Reducing Energy Losses at the Organic-anode-buffer Interface of Organic Photovoltaics

Understanding the sources of energy loss in organic photovoltaic cells can ultimately lead to increasing their power-conversion efficiency. Here we explore energy losses at the interface between an archetype structure comprising a vacuum-deposited tetraphenyldibenzoperiflanthene/C-70 bulk-heterojunction active layer and several anode buffer layers (ABLs) using a combination of experimental and computational tools. The composition of the active organic region adjacent to the ABL interface is controlled independently from that in the bulk to determine energy losses that are attributed to charge transfer between the organic active region and the ABL. The open-circuit voltage (V-OC) can be varied over a range of 120 meV without affecting the short-circuit current and fill factor by growing an interface layer whose composition ranges from 100% C-70 to 0% C-70 in tetraphenyldibenzoperiflanthene at the active-region-ABL interface. Kinetic Monte Carlo simulations are used to quantitatively evaluate the magnitude of the interfacial energy loss and change in V-OC with various interface-layer compositions. When the interface layer consists of neat C-70 that permits nondissipative hole tunneling into the ABL, the interfacial energy loss is reduced and V-OC increases by 20-40 mV compared with conventional devices with a homogenous mixed active region.