Enhanced stability of lead-free perovskite heterojunction for photovoltaic applications

A low-cost and simple solution-based method is employed to prepare cesium tin tri-iodide (CsSnI₃) thin films. The as-prepared dense CsSnI₃ thin films are confirmed belong to orthorhombic structure (black-γ phase) of CsSnI₃ (B-γ-CsSnI₃), which deposited via spin coating technique at 3000 r/min. X-ray photoelectron spectroscopy (XPS) results reveal that Sn is + 2 valence and no other states of Sn can be observed in the thin film. Hall effect measurements of the B-γ-CsSnI₃ thin film indicate that it is a p-type direct band gap semiconductor with carrier density at room temperature of ~ 10¹⁹ cm⁻³ and a hole mobility increases from ~ 2 to 19 cm² V⁻¹s⁻¹ with the film thickness increasing from 200 to 800 nm. Moreover, the n-type Cs₂SnI₆, a kind allotrope of CsSnI₃, is deposited on B-γ-CsSnI₃ thin film to form a p–n heterojunction, and the photoelectric conversion efficiency (PCE) of this lead-free device reaches 1.1%. Although the PCE is low, the Cs₂SnI₆ layer effectively prevents the degradation of the B-γ-CsSnI₃ thin film, and 90% of the initial performance is retained after the device stored in ambient air for 20 h, which significantly enhanced the stability of lead-free B-γ-CsSnI₃-based perovskite solar cells (PSCs).