Two-dimensional Bi2OS2 doping improves the performance and stability of perovskite solar cells

Additive engineering has been proved to be an effective method to increase the performance of perovskite solar cells (PVSCs) by increasing the quality of the perovskite thin film. Two-dimensional (2D) Bi2OS2 nanomaterial is an excellence optoelectronic material because of its tunable direct band gap, ultra-high charge mobility, wide absorption range and high absorption coefficient. In this work, 2D Bi2OS2 nanosheets are synthesized and introduced as additives into the perovskite precursor solution to adjust the film formation process for the first time. Serving as heterogeneous nucleation site, 2D Bi2OS2 can reduce the nucleation barrier and assist the formation of highly crystalline and crystal orientation along the (1 1 0) plane perovskite thin film, which is beneficial to improve the mobility and lifetime of charge carriers. Besides, X-ray photoelectron spectroscopy (XPS) measurement reveals that 2D Bi2OS2 nanosheets can interact with Pb2+ within the perovskite through Pb-S bond, thus suppressing the uncoordinated Pb2+ ions trap states. Scanning electron microscope (SEM) demonstrates 2D Bi2OS2 nanosheets can fill up the pin-holes and grain boundaries on the perovskite film surface. Ultraviolet photoelectron spectroscopy (UPS) measurement indicates that the valence band (VB) of optimized perovskite film is 0.28 eV upshifted closer to the highest occupied molecular orbital (HOMO) of the hole transport layer (HTL), which accelerates the charge transportation at perovskite/HTL interface. Consequently, the average power conversion efficiency (PCE) of 18.96% (highest PCE of 19.89%) is achieved upon incorporating 2D Bi2OS2 nanosheets with an optimized concentration of 0.02 mg/ml into MAPbI3 precursor solution, which is greatly improved compared to the average PCE of 15.71% for the control devices (highest PCE of 16.77%). Besides, the lifetime of optimized PVSCs is also prolonged due to the less defect states and better perovskite film quality. This work not only provides a facile method to increase the PCE and stability of PVSCs, but also reveals 2D Bi2OS2 material has potential applications in PVSCs.