Electronic states and electronic excitations in a molecular solid such as an organic bulk heterojunction solar cell either may reflect the properties of individual molecules or may be delocalized over several molecules, exhibiting alloy properties of the average composition. Measurements of a variety of dilute ternary blend organic solar cells based on either two polymer donors and one fullerene acceptor or one polymer donor and two fullerene acceptors provide information about the degree of localization in different situations. In the two polymer case, where the polymers are well intermixed, excitons have molecular characteristics. Despite their localization, excitons from the dilute low band gap component readily diffuse to the heterojunction interface and generate mobile charge, and their diffusion is attributed to rod percolation. Mobile holes are delocalized, and the blend concentration dependence suggests delocalization over about 10 polymer molecules. In contrast, with poorly intermixed polymers, low band gap excitons are unable to diffuse and exhibit no charge generation. With fullerene mixtures, two different behaviors are also observed. Mixtures of PC 61 BM and ICBA exhibit delocalized alloy states, while dilute PC 84 BM in PC 61 BM mixtures exhibits localized trap states. The difference is attributed to the size mismatch of the larger PC 84 BM molecule.
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Street et al. (2014) studied this question.