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Metal halide perovskites are attractive for highly efficient solar cells. As most perovskites suffer large or indirect bandgap compared with the ideal bandgap range for single-junction solar cells, bandgap engineering has received tremendous attention in terms of tailoring perovskite band structure, which plays a key role in light harvesting and conversion. In this Review, various reported bandgap engineering strategies are summarized. The recently widely used two main strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap engineering are also investigated. Moreover, future research directions are outlined to guide the further investigation.

Bismuth-based solar cells have been under intensive interest as an efficient non-toxic absorber in photovoltaics. Within this new family of semiconductors, we herein report a new, long-term stable copper bismuth iodide (CuBiI ). A solutionprocessed method under air atmosphere is used to prepare the material. The adopted HI-assisted dimethylacetamide (DMA) co-solvent can completely dissolve CuI and BiI powders with high concentration compared with other organic solvents. Moreover, the high vapor pressure of tributyl phosphate, selected for the solvent vapor annealing (SVA), enables complete low-temperature (≤70 °C) film preparation, resulting in a stable, uniform, dense CuBiI film. The average grain size increases with the precursor concentration, greatly improving the photoluminescence lifetime and hall mobility; a carrier lifetime of 3.03 ns as well as an appreciable hall mobility of 110 cm V s were obtained. XRD illustrates that the crystal structure is cubic (space group Fd3m) and favored in the [1 1 1] direction. Moreover, the photovoltaic performance of CuBiI was also investigated. A wide bandgap (2.67 eV) solar cell with 0.82 % power conversion efficiency is presented, which exhibits excellent long-term stability over 1008 h under ambient conditions. This air-stable material may give an application in future tandem solar cells as a stable short-wavelength light absorber.

High-efficiency perovskite solar cells (PSCs) need to be fabricated in the nitrogen-filled glovebox by the atmosphere-controlled crystallization process. However, the use of the glovebox process is of great concern for mass level production of PSCs. In this work, notable efficient CHNHPbI solar cells can be obtained in high humidity ambient atmosphere (60-70% relative humidity) by using acetate as the antisolvent, in which dependence of methyl, ethyl, propyl, and butyl acetate on the crystal growth mechanism is discussed. It is explored that acetate screens the sensitive perovskite intermediate phases from water molecules during perovskite film formation and annealing. It is revealed that relatively high vapor pressure and high water solubility of methyl acetate (MA) leads to the formation of highly dense and pinhole free perovskite films guiding to the best power conversion efficiency (PCE) of 16.3% with a reduced hysteresis. The devices prepared using MA showed remarkable shelf life stability of more than 80% for 360 h in ambient air condition, when compared to the devices fabricated using other antisolvents with low vapor pressure and low water solubility. Moreover, the PCE was still kept at 15.6% even though 2 vol % deionized water was added in the MA for preparing the perovskite layer.

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