Pure 2H phase MoSe2 nanosheets promote the formation of a porous PbI2 film and modulate residual stress for highly efficient and stable perovskite solar cells

The above 25% certified efficiency had been achieved for perovskite solar cells (PSCs) with regular n-i-p device architecture based on two-step processes in recent years. However, the perovskite films prepared by two-step processes still have an incomplete PbI2 reaction, poor crystallization quality and residual stress. Herein, an excellent strategy, i.e. introducing 2D MoSe2 nanosheets as additives into PbI2 precursor solution in the first step of two-step processes, is proposed to increase the conversion of PbI2 and improve crystallinity as well as reduce residual stress simultaneously. It is revealed that MoSe2 nanosheets prompt the formation of a porous PbI2 film and then achieve a high quality perovskite film. Interestingly, MoSe2 nanosheets play the role of a "lubricant" to relax residual stress. In addition, MoSe2 nanosheets also result in a more suitable energy-level alignment at the perovskite/electron transport layer interface. Consequently, the average power conversion efficiency (PCE) of 21.76% and the highest PCE of 22.80% are achieved upon incorporating 2D MoSe2 nanosheets, which are greatly improved compared to the average PCE of 19.40% and the highest PCE of 20.92% for the control PSCs. Besides, the MoSe2-doped PSC could maintain 80% of its initial efficiency under 15-20% RH at room temperature for 1000 hours in air. This work not only provides a facile and efficient method to increase the PCE and stability of PSCs but also reveals the application potential of 2D MoSe2 in PSCs and other optoelectronic devices.