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.