Surface Evaporation Protocol for Solution-Processed Bulk-Heterojunction Films via Coarse-Grained Simulations

Abstract We present a coarse-grained molecular dynamics protocol for simulating solvent evaporation applied to P3HT:PCBM bulk-heterojunction (BHJ) films. Prior reported protocols lacked an explicit air–film interface or resorted to monatomic solvents with heavily coarse-grained solute models to work. Our approach maintains an explicit air–film interface, handles polyatomic solvents, and is better suited for complex molecular models. The resulting films reproduce experimentally reported vertical segregation (a P3HT-enriched skin over a PCBM-enriched subsurface) and an in-plane preferential orientation of P3HT backbones and π-stacking that originates at the interface and propagates downward with the drying front. A Marcus-type charge extraction model applied to the dried films revealed faster in-plane hole transport in the P3HT domains than through the film thickness, while electron transport in PCBM domains suggested the opposite trend. The results suggest an active upper surface with a large capacity for generating free charges emerges during the film’s formation that might benefit OPVs with an inverted architecture. Both the evaporation protocol and the charge transport model are formulated in general terms with the purpose of enabling the transfer of the algorithms to other systems.