Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers

TSF-33339; No of Pages 9 Thin Solid Films xxx (2014) xxx–xxx

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Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers Jiefeng Hai a, Wei Yu b, Enwei Zhu a, Linyi Bian a, Jian Zhang b,⁎, Weihua Tang a,⁎ a

Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education of China, Nanjing University of Science and Technology, Nanjing 210094, People's Republic of China State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Zhongshan Road 457, Dalian 116023, People's Republic of China

b

a r t i c l e

i n f o

Article history: Received 27 October 2013 Received in revised form 22 March 2014 Accepted 26 March 2014 Available online xxxx Keywords: Polymer solar cells Thiadiazole Benzodithiophene Conjugated polymers

a b s t r a c t Two copolymers comprised of different fused thiadiazole electron withdrawing units and benzo[1,2-b:4,5-b′] dithiophene electron donating unit have been synthesized by Stille reaction. The structural, optical, electrochemical and photovoltaic properties of the copolymers were investigated. Both copolymers in the film state exhibited abroad absorption spectra with an extremely low bandgap of ~1.2 eV. Bulk heterojunction solar cells were further fabricated and the best device delivered a power conversion efficiency of 0.52% when thermal annealed at 90 °C. © 2014 Elsevier B.V. All rights reserved.

1. Introduction During the past decade, polymers solar cells (PSCs) have attracted researchers' extensive attention in the field of renewable energy technologies due to their unparalleled features such as light-weight, low fabrication cost and great potential for the fabrication of large-area flexible devices with high speed [1–3]. PSCs based on bulk-heterojunction (BHJ) architecture are of particular interests since high performance devices with power conversion efficiencies (PCEs) that exceed 9% can be achieved [4–7] with the art of level of 10.6% [6]. Typical BHJ devices comprise a phase separated donor/acceptor blend layer with low bandgap conjugated polymers as the electron donor, and fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor. Nevertheless, the low PCE, dependable encapsulation technology to secure long-life working devices, and robust large-area processing for device making have become the most significant obstacle in the field of PSC commercial application [8–10]. To improve the PCE, an ideal donor polymer material requires the following ideal conditions: (i) optimal optical bandgap (Eg) with wide absorbance area and high extinction coefficient [11]; (ii) good solubility and appropriate compatibility with fullerene derivatives; (iii) highest occupied molecular orbital (HOMO) energy level for high open-circuit voltage (Voc) [12]; and (iv) high hole mobility. To fulfill these criteria, ⁎ Corresponding authors. Fax: +86 25 84317311. E-mail addresses: [email protected] (J. Zhang), [email protected] (W. Tang).

a large library of a so-called D–A copolymers designed by alternating electron-rich (D) and electron-deficient (A) units along the conjugated backbone have been developed for highly efficient PSCs more than 8% [4–7,13–16]. By incorporating a weakly electron donating donor unit and a strongly electron accepting unit in the copolymer backbone, the electrical and optical properties can be tuned. Furthermore, intramolecular charge transfer (ICT) from the donor to the acceptor unit can enhance the π–π stacking between copolymer chains and effectively reduce the bandgap [17]. To ensure suitable charge transport properties among the polymers, the close packing between polymers should be increased through the reduction of the energetic disorder of polymers, which is caused by the increase in the coplanarity and π–π stacking of polymers. The morphology of the BHJ blend of an electron donor (such as a conjugated polymer) and an electron acceptor (such as the derivative of fullerene, i.e., PC61BM) is also of great importance to photovoltaic performance of PSCs [18]. Benzo[1,2-b:4,5-b′]dithiophene (BDT) has been demonstrated to be one of the most excellent building blocks for the synthesis of highly efficient photovoltaic polymers [19–24]. The BDT units possess symmetric and rigid planar structures with extended π-conjugation. They facilitate π-electron delocalization in polymer chains and improve π–π interactions between polymer chains in solid states [24,25]. Therefore, BDTbased polymers usually have shown broad absorption properties, high charge carrier mobility and deep highest occupied molecular orbital (HOMO) energy position compared with the polymers based on other electron donor units [24]. In the past several years, 4,7-dithienyl-2,1,3-benzothiadiazole (DTBT) has emerged as a promising acceptor moiety for D–A polymers

http://dx.doi.org/10.1016/j.tsf.2014.03.087 0040-6090/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

