 Hz, 8H), 7.15 (d, J = 7.6 Hz, 12H), 7.05 (d, J = 7.2 Hz, 4H), 4.19 (t, J = 7.2 Hz, 4H), 2.00 (m, 4H), 1.52 (t, J = 8.2 Hz, 4H), 1.25–1.36 (m, 16H), 0.87 (t, J = 7.4 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): d 151.77, 150.92, 147.66, 147.38, 145.32, 139.76, 134.28, 133.70, 132.95, 131.99, 129.34, 127.51, 126.98, 126.19, 124.55, 123.84, 123.06, 117.50, 74.52, 31.88, 30.54, 29.63, 29.38, 26.17, 22.73, 14.15. HRMS (MALDI–TOF): 1195.4960, [M+H+] (calcd for C78H74N4O2S3: 1195.5052). 2.4.9. Compound BDATBT A mixture of compound D1 (235 mg, 0.5 mmol), compound A (179 mg, 0.25 mmol), Na2CO3 (848 mg, 8.00 mmol) and Pd(PPh3)4 (4.00 mg, 0.013 mmol) in the solution of toluene (8 mL), H2O (4 mL) and EtOH (2 mL) was heated at 110 °C under a nitrogen atmosphere for 24 h. After being cooled to room temperature, 50 mL water was poured into the solution, and the mixture was extracted with dichloromethane (3  20 mL). The combined organic phase was dried over anhydrous Na2SO4 and then evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel column eluting with petroleum ether:dichloromethane (v/v, 3:1) to give an orange red solid (264 mg, yield: 85%),

Fig. 1. TGA plots of BDPTBT, BDETBT, BDATBT and BDCTBT recorded at a heating rate of 10 °C min1 under a nitrogen atmosphere.

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M.p.: 145–148 °C. 1H NMR (400 MHz, CDCl3, ppm): d 8.54 (d, J = 4.0 Hz, 2H), 7.70 (d, J = 8.4 Hz, 4H), 7.54 (d, J = 8.0 Hz, 4H), 7.47 (d, J = 4.0 Hz, 2H), 7.40 (d, J = 8.8 Hz, 4H), 7.29 (t, J = 7.8 Hz, 8H), 7.12 (d, J = 7.6 Hz, 8H), 7.07 (t, J = 7.4 Hz, 4H), 7.02 (d, J = 8.8 Hz, 4H), 4.18 (t, J = 7.0 Hz, 4H), 1.99 (t, J = 7.6 Hz, 4H), 1.47–1.53 (m, 4H), 1.35–1.29 (m, 16H), 0.85 (t, J = 6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm): d 151.85, 150.86, 147.99, 147.22, 144.90, 134.23, 133.88, 132.57, 131.99, 129.42, 125.56, 125.03, 123.59, 123.47, 122.71, 122.32, 117.48, 116.08, 90.94, 88.73, 74.56, 31.86, 30.51, 29.60, 29.35, 26.14, 22.70, 14.13. HRMS(MALDI-TOF): 1243.5040, [M+H+] (calcd for C82H74N4O2S3: 1243.5052).

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3. Results and discussion

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3.1. Synthesis and thermal stability

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The synthetic route of BDPTBT and BDATBT was shown in Scheme 2. The DOBT based acceptor moiety A was synthesized in six steps starting from commercially available catechol according to the reported methods [34]. Compound (3) was prepared with 4-iodo-N,N-diphenylaniline as a starting material [36]. Sonogashira cross-coupling of 4-iodo-N,N-diphenylaniline with trimethylsilylacetylene afforded compound (1) in a yield of 90%. Cleavage of trimethylsilyl group of compound (1) with K2CO3 in the mixture of methanol and THF solution afforded compound (2) with a yield of 95%. Sonogashira cross-coupling of compound (2) with 1-bromo-4-iodobenzene gave the compound (3) in a yield of 93%. The compound (5) was prepared with 4-bromo-N,N-diphenylaniline as a starting material and through two typical reactions of Miyaura borylation and Suzuki coupling gave the compound (5) in a yield of 65%. The donor units (compounds D1 and D2) were synthesized by Miyaura borylation in the yields of 73% and 60%, respectively. Finally, small molecules BDPTBT and BDATBT were prepared by Pd(PPh3)4-assisted Suzuki coupling reaction in high yields of 96% and 85%, respectively. The chemical structures of these synthesized compounds were identified by 1H NMR,13C NMR spectra and HRMS. These obtained materials are readily dissolved in common organic solvents, such as chloroform, dichloromethane, toluene, chlorobenzene and dichlorobenzene, owing to the alkoxy-substituted DOBT as central unit. The thermal stability of four materials was first investigated with thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 °C min1, as shown in Fig. 1, and the thermal stability data of these materials were summarized in Table 1. The onset decomposition temperature (Td, at 5% weight loss) of BDPTBT, BDETBT, BDATBT and BDCTBT are 318 °C, 336 °C, 331 °C and 343 °C, respectively. The results show that these materials are fairly stable for their long-term PV applications.

