Ionic Charge Imbalance in Perovskite Solar Cells

Abstract

Ion migration in perovskite solar cells is usually analyzed and understood in terms of the charge neutrality condition. However, recent reports indicate the possibility of ionic imbalance in the active layer due to external ion migration and/or chemical reactions, with several puzzling experimental trends not easily amenable to conventional analysis. In this context, here, we critically examine the prevalent assumption of ionic charge neutrality in perovskite solar cells. Our results indicate that ionic imbalance results in an asymmetry in the device electrostatics, which contributes to interesting experimental observations like double diode characteristics, S kink, anomalous open circuit voltage (V OC ) improvement, etc. Coupled with device architecture and incident spectrum-induced asymmetries, our simulations indicate that ionic imbalance has nontrivial impact on other key characteristics like hysteresis, temperature coefficients, and aging-induced material/lifetime degradation in NIP vs PIN devices.