Side‐chain engineering of perylene diimide dimers: Impact on morphology and photovoltaic performance

Morphology evolution of conjugated organic molecular materials in the solid state has important implications for optoelectronic applications. Two aspects that have a large impact on materials self‐assembly in the film are the nature of the side‐chains and the processing conditions. Films of N‐annulated perylene diimide (PDI) dimer derivatives with different side‐chains at the pyrrolic and imide N‐positions are herein studied after solution processing using the solvent additives 1,8‐diiodooctane (DIO) or diphenyl ether (DPE). Branched or cyclohexyl side‐chains are investigated at the imide position (compounds 1 vs. 2 ) while linear, cyclic or branched alkyl side‐chains are investigated at the pyrrolic position (compounds 1 vs. 3 vs. 4 ). Film morphology of the four materials were examined using polarized light, fluorescence, and atomic force microscopy (POM, FM, and AFM, respectively). Organic photovoltaic (OPV) devices using the donor polymer PTB7‐Th with each PDI dimer as a non‐fullerene acceptor (NFA) were fabricated and tested to correlate morphology to electronic performances. Cyclohexyl side‐chains at the imide position reduced PDI dimer ( 2 ) solubility, gave flower like crystals in the film, and overall poor OPV performance regardless of processing. Cyclic alkyl side‐chains at the pyrrolic position impeded PDI dimer ( 3 ) crystallization and lead to moderate OPV performance. A combination of branched and linear side‐chains on the PDI dimer (compounds 1 and 4 ) provided sufficient solubility in non‐halogenated solvents and best OPV performance without the use of processing additives. For 1 with linear side‐chains at the pyrrolic position, processing with additives lead to moderate crystallization and improved OPV performance. For 4 with branched‐side chains, processing with additives lead to major crystallization and poor OPV performance. Side‐chain engineering is critical for controlling the self‐assembly of N‐annulated PDI dimers and it was shown that the choice and volume of processing additive must be evaluated systematically for maximizing electronic performance.