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3.2. Photophysical properties

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The normalized absorption spectra of these small molecules in chloroform solution and in film were depicted in Fig. 2, and the corresponding absorption data were listed

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Table 1 The photophysical, electrochemical, and thermal stability properties as well as theoretically calculated data of four compounds.

BDPTBT BDETBT BDATBT BDCTBT

ksol max (nm)

kfilm max (nm)

(eV) Eopt g

Td (°C)

HOMOCV (eV)

LUMOCV (eV)

ECV g (eV)

HOMODFT (eV)

LUMODFT (eV)

(eV) EDFT g

488 496 487 484

501 511 500 504

2.07 2.01 2.05 2.05

318 336 331 343

5.24 5.12 5.32 5.35

3.13 3.18 3.20 3.20

2.11 1.94 2.12 2.15

4.78 4.63 4.73 4.87

2.42 2.44 2.48 2.52

2.36 2.19 2.25 2.35

Fig. 2. Normalized UV–Vis absorption spectra of BDPTBT, BDETBT, BDATBT and BDCTBT in chloroform solution and in thin film.

BDPTBT, BDETBT, BDATBT and BDCTBT were 1937 M1, 3260 M1, 2067 M1, and 3320 M1, respectively. High values of Ksv for BDETBT and BDCTBT were corresponding to the higher PCEs of their PV devices due to more efficient charge dissociation process.

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3.3. Electrochemical properties

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In order to investigate the electrochemical properties of these materials, CV measurement was conducted with these small molecules in 0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4) solution of anhydrous dichloromethane (CH2Cl2) at a scan rate of 100 mV s1 under a nitrogen atmosphere at room temperature. The HOMO energy levels (HOMOCV), LUMO energy levels (LUMOCV) and the electrochemical band gaps (ECV g ) of these compounds were calculated from the onset oxidation potentials (Eox) and onset reduction potentials (Ered) according to the following equations [42]:

442

406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435

in Table 1. All small molecules showed the strong absorption characteristics in the visible region covering the 300–700 nm wavelength. The maximum absorption peaks of BDPTBT, BDETBT, BDATBT and BDPTBT in chloroform (ksol max ) were 501, 511, 500 and 504 nm, respectively, corresponding to the ICT transition between the donor unit TPA and the acceptor unit DOBT [37]. In comparison with their absorption spectra in solution, the maximum absorption peaks of the four small molecules in film (kfilm max ) were red-shifted by 13, 15, 13 and 20 nm, respectively, showing an increased p–p intermolecular interaction in the solid state [38]. As a result, the optical band gaps (Eopt g ) of BDPTBT, BDETBT, BDATBT and BDCTBT calculated from their absorption edges in film were 2.07, 2.01, 2.05 and 2.05 eV, respectively. The fluorescence quenching experiments were performed for investigation of the photoinduced charge separation process, and the fluorescence emission spectra were shown in Fig. 3. The emission intensity of small molecules was gradually decreased as the concentration of PC61BM was increased in CHCl3, indicating an efficient photoinduced charge dissociation process between the OSM donors and the acceptor materials PC61BM. Furthermore, the fluorescence quenching phenomenon was described by the Stern–Volmer equation and a linear relationship was observed: F0/F = 1 + KSV [C], where F0 and F represent the intensity of fluorescence in the absence and presence of PC61BM, respectively, Ksv is the quenching constant, and [C] is concentration of PC61BM [39–41]. The fitting plots by Stern–Volmer equation were shown in insets of Fig. 3. According to the fitting plots, the Ksv of