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due to its strong electron-drawing ability, which lowers the bandgap through ICT [26]. As is known, DTBT and its derivatives have been broadly used as building blocks in photovoltaic materials. Several important polymers have been developed with PCEs reaching 4–7% in PSCs [27–30]. Fused heteroaromatic units with extended πconjugation and rigid coplanar structure can enhance π-conjugation within polymer chains and promote interaction and stacking between polymer chains. Conjugated polymers based on fused heteroaromatic repeating units frequently show improved absorption properties and high charge mobilities [31,32]. So we tried to enlarge the conjugated framework of benzothiadiazole via fusing quinoxaline or phenazine and developed accepting units called thiadiazolo[3,4-g]quinoxaline (DTBTQx) and thiadiazolo[3,4-i]phenazine (DTBTBPz). DTBTQx has been a promising building block for the synthesis of low bandgap polymers because of the strong electron with-drawing property of four imine nitrogen in the DTBTQx unit. However, most of the alternating copolymers composed of DTBTQx derivatives as electron acceptor and thiophene [33], fluorene [34], phenylene [35] or carbazole [36] as electron donor have shown poor photovoltaic performance with the PCE lower than 2% [37,38]. This low PCE is mainly attributed to the low molecular weights (Mn b 10 kDa) of the copolymers because of the lack of solubility. DTBTBPz derivatives have been developed for optoelectronic applications, but no report on PSC application [39,40]. In this study, two alternating copolymers of BDT with DTBTQx or DTBTBPz were designed and synthesized two narrow bandgap copolymers, i.e., poly[4-(5-(4,8-bis(dodecyloxy)-4,8-dihydrobenzo [1,2-b:4,5-b′]dithiophen-2-yl)-alt-5,8-bis-(thiophen-2-yl)-6,7-bis(3,4bis(dodecyloxy)phenyl)-[1,2,5]-thiadiazolo[3,4-g]quinoxaline] (PBDTDTBTQx) and poly[4-(5-(4,8-bis(dodecyloxy)-4,8-dihydrobenzo[1,2b:4,5-b′]dithiophen-2-yl)-alt-10,14-bis(4-(2-ethylhexyl)thiophen-2yl)dibenzo[a,c][1,2,5]thiadiazolo[3,4-i]phenazine] (PBDT-DTBTBPz). The correlation of optical, electrochemical and PSC device performance of the resulted polymers with different fused accepting units was investigated. PBDT-DTBTQx exhibits good solubility with introduced peripheral tetradodecyloxybenzyl side chains of DTBTQx unit. A recent publication discussed the photovoltaic performance with the DTBTQx derivative as acceptor unit and BDT as donor unit just when we were preparing this paper [41]. The device of bulk heterojunction solar cells by blending PBDT-DTBTQx with PC61BM exhibits a maximum power conversion efficiency of 0.52%, with a short circuit current of 1.44 mA/cm2, an open-circuit voltage of 0.69 V and a fill factor of 0.40. Our results indicate that these copolymers show promising photovoltaic properties after careful structure design. 2. Experimental section 2.1. Materials All chemicals and solvents were reagent grade and purchased from Aldrich, Acros or TCI companies and used without further purification. Toluene, tetrahydrofuran (THF) and diethyl ether were freshly distilled before use. Unless otherwise specified, all reactions were conducted under a nitrogen atmosphere. The alternating polymers of BDT with DTBTQx or DTBTBPz were synthesized according to the procedure outlined in Scheme 1. The synthesis detail of monomers and targeted polymers are described as follows. 2.1.1. 4,7-Dibromobenzo[c][1,2,5]thiadiazole (1) [42] 2,3,1-Benzothiadiazole (5.00 g, 36.72 mmol) was dissolved in HBr aqueous solution (100 mL, 47%). Bromine (17.60 g, 110.15 mmol) in HBr aqueous solution (50 mL of 47%) was slowly added to the solution. The mixture was refluxed overnight. After the mixture was cooled to room temperature, an aqueous solution of Na 2S 2O 3 ·5H2 O was added and the product was extracted with CH2Cl2. The organic layer was washed with water and brine, dried with MgSO 4, filtered, concentrated via rotary evaporation and

purified by column chromatography on silica gel eluting with CH 2Cl2 to give pure compound 1 as pale yellow needles (8.50 g, 79%). 1H NMR (500 MHz, CDCl3) δ 7.73 (s, 2H). 2.1.2. 4,7-Dibromo-5,6-dinitrobenzo[c][1,2,5]thiadiazole (2) [42] Trifluoromethanesulfonic acid (7.5 mL) and HNO3 (8 mL) were added dropwise to concentrated H2SO4 (10 mL) at 0 °C. Compound 1 (2.00 g, 6.85 mmol) was added to the acid mixture at 0 °C. The mixture was kept at room temperature for 4 h before being poured into ice-water (100 mL) and then extracted with CH2Cl2. The organic layer was washed with water and brine, dried with MgSO4, filtered, concentrated via rotary evaporation and purified by column chromatography on silica gel with ethyl acetate:petroleum ether (1:10, v/v) as eluent to give pure compound 2 as a pale yellow solid (1.58 g, 40%). mp: 190–195 °C. 2.1.3. 5,6-Dinitro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (3) [43] PdCl2(PPh3)2 (78.2 mg, 0.11 mmol) was added to a solution of 2tributylstannylthiophene (2.40 g, 6.41 mmol) and 2 (1.07 g, 2.79 mmol) in anhydrous THF (40 mL). The reaction mixture was then heated at 80 °C for 20 h. After cooling to room temperature, the solvent was evaporated and the crude product was washed with hexane and dried to give pure compound 3 as an orange solid (0.72 g, 67%). 1H NMR (500 MHz, CDCl3) δ 7.74 (d, J = 4.9 Hz, 2H), 7.52 (d, J = 3.5 Hz, 2H), 7.24 (dd, J = 5.0 Hz, 3.9 Hz, 2H). 2.1.4. 4,7-Bis(4-(2-ethylhexyl)thiophen-2-yl)-5,6-dinitrobenzo[c][1,2,5] thiadiazole (4) [43] PdCl2(PPh3)2 (144 mg, 0.21 mmol) was added to a solution of tributyl(4-(2-ethylhexyl)thiophen-2-yl)stannane (4.59 g, 9.46 mmol) and 2 (1.58 g, 4.11 mmol) in anhydrous THF (40 mL). The reaction mixture was then heated at 80 °C for 20 h. After cooling to room temperature, the solvent was evaporated and the crude product was washed with hexane and dried to give pure compound 4 as an orange solid (2.04 g, 81%). 1H NMR (500 MHz, CDCl3) δ 7.30 (s, 2H), 7.29 (s, 2H), 2.61 (d, J = 6.8 Hz, 4H), 1.60–1.56 (m, 2H), 1.29 (m, 16H), 0.90 (t, J = 7.4 Hz, 12H). 2.1.5. 4,7-Di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-5,6-diamine (5) [44] Iron dust (1.23 g, 20.01 mmol) was added to a solution of compound 3 (0.72 g, 1.83 mmol) in acetic acid (50 mL). The mixture was heated at 80 °C for 5 h before being cooled to room temperature, poured into the water, and extracted with ether. The organic layer was washed with water and brine, dried with MgSO4, filtered, concentrated via rotary evaporation to yield compound 5 as a brown solid (0.51 g, 84%). 1H NMR (500 MHz, CDCl3) δ 7.56 (dd, J = 5.2, 1.0 Hz, 2H), 7.37 (dd, J = 3.5, 1.0 Hz, 2H), 7.26 (m, 2H), 4.40 (s, 4H). 2.1.6. 4,7-Bis(4-(2-ethylhexyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole5,6-diamine (6) [44] Iron dust (1.85 g, 33.15 mmol) was added to a solution of compound 4 (1.70 g, 2.76 mmol) in acetic acid (50 mL). The mixture was heated at 80 °C for 5 h before being cooled to room temperature, poured into the water, and extracted with ether. The organic layer was washed with water and brine, dried with MgSO4, filtered, concentrated via rotary evaporation to yield compound 6 as an orange liquid (1.17 g, 76%). 1H NMR (500 MHz, CDCl3) δ 7.17 (s, 2H), 7.11 (s, 2H), 4.40 (s, 4H), 2.64 (d, J = 6.8 Hz, 4H), 1.63 (m, 2H), 1.39– 1.25 (m, 16H), 0.92–0.87 (m, 12H). 2.1.7. 6,7-Bis(3,4-bis(dodecyloxy)phenyl)-4,9-di(thiophen-2-yl)-[1,2,5] thiadiazolo[3,4-g]quinoxaline (7) [45] Compound 5 (510 mg, 1.54 mmol) and 1,2-bis(3,4-bis(dodecyloxy) phenyl)ethane-1,2-dione [39] (1.61 g, 1.70 mmol) were dissolved in acetic acid (80 mL). The reaction mixture was purged with nitrogen and heated at 90 °C for 12 h. After cooling to room temperature, the

