Vacuum-deposited small-molecule organic solar cells based on DTDCTB (donor) and C70 (acceptor) molecules were systematically investigated for their solar-cell parameters and also the influence of thermal annealing on the morphology of the active layer. A detailed analysis is presented herewith, to understand the impact of thermal annealing on active layer nano-morphology and the origin of improved device performances via control over the trap-assisted charge recombination that directly affects the short-circuit current. © 2021 Elsevier B.V.