438 439 440

443 444 445 446 447 448 449 450 451 452

  ¼  Eox  Eferrocene þ 4:8 eV 1=2

453

  LUMOCV ¼  Ered  Eferrocene þ 4:8 eV 1=2

456

HOMOCV 405

437

455

458 459

CV Ecv  HOMOCV g ¼ LUMO

where Eox and Ered are measured onset potentials relative to Ag/Ag+, respectively, Eferrocene = 0.05 eV versus Ag/Ag+. 1=2 The CV curves of these small molecules were shown in Fig. 4 and the corresponding electrochemical data (HOMOCV, LUMOCV, ECV g ) were summarized in Table 1. The LUMOCV of BDPTBT, BDETBT, BDATBT and BDCTBT were 3.13, 3.18, 3.20 and 3.20 eV, and the HOMOCV of them were 5.24, 5.12, 5.32 and 5.35 eV, respectively. The results indicated that these compounds as donor materials contained appropriate energy levels to satisfy the requirement of solution-processable OSCs with PC61BM as acceptor materials [43]. Encouragingly, BDATBT and BDCTBT presented fairly deep-lying HOMOCV of 5.32 and 5.35 eV, respectively, which guaranteed high Voc in small molecular BHJ solar cells. In comparison with P3HT (4.99 eV, CV curve was shown in Fig. S2 of supplementary materials, and P3HT was purchased from Sigma Aldrich Inc.), these materials gave rise to relatively low HOMO energy levels. It is well known that, the deep-lying HOMO level is desirable for obtaining high Voc. As expected, plinkage units of ethynylbenzene and 2-phenylacrylonitrile with electron-withdrawing effect successfully deepened the HOMOCV level due to their increased oxidation potentials. Moreover, in comparison with the others, a narrow band gap of BDETBT of 1.94 eV was realized with p-linkage

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Fig. 3. Fluorescence emission spectra of BDPTBT, BDETBT, BDATBT and BDCTBT in CHCl3 (1.0  105 M) with the increasing concentration of PC61BM (105 M): 0.0(1), 3.0(2), 5.0(3), 8.0(4), 10.0(5), 15.0(6), 20.0(7), 25.0(8); the insets are fitting plots by the Stern–Volmer equation for four compounds respectively.

Fig. 4. Cyclic voltammogram of BDPTBT, BDETBT, BDATBT and BDCTBT in 0.1 M Bu4NBF4/CH2Cl2 solution at a scan rate of 100 mV s1 under a nitrogen atmosphere.

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of styrene due to the better rigidity and coplanarity of the molecular backbone. Therefore, the energy levels and band gaps of these materials were effectively tuned by incorporating various p-linkage groups, which revealed an obvious relationship between molecular structure and property.

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3.4. Theoretical calculations

493

In order to deeply understand the p-linkage effect of these materials, the ground-state geometry and the

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electron density distribution of HOMO and LUMO of them have been fully optimized by Gaussian 09 software at the Becke’s three-parameter gradient-corrected functional (B3LYP) with a polarized 6–31G (d) basis. The density functional theoretical (DFT) calculated HOMO and LUMO energy levels, as well as electronic-density distribution were sketched in Fig. 5a. The electron density of the HOMO is distributed over the whole molecules, while that of the LUMO is localized on the central DOBT groups, indicating an effective ICT transition from the TPA groups to the central DOBT acceptor [44–46]. The LUMO of BDPTBT, BDETBT, BDATBT and BDCTBT were 2.42, 2.44, 2.48 and 2.52 eV, and the HOMO of them were 4.78, 4.63, 4.73 and 4.87 eV, respectively. Notably, the calculated energy levels are higher than experimental data, because solvent effect and intermolecular interaction cannot be considered in theoretical calculation [47]. To our knowledge, this is a common phenomena observed in most of the organic PV materials as reported in the literature [48–50]. Despite the discrepancies existed between the calculation and experimental results, the theoretically calculated results could give an important guide for designing new materials. Small molecules BDCTBT showed a fairly deep-lying HOMO levels, indicating a high Voc of the corresponding device. In addition, a narrow band gap of BDETBT was obtained in comparison with the others. Therefore, all the results above demonstrated that the type of the p-linkage has a great effect on the HOMO and LUMO level and suggested that the Voc could be varied by choosing appropriate p-linkage in small molecular donor materials [51]. The molecular structure of these materials and dihedral angle data were shown in Fig. 5b, where h1 is a dihedral angle between TPA unit and p-linkage group, and h2 is a dihedral angle between DOBT and the p-linkage. According to the dihedral angle data, bow shape is a typical structure for these TPA–DOBT based D–p-A–p-D typed small

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Fig. 5. (a) Theoretical calculated HOMO and LUMO energy levels, as well as electronic-density distribution of BDPTBT, BDETBT, BDATBT and BDCTBT; (b) Calculated dihedral angles of four compounds.

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molecules due to their similar dihedral angle h2 (22° < h2 < 25°) [52,53]. The dihedral angles h1 of BDETBT and BDCTBT are less than 4°, indicating that the terminal TPA groups are nearly coplanar with the p-linkage groups of styrene or phenylacrylonitrile. In comparision with them, the corresponding dihedral angles of BDPTBT and BDATBT show relatively larger angles above 30°, indicating a relatively poor coplanarity. As a result, the linkage groups styrene and 2-phenylacrylonitrile show a remarkable ability to improve the planar conformation of the TPA–DOBT based small molecules, resulting a higher PV performance due to the effective intermolecular interactions [54]. Meanwhile, the ethynylbenzene and 2-phenylacrylonitrile as linkage groups possess an ability to improve the oxidation potential of materials due to their electron-withdrawing effect, thus increasing Voc of these PV materials.

