bulkheterojunction

A bulk heterojunction is a type of active layer architecture used in organic solar cells in which donor and acceptor materials are intimately mixed to form an interpenetrating network. This continuous blend creates numerous donor–acceptor interfaces throughout the film, enabling efficient separation of bound electron–hole pairs (excitons) into free charges.

In common implementations, a conjugated polymer or small molecule acts as the electron donor, while a fullerene derivative or a non-fullerene acceptor serves as the electron acceptor. The resulting nanoscale phase separation typically yields domain sizes on the order of 10 to 20 nanometers, which balance exciton diffusion to interfaces with continuous pathways for charge transport.

Operation relies on photoexcitation generating excitons within the donor or acceptor. Because exciton diffusion lengths are short, the high interfacial area provided by the bulk heterojunction facilitates exciton dissociation into free electrons and holes. The electrons and holes then percolate through their respective transport channels toward the electrodes, driven by built-in fields and energy-level alignment.

Fabrication generally involves dissolving donor and acceptor materials in a common solvent to form a blend, followed by solution deposition and post-deposition treatment such as thermal or solvent annealing. The processing conditions strongly influence morphology, electronic coupling, and device performance.

Advantages of bulk heterojunctions include simplified device structure and the ability to harvest light across a broad spectrum. Challenges include precise morphology control to prevent excessive phase separation or crystallinity that impedes charge transport, as well as stability concerns under operation. Advances with non-fullerene acceptors have driven significant efficiency gains while maintaining the bulk heterojunction paradigm.