New insight of molecular interaction, crystallization and phase separation in higher performance small molecular solar cells via solvent vapor annealing

Abstract

Solvent vapor annealing (SVA) studies on the morphology and performance of a porphyrin-based deep-absorption organic solar cells consisting of a strongly segregated bulk heterojunction (BHJ) blend, are presented. It is seen that the solvent vapor annealing of a well-mixed BHJ blends induces molecular motion, leading to a phase separated morphology governed by a spinodal decomposition mechanism. The earlier stage of solvent vapor swelling (<10 s) led to an obvious phase separation but not device performance. The device performance showed a dramatic increase in short circuit current and fill factor between 15 and 20 s of SVA. Thus, phase purity is a critical parameter in determining the performance of this binary blend. SVA on a thermally annealed BHJ thin film showed two distinctive processes, a crystal dissolution and a recrystallization, accompanied by phase mixing and then phase separation. The final morphology of SVA films that were initially thermally annealed showed a reduced length scale of phase separation, in comparison to SVA on as-cast films. Thus preformed donor crystallites appear to lock-in the morphology, even in a small molecule blend setting. The best performing device was obtained by a slight SVA (10 s) of films that were initially thermally annealed, reaching a power conversion efficiency of 8.48%. This suggests that the localized morphological optimization and domain size reduction are most important factors in dictating organic photovoltaic device efficiencies.