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3.5. Hole mobility

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To determine the hole mobility (lh) of these materials, current density–voltage (J–V) characteristic of each material is measured and the space-charge-limited currents (SCLC) model is fitted according to the Mott–Gurney law [55]:

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553 3

555

J ¼ ð9=8Þe0 er lh ðV 2 =d Þ

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where J is the current density, e0 is the permittivity of free space, er is the relative permittivity and assumed as approximately 3.0, lh is the hole mobility, V is the effective voltage, and d is the thickness of the active layer. The J–V curves of hole-only devices with a structure of ITO/PEDOT:PSS/OSMs:PC61BM/Au were measured in the dark. By fitting the JV curves in a double logarithmic scale to a SCLC model, hole mobility of these compounds was obtained, and the fitting plots of the data points was shown in Fig. 6c. The hole mobility values were 1.38  104, 3.28  104, 2.91  105 and 9.20  104 cm2 V1 s1 for BDPTBT, BDETBT, BDATBT and BDCTBT, respectively, and the corresponding data were summarized in Table 2. We found that the lh values of these materials were comparable with P3HT [56] except BDATBT. This method estimated the material’s hole mobility in the bulk, which was a key parameter for both material design and device fabrication.

557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572

The charge mobility in the BHJ film might be closely related to the charge stability, the morphology of the active layer as well as the intermolecular interaction. As a result, it was interesting to note that the trend of lh of these compounds fitted well with their PCE values. In comparison, BDCTBT based device exhibited the highest hole mobility among these devices probably due to the strong intermolecular interaction, which was consistent with its best PV performance.

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3.6. Photovoltaic properties

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To investigate the p-linkage effect on PV properties of these small molecules, BHJ devices with a configuration of ITO/PEDOT:PSS/OSMs:PC61BM/Al were fabricated. The electrochemical energy-level diagram of these small molecules together with PC61BM was schematically shown in Fig. 6a. It was worth noting that the PV performance data of them were obtained without any other common treatment, such as thermal annealing or addition of additives. The current density–voltage (J–V) characteristics of PV devices under an illumination of AM 1.5 G at 100 mW cm2 were shown in Fig. 6b and the PV performance data of the devices were summarized in Table 2. The device based on BDPTBT:PC61BM with the weight ratio of 1:2 gave a Voc of 0.94 V, a short circuit current density (Jsc) of 9.68 mA cm2, a fill factor (FF) of 0.33 and a PCE of 2.99%. The device based on BDATBT:PC61BM displayed a Jsc of 6.56 mA cm2, a FF of 0.30, a PCE of 2.03% and a Voc of 1.03 V, and the high Voc of BDATBT based device was consistent with its deep-lying HOMO level caused by the electron-withdrawing effect of ethynylbenzene linkage group. As a reference device, P3HT-based device was fabricated under the same conditions and showed a moderate Voc of 0.52 V (J–V curve was shown in Fig. S1 of supplementary materials). In comparison with BDPTBT, BDATBT based device exhibited a relatively low values of Jsc, FF and PCE, which was consistent with its low lh value. Consideration of the high Ksv values of BDETBT and BDCTBT owing to their high photoinduced charge separation efficiency with PC61BM, further optimized devices of them were investigated with an increased D/A weight ratio of 1:1 [57]. Notably, the BDCTBT:PC61BM based device

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Fig. 6. (a) Schematic energy-level diagram of the materials for BHJ solar cells, (b) J–V characteristics of PV devices with a configuration of ITO/PEDOT:PSS/ OSMs:PC61BM/Al under an illumination of AM 1.5G (100 mW cm2). Inset: the device structure, (c) J–V curves of the hole-only devices with a structure of ITO/PEDOT:PSS/OSMs:PC61BM/Au in a double logarithmic scale, and the sold lines are fitting plots of the data points by a SCLC model, and (d) IPCE spectra of the devices.