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

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Scheme 1. Synthesis route to PBDT-DTBTQx and PBDT-DTBTBPz.

mixture was then poured into water and extracted with CH2Cl2. The organic layer was washed with water and brine, dried with MgSO4, filtered, concentrated via rotary evaporation and purified by column chromatography on silica gel with ethyl acetate:petroleum ether (1:20, v/v) as eluent to give pure compound 7 as a purple black solid (1.10 g, 50%). 1H NMR (500 MHz, CDCl3) δ 8.99 (d, J = 2.9 Hz, 2H), 7.63 (d, J = 4.0 Hz, 2H), 7.60 (d, J = 1.8 Hz, 2H), 7.34–7.30 (m, 4H), 6.85 (d, J = 8.4 Hz, 2H), 4.05 (t, J = 6.6 Hz, 4H), 4.00 (t, J = 6.7 Hz, 4H), 1.89–1.78 (m, 9H), 1.51–1.42 (m, 8H), 1.40–1.22 (m, 64H), 0.90– 0.86 (m, 12H); 13C NMR (125 MHz, CDCl3) δ 152.72, 151.71, 150.68, 148.65, 135.89, 134.57, 132.84, 130.76, 130.61, 126.68, 124.15, 120.69, 115.87, 112.41, 69.24, 69.08, 31.92, 29.67, 29.46, 29.37, 29.23, 26.11, 26.05, 22.67, 14.08. 2.1.8. 10,14-Bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c][1,2,5] thiadiazolo[3,4-i]phenazine (8) [44] Compound 6 (1.17 g, 2.11 mmol) and phenanthrene-9,10-dione (436 mg, 2.11 mmol) were dissolved in acetic acid (80 mL). The reaction mixture was purged with nitrogen before heating at 90 °C for 12 h. After cooling to room temperature, the mixture was then poured into water and extracted with CH2Cl2. The organic layer was

concentrated via rotary evaporation. The crude residue was purified by column chromatography on silica gel (ethyl acetate/petroleum ether, 1:15 v/v) to give pure compound 7 as a green black solid (1.25 g, 82%). 1H NMR (500 MHz, CDCl3) δ 9.03 (dd, J = 7.8, 1.1 Hz, 2H), 8.66 (s, 2H), 8.07 (d, J = 7.8 Hz, 2H), 7.47 (t, J = 7.4 Hz, 2H), 7.40 (t, J = 7.3 Hz, 2H), 7.12 (s, 2H), 2.61 (d, J = 6.8 Hz, 4H), 1.76– 1.69 (m, 2H), 1.48–1.35 (m, 16H), 1.03–0.94 (m, 12H); 13 C NMR (125 MHz, CDCl3) δ 150.52, 142.30, 140.60, 135.34, 135.02, 134.91, 131.91, 129.90, 129.56, 128.36, 127.18, 121.82, 119.96, 40.28, 34.38, 32.57, 28.96, 25.63, 23.23, 14.30, 10.92. 2.1.9. 6,7-Bis(3,4-bis(dodecyloxy)phenyl)-4,9-bis(5-bromothiophen-2yl)-[1,2,5] thiadiazolo[3,4-g]quinoxaline (9) N-bromosuccinimide (NBS) (168 mg, 0.95 mmol) in one portion was added to a solution of compound 7 (534 mg, 0.43 mmol) in anhydrous THF (100 mL). The mixture was stirred in the dark at 0 °C overnight. The mixture was then poured into water and extracted with CH2Cl2. The organic layer was concentrated via rotary evaporation. The crude residue was purified by column chromatography on silica gel (ethyl acetate/petroleum ether, 1:20 v/v) to give pure compound 9 as a purple black solid (538 mg, 89%). 1H NMR

