The alkyl chain positioning of thieno[3,4-c]pyrrole-4,6-dione (TPD)-Based polymer donors mediates the energy loss, charge transport and recombination in polymer solar cells

Abstract Tailoring the alkyl chains of polymeric photovoltaic materials is a cost-effective strategy towards efficient polymer solar cells (PSCs). However, the alkyl chain positioning effect of polymers on the photovoltaic performance remains a crucial issue to be examined. In this contribution, three thieno [3,4-c]pyrrole-4,6-dione (TPD)-based donor polymers 2HD/C8, C8/2BO/C6, and 2BO/C6/C6, in which alkyl side chains are appended at different positions, are developed and examined. Studies show that polymer 2HD/C8 largely outperforms its other two analogues C8/2BO/C6 and 2BO/C6/C6. Results make clear that (i) dispersing alkyl side-chain from the N-site of thieno [3,4-c]pyrrole-4,6-dione (TPD) motifs to the 4,8-di(thiophen-2-yl)benzo [1,2-b:4,5-b']dithiophene (BDTT) or to the backbone thiophene units leads to less crystallinity and decreased face-on packing ratio in correlated BHJ films; (ii) the hole (μh) and electron mobility (μe) show gradual reduction from 2HD/C8- to 2BO/C6/C6-, and further to 2BO/C8/C6-based BHJ devices, which result in serious charge recombination, decreased FF and JSC in the corresponding BHJ devices; (iii) the alkyl chain positioning also impacts the energy levels due to the hyperconjugation effect of alkyl chains, consequently, distinct VOC and energy loss are observed. This examination sheds light on that meticulous tailoring the alkyl chain positions should be considered when developing high-performing polymeric photovoltaic materials.