Organic photovoltaics: 30 years of the bulk heterojunction

Over the last 30 years, the bulk heterojunction (BHJ) has been the central architecture in organic photovoltaics (OPVs), enabling high interfacial area for efficient exciton dissociation and driving major improvements in power conversion efficiencies. This mini-review traces the development of the BHJ and the parallel evolution of OPV materials, from the first fullerene acceptors to today's highly engineered non-fullerene acceptors. Materials advances have significantly improved control over molecular packing, energetics and absorption, but have also highlighted the challenges of stabilising the inherently multicomponent BHJ morphology. We summarise current understanding of how donor–acceptor interactions, miscibility, crystallisation and film-formation kinetics shape morphology across length scales, and how these features govern charge generation, transport, recombination and operational degradation. The review also explores emerging alternatives to the conventional BHJ, including single-component and layer-by-layer systems, which challenge conventional assumptions about the need for a bicontinuous donor:acceptor network. Together, these systems reveal several ongoing challenges that need to be addressed to achieve OPVs that are not only efficient but also stable, scalable and mechanically robust.