The effect of inclusion of alkylthiophene and fluorine on the optoelectronic properties of copolymers from dithienobenzodithiophene and benzothiadiazole derivatives is systematically investigated.

Two low band gap alternating copolymers, namely PTTBDT-DTBT and PTTBDT-DTFBT, were designed and synthesized, respectively, utilizing benzo[1,2-b:4,5-b′]dithiophene (BDT) modified by alkylthieno[3,2-b]thiophene as donor unit, benzothiadiazole (BT) and/or fluoro-substituted benzothiadiazole (FBT) as acceptor moiety and alkylthiophene as conjugated π-bridge.Compared to PTTBDT-DTBT, fluoro-substituted PTTBDT-DTFBT showed poorer solubility in common organic solvents, higher thermostability and blue-shifted absorption, more decreased trend of the highest occupied molecular orbital (HOMO) energy level.Inverted bulk hetero-junction polymer solar cells (PSCs) devices were fabricated, based on PTTBDT-DTBT and PTTBDT-DTFBT as donor and PC61BM as acceptor under the illumination of AM1.5, 100 mW·cm-2.The device measurement suggested that PTTBDT-DTFBT exhibited an enhanced PCE (2.81% vs.2.67%), an enlarged open-circuit voltage (VOC) (0.82 vs.0.76 V) and a slightly raised fill factor (FF) (54.52% vs.53.04%) than those of PTTBDT-DTBT, in spite of a decreased short-circuit current density (JSC) (6.28 vs.6.62 mA cm-2), respectively.These results exhibited that adding the fluorine onto the backbone of the conjugated polymers was an effective strategy for improving the efficiency of PSCs.

This journal is © The Royal Society of C phene spacers and fluorine on the optoelectronic properties of 5,10-bis(dialkylthien2-yl)dithieno[2,3-d:20,30-d0]benzo[1,2-b:4,5-b0] dithiophene-alt-benzothiadiazole derivative copolymers† Pengzhi Guo, Jingbiao Sun, Shuo Sun, Jianfeng Li, Junfeng Tong, Chuang Zhao, Liangjian Zhu, Peng Zhang, Chunyan Yang and Yangjun Xia *ac Alternating conjugated copolymers based on 5,10-bis(dialkylthien-2-yl)dithieno[2,3-d:20,30-d0]benzo[1,2b:4,5-b0]dithiophene (DTBDT) and 2,1,3-benzothiadiazole (BT) or 5,6-difluoro-2,1,3-benzothiadiazole (FBT) with alkylthiopene spacers were synthesized, and the effect of insertion of alkylthiophene spacers and fluorine atoms on the characteristics of the copolymers, such as the energy levels, intrachain p–p interaction, dielectric constants, photovoltaic properties, etc., were systematically investigated. It has been found that: (i) the introduction of alkylthiophene spacers not only led to an increase in the intrachain interaction of the copolymers, but also resulted in an increase in the highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels, and (ii) the inclusion of fluorine atoms resulted in a decrease in both HOMO and LUMO energy levels with enhancement of the planarity and hole mobility. However, the inclusion of fluorine atoms had little effect on the LUMO levels relative to the decrease in the HOMO levels, and almost did not affect the dielectric constant of the copolymers. Use of the materials in polymeric photovoltaic cells led to high performance photovoltaic cells (PVCs) with power conversion efficiencies of 6.04–7.12%. The results demonstrated that the optoelectronic and aggregation properties of the 5,10-bis(alkylthien-2-yl)dithieno[2,3-d:20,30-d0]benzo[1,2-b:4,5-b0]dithiophene-alt-benzothiadiazole derivative copolymers can be effectively regulated by the introduction of alkylthiophene spacers and/or fluorine atoms into the backbone.

Two donor-p-acceptor (D-p-A) type low bandgap (LBG) conjugated polymers, namely, PT-DTNT-DT and PTT-DTNT-DT, consisting of thiophene (T) and/or thieno[3,2-b] thiophene (TT) as donor moiety, two flanking 2-decyltetradecylthienyl as pi-bridges and large planarity and strong electron-withdrawing naphtho[1,2-c:5,6-c']bis[1,2,5] thiadiazole (NT) as acceptor moiety have been developed and systematically investigated. Both copolymers exhibited strong absorption in the range of 300-850 nm, whilst possessed the low-lying highest occupied molecular orbital (HOMO) energy level of ca. -5.38 eV and the matched lowest unoccupied molecular orbital (LUMO) energy level of -3.90 eV with PC71BM. Compared with NT-based star material PNTz4T using 2,2'-bithiophene (2T) as the linker, the HOMO energy level of the resultant two polymers have been significantly lowered while the absorption spectra range were obviously broadened by replacing of 2T with T and TT. PTT-DTNT-DT showed better coplanarity, more pronounced aggregation tendency in chlorobenzene and higher hole mobility of space-charge-limited current (SCLC) model than those of PT-DTNT-DT. Inverted bulk heterojunction polymer solar cells (PSCs), with these NT-based copolymers as electron donors, respectively, and PC71BM as electron acceptor were fabricated. The optimized cell based on PT-DTNT-DT exhibited the power conversion efficiency (PCE) of 5.10%, with an open-circuit voltage (V-OC) of 0.76 V, promising short-circuit current (J(SC)) of 11.24 mA cm(-2) and fill factor (FF) of 59.88%, whereas, the device based on PTT-DTNT-DT showed relatively higher PCE of 6.88% which was slightly higher than that of PNTz4T (6.72%), with an increased JSC of 14.74 mA cm(-2) and FF of 64.00%, despite the slightly decreased V-OC of 0.73 V, under the same experimental conditions. Therefore, these results indicated the oligothiophene-NT based copolymers were promising candidates aimed at achieving efficient PSCs. (C) 2017 Elsevier Ltd. All rights reserved.