Mismatch of Quasi-Fermi Level Splitting and V_oc in Perovskite Solar Cells

Perovskite solar cells have demonstrated low non-radiative voltage losses and open-circuit voltages (V(OC)s) that often match the internal voltage in the perovskite layer, i.e. the quasi-Femi level splitting (QFLS). However, in many cases, the V-OC differs remarkably from the internal voltage, for example in devices without perfect energy alignment. In terms of recombination losses, this loss often outweighs all non-radiative recombination losses observed in photoluminescence quantum efficiency measurements by many orders of magnitude. As such, understanding this phenomenon is of great importance for further perovskite solar cell development and tackling stability issues. The classical theory developed for Si solar cells explains the QFLS-V-OC mismatch by considering the partial resistances/conductivities for majority and minority carriers. Here, the authors demonstrate that this generic theory applies to a variety of physical mechanisms that give rise to such a mismatch. Additionally, it is found that mobile ions can contribute to a QFLS-V-OC mismatch in realistic perovskite cells, and it is demonstrated that this can explain various key observations about light soaking and aging-induced V-OC losses. The findings in this paper shine a light on well-debated topics in the community, identify a new degradation loss, and highlight important design principles to maximize the V-OC for improved perovskite solar cells.