The stabilization of black-phase formamidinium lead iodide (α-FAPbI3) perovskite under various environmental conditions is considered necessary for solar cells. However, challenges remain regarding the temperature sensitivity of α-FAPbI3 and the requirements for strict humidity control in its processing. Here we report the synthesis of stable α-FAPbI3, regardless of humidity and temperature, based on a vertically aligned lead iodide thin film grown from an ionic liquid, methylamine formate. The vertically grown structure has numerous nanometer-scale ion channels that facilitate the permeation of formamidinium iodide into the lead iodide thin films for fast and robust transformation to α-FAPbI3. A solar cell with a power-conversion efficiency of 24.1% was achieved. The unencapsulated cells retain 80 and 90% of their initial efficiencies for 500 hours at 85°C and continuous light stress, respectively.

Electron-transport-layer free perovskite solar cells (ETL-free PSCs) have attracted great attention due to their low cost and simple manufacturing process. However, an additional interface layer has to be introduced, and the currently achieved efficiency remains far from full-structure PSCs. Here, we report an in situ interface engineering strategy by the methylammonium acetate (MAAc) ionic liquid perovskite precursor. We found that a dipole layer was in situ constructed through the physical adsorption of the residual MAAc polar molecules on the indium tin oxide electrode, which is significantly different from the treatment by the interface layer in previous reports. This allows a decrease of the effective work function and enables in situ band bending in the perovskite semiconductor. The in situ band bending facilitates charge collection and hinders interfacial charge recombination, leading to ETL-free PSCs with a maximum power conversion efficiency of 21.08%, which is the highest report to date.

Metal halide perovskites (MHPs), emerging as innovative and promising semiconductor materials with prominent optoelectronic properties, has been pioneering a new era of light management (ranging from emission, absorption, modulation, to transmission) for next-generation optoelectronic technology. Notably, the exploration of fundamental characteristics of MHPs and their devices is the main research theme during the past decade, while in the next decade, it will be primarily critical to promote their implantation in the next-generation optoelectronics. In this review, we first retrospect the historical research milestones of MHPs and their optoelectronic devices. Thereafter, we introduce the origin of the unique optoelectronic features of MHPs, based on which we highlight the tunability of these features via regulating the phase, dimensionality, composition, and geometry of MHPs. Then, we show that owing to the convenient property control of MHPs, various optoelectronic devices with target performance can be designed. At last, we emphasize on the revolutionary applications of MHPs-based devices on the existing optoelectronic systems. This review demonstrates the key role of MHPs played in the development of modern optoelectronics, which is expected to inspire the novel research directions of MHPs and promote the widespread applications of MHPs in the next-generation optoelectronics.

during the past few years, researchers have focused on developing alternative Pbfree perovskites by replacing Pb with other metals, including tin (Sn), germanium (Ge), bismuth (Bi), stibium (Sb), and copper (Cu). [11][12][13][14][15][16] Among these alternatives, Sn-based perovskites have made an emerging breakthrough, and their PSCs with highest PCE of 14.81% have been recently reported, demonstrating the great potential. [17][18][19] Despite the great progress in performance improvement of Sn-based PSCs, PCE and stability of Sn-based PSCs are still far behind those of their Pb counterparts. The poor device performance mainly stems from the poor quality of solution processed Sn-based perovskite film, which is attributed to the hardly controllable crystallization process of Sn-based perovskites. [12,20] It should be noted that the uncontrollable crystallization process of Sn-based perovskites does not directly lead to poor solar cell performance, but the formed defects, film morphology, crystallinity and orientation, structural distribution, and residual strains induced during crystallization do, which are the direct factors affecting the performance of PSCs. [21][22][23][24][25] For Sn-based perovskite, the uncontrollable crystallization process mainly originates from the unique property of Sn 2+ , including high Lewis acidity and easy oxidation of Sn 2+ . [26,27] On the one hand, due to the higher energy of 5p orbital compared to 6p orbital, the Lewis acidity of Sn 2+ is higher than that of Pb 2+ , resulting in the fast reaction rate of SnI 2 with MAI or FAI. On the other hand, because the two electrons on 5s orbital of Sn 2+ are active and easy to lose, giving rise to the easy oxidation of Sn 2+ , the formation of Sn vacancy Tin-based perovskites show great potential in photovoltaic applications, and the development of the corresponding solar cells (PSCs) has made exciting progress during the past few years. However, owing to the high Lewis acidity and easy oxidation of Sn 2+ , Sn-based perovskite films suffer from fast crystallization and easy formation of vacancy defects with low activation energy during the solution film-forming process, resulting in poor film quality and inferior device performance. Therefore, an in-depth understanding and rational control of film-forming dynamics of Sn-based perovskites is essential to improve the photovoltaic performance of their PSCs. In this review, the state-of-the-art developments in crystallization dynamics control for Sn-based perovskites and their impact on the photovoltaic performance of PSCs are systematically summarized. The review begins with the introduction of fundamentals and key difficulties for the control of the crystallization process of Sn-based perovskites. Then, the advanced strategies that focus on regulating the crystallization process of Sn-based perovskite films are comprehensively reviewed, including solvent engineering, additive engineering, cation engineering, and film-forming technique engineering. Finally, future perspectives and research di...

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