High-efficiency hybrid planar/bulk heterojunction organic solar cells
Organic solar cells (OSCs) require both a high donor/acceptor (D/A) interfacial area for efficient exciton dissociation and a vertically phase-separated morphology for efficient charge transport. Traditional bulk heterojunctions (BHJs) provide large interfacial areas but lack vertical phase separation, while quasi-planar heterojunctions (QPHJs) achieve vertical separation at the cost of reduced interfacial contact. Here, we introduce an in situ pore-forming strategy for polymer thin films. By incorporating an excess of additives as pore-forming agents into the donor layer, a nanoporous film with a fibrous nano-network is generated. Subsequent deposition of acceptor molecules fills these nanopores, creating a hybrid planar/bulk heterojunction (HP/BHJ) that synergizes the strengths of both architectures. This design enhances performance by: (1) increasing the D/A interfacial area via nanopores, forming a three-dimensional network that accelerates exciton dissociation; (2) promoting close molecular packing that minimizes carrier recombination and establishes low-defect charge transport channels; and (3) fostering vertical phase separation through layer-by-layer deposition. Binary OSCs fabricated with this strategy achieve a power conversion efficiency (PCE) of 20.0%, surpassing conventional BHJ and QPHJ devices by a significant margin. The approach demonstrates general applicability, with analogous improvements observed in D18/BTP-eC9-4F and PM6/L8-BO systems, underscoring its potential for advancing OSC performance.