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

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(500 MHz, CDCl3) δ 8.74 (d, J = 4.2 Hz, 2H), 7.69 (d, J = 1.4 Hz, 2H), 7.13 (d, J = 4.2 Hz, 2H), 7.05 (dd, J = 8.3, 1.6 Hz, 2H), 6.78 (d, J = 8.4 Hz, 2H), 4.17 (t, J = 6.3 Hz, 4H), 4.06 (t, J = 6.5 Hz, 4H), 1.95– 1.84 (m, 8), 1.61–1.48 (m, 8H), 1.44–1.20 (m, 64H), 0.92–0.85 (m, 12H); 13C NMR (125 MHz, CDCl3) δ 152.77, 150.79, 150.61, 149.10, 137.15, 133.51, 132.88, 130.32, 129.31, 124.56, 119.89, 115.41, 111.95, 69.37, 69.07, 31.94, 29.71, 29.39, 26.34, 26.13, 25.99, 22.69, 14.10. 2.1.10. 10,14-Bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c] [1,2,5]thiadiazolo[3,4-i]phenazine (10) N-bromosuccinimide (NBS) (279 mg, 1.57 mmol) in one portion was added to a solution of compound 8 (542 mg, 0.75 mmol) in anhydrous THF (100 mL). The mixture was stirred in the dark at 0 °C overnight. The mixture was then poured into water and extracted with CH2Cl2. The organic layer was concentrated via rotary evaporation. The crude residue was purified by column chromatography on silica gel (ethyl acetate/petroleum ether, 1:20 v/v) to give pure compound 10 as a green black solid (600 mg, 91%). 1H NMR (500 MHz, CDCl3) δ 8.46 (d, J = 7.7 Hz, 2H), 8.30 (s, 2H), 7.78 (d, J = 8.1 Hz, 2H), 7.27 (t, J = 7.1 Hz, 2H), 7.11 (t, J = 7.2 Hz, 2H), 2.32 (d, J = 7.3 Hz, 4H), 1.70–1.62 (m, 2H), 1.47–1.29 (m, 16H), 1.03–0.92 (m, 12H); 13 C NMR (125 MHz, CDCl 3 ) δ 149.38, 141.85, 139.37, 134.54, 134.03, 133.77, 131.45, 129.76, 128.59, 128.17, 126.82, 121.37, 118.27, 118.16, 77.00, 39.72, 33.30, 32.45, 28.75, 25.56, 23.27, 14.32, 10.89. 2.1.11. PBDT-DTBTQx Monomer 9 (233 mg, 0.17 mmol), BDT-Sn (148 mg, 0.17 mmol), and anhydrous toluene (6 mL) were added to a two-necked flask (10 mL). The mixture was purged with nitrogen for 15 min, and then Pd(PPh3)4 (10 mg, 0.08 mmol) was added. After being purged for 15 min, the reaction mixture was heated at 105 °C for 48 h. After being cooled to room temperature, the reaction mixture was added dropwise to methanol (200 mL) to precipitate out the crude product. The collected solid was then subjected to Soxhlet extraction with methanol, acetone and chloroform. Subsequently, the fraction washed by chloroform was concentrated. By re-precipitating from methanol, the title polymer was obtained as a shiny black solid (250 mg, yield 83%). 1 H NMR (500 MHz, CDCl 3 ) δ 9.03 (s, 2H), 7.85–7.28 (m, 6H), 7.24–6.71 (m, 4H), 4.54–3.68 (m, 12H), 2.11–0.72 (m, 138H); 13C NMR (125 MHz, CDCl3) δ 152.10, 151.65, 150.92, 148.82, 144.06, 143.62, 142.25, 136.97, 134.71, 134.16, 132.75, 132.24, 130.97, 129.18, 125.32, 124.39, 120.46, 119.95, 116.19, 112.83, 73.82, 69.41, 31.97, 30.65, 29.79, 29.44, 26.19, 22.70, 14.09; Mn = 10.5 kDa, Mw = 41.0 kDa, PDI = 3.9. 2.1.12. PBDT-DTBTBPz Similar procedure as for PBDT-DTBTQx was employed for the preparation of the title polymer by the Pd(0)-mediated cross-coupling between monomer 10 (182 mg, 0.21 mmol) and BDT-Sn (182 mg, 0.21 mmol). The pure product was obtained as a shiny black solid (234 mg, yield 88%). 1H NMR (500 MHz, CDCl3) δ 9.79–7.31 (m, 12H), 4.34 (s, 2H), 3.07–2.45 (m, 4H), 2.08–0.59 (m, 78H); 13C NMR (125 MHz, CDCl3) δ 151.14, 144.19, 143.67, 143.38, 143.19, 142.70, 140.36, 138.06, 137.43, 135.70, 134.36, 132.33, 130.09, 128.01, 127.72, 122.68, 73.24, 39.76, 37.43, 33.76, 31.97, 29.71, 26.22, 23.27, 22.72, 14.12, 10.86; Mn = 3.4 kDa, Mw = 5.1 kDa, PDI = 1.5. 2.2. Measurement and characterization The nuclear magnetic resonance (NMR) spectra were obtained on a Bruker AVANCE 500 MHz (Bio-Spin Corporation, Europe) spectrometer using d-chloroform (CDCl3) as the solvent and the chemical shifts were recorded in ppm unit with tetramethyl silane (TMS) as the internal standard. The average molecular weight and polydispersity (PDI) of

