Visualising ionic screening in perovskite solar cells: a bumpy ride along the J-V curve.

The current density-voltage (J-V) curve of perovskite solar cells (PSCs) commonly depends on the voltage scanning rate and direction, due to the presence of mobile ionic charges which screen the electric field, lowering the total driving force for charge extraction. In this study, experimental data and drift-diffusion simulations are combined to provide new insights into scan rate dependent J-V curves, focusing on triple mesoscopic carbon-based PSCs (CPSCs), which show a distinct current overshoot ('bump') in the backward scan which had not been fully explained until now. Additionally, the thickness optimisation problem in CPSCs is shown to be governed by the ionic distribution, which determines the ability to collect charge photogenerated in the ZrO₂ layer. Using simulations, we provide intuitive visual representations of the changes in electric field across the perovskite absorber during voltage scans at different rates, which determine the hysteresis and occurrence of the bump as a result of the polarity inversion of ionic space charge layers. The spatial maps obtained are directly correlated with experimental temperature- and voltage-dependent measurements of external quantum efficiency (EQE), offering an innovative and effective method to visualise ionic screening. This study introduces significant insights for the design and optimisation of CPSC devices considering ionic effects and presents a versatile characterisation approach applicable to all PSC architectures.