Methyl Thioether Functionalization of a Polymeric Donor for Efficient Solar Cells Processed from Non-Halogenated Solvents

Polymer solar cells (PSCs) processed from non-halogenated solvents are favorable for large scale production. However, the photovoltaic performance of devices from non-halogenated solvents is generally inferior to that of the counterparts from halogenated solvents. Herein, we report the utilization of 5-alkyl-4-(methylthio)thiophene (MTT) as a conjugated side chain of the polymeric donor to achieve efficient PSCs processed from toluene. The derived polymer PMTT56 exhibits lower energy levels than the 5-alkyl-4-methoxythiophene (MOT) counterpart (PMOT39) and 5-(alkylthio)thiophene counterpart (PEHTT). PMTT56 is superior to PMOT39 with an enhanced interchain interaction and hole mobility, and also shows better solubility than PEHTT in toluene. In toluene-processed PSCs, the PMTT56 devices deliver a power conversion efficiency (PCE) of up to 12.6% with IT-2F as the acceptor. By introducing [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the second acceptor, a remarkable PCE of 13.2% is further achieved in the ternary devices, which is among the best for PSCs processed from toluene. Our study presents a fine-tuning approach to develop efficient organic photovoltaic materials processed from non-halogenated solvents.