All-inorganic CsPbI 3 perovskite is emerging to be an alternative light-harvesting material in solar cells owing to the enhanced stability and comparable photovoltaic performance compared to organic−inorganic hybrid perovskites. However, the desirable black phase α-CsPbI 3 is not stable at room temperature and degrades rapidly to a nonperovskite yellow phase δ-CsPbI 3 . Herein, we introduce a compositional engineering approach via incorporating Bi 3+ in CsPbI 3 to stabilize the α-phase at room temperature. Fully inorganic solar cells based on the Bi-incorporated α-CsPb 1−x Bi x I 3 compounds demonstrate a high PCE of 13.21% at an optimal condition (incorporation of 4 mol % Bi 3+ ) and maintain 68% of the initial PCE for 168 h under ambient conditions without encapsulation. This is the first attempt of partial substitution of the "B"-site of the perovskite to stabilize the α-CsPbI 3 , which paves the way for further developments of such perovskites and other optoelectronic devices.O rganic−inorganic halide perovskite materials have attracted tremendous research interest owing to their intriguing optical characteristics as well as promising application in next-generation optoelectronic devices. 1−6 Among the various hybrid halide perovskites, CH 3 NH 3 PbI 3 (MAPbI 3 ) and HC(NH 2 ) 2 PbI 3 (FAPbI 3 ) have been frequently studied and have achieved power conversion efficiencies (PCEs) exceeding 20% in solar cells. 7−15 However, due to the hygroscopicity and thermally unstable nature of organic cation MA + , MAPbI 3 is thermally unstable and vulnerable to moisture. 16−20 Even for the more thermostable FAPbI 3 , the presence of hygroscopic FA + also makes it suffer from the moisture stability issue. 8,21−23 In order to improve the stability and photovoltaic performance of the devices, a series of Cs-incorporated systems have been developed, 24−34 such as Cs x MA 1−x PbI 3 , 24 Cs x FA 1−x PbI 3 , 27,28,30 FA 0.83 Cs 0.17 Pb-(I 1−x Br x ) 3 , 31 and Cs x (MA 0.17 FA 0.83 ) 1−x Pb(I 0.83 Br 0.17 ) 3 . 29,32 However, these Cs-incorporated systems still face big challenges for the long-term stability due to the remaining organic components.Recently, all-inorganic cesium lead halide perovskites (CsPbX 3 ) are emerging to be alternative light-harvesting materials in solar cells and have exhibited excellent ability to resist moisture and heat. 35−39 Nevertheless, CsPbBr 3 has a very large band gap of 2.3 eV, which is unable to absorb light with long-range wavelengths and usually results in low PCE of the solar cells. 38−40 Compared to CsPbBr 3 , black phase α-CsPbI 3 (Figure 1a) has a more suitable band gap of 1.73 eV for solar

Heterovalent Bi-doped CsPb1−xBixBr3 perovskites greatly improve the light absorption and photostriction of CsPbBr3 crystals.

Organic–inorganic hybrid lead halide perovskites are emerging as highly promising candidates for highly efficient thin film photovoltaics due to their excellent optoelectronic properties and low‐temperature process capability. However, the long‐term stability in ambient air still is a key issue limiting their further practical applications. Herein, the enhancement of both performance and stability of perovskite solar cells is reported by employing 2D and 3D heterostructured perovskite films with unique nanoplate/nanocrystalline morphology. The 2D/3D heterostructured perovskites combine advantages of the high‐performance lead‐based perovskite 3D CH3NH3PbI3 (MAPbI3) and the air‐stable bismuth‐based quasi‐perovskite 2D MA3Bi2I9. In the 2D/3D heterostructure, the hydrophobic MA3Bi2I9 platelets vertically situate between the MAPbI3 grains, forming a lattice‐like structure to tightly enclose the 3D MAPbI3 perovskite grains. The solar cell based on the optimal 2D/3D (9.2%) heterostructured film achieves a high efficiency of 18.97%, with remarkably reduced hysteresis and significantly improved stability. The work demonstrates that construction of 2D/3D heterostructured films by hybridizing different species of perovskite materials is a feasible way to simultaneously enhance both efficiency and stability of perovskite solar cells.

COMMUNICATION 1700131 (1 of 8)

The partial substitution of B-site Pb2+ by Bi3+ in FAPbI3 perovskites greatly improved the moisture stability and photovoltaic performance.