High open-circuit voltage in perovskite solar cells: The role of hole transport layer

Using the drift-diffusion model, a new expression for the open-circuit voltage (Voc) in perovskite solar cells is derived. The Voc increases with the ratio of the charge carriermobilities (μe/μh) and by lowering the HOMO energy level of the hole transport layer (HTL). Using the derived Voc , we have found an analytical expression for the bimolecular recombination coefficient which decreases exponentially with increasing temperature. We have shown that the recombination coefficient thus derived is reduced by the formation of polarons, which may be expected to increase the photocurrent and hence power conversion efficiency in perovskite solar cells.