Synthesis of thienoselenadiazole-containing conjugated copolymers and their application in polymer solar cells

INTRODUCTION In recent years, polymer solar cells (PSCs) have attracted great attention due to the energy crisis all over the world. Compared with inorganic solar cells, PSCs have many advantages, such as light weight, solution processability, mechanical flexibility and low cost.1 The stateof-the-art of PSCs are based on a fullerene-derivative acceptor, such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]phenyl-C71-butyric acid methyl ester (PC71BM), and a conjugated polymer donor, such as poly(3-hexylthiophene) (P3HT).2 Power conversion efficiency (PCE) of 4–5% has been achieved for P3HT/ PCBM solar cells.3 To obtain high-performance PSCs, a variety of low-bandgap conjugated polymers have been synthesized through donor–acceptor (D–A) copolymerization.4,5 A number of electronrich heterocycles, such as benzodithiophene (BDT),6 dithieno[3,2b:20,30-d]silole (DTS)7 and indacenodithiophene,8 have been widely used as donor units, whereas benzothiadiazole,9 thieno[3,4-c]pyrrole-4,6-dione10 and diketopyrrolo[3,4-c]-pyrrole-1,4-dione11 have been used as acceptor units. Recently, remarkable progress has been made by using a new copolymer, PBDTTT-CF, yielding a high PCE up to 7.73%.12 To further improve the device performance, some heavy-atom-containing heterocycles, such as Seand Ge-containing heterocycles, have also been explored for developing novel D–A copolymers.13,14 The influence of the selenium atom on the electron distribution and the charge transport property has been investigated.15 Selenium shows a stronger heteroaromatic interaction than sulfur, and it may enhance the inter-chain interaction and free charge carrier transportation.16 To the best of our knowledge, the novel selenium-heterocycle thienoselenadiazole, has never been synthesized and investigated as the building block for conjugated polymers. In this work, we report for the first time the synthesis of a thienoselenadiazole derivative 4,6-Bis(40-hexylthiophen-20yl)thieno[3,4-c][1,2,5]selenadiazole (DTTSe) through the condensation of the 4,400-dihexyl-3,40-diamino-2,20:50,200-terthiophene and SeO2. Furthermore, we synthesized two new Se-containing conjugated copolymers PBDTDTTSe and PDTSDTTSe by copolymerization of DTTSe with BDT and DTS. The optical, electrochemical and photovoltaic properties of the polymers were investigated.