Improved bulk-heterojunction polymer solar cell performance through optimization of the linker groupin donor–acceptor conjugated polymer

Two donor–acceptor conjugated polymers PCTBTC8 and PCTTBTC12 have been prepared from Suzuki coupling reactions between the 2,7-carbazole and alkoxy substituted 2,1,3-benzothiadiazole units with two different linker groups thiophene and thieno[3,2-b] thiophene, respectively. The polymer PCTTBTC12 with the thieno thiophene linker group possessed a red-shifted UV-vis absorption spectrum and similar HOMO level in comparison with PCTBTC8. BHJ polymer solar cells were fabricated to investigate the photovoltaic properties of the two polymers and the polymer PCTTBTC12 showed superior device performance than PCTBTC8 with a significantly improved current density. In addition, the hole only devices tests results indicated that the hole mobility of the polymer was increased by replacing the thiophene group with fused thieno thiophene as the linker. The relationships between the device performance with the light absorption, energy levels, BHJ film morphology and hole mobility are discussed in details. And the improved photovoltaic property of the polymer PCTTBTC12 probably can be contributed to the better light absorption spectrum and enhanced hole transporting ability related to the more electron rich nature and planar structure of the thieno[3,2-b] thiophene group.