Fine-Tuning Miscibility and π–π Stacking by Alkylthio Side Chains of Donor Molecules Enables High-Performance All-Small-Molecule Organic Solar Cells

Optimization of morphology and precise control of miscibility between donors and acceptors play an important role in improving the power conversion efficiencies (PCEs) of all-small-molecule organic solar cells (SM-OSCs). Besides device optimization, methods such as additives and thermal annealing are applied for finely tuning bulk-heterojunction morphology; strategies of molecular design are also the key to achieve efficient phase separation. Here, a series of A-D-A-type small-molecule donors (SM4, SM8, and SM12) based on benzodithiophene units were synthesized with different lengths of alkylthio side chains to regulate crystallinity, and their miscibility with the acceptor (BO-4Cl) was investigated. Consequently, SM4 with a short alkylthio substituent had a high crystallization propensity, leading to the oversized molecular domains and the poor morphology of the active layer. Meanwhile, SM12 with a longer alkylthio substituent showed weak crystallinity, causing a relatively looser π-π stacking and thus adversely affecting charge-carrier transport. The SM-OSC based on the small-molecule donor SM8 with a mid-length alkylthio substituent achieved a better PCE over 13%, which was attributed to a more harmonious blend miscibility without sacrificing carrier-charge transport. Eventually, the modulation of phase separation and miscibility via controlling the lateral side chains has proven its potential in optimizing the blend morphology to aid the development of highly efficient SM-OSCs.