Materials Design Considerations for Charge Generation in Organic Solar Cells

This article reviews some of our recent progress on materials design guidelines for photoinduced charge generation in bulk-heterojunction organic solar cells. Over the last 7 years, our group has employed transient absorption measurement to determine the relative quantum yields of long-lived polaron pairs for over 300 different organic Donor/Acceptor blend films. We have shown that this optical assay of charge separation can be a strong indicator of photocurrent generation efficiency in complete devices. In this review, we consider the lessons that can be drawn from these studies concerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistently find, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiency of this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offset required to drive charge separation, and the strength of this energetic dependence, varies substantially between materials classes. In particular, copolymers such as diketopyrrolopyrrole- and thiazolothiazole-based polymers are found to be capable of driving charge separation in blends with PCBM at much lower energy offsets than polythiophenes, such as P3HT, while replacement of PCBM with more crystalline perylene diimide acceptors is also observed to reduce the energy offset requirement for charge separation. We go on to discuss the role of film microstructure in also determining the efficiency of charge separation, including the role of mixed and pure domains, PCBM exciton diffusion limitations and the role of material crystallinity in modulating material energetics, thereby providing additional energy offsets that can stabilize the spatial separation of charges. Other factors considered include the role of Coulombically bound polaron pair or charge transfer states, device electric fields, charge carrier mobilities, triplet excitons, and photon energy. We discuss briefly a model for charge separation consistent with these and other observations. We conclude by summarizing the materials design guidelines for efficient charge photogeneration that can be drawn from these studies.