the polymers were determined by Waters 717-2410 gel permeation chromatography (GPC) analysis with THF as eluent and polystyrene as standard. Ultraviolet–visible (UV–vis) absorption spectra were measured on a UV–vis instrument Evolution 220. Differential scanning calorimetry (DSC) measurements were performed on a DSC instrument DSC823. Thermogravimetric analyses (TGA) were conducted on a TA instrument TGA/SDTA851e under nitrogen at a heating rate of 20 °C/min. The electrochemical cyclic voltammetry (CV) measurements were conducted on an electrochemistry workstation (CHI660D, Chenhua Shanghai) with the polymer film on Pt plate as working electrode, Pt slice as the counter electrode, and saturated calomel electrode (SCE) as reference electrode in a 0.1 mol/L tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) anhydrous nitrogen saturated acetonitrile solution at a scan rate of 50 mV/s. The copolymers were coated on the platinum plate working electrodes from dilute chloroform solution. 2.3. Fabrication and characterization of polymer solar cells The photovoltaic cells were constructed in the traditional sandwich structure through several steps. Poly(3,4-ethylene-dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS, from H. C. Starck) was spin coated from an aqueous solution on a cleaned (indium tin oxide) ITO glass substrate with a thickness of 40 nm, and then baked at 150 °C for 15 min. With a nominal thickness of ~ 80 nm, the photosensitive blend layer was prepared by spin-coating the o-dichlorobenzene (o-DCB) solution of the polymers and PC61BM (with different weight ratios) with the polymer concentration of the 10 mg/mL on top of the ITO/PEDOT:PSS electrode, and dried at 85 °C for 30 min in a nitrogen-filled glove box. Finally, Al layer was subsequently evaporated through a shadow mask to define the active area of the devices (~ 2 × 8 mm2) and form a top anode to complete the photovoltaic device fabrication. In this study, all the devices were fabricated with the following structure: ITO glass/ PEDOT:PSS/polymer:PC61BM/Al encapsulation glass. The current–voltage (J–V) curves of photovoltaic devices were measured with a Keithley 2400 SourceMeter under 1 sun, AM 1.5G spectrum from a solar simulator (Oriel model 91192, 100 mW/cm2) at room temperature in a nitrogen filled glove-box. Solar simulator illumination intensity was calibrated using a standard Si photodiode detector equipped with a KG-5 filter. 3. Results and discussion 3.1. Synthesis and characterization The synthetic routes for the monomers and copolymers are outlined in Scheme 1. Compound 2 was obtained through bromination and nitration of 2,1,3-benzothiadiazole. The non-alkylated compound 3 and alkylated compound 4 were prepared with nitrated compound 2 and non-alkylated or alkylated thiophene. After further reduction with iron power, condensation with diketones and further bromination, the targeted brominated monomers 9 and 10 were obtained in good yields. The targeted alternating polymers PBDT-DTBTQx and PBDTDTBTBPz were prepared via Pd(0)-mediated Stille coupling reaction between stannylated BDT monomer and brominated monomers (9 or 10) with yields ranging from 83% to 88% (Scheme 1). Crude polymers were Soxhlet-extracted with methanol, acetone, hexane and chloroform in sequence. The title polymers were obtained by reprecipitation of their concentrated chloroform solutions from methanol. The structures of the obtained copolymers were confirmed by 1H NMR and 13C NMR spectroscopy. The two copolymers possess excellent solubility in common organic solvents, including tetrahydrofuran, chloroform, odichlorobenzene and toluene, which is attributed to the incorporation of solubilizing alkoxyl chains attached on benzene rings or branch alkyl chains adjoined to thiophene flank of polymer backbone. Molecular weight and polydispersity index (PDI) of the polymers were measured by gel permeation chromatography (GPC) using

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

J. Hai et al. / Thin Solid Films xxx (2014) xxx–xxx Table 1 Characterization of the polymers. Polymer

Yield (%)

Mwa

Mn

PDI

Tdb (°C)

PBDT-DTBTQx PBDT-DTBTBPz

83 88

41 500 5200

10 500 3400

3.9 1.5

348 325

a Mw, Mn, and PDI of polymers were determined by GPC using polystyrene standards in THF. b The 5% weight-loss temperatures under N2 and heating from 50 to 800 °C at 20 °C/ min rate.

polystyrene as standard and THF as eluent. The GPC results indicated that two copolymers have the number-average molecular weight (Mn) of 4–11 kDa with a PDI of 1.5–3.9 (Table 1). PBDT-DTBTBPz showed a low Mn, probably due to the steric hindrance resulted by the too rigid planarity structure of acceptor unit. This finding is in good agreement with previous reports by other group [31,46,47]. Lee et al. reported that TBPz has a more planar structure which more effectively induces interchain interaction of the polymers chains, resulting in a lower Mn with less solubility [31]. The degree of polymerization for PBDT-DTBTBPz was lower than PBDT-DTBTQx because of phenazine rigid backbone structure and less effective polymerization. And PBDTDTBTQx harvested much improved Mn (10.5 kDa) with side chains in benzene ring suspended of DTBTQx and its Mw (41.5 kDa) is higher than similar structure reported BDT-alt-DTBTQx polymer (Mw 38.4 kDa) by Keshtov et al. [41], whose PCE reached 5.12%. PBDTDTBTQx exhibited a Mn of 10.5 kDa, higher than other DTBTQx-based polymers such as P(3HT-PhTDQ) [33] (Mn 7.2 kDa), P1–P3 [36] (Mn 2.6–3.3 kDa), and PBTTQ [38] (Mn 9.3 kDa). So the introduction of side chains into DTBTQx is a useful strategy to increase solubility and Mn of copolymers. The thermal stability of polymers was characterized with thermogravimetric analyses (TGA) (Fig. 1). Decomposition temperatures (Td) (5% weight loss) were 348 °C and 325 °C for PBDT-DTBTQx and PBDTDTBTBPz, respectively. Good thermal stability of the photovoltaic polymers is necessary for device fabrication and applications. Differential scanning calorimetry (DSC) experiments, however, did not show any glass transition and crystallization behavior within the temperature range (0–280 °C), indicating the amorphous nature of these polymers. 3.2. Optical and electrochemical properties The UV–vis spectra of two monomers in CHCl3 solutions and two copolymers in both CHCl3 solutions and thin films are shown in Fig. 2. Two monomers DTBTQx and DTBTBPz displayed a broad absorption band in the range of 500–800 nm. DTBTQx solution shows a λmax at 532 nm while DTBTBPz solution shows a λmax at 730 nm, which shows strong electron drawing ability than our previous synthesized triazoloquinoxaline (BTzQx) units [44]. The result illustrates

