Charge-transfer electronic states in organic solar cells

In organic solar cells, the charge-transfer (CT) electronic states that form at the interface between the electron-donor (D) and electron-acceptor (A) materials have a crucial role in exciton-dissociation, charge-separation and charge-recombination processes. Since the introduction of active layers consisting of D–A bulk heterojunctions, CT states have been the focus of extensive experimental and theoretical studies. In this Review, we assess the current understanding of CT states and describe how factors such as the geometry of the D–A interface, electronic polarization and the extent of electron delocalization affect their nature and influence the radiative and non-radiative decay processes. We focus on the description and application of fundamental concepts, which provides the framework to discuss the path to organic solar cells with efficiencies comparable to those in inorganic photovoltaic technologies.