Influence of alkyl chain length in S,N-heteropentacenes on the performance of organic solar cells

Abstract

A novel family of alkyl-substituted S,N-heteropentacenes SN5 4–6 with a systematic variation of the alkyl side chains from methyl to hexyl is presented. The parent heteroacene series 4 and the acceptor end-capped series 5 and 6 were synthesized and characterized with respect to their structure–property relationships. The dicyanovinyl (DCV) end-capped derivatives 6a–f were implemented as donors in vacuum-processed bulk-heterojunction solar cells with fullerene C₆₀ as acceptor, their photovoltaic behaviour studied, and structure–device relationships discussed. The devices exhibited power conversion efficiencies (PCE) in the range of 4–6% and interestingly a correlation between the length of the alkyl side chains and the photovoltaic performance of the resulting devices was discovered: an odd–even effect with higher values for the even-numbered alkyl chains. Atomic force microscopy (AFM) studies on the active layer surfaces gave hints for the explanation of this effect.