Table 2 The PV Properties and hole mobility of the small molecules based on BDPTBT, BDETBT, BDATBT, BDCTBT and P3HT.

lh (cm2 V1 s1) BDPTBT BDETBT BDATBT BDCTBT P3HT a b

614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629

4

1.38  10 3.28  104 2.91  105 9.20  104 3.0  104a

Voc (V)

Jsc (mA cm2)

FF

PCE (%)b

0.94 0.92 1.03 1.08 0.52

9.68 8.77 6.56 14.0 15.55

0.33 0.42 0.30 0.32 0.47

2.99 ± 0.22 3.40 ± 0.05 2.03 ± 0.12 4.84 ± 0.15 3.82 ± 0.16

Hole mobility data of P3HT-based reference device[56]. Error analysis over 20 devices.

displayed an impressive PV performance with a Voc of 1.08 V, a Jsc of 14.0 mA cm2, a FF of 0.32 and a PCE of 4.84%. Up to date, the PCE of 4.84% is the highest efficiency for D–A–D-typed BT–TPA based solution-processed OSM devices. In addition, the Voc of 1.08 V is the highest voltage for solution-processable OSM donors reported so far. The BDETBT:PC61BM based SMOSCs yielded a Voc of 0.92 V, a Jsc of 8.77 mA cm2, a FF of 0.42 and a PCE of 3.40%. The results demonstrated that the significantly increased PCEs were obtained with the decreased concentration of PC61BM. In comparison, BDCTBT exhibited the highest Jsc and PCE values owing to the highest hole mobility among these materials. As expected, SMOSCs based on these small molecules yielded relatively larger Voc values above 0.92 V, which were consistent with their deep-lying HOMO levels.

Encouragingly, SMOSCs based on BDCTBT and BDATBT exhibited high Voc up to 1.03 V and 1.08 V, respectively, which were benefiting from the electron-withdrawing effect of linkage group of ethynylbenzene and 2-phenylacrylonitrile. It was shown that the SMOSCs based on BDCTBT and BDETBT showed relatively high PV performance, attributing to their better planarity, larger Ksv values and higher hole mobility. The monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra of these devices were measured as well. As shown in Fig. 6d, all devices based on these materials exhibited significant photon-to-current responses in the range of 300–700 nm with the maximum IPCE of 33% at 473 nm, 35% at 417 nm, 31% at 400 nm and 44% at 437 nm for BDPTBT, BDETBT, BDATBT and BDCTBT, respectively, which were consistent with their trend of

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their PCEs. In comparison, the BDCTBT:PC61BM based device showed relatively high IPCE response in agreement with its best PCE performance, which was attributed to its high hole mobility and high charge dissociation, as well as strong and broad absorption in visible region.

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4. Conclusion

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In conclusion, two novel small molecules BDPTBT and BDATBT with p-linkage of benzene and ethynylbenzene were synthesized, respectively. To study the linkage effect on the PV properties, the corresponding small molecules BDETBT and BDCTBT with p-linkage of styrene and 2-phenylacrylonitrile were also investigated. BHJ solar cells based on BDPTBT and BDATBT yielded the PCEs of 2.99% and 2.03%, respectively. Notably, BDATBT based device showed a high Voc of 1.03 V. The reference devices based on BDCTBT and BDETBT with the optimized weight ratio showed dramatically enhanced PCEs of 4.84% and 3.40%, respectively, and BDCTBT based device showed a high Voc of 1.08 V. The high PCEs of devices based on BDETBT and BDCTBT could be caused by the linkage group of styrene and 2-phenylacrylonitrile which could not only improve the molecular planarity but also facilitate the photoinduced charge dissociation and hole mobility. The high Voc of BDATBT and BDCTBT were benefiting from the deep-lying HOMO level caused by the electron-withdrawing effect of linkage groups of ethynylbenzene and 2-phenylacrylonitrile. To our knowledge, the Voc of 1.08 V is the highest voltage reported to date for solution-processed BHJ OSCs based on OSM donors, and the PCE of 4.84% is the highest efficiency reported so far for D–A–D-typed BT–TPA based solution-processed OSM devices. As a result, the p-linkage units, benzene, styrene, ethynylbenzene and 2-phenylacrylonitrile played an important role in modifying molecular structure and improving the PV performance.

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Acknowledgements

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Q7 The authors would like to thank the NSFC (21102013), Q5 the Dalian Committee of Science and Technology (2011 J21DW001), the SRF for ROCS-SEM (Nos. 2010014 38 and 201001439), the Fundamental Funds for the Central Universities (DUT11LK20), and the RFDP (No. 20090041 120017) for financial support. We are very grateful for the useful discussion with Prof. Yue Wang and Prof. Wenjing Tian.

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Appendix A. Supplementary material

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.03.013.

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Please cite this article in press as: L. wang et al., High open-circuit voltage of the solution-processed organic solar cells based on benzothiadiazole–triphenylamine small molecules incorporating p-linkage, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.03.013