Spatial Collection in Colloidal Quantum Dot Solar Cells

In thin-film photovoltaic (PV) research and development, it is of interest to determine where the chief losses are occurring within the active layer. Herein, a method is developed and presented by which the spatial distribution of charge collection, operando, is ascertained, and its application in colloidal quantum dot (CQD) solar cells is demonstrated at a wide range of relevant bias conditions. A systematic computational method that relies only on knowledge of measured optical parameters and bias-dependent external quantum efficiency spectra is implemented. It is found that, in CQD PV devices, the region near the thiol-treated hole-transport layer suffers from low collection efficiency, as a result of bad band alignment at this interface. The active layer is not fully depleted at short-circuit conditions, and this accounts for the limited short-circuit current of these CQD solar cells. The high collection efficiency outside of the depleted region agrees with a diffusion length on the order of hundreds of nanometers. The method provides a quantitative tool to study the operating principles and the physical origins of losses in CQD solar cells, and can be deployed in thin-film solar cell device architectures based on perovskites, organics, CQDs, and combinations of these materials.