5

that DTBTQx and DTBTBPz are the stronger electron acceptor units than BTzQx due to the electron-drawing ability of benzothiadiazole stronger than benzotriazole. The fused heteroaromatic unit (DTBTBPz) shows more broaden absorption and narrower Eopt than DTBTQx. Two polyg mers films exhibit a broad absorption band in the range of 600– 1000 nm, which is 50–60 nm red-shifted in comparison to their absorption spectra in solution. PBDT-DTBTQx film shows a λmax at 864 nm, which is 66 nm red-shifted than its solution. PBDT-DTBTBPz film exhibits a λmax at 913 nm, 53 nm red-shifted than its solution. The redshifted absorption of polymers film indicated that strong intermolecular interaction and aggregation exist in the solid-state of these polymers, which is probably related to the increased extent of π–π stacking of the backbones and increased polarizability of the solid-state. PBDTDTBTQx and PBDT-DTBTBPz exhibited two evident absorption bands, the first absorption band at the shorter wavelength was attributed to the π–π transition of the conjugated main chain and the second band at the longer wavelength was owed to the ICT interaction between the BDT donating unit and DTBTQx or DTBTBPz accepting unit. PBDTDTBTQx has more broad spectrum absorption than another BDT-altDTBTQx based polymer P1 (λmax at 764 nm) reported by Keshtov et al. [41]. It was revealed that conjugated phenyl attached to DTBTQx broaden spectrum absorption stronger than alkyl chains group. The λmax of PBDT-DTBTQx film (864 nm) is much red-shifted when comparing other reported DTBTQx alternating polymers with carbazoler [36], thiophene [33] and fluorene [34]. It was revealed that BDT unit maybe a better choice than thiophene, fluorene, and carbazole, in constructing DTBTQx-based polymer. Compared with PBDT-DTBTQx, PBDT-DTBTBPz with conjugated refused phenazine, could further add spectrum absorption at longer wavelength. This phenomenon that the greater conjugated fused acceptor unit could reduce the bandgap of copolymers was also found by other group [31,32,46,48]. Meanwhile, by introducing more alkoxy chains, the electron-withdrawing characteristics of the acceptor were weakened and thus the bandgap of PBDT-DTBTQx was increased in comparison to PBDT-DTBTBPz. The Eopt g of PBDT-DTBTQx and PBDT-DTBTBPz was determined with absorption onset to be 1.16 and 1.14 eV, respectively. From the Eopt g data of our polymers, one could conclude that the Eopt g of polymers is dependent upon their electron-withdrawing ability of polymer backbone. The fused heteroaromatic unit in backbone of PBDT-DTBTBPz shows more broaden absorption and narrower bandgap than PBDT-DTBTQx. Compared with our earlier reported BDT-BTzQx copolymers [44], PBDTDTBTQx and PBDT-DTBTBPz presented narrower bandgap, indicating stronger electron-accepting capability of DTBTQx and DTBTBPz than BTzQx unit due to its benzothiadiazole unit [44]. Compared with other reported DTBTQx-based polymers, PBDT-DTBTQx (1.16 eV) presented narrower bandgap than literature reported DTBTQx alternating polymers with thiophene [33], fluorene (1.20 eV) [34], carbazole [36] and BDT [41]. We could conclude that different electron donating units could also effectively tune the bandgap of copolymers (Table 2). 3.3. Electrochemical properties To reveal the effect of fused units on the energy levels, electrochemical cyclic voltammetry (CV) was conducted to determine the energy levels of the as-prepared polymers. Fig. 3 shows monomers and polymers film on Pt electrode in 0.1 M Bu4NPF6 acetonitrile solutions. The potential of ferrocene 0.40 V vs SCE is used as internal standard. On the basis of 4.8 eV below vacuum for the energy level of Fc/Fc+, the HOMO and lowest unoccupied molecular orbital (LUMO) level of polymers are calculated from the onset oxidation potentials (Eox) and the onset reduction potentials (Ered) according to the following equations [49]:

Fig. 1. TGA traces of the resulted polymers.

EHOMO ¼ −ðEox þ 4:4ÞðeVÞ ELUMO ¼ −ðEred þ 4:4ÞðeVÞ:

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

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J. Hai et al. / Thin Solid Films xxx (2014) xxx–xxx

Fig. 2. UV–vis spectra of (a) PBDT-DTBTQx solution and films together with DTBTQx in CHCl3 solution; and (b) PBDT-DTBTBPz in CHCl3 solutions and solid films together with DTBTBPz in solution.

