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-gamma phase) of CsSnI3 (B-gamma-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-gamma-CsSnI3 thin film indicate that it is a p-type direct band gap semiconductor with carrier density at room temperature of 10(19) cm(-3) and a hole mobility increases from 2 to 19 cm(2) V(-1)s(-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-gamma-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-gamma-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-gamma-CsSnI3-based perovskite solar cells (PSCs).