Methyl functionalization on conjugated side chains for polymer solar cells processed from non-chlorinated solvents

Structure fine-tuning is effective to optimize the energy level matching and morphology in polymer solar cells (PSCs), and is important to enhance the photovoltaic performance. Here, we report a fine-tuning approach by introducing a methyl group in the 4-position of the conjugated thiophene side chains. A series of polymeric donors are synthesized based on benzo[1,2-b:4,5-b′]dithiophene (BDT) units with 4-alkyl-5-(2-ethylhexyl)thiophene conjugated side chains. Methyl, ethyl, and n-hexyl are employed as the 4-alkyl groups to afford polymers PMT49, PET52, and PHT53, respectively, and PTh37 is the control polymer without 4-alkyl groups. It is found that the introduction of a 4-alkyl group can lower the energy levels, leading to enhanced open-circuit voltage (VOC) of PSCs. PMT49 shows higher charge mobilities in blend films and more optimized morphology with smaller domain size and little decrease of domain purity than other alkyl group modified polymers. As a result, PMT49:ITIC PSCs processed from toluene exhibit a better average power conversion efficiency (PCE) of 12.0% than the PTh37 (10.6%), PET52 (9.68%) and PHT53 (7.85%) based PSCs. Furthermore, PMT50 with the 5-(2-ethylhexylthio)-4-methylthiophene side chains is synthesized. PMT50:Y6(BO) PSCs exhibit an impressive average PCE of 15.1% when processed from 1,2,4-trimethylbenzene, significantly higher than the devices processed from PEHTT:Y6(BO) without methyl functionalization (12.8%). Our studies suggest that methyl functionalization on conjugated side chains is a simple but effective fine-tuning strategy to develop conjugated polymers for high performance PSCs processed from non-chlorinated solvents.