Effects of Short-Axis Alkoxy Substituents on Molecular Self-Assembly and Photovoltaic Performance of Indacenodithiophene-Based Acceptors

The effects of central alkoxy side chain length of a series of narrow bandgap small molecule acceptors (SMAs) on their physicochemical properties and on the photovoltaic performance of the SMA-based polymer solar cells (PSCs) are systematically investigated. It is found that the ordered aggregation of these SMAs in films is enhanced gradually with the increase of alkoxy chain length. The single-crystal structures of these SMAs further reveal that small changes in the side chain length can have a dramatic impact on molecular self-assembly. The short-circuit current density and power conversion efficiency values of the corresponding PSCs increase with the increase of the side chain length of the SMAs. The pi-pi coherence length of the SMAs in the active layers is increased with the increase of the side chain length, which could be the reason for the increase of the J(sc) in the PSCs. The results indicate that small changes in side chain length can have a dramatic impact on the molecular self-assembly, morphology, and photovoltaic performance of the PSCs. The structure-performance relationship established in this study can provide important instructions for the side chain engineering and for the design of efficient SMAs materials.