From CV measurement, both clear oxidation peaks and reduction peaks were observed for monomers DTBTQx and DTBTBPz. Their HOMO levels were determined to be −5.44 and −5.43 eV for DTBTQx and DTBTBPz, respectively. Their LUMO energy level was determined to be −3.69 and −4.00 eV for DTBTQx and DTBTBPz, respectively. The electrochemical bandgap (Eg) of DTBTQx and DTBTBPz was thus calculated to be 1.75 and 1.43 eV. The Eox was observed to be 0.87 and 0.91 V for PBDT-DTBTQx and PBDT-DTBTBPz, respectively. Accordingly, the HOMO energy levels were calculated to be − 5.27 and − 5.35 eV for PBDT-DTBTQx and PBDT-DTBTBPz, which are in good agreement with the ideal HOMO energy levels to ensure good air stability and high open circuit voltage (Voc) in PSCs [50]. It is obvious that the HOMO energy levels of the two polymers are almost identical due to the presence of the same backbone, which is dependent upon the donating unit. PBDT-DTBTQx exhibited a HOMO level higher than copolymers of quinoxaline alternating with fluorene [34], cabazole [36], thiophene [33] and BDT [41,44]. PBDT-DTBTBPz exhibited a HOMO level higher than literature reported PIDT-phanQ [32], poly(9,9dioctylfluorene-alt-10,13-bis-(4-octyl-thiophene-2-yl)-dibenzo[a,c] phenazine) [31], poly(N-9′-heptadecanyl-2,7-carbazole-alt-10,13bis-(4-octyl-thiophene-2-yl)-dibenzo[a,c]phenazine) [31], poly(4,8bisethylhexyloxy-benzo[1,2-b:4,5-b′]-dithiophene-10,13-)dibenzo[a,c] phenazine (PBDT-DBPz) and poly(4,8-bisethylhexyloxy-benzo[1,2b:4,5-b′]dithiophene-10,13-di(thiophen-2-yl)dibenzo[a,c]phenazine) (PBDT-DTDBPz) [48]. The Ered was observed to be −0.74 and −0.60 V for PBDT-DTBTQx and PBDT-DTBTBPz, respectively. Accordingly, the LUMO energy levels were calculated to be − 3.66 and − 3.80 eV for PBDT-DTBTQx and PBDT-DTBTBPz. The HOMO–LUMO energy diagrams of the polymers and PC61BM are shown in Fig. 4. It is obvious that the LUMO energy levels of the four polymers are strongly dependent upon the accepting unit. The LUMO energy levels were located at −3.66 eV for PBDT-DTBTQx, and −3.80

for PBDT-DTBTBPz, which are much higher than that of PC61BM (− 4.2 eV) to guarantee an energetically favorable electron transfer. PBDT-DTBTBPz has the lower lying LUMO level due to the stronger electron deficiency of DTBTBPz. Compared other groups reported polymers [31–34,36,41,44,48], the resulted polymers have suitable HOMO and LUMO energy levels. 3.4. Photovoltaic properties The photovoltaic properties of two copolymers in BHJ solar cells were evaluated by blending polymers with PC61BM acceptor as active layer. The thickness of the active layers was controlled by changing the spin speed during the spin-coating process. The photoactive layers were prepared by spin-coating of the blend solution of polymer: PC61BM dissolved in o-DCB, in which two copolymers show good solubility, on ITO/PEDOT:PSS patterned substrates (40 nm). The thickness of all the blends is around 80 nm. The active area of the cells is 0.16 cm2. All devices were prepared under an inert nitrogen atmosphere and characterized in air without encapsulation, where current–voltage (J–V) measurements were performed under simulated AM 1.5G illumination (100 mW/cm2). The photovoltaic properties of the fabricated PSCs with a configuration ITO/PEDOT:PSS/polymer:PC61BM/Al are summarized in Table 3. The corresponding photovoltaic performances of devices with different ratios are summarized in Table 3. Different PBDT-DTBTQx or PBDT-DTBTBPz/PC61BM weight ratios, such as 1:1, 1:2, and 1:3 have been investigated to optimize the photovoltaic properties. The polymer/PC61BM weight ratio of 1:2 showed the best device performances. The devices were further explored with thermal annealing at 90 °C for the active layer (Fig. 5). The thermal annealing had positive effect on device performance by increasing Jsc. The device obtained from the PBDT-DTBTQx:PC61BM (weight ratios, 1:1), showed a PCE of 0.29% with a short circuit current (Jsc) of

Table 2 Optoelectronic data of monomers and polymers. Polymer

UV–vis absorption

Cyclic voltammetry

Solution

DTBTQx DTBTBPz PBDT-DTBTQx PBDT-DTBTBPz a b

Film

λmax (nm)

λonset (nm)

λonset (nm)

Eopt g (eV)

Eox (V)

Ered (V)

HOMO (eV)a

LUMO (eV)b

532 730 800 861

– – 998 1040

– – 1070 1090

– – 1.16 1.14

1.04 1.03 0.87 0.91

−0.71 −0.40 −0.74 −0.60

−5.44 −5.43 −5.27 −5.31

−3.69 −4.00 −3.66 −3.80

HOMO = −(Eox + 4.4) (eV). LUMO = −(Ered + 4.4) (eV).

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

J. Hai et al. / Thin Solid Films xxx (2014) xxx–xxx

7

Fig. 3. CVs of (a) monomers and (b) polymer thin films.

