Enhanced Organic Solar Cell Stability by Polymer (PCPDTBT) Side Chain Functionalization

Abstract

Organic photovoltaics represent a promising thin-film solar cell technology with appealing mechanical, aesthetical, and cost features. In recent years, a strong growth in power conversion efficiency (to over 10%) has been realized for organic solar cells through extensive material and device research. To be competitive in the renewable energy market, further improvements are mandatory though, both with respect to efficiency and lifetime. High intrinsic stability of the photoactive layer is obviously a crucial requirement for long lifetimes, but the generally applied bulk heterojunction blends and their components are prone to light-induced and thermal degradation processes. In the present contribution, the high-T-g polymer strategy is combined with specific side chain functionalization to address the thermal stability of polymer solar cells. These two design concepts are applied to a prototype low bandgap copolymer, PCPDTBT. Accelerated aging tests (at 85 degrees C) indicate an improved thermal durability of the PCPDTBT:PC71BM blends and the resulting devices by the insertion of ester or alcohol moieties on the polymer side chains. The different stages in the efficiency decay profiles are addressed by dedicated experiments to elucidate the (simultaneously occurring) degradation mechanisms.