Ladder-type high gap conjugated polymers based on indacenodithieno[3,2-b]thiophene and bithiazole for organic photovoltaics

Abstract

Two push-pull type conjugated polymers - PIDTT-BTz and PIDTT-DTBTz, based on the ladder-type donor unit indacenodithieno[3,2-b]thiophene (IDTT) and bithiazole (BTz) as acceptor component - are designed and synthesized for photovoltaic applications. The polymers exhibit relatively high optical gaps of similar to 2.0 eV with strong absorption in the range of 400-600 nm, rendering them of particular interest for the harvesting of indoor light and/or multijunction devices. Electrochemical investigations indicate a lower highest occupied molecular orbital energy level (-5.44 eV) for PIDTT-BTz as compared to PIDTT-DTBTz (-5.36 eV), enabling to achieve a higher open-circuit voltage. Under solar illumination, the best power conversion efficiency (5.1%) is achieved for the combination PIDTT-DTBTz:PC71BM (compared to 4.6% for PIDTT-BTz:PC71BM).