Photostability improvement of wide bandgap mixed-halide perovskite solar cells via ion migration reduction

Abstract

Efficiency of one junction perovskite solar cells has almost reached the theoretical limit, and tandem devices are the possible solution to surpass it. To fabricate all perovskite tandem solar cells, it is required to have efficient and stable wide bandgap absorbing material as a top cell. Mixed-halide MAPbBr₂I perovskite has bandgap of 2 eV, which makes this material a perfect pair to conventional MAPbI₃ in two junction tandem solar cell. However, this anion composition is highly sensitive to ion migration, which significantly limits the photostability of fabricated devices. Under external influences ions start to migrate across the device, which leads to Hoke and Schottky effects, and consequent efficiency losses during sun exposure. In this work we investigate the effect of incorporation of cesium as additional cation in MAPbBr₂I perovskite composition. We demonstrate that 20% of Cs leads to significantly improved photostability of the films and devices due to ion migration reduction, which was found from impedance spectroscopy and capacitance measurements of the samples. The shift of photoluminescence peak position under light soaking was ten times lower for MA_0.8Cs_0.2PbBr₂I, and T₈₀ time increased up to 38 times, showing significant decreased only after storing the devices for 1500 h.