Probing the Interface: Unveiling Ionic Accumulation Dynamics in Perovskite Devices via Rapid Voltage Pulses

Abstract

Perovskite solar cells have been demonstrated to achieve power conversion efficiencies of over 25% as of 2023. Their remarkable low-cost, solution processability makes integration of perovskite solar cells a favourable strategy for large-scale commercialisation. Recent performance improvements have been seen, however, the material possesses intrinsic mobile defects, which can hamper the performance and lead to anomalous effects. To address the challenges presented by mobile ions, a new set of novel measurements are required to understand the effects they have at the interfaces of perovskite devices. In this thesis, a systematic approach is taken to understand the role of ion migration in perovskite devices. The origin of ’s-shaped’ J-V curves is explored and found to be caused by slow-moving ions, through drift-diffusion simulations of highly-orientated crystal grain PSCs. A novel method is used to explore how the distribution of mobile ions at the perovskite interfaces changes the s-shaped behaviour. Ionic-electronic effects are detangled by using stabilising voltages and rapid voltage pulses. An empirical data analysis approach is then used to quantify a fundamental device parameter, the built-in voltage, through knowledge of the intersection of two characteristic J-V shapes. 2-Dimensional layered perovskites are also investigated and their ionic-electronic transport behaviour is detangled allowing the quantification of the ionic activation energy in perovskites which are found to be heavily influenced by measurement orientation and crystalline morphology. These findings present future researchers with a guide to examine the effects of mobile ions in perovskite devices. The voltage sweep rate is shown to be highly influential in the magnitude of the s-shaped J-V curves, highlighting the importance of its consideration. The stabilise-and-pulse measurement provides scientists’ toolbox with a technique to systematically improve perovskite solar cells through measurement of energy band alignment when modified with interfacial layers – all whilst using standard laboratory equipment. Lastly, ionic activation energy measurements of layer perovskites reveal the importance of measurement orientation, grain morphology and light intensity when using such materials in memristor devices, which fundamentally operate due to the accumulation of ions at the interface and hence opening up a way for operationally tunable devices.