Guanidinium pseudo-halide salt vapor post-treatment of 3D/2D Sn-based perovskite heterostructure thin films prepared by sequential physical vapor deposition for solar cell applications

Herein, 3D/2D-Sn based perovskite heterostructure thin films were prepared by sequential physical vapor deposition (SPVD) and vacuum passivated by physical vapor treatment using Guanidinium tetrafluoroborate (GuaBF 4 ). The effect of GuaBF 4 surface treatment on the structural, optical, morphological and electrical properties of 3D/2D-Sn perovskite thin films as well as solar cell device performance was investigated. XRD results revealed a better film quality with high degree of crystallinity in GuaBF 4 treated films. Morphological studies showed that the introduction of GuaBF 4 led to an increase in grain size from 320.6 to 388.7 nm with a reduction in surface roughness. The grain growth and smoother films were attributed to secondary crystallization of the perovskite and the effective passivation of defects respectively induced by GuaBF 4 . Furthermore, the treated films exhibited a much higher PL intensity than the pristine films demonstrating the suppression of non-radiative recombination through passivation of surface defects and control of Sn 2+ oxidation. As a result, the GuaBF 4 device showed an improvement in the power conversion efficiency (PCE) of 5.4 % with low hysteresis compared to the pristine devices with PCE of 3.9 %. Notably, stability studies on the films and the devices revealed an improved stability of both the films and the device with GuaBF 4 . This was attributed to the synergistic contributions of Gua + and B F 4 − towards defect passivation and suppression of Sn 2+ oxidation. Our study suggests a solvent-free preparation and treatment process of 3D/2D-Sn based perovskite heterostructures for improved film quality and solar cell performance.