Diketopyrrolopyrrole-based conjugated polymers containing alkyl and aryl side-chains for bulk heterojunction solar cells

•DPP-based polymers were synthesized with varied side groups on the DPP core.•The highest PCE was 2.02% with Voc = 0.68 V, Jsc = 5.31 mA cm−2, and FF = 56.0%.•PDPPT-as having asymmetrical side groups also showed good solar cell performance.

A series of diketopyrrolopyrrole (DPP)-based conjugated polymers were synthesized with varied side groups such as alkyl and aryl side-chains on a DPP backbone. Their thermal, optical, and photovoltaic properties were investigated. After adding 2% of 1,8-diiodooctane (DIO) as a processing additive in active layers for the bulk heterojunction device, we could achieve the highest PCE of 2.02% with Voc = 0.68 V, Jsc = 5.31 mA cm−2, and FF = 56.0% for PDPPT-s1:[70]PCBM (1:2). The device for PDPPT-as designed with asymmetrical side groups on the DPP units also showed good solar cell performance comparable with PDPPT-s1. These results suggest that the DIO additives can control side-chain interference on DPP units.