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An organic solar cell is a photovoltaic device in which light is absorbed and charge is separated by molecular or polymeric semiconductors rather than by an inorganic crystal. Absorption creates a bound exciton, so a donor-acceptor interface is needed to split it; in the bulk heterojunction the two materials are cast from a common solution and interpenetrate on a scale of 10 to 20 nanometres, which sets how many excitons reach an interface before they decay ^[1]; in an oligothiophene-fullerene blend it is this interpenetrating structure that carries the photocurrent ^[2]. The morphology of the blend depends on the solvent, the drying rate and any thermal or solvent annealing, and small changes in domain purity and size can move the power conversion efficiency by several percentage points ^[1], the domain structure entering the optical and photovoltaic response directly ^[3]. Early devices used fullerene derivatives as the acceptor, and laboratory cells of that type reached efficiencies approaching 10 percent ^[2]. Non-fullerene acceptors, whose absorption and energy levels can be tuned independently of the donor, were introduced to raise both the current and the voltage ^[4]. Certified efficiencies in single junctions now exceed 19 percent where wide-bandgap donors are matched to them to keep the open-circuit voltage high ^[5]. Stability under illumination and in air, and the difficulty of reproducing the blend morphology outside spin coating, remain the main obstacles ^[3].