Enhancing Charge Transfer in Perovskite-Inspired Silver Iodobismuthate-Based Solar Cells via Cesium Iodide Interlayer

Ag₃BiI₆ (ABI) is one of the most widely explored lead-free perovskite-inspired materials for eco-friendly solar cell applications. However, despite the intense research efforts, the photovoltaic performance of ABI-based devices remains very modest, primarily due to poor film morphology and ineffective charge extraction. This work aims at investigating the potential benefits of a thermally evaporated cesium iodide (CsI) interlayer on the performance of ABI-based solar cells. Upon the addition of CsI atop the ABI layer in the device stack, the solar cells deliver a power conversion efficiency (PCE) of 2.27%. This is the highest efficiency reported for ABI solar cells employing a similar device architecture. It is found that the enhancement in PCE is largely due to improvement in the ABI|hole transport layer interface upon the introduction of CsI interlayer. The improvement is largely ascribed to enhanced surface coverage upon introduction of CsI interlayer, as evidenced by our comprehensive microscopy studies. Furthermore, impedance spectroscopy analysis is employed to provide further insights into the changes in charge transfer dynamics interlayer that dictate the enhancement of fill factor and short-circuit current density in the devices. The findings indicate that the addition of CsI promotes charge transfer and minimizes recombination losses.