New Research in Solar Cells: Urbach Tails and Open Circuit Voltage

Abstract

The development of various emerging photovoltaic technologies, as classified by the National Renewable Energy Laboratory, has experienced very different rates of progress over the last five to ten years. In particular, two thin-film technologies, quantum dot solar cells and perovskite-based solar cells, have achieved drastically different values for power conversion efficiency as quantum dots have failed to keep up with perovskites. Clearly, the materials are very different, but in order to quantify the discrepancy in efficiency, we focus on measuring the tail of the absorption past the nominal band edge and therefore the Urbach energy of the two materials. We discuss a theoretical relationship between Urbach energy and open circuit voltage based on the original considerations by Shockley and Queisser on the efficiency limits of solar cells.