Organic–inorganic metal halide perovskite solar cells (PSCs) have achieved certified power conversion efficiency (PCE) of 25.2% with complex compositional and bandgap engineering. However, the thermal instability of methylammonium (MA) cation can cause the degradation of the perovskite film, remaining a risk for the long‐term stability of the devices. Herein, a unique method is demonstrated to fabricate highly phase‐stable perovskite film without MA by introducing cesium chloride (CsCl) in the double cation (Cs, formamidinium) perovskite precursor. Moreover, due to the suboptimal bandgap of bromide (Br−), the amount of Br− is regulated, leading to high power conversion efficiency. As a result, MA‐free perovskite solar cells achieve remarkable long‐term stability and a PCE of 20.50%, which is one of the best results for MA‐free PSCs. Moreover, the unencapsulated device retains about 80% of the original efficiencies after a 1000 h aging study. These results provide a feasible approach to enhance solar cell stability and performance simultaneously, paving the way for commercializing PSCs.

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Despite the excellent photovoltaic properties achieved by perovskite solar cells at the laboratory scale, hybrid perovskites decompose in the presence of air, especially at high temperatures and in humid environments. Consequently, high‐efficiency perovskites are usually prepared in dry/inert environments, which are expensive and less convenient for scale‐up purposes. Here, a new approach based on the inclusion of an in situ polymerizable ionic liquid, 1,3‐bis(4‐vinylbenzyl)imidazolium chloride ([bvbim]Cl), is presented, which allows perovskite films to be manufactured under humid environments, additionally leading to a material with improved quality and long‐term stability. The approach, which is transferrable to several perovskite formulations, allows efficiencies as high as 17% for MAPbI3 processed in air % relative humidity (RH) ≥30 (from an initial 15%), and 19.92% for FAMAPbI3 fabricated in %RH ≥50 (from an initial 17%), providing one of the best performances to date under similar conditions.

Tuning the band alignment is proved to be an effective way to facilitate carrier transportation and thus enhance the power conversion efficiency (PCE) of solar cells. Doping the compact layer with metal ions or modifying the interfaces among functional layers in perovskite solar cells (PSCs) can appreciably improve the PCE of PSCs. Inspired by the rare earth elemental doping of TiO, which has witnessed the success in photocatalysis and dye-sensitized solar cells, we firstly demonstrated here that La doping in the mesoporous TiO layer of a mesostructured PSC can tune its Fermi level and thus significantly enhance the device PCE. Systematic analysis reveals that doping La into TiO raises the Fermi level of TiO through scavenging oxygen and inducing vacancies, which subsequently increases the open circuit voltage and the fill factor while reducing the series resistance of the PSC using La-doped TiO as a mesoporous layer. As a result, a PCE of 15.42% is achieved, which is appreciably higher than the PCE of a device with undoped TiO (12.11%).

Water vapor modulated post-annealing is a reproducible and easily-scalable method to promote the grain growth and heal the pinholes for efficient PSCs.

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Methylammonium-free perovskite was prepared by using a (PEA)2PbI4 nanosheet/chlorobenzene suspension as the anti-solvent for enhanced device stability and photovoltaic performances.