Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells,b ut the overall power conversion efficiencies (PCEs) have been limited by the high carrier concentration from the facile oxidation of Sn 2+ to Sn 4+ .N ow ac hemical route is developed for fabrication of high-quality methylammonium tin iodide perovskite (MASnI 3 )f ilms:h ydrazinium tin iodide (HASnI 3 ) perovskite film is first solution-deposited using presursors hydrazinium iodide (HAI) and tin iodide (SnI 2 ), and then transformed into MASnI 3 via acation displacement approach. With the two-step process,adense and uniform MASnI 3 film is obtained with large grain sizes and high crystallization. Detrimental oxidation is suppressed by the hydrazine released from the film during the transformation. With the MASnI 3 as light harvester,m esoporous perovskite solar cells were prepared, and amaximum power conversion efficiency (PCE) of 7.13 %i sdelivered with good reproducibility.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

Defects from grain interiors and boundaries of perovskite films cause significant nonradiative recombination energy loss, and thus perovskite films with controlled crystallinity and large grains is critical for improvement of both photovoltaic performance and stability for perovskite-based solar cells. Here, a methylamine (MA0) gas-assisted crystallization method is developed for fabrication of methylammonium lead iodide (MAPbI3) perovskite films. In the process, the perovskite film is formed via controlled release of MA0 gas molecules from a liquid intermediate phase MAPbI3·xMA0. The resulting perovskite film comprises millimeter-sized grains with (110)-uniaxial crystallographic orientation, exhibiting much low trap density, long carrier lifetime, and excellent environmental stability. The corresponding perovskite solar cell exhibits a power conversion efficiency (PCE) of ~ 21.36%, which is among the highest reported for MAPbI3-based devices. This method provides important progress towards the fabrication of high-quality perovskite thin films for low-cost, highly efficient and stable perovskite solar cells.

toxicity of lead (Pb) is considered one of the main obstacles to the commercialization of this technology. Tin (Sn) is among the most promising candidates to replace Pb in the perovskites as both elements belong to the same group of the periodic table with the same outer electron configuration of ns 2 np 2 . In addition, Sn-based perovskites have the advantages of smaller optical bandgaps and greater charge mobility than the lead-based perovskites. However, the PCEs obtained so far are far below those of the lead counterparts. The inferior performance is mainly ascribed to the facile oxidation of Sn 2+ to Sn 4+ and poor film formability. [5][6][7][8][9][10] The excess Sn 4+ ions can produce high carrier concentration and thus more recombination loss, severely deteriorating the semiconductor behavior and device performance. In addition, tin perovskite films usually suffer from low surface coverage due to fast crystallization during the solution deposition. [6][7][8][9][10] In the past years, great efforts have been devoted to suppress the oxidation for high-efficiency Sn-based PSCs. In most of the efficient devices, SnF 2 or other tin (II) halides is usually incorporated as an indispensible additive during the film formation of the tin perovskites. Especially, SnF 2 can function as both Sn 2+ compensator and reducing agent in the Sn-based The low toxicity, narrow bandgaps, and high charge-carrier mobilities make tin perovskites the most promising light absorbers for low-cost perovskite solar cells (PSCs). However, the development of the Sn-based PSCs is seriously hampered by the critical issues of poor stability and low power conversion efficiency (PCE) due to the facile oxidation of Sn 2+to Sn 4+ and poor film formability of the perovskite films. Herein, a synthetic strategy is developed for the fabrication of methylammonium tin iodide (MASnI 3 ) film via ion exchange/insertion reactions between solid-state SnF 2 and gaseous methylammonium iodide. In this way, the nucleation and crystallization of MASnI 3 can be well controlled, and a highly uniform pinhole-free MASnI 3 perovskite film is obtained. More importantly, the detrimental oxidation can be effectively suppressed in the resulting MASnI 3 film due to the presence of a large amount of remaining SnF 2 . This high-quality perovskite film enables the realization of a PCE of 7.78%, which is among the highest values reported for the MASnI 3 -based solar cells. Moreover, the MASnI 3 solar cells exhibit high reproducibility and good stability. This method provides new opportunities for the fabrication of low-cost and lead-free tin-based halide perovskite solar cells.