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An organic solar cell converts light into electrical output using organic semiconductors, such as conjugated molecular or polymeric absorbers. Light absorption commonly produces a bound exciton, so useful charge generation requires more than simply creating an excited molecule.

In a donor–acceptor design, an exciton reaches an interface where charge transfer can separate its electron and electron hole. Selective contacts collect the separated charge carriers before they undergo recombination. A bulk heterojunction distributes donor–acceptor interfaces throughout the absorber, shortening the distance an exciton must travel. Its morphology must also supply connected transport pathways to the contacts. Absorption, exciton diffusion length, separation and charge collection therefore constrain performance together.

A suitable donor-acceptor heterojunction provides an energetically favourable route from a bound excitation to an interfacial charge-transfer state and then separated carriers. The electron is preferentially transported in the acceptor and the hole in the donor. The interface facilitates dissociation that is inefficient in a single strongly excitonic material; it must also avoid excessive energy loss and rapid interfacial recombination.

An idealized bulk heterojunction places most generated excitons within roughly a diffusion length of an interface while maintaining continuous donor paths to the hole contact and acceptor paths to the electron contact. Domain dimensions should therefore be comparable to the material's exciton transport scale, not arbitrarily large. The total absorbing thickness should capture substantial light yet remain within the charge-collection capability set by mobility, field and recombination. Excessive mixing can impair transport or increase recombination; very coarse domains waste excitons before they reach an interface.^[ref]