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ABX3‐type halide perovskite nanocrystals (NCs) have been a hot topic recently due to their fascinating optoelectronic properties. It has been demonstrated that A‐site ions have an impact on their photophysical and chemical properties, such as the optical band gap and chemical stability. The pursuit of halide perovskite materials with diverse A‐site species would deepen the understanding of the structure–property relationship of the perovskite family. In this work we have attempted to synthesize rubidium‐based perovskite NCs. We have discovered that the partial substitution of Rb+ by Cs+ help to stabilize the orthorhombic RbPbBr3 NCs at low temperature, which otherwise can only be obtained at high temperature. The inclusion of Cs+ into the RbPbBr3 lattice results in highly photoluminescent Rb1−xCsxPbBr3 NCs. With increasing amounts of Cs+, the band gaps of the Rb1−xCsxPbBr3 NCs decrease, leading to a redshift of the photoluminescence peak. Also, the Rb1−xCsxPbBr3 NCs (x=0.4) show good stability under ambient conditions. This work demonstrates the high structural flexibility and tunability of halide perovskite materials through an A‐site cation substitution strategy and sheds light on the optimization of perovskite materials for application in high‐performance optoelectronic devices. Adding stability: Lead halide perovskite nanocrystals (NCs) attract tremendous attention due to their extraordinary optical properties. Herein, we show that the partial substitution of Rb+ by Cs+ helps to stabilize the orthorhombic RbPbBr3 NCs at low temperature, which otherwise could only be obtained at high temperature (above 300 °C, see figure). [ABSTRACT FROM AUTHOR]

The halide perovskite (PVSK) materials (with ABX3 formulation) have emerged as 'dream materials' for photovoltaic (PV) applications due to their remarkable physical properties such as high optical absorption coefficient, carrier mobility, long carrier diffusion lengths, etc. These properties have enabled the PV devices to reach higher than 20% power conversion efficiencies (PCE) in record time. The further pursuit of higher PCE and improved stability brings forth increasing interests in so-called 'mixed composition' PVSK materials, consisting of partial substitution of the A, B, and/or X-sites with alternative elements/molecules of similar size. Herein, we highlight the recent advances in developing mixed PVSK for PVs and their relevant optoelectronic properties. We mainly focus on mixed PVSK materials in the form of polycrystalline thin films, but also discuss nanostructured and two-dimensional (2D) PVSK materials due to the increasing interest of broad readership. Efforts are exerted to elucidate the design principles of mixed PVSK and fabrication techniques for high performance optoelectronic devices, which help deepen our fundamental understanding of mixed PVSK systems. We hope this review will shed light onto the design and synthesis of mixed PVSK materials to further the progress of PVSK photovoltaics towards higher efficiencies and longer lifetimes. [ABSTRACT FROM AUTHOR]