Enhancing Indacenodithiophene Acceptor Crystallinity via Substituent Manipulation Increases Organic Solar Cell Efficiency

Abstract

The post-fullerene indacenodithiophene acceptor ITIC is a highly effective n-type component of high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs) for reasons that are not well-understood. Here, the impact of the ITIC alkyl substituent architecture on PSC active layer film morphology, charge transport, and photovoltaic (PV) performance is investigated with the donor polymers PBDB-T and PBDB-TF. On progressing from n-propyl to n-hexyl to n-nonyl ITIC substituents, PSC power conversion efficiency (PCE) increases from <0.1 % to 9.31% to 10.24%, respectively. BHJ blend morphology, carrier recombination dynamics, and PV performance with both donor polymers as probed by AFM, XRD, GIWAXS, and light intensity dependence correlate with marked differences in ITIC acceptor crystallinity. The DSC cold crystallization temperatures of the nhexyl and n-nonyl-functionalized acceptors are found to closely track the annealing temperatures for optimum PSC performance. These results identify a promising strategy for optimizing the performance of post-fullerene acceptor PSCs.