"Twisted" small molecule donors with enhanced intermolecular interactions in the condensed phase towards efficient and thick-film all-small-molecule organic solar cells

How to achieve high crystallinity and suitable phase scale simultaneously from the perspective of molecular engineering remains a long-standing challenge for all-small-molecule organic solar cells (ASM-OSCs). Herein, two small molecule donors, named DRTT-2Se (with two alkylselenophenyl chains) and DRTT-6Se (with six alkylselenophenyl chains), were designed and synthesized by introducing "twisted" backbones and building units with strong intermolecular interactions. The "twisted" skeletons of the molecules can suppress the molecular crystallization and avoid excessive aggregation during the film forming process, while the strong intermolecular interactions of the building units can encourage the formation of ordered molecular packing and nanoscale phase separation via post-treatments. Owing to the enhanced intermolecular interactions induced by alkylselenophenyl side chains, DRTT-2Se and DRTT-6Se with a "twisted" structure displayed more ordered pi-pi packing, and retained the similar nanoscale microstructures in the blend films in comparison to the "twisted" molecule with alkylthiophenyl side chains (DRTT-T), yielding improved hole transport and reduced charge recombination in OSCs. In addition, the alkylselenophenyl substituted molecules showed higher HOMO energy levels, which caused the larger HOMO offset to drive exciton dissociation, but did not increase the energy loss. As a result, the ASM-OSCs based on the selenophene-containing donors exhibited higher FF and J(sc), and similar V-oc, yielding superior PCEs (14.79% and 15.03%) compared to the DRTT-T based device (13.37%). More importantly, the appropriate film morphology made the device efficiency of DRTT-2Se and DRTT-6Se less sensitive to the thickness of active layers. This study provides some implications in designing small molecule donors with suitable crystallization behavior towards high-performance ASM-OSCs.