Morphology of small molecular donor/polymer acceptor blends in organic solar cells: Effect of the π-π Stacking capability of the small molecular donors

Organic solar cells (OSCs) based on small molecular donor and polymer acceptor (M_D/P_A) blends display inferior device performance due to poor active layer morphology. However, the phase separation behaviour and morphology evolution process of M_D/P_A-blends are scarcely investigated and less understood. Here, the morphology of M_D/P_A-blends based on two small molecular donors with different π-π stacking capability was systemically investigated. The small molecular donor with strong π-π stacking crystallizes into discontinuous large domains, which disturbs the ordered aggregation of the polymer acceptor. In contrast, the small molecular donor with weak π-π stacking remains amorphous and the polymer acceptor forms ordered aggregates in the as-cast film, while post-annealing promotes the small molecular donor to crystallize into nano-sized domains, which results in interpenetrating donor/acceptor networks. Due to the improved exciton dissociation and charge transport, the M_D/P_A-type OSC from the small molecular donor with weak π-π stacking shows superior photovoltaic performance. These results suggest that the π-π stacking capability of small molecular donors is critical to the active layer morphology of M_D/P_A-type OSCs.