Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.