Side Chain and Solvent Direction of Film Morphology in Small-Molecule Organic Solar Materials

Film morphology control is essential to developing materials for efficient organic solar cells (OSCs). Unlocking methods to optimize the film morphology without changing the fundamental electronic characteristics can enable previously unacceptable materials to achieve high performance. Here, a series of acceptor/donor/acceptor small-molecule p-type OSC semiconductors having benzo­[1,2-b:6,5-b0]­dithiophene (bBDT) donor units and thiophene-capped diketopyrrolopyrrole (TDPP) acceptor units are examined, where the sole molecular design variation is the electronically inactive side chains. Four bBDT­(TDPP) 2 molecules with all possible combinations of either 2-ethylhexyl (EH) or 3,7-dimethyloctyl (DMO) side chains on the bBDT core and TDPP units are discussed. When processed from chloroform-only solutions, blend films of the molecules, with the PC71BM electron acceptor, exhibit similar thin-film properties and photovoltaic performance, regardless of the side chain composition. However, when the additive 1,8-diiodooctane (DIO) is included in the processing solution, the morphological properties and OSC performance diverge dramatically, with the side chain variations now dictating the film morphology, resulting in a >400% spread in power conversion efficiency. Donor molecules with a mixed side chain motif having the larger DMO side chain on the bBDT core unit deliver superior performance owing to greater donor/acceptor phase intermixing while maintaining contiguous and efficient charge transport pathways. In situ grazing incidence wide-angle X-ray scattering data acquired during spin-coating show that DIO extends crystallization times, thereby promoting more thermodynamically driven structures, where the side chain compositions have greater impact. Finally, general guidelines are presented for achieving morphological optimization via combined side chain engineering and processing additives.