Exploiting the impact of Ionic Liquids and light exposure on performance of fully inorganic CsPbBr₃ semi-transparent perovskite solar cells

Inorganic wide-bandgap CsPbBr₃ perovskite is an interesting material to investigate for several opto-electronic devices such as solar cells, detectors and light emitting diodes. Although the 2.3 eV-bandgap limits the achievement of power conversion efficiencies (PCEs) larger than 10 % in opaque perovskite solar cells (PSC), these materials are still promising in several photovoltaic (PV) fields, such as Building Integrated PV and agri-PV, where the semi-transparency of the device stack can represent an additional value. In this work, we developed a deposition process where the use of BMIM-BF₄ ionic liquid in PbBr₂ deposition helps the formation of CsPbBr₃ with improved uniformity and reduced presence of competitive perovskite phases. Finally, we investigated thank to photoemission spectroscopy and in-situ XRD analysis the impact of the light exposure in the bulk and/or at the surface of thin film. Thanks to the ionic liquid addition and light soaking, the PCE was drastically improved up to 6.11 % using transparent top contact. The Average Visible Transmittance of the semi-transparent PSC device stack exceeded 58 % with Light Utilization Efficiency greater than 3.53 %. Finally, we performed different ageing tests in air such as shelf-life, light soaking at MPP and dry-heat demonstrating encouraging results in terms of the long-term stability under operative working conditions.