Lead‐based organic–inorganic hybrid perovskite materials are widely used in optoelectronic devices due to their excellent photophysical properties. However, the main issues which hinder its commercialization are the toxicity caused by lead and the intrinsic instability of the material. Recently, many lead‐free halide materials with good intrinsic stability have been reported, among which bismuth‐based halide materials have attracted extensive research due to their structure and promising optoelectronic properties. In this review, bismuth‐based materials are divided into binary BiX3 (X = I, Br, Cl), ternary AaBibXa+3b (A = Cs, Rb, MA, Ag, etc.), and quaternary A2AgBiX6 (A = Cs, Rb, MA, etc.) according to its elemental composition. The structure and optoelectronic properties of bismuth‐based halide materials, which may be helpful for the development of bismuth‐based halide materials and lead‐free perovskites in the future, are summarized and highlighted.

Compared with silicon-based solar cells, organic-inorganic hybrid perovskite solar cells (PSCs) possess a distinct advantage, i.e., its application in the flexible field. However, the efficiency of the flexible device is still lower than that of the rigid one. First, it is found that the dense formamidinium (FA)-based perovskite film can be obtained with the help of N-methyl-2pyrrolidone (NMP) via low pressure-assisted method. In addition, CH 3 NH 3 Cl (MACl) as the additive can preferentially form MAPbCl 3−x I x perovskite seeds to induce perovskite phase transition and crystal growth. Finally, by using FAI·PbI 2 ·NMP+x%MACl as the precursor, i.e., ligand and additive synergetic process, a FA-based perovskite film with a large grain size, high crystallinity, and low trap density is obtained on a flexible substrate under ambient conditions due to the synergetic effect, e.g., MACl can enhance the crystallization of the intermediate phase of FAI·PbI 2 ·NMP. As a result, a record efficiency of 19.38% in flexible planar PSCs is achieved, and it can retain about 89% of its initial power conversion efficiency (PCE) after 230 days without encapsulation under ambient conditions. The PCE retains 92% of the initial value after 500 bending cycles with a bending radii of 10 mm. The results show a robust way to fabricate highly efficient flexible PSCs.

All‐inorganic CsPbI2Br perovskite has attracted increasing attention, owing to its outstanding thermal stability and suitable bandgap for optoelectronic devices. However, the substandard power conversion efficiency (PCE) and large energy loss (Eloss) of CsPbI2Br perovskite solar cells (PSCs) caused by the low quality and high trap density of perovskite films still limit the application of devices. Herein, the post‐treatment of evaporating cesium bromide (CsBr) is utilized on top of the perovskite surface to passivate the CsPbI2Br–hole‐transporting layer interface and reduce Eloss. The results of microzone photoluminescence indicate that the evaporated CsBr gathered at the grain boundaries of CsPbI2Br layers and Br‐enriched perovskites (CsPbIxBr3−x, x < 2) are formed, which can provide protection for CsPbI2Br. Therefore, the gaps between crystal grains are filled up, and the recombination loss of the all‐inorganic CsPbI2Br PSCs is reduced accordingly. The champion device exhibits high open‐circuit voltage and a PCE of 1.271 V and 16.37%, respectively. This is the highest reported PCE among all‐inorganic CsPbI2Br PSCs reported so far. In addition, the stability of CsPbI2Br PSCs is effectively improved by CsBr passivation, and the device without encapsulation can retain 86% of its initial PCE after 1368 h of storage, which is beneficial for practical applications.

Flexible perovskite solar cells (PSCs) were ideal candidates for wearable devices due to the merits of flexibility, high efficiency, and being lightweight, and they could be fabricated in a continuous roll-to-roll production process to achieve large-area and low cost devices. Herein, the high efficiency (up to 18.53%) and fill factor (0.81) of flexible PSCs (ITO/SnO2/KCl/MAPbI3/spiro-OMeTAD/Ag) were achieved by low-pressure assisted solution processing under low temperature (⩽100 °C). The surface morphology and crystallinity of perovskite films were effectively promoted by the KCl modification and the defect density of perovskite films as well as the hysteresis of the corresponding devices was reduced accordingly. In addition, the stability and bendability of the KCl-modified flexible PSCs were improved simultaneously. To the best of our knowledge, both the efficiency and fill factor are the best among all flexible PSCs reported to date. Therefore, the insertion of KCl between SnO2 and MAPbI3 layers provided a promising strategy for highly efficient flexible PSCs fabricated in low temperature (⩽100 °C) conditions.

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The toxicity of lead-based halide perovskites hampers broad application in optoelectronics. Lead-free perovskite Cs 2 AgBiBr 6 is considered a promising candidate, owing to the long carrier lifetime and great stability. However, the relatively large bandgap of 1.98 eV limits its absorption in the visible region. Herein, Fe 2+ is chosen as the dopant to alloy into Cs 2 AgBiBr 6 single crystals, and results in an absorption range broadening to ≈1350 nm, which is the longest near-infrared (NIR) response recorded among lead-free perovskites. About 1% of Fe ions are alloyed into the Cs 2 AgBiBr 6 lattice to cause lattice shrink age. Instead of narrowing the bandgap, Fe doping would introduce a new intermediate band inside the pristine bandgap of Cs 2 AgBiBr 6 to strongly absorb NIR light, as confirmed by third harmonic generation results. Moreover, considerable photogenerated carriers are produced in Fe doped Cs 2 AgBiBr 6 crystals with NIR irradiation. This work has provided a new way to extend the optical response of lead-free perovskites for NIR photodetectors and intermediate band photovoltaics.

Although the power conversion efficiency of perovskite solar cells has reached 25.5%, their long-term stability is still a barrier to commercialization. In this work, 1-methyl-3-(3′,3′,4′,4′,4′-pentafluorobutyl)imidazolium tetrafluoroborate (MFIM-2) ionic liquid and another two analogues were used as additives to study their interaction mechanism with the FAPbI 3 perovskite layer. The results reveal that MFIM-2 suppressed the formation of PbI 2 crystals during crystallization, enlarged the grain size, and reduced the defect density, which led to an increased photovoltage of 1.12 V and efficiency of 19.4%. Furthermore, the moisture stability of the solar cell devices was also improved. Devices with MFIM-2 retained above 83% of the original value after 35 days in an atmosphere with about 25% relative humidity, and the perovskite film with MFIM-2 showed no phase transition in a 10 month aging process. These results demonstrate that the additive strategy of the polyfluoroalkylated imidazolium salt is a promising way for simultaneously extending the lifetime and improving the device performance of the perovskite solar cells.

Perovskite solar cells have attracted great research interest as a promising candidate for silicon solar cells. Plenty of work has been reported to use perovskites to semitransparent windows and transparent photovoltaic (TPV) devices to obtain multifunctional systems. However, the narrow bandgap and sharp absorption edge of the typical perovskites prevent them from achieving the highest transparency to satisfy the requirements of aesthetic and integration, and the poor stability and toxic Pb compositions hinder their practical application. Herein, lead‐free halide double perovskites with a wide bandgap and indirect bandgap characteristics is introduced to fabricate long‐term stable transparent photovoltaic devices exhibiting high visible transmittance (73%) and considerable energy conversion efficiency (1.56%). Through further theoretical calculation and evaluation, a new strategy using indirect bandgap material on TPV devices is proposed to combine the enhancement of these two parameters. This approach will be a significant compliment to near‐infrared‐absorbing solar cells to selectively harvest light in the invisible region to obtain highly performing multi‐junction smart windows on buildings, vehicles and mobile electronics, providing a new reasonable idea to realize TPVs with high efficiency and transparency simultaneously.