1.22 mA/cm2, open circuit voltage (Voc) of 0.68 V, and a fill factor (FF) of 0.35. The device obtained from the PBDT-DTBTQx:PC61BM (weight ratios, 1:3), showed a PCE of 0.30%, a Jsc of 1.33 mA/cm2, a Voc of 0.65 V, and a FF of 0.35. The best PSC performances were obtained from the PBDT-DTBTQx:PC61BM (weight ratios, 1:2) devices, which reached a PCE of 0.52% with a Jsc of 1.44 mA/cm2, a Voc of 0.69 V, and a FF of 0.40 after thermal annealing at 90 °C, whose PCE increased 33% compared with no annealing device. The device based PBDT-DTBTBPz:PC61BM (weight ratios, 1:1) as the active layer gave a Voc of 0.72 V, a Jsc of 0.61 mA/cm2, a FF of 35%, and resulted in a PCE of 0.15%. The device based PBDT-DTBTBPz:PC61BM (weight ratios, 1:3) as the active layer gave a Voc of 0.72 V, a Jsc of 0.74 mA/cm2, a FF of 34%, and resulted in a PCE of 0.18%. The best PSC performances were obtained from the PBDT-DTBTBPz:PC61BM (weight ratios, 1:2) devices, which reached a PCE of 0.44% with a Jsc of 1.32 mA/cm2, a Voc of 0.73 V, and a FF of 0.46 after thermal annealing at 90 °C, whose PCE increased 29% compared with no annealing device. The Voc value is closely related to the energy difference between the HOMO energy level of the polymer and the LUMO energy level of electron acceptor, PC61BM. As expected from the HOMO energy levels of the polymers, the Voc for PBDT-DTBTQx and PBDT-DTBTBPz was correlated with their electrochemical properties and the Voc of PBDT-DTBTBPz is higher than PBDT-DTBTQx, which is keeping with the value of electrochemical energy levels. In general, the PCE of PBDT-DTBTQx is better than that of the reported BTBTQx alternating copolymer with fluorene (PCE of 0.37%) [34], bithiophene (PCE of 0.08%) [38], thiophene (PCE of 0.14%) [33] and benzene (PCE of 0.38%) [35] due to its excellent solubility and high Mn. The PCE of PBDT-DTBTBPz is much lower than PIDT-phanQ (PCE of 5.69%) [32], PFTBPz (PCE of 2.61%) [31], PCTBPz (PCE of 3.80%) [31], PBDT-DTDBPz (PCE of 4.75%) [48], PBDT-DBPz (PCE of 0.46%) [48] due to its large space steric effect and low Mn. From above, the devices based on PBDT-DTBTBPz had larger Voc (0.73 eV) than the devices based on PBDT-DTBTQx (0.69 eV), mainly due to their different HOMO energy levels. The low Jsc and FF of PBDTDTBTQx were due to its bulk side chains which hindered π–π stacking and ICT. The low Jsc and FF of PBDT-DTBTBPz were due to its low

molecular weight which hinders the effective conjugated length. All devices could not form uniform and bicontinuous network blending film with PC61BM.

4. Conclusion In summary, two electron withdrawing fuse thiadiazole units, DTBTQx and DTBTBPz were developed. DTBTBPz acceptor unit was used for the design of D–A alternating copolymer for PSC application. By fusing quinoxaline or phenazine with benzothiadiazole unit, triadiazolo[3,4-g]quinoxaline (DTBTQx) and thiadiazolo[3,4-i]phenazine (DTBTBPz) feature excellent solubility and strong electron accepting ability. The two polymers have narrow optical bandgap (~1.2 eV) and good thermal stability. Preliminary tests on BHJ devices showed a best PCE of 0.52%, with a Jsc of 1.44 mA/cm2, a Voc of 0.69 V, and a FF of 0.40, after thermal annealing at 90 °C. There is more space for PCE of BHJ devices after sophisticated changed processing methods i.e. solvent annealing and solvent additives or added cathode interfacial layer. Our results show that DTBTQx and DTBTBPz units could become the choices of acceptors to construct D–A copolymers for PSCs after careful structure design.

Acknowledgments We gratefully acknowledged the financial support from the National Natural Science Foundation of China (Grant No. 21074055), Program for New Century Excellent Talents in University (NCET-12-0633), Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20130032), Doctoral Fund of Ministry of Education of China (No. 20103219120008), and the Fundamental Research Funds for the Central Universities (3092013111006).

Table 3 Photovoltaic performance of polymer:PC61BM cells at different processing conditions. Polymer

D/A

Voc [V]

Jsc [mA/cm2]

FF

PCE [%]

PBDT-DTBTQx

1:1 1:2a 1:2b 1:3 1:1 1:2a 1:2b 1:3

0.68 0.68 0.69 0.65 0.72 0.73 0.73 0.72

1.22 1.35 1.44 1.33 0.61 1.01 1.32 0.74

0.35 0.42 0.40 0.35 0.35 0.46 0.46 0.34

0.29 0.39 0.52 0.30 0.15 0.34 0.44 0.18

PBDT-DTBTBPz

a

Fig. 4. The HOMO and LUMO energy levels of polymers.

b

No annealing. Annealing at 90 °C.

Please cite this article as: J. Hai, et al., Synthesis and photovoltaic characterization of thiadiazole based low bandgap polymers, Thin Solid Films (2014), http://dx.doi.org/10.1016/j.tsf.2014.03.087

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J. Hai et al. / Thin Solid Films xxx (2014) xxx–xxx

Fig. 5. J–V characteristics of (a) PBDT-DTBTQx:PC61BM and (b) PBDT-DTBTBPz:PC61BM devices at different weight ratios with/without annealing.

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