Entropy Promotes Charge Separation in Organic Photovoltaics

Morphology of organic photovoltaics (OPVs)—phase separation and miscibility of organic semiconductors—is considered as a key factor to improve device performance. For example, accessibility to interfaces and electrodes—the transport properties—determine exciton dissociation and charge collection efficiencies, respectively. The morphological effects, however, are unclear.
We applied graph algorithm to bulk heterojunction morphologies generated by reputation to quantitatively evaluate transport properties and entropic effects on charge separation. The shortest distances, e.g., those between interfaces and electrodes, were obtained by Dijkstra’s algorithm. A morphology was divided into isolated donor and acceptor domains by depth-first search, and entropy S and Helmholtz energy F were calculated for the donor–acceptor domain pairs, which revealed that F, decreased by S, had global maxima around electron–hole distance of 6 nm. Charge separation efficiencies at the interfaces were evaluated by dynamic Monte Carlo simulations, elucidating that lower barrier, i.e., the energy required to overcome the maximum of F, promoted separation and that small acceptor domains in the large donor domain were recombination centers.