We show the incorporation of europium into CsPbI 2 Br inorganic perovskite lattice. With the optimization of the doping concentration of europium, we obtained a high power-conversion efficiency of 13.71%. We found that incorporation of europium reduces non-radiative recombination to achieve a high open-circuit voltage of 1.27 V. The exceptional stability of such a device was demonstrated by retaining 93% of the initial efficiency under 100 mW cm À2 continuous illumination for 370 hr.

perovskite-based solar cells (PSCs) have reached a certified power conversion efficiency (PCE) of 24.2% in 2019, [19] which is comparable with that of copper indiumgallium diselenide (CIGS) [20] and siliconbased solar cells.The general chemical formula of a perovskite compound can be described as ABX 3 , where A is a monovalent cation (methylammonium (CH 3 NH 3 + (MA + )), formamidinium (CH(NH 2 ) 2 + (FA + )), cesium, etc.), B is a divalent metal cation (Pb 2+ , Sn 2+ , etc.), and X is occupied by a halide counterion (Cl − , Br − , and I − ). It is possible to form mixed compounds with respect to each site. The commercialization of hybrid perovskites requires materials that are thermodynamically stable and can withstand various thermal stressing tests, including natural daynight cycling and exposure to full sunlight. The organic parts in perovskites (such as MA + or FA + ) cannot endure high temperature and thus presents a longterm stability issue for devices under operation. Among the various degradation and decomposition pathways of hybrid perovskite, which depend on environmental factors such as humidity and illumination, a common theme is the low thermal stability of these perovskites, even in inert atmosphere. [21][22][23] For example, release of CH 3 NH 2 has been reported in MA-based perovskite films at temperatures of 80 °C, indicating decomposition of MA. [24] However, standard operational conditions for photovoltaics require materials and devices to be stable at this temperature. Therefore, the replacement of the organic parts by inorganic components (Cs + ) is expected to dramatically increase the long-term stability. [23,25,26] It has been widely reported that even a small amount of cesium can greatly enhance the thermal stability of organic-inorganic hybrid perovskites. [27,28] For example, FA 0.83 Cs 0.17 Pb(I 0.6 Br 0.4 ) 3 demonstrates both thermal and moisture stability, and stability to operation in the presence of oxygen. [29] An early report by Hodes [30] shows that thin films of the inorganic perovskite CsPbBr 3 can be prepared with a two-step method. The solar cells employing these layers yield an impressive high open circuit voltage (V oc ) of 1.32 V. These results, as the authors emphasize, indicate that the organic moiety is not an essential component in constructing high-performance perovskite materials and devices. Since then, with numerous publications each year, the inorganic PSCs research community is keeping highly active, including novel deposition methods All-inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic-inorganic hybrid counterparts. The power-conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high-quality perovskite films, and avoidance of phase in...

Iodine vacancies (V I ) and undercoordinated Pb 2+ on the surface of all-inorganic perovskite films are mainly responsible for nonradiative charge recombination. An environmentally benign material, histamine (HA), is used to passivate the V I in perovskite films. A theoretical study shows that HA bonds to the V I on the surface of the perovskite film via a Lewis base-acid interaction; an additional hydrogen bond (H-bond) strengthens such interaction owing to the favorable molecular configuration of HA. Undercoordinated Pb 2+ and Pb clusters are passivated, leading to significantly reduced surface trap density and prolonged charge lifetime within the perovskite films. HA passivation also induces an upward shift of the energy band edge of the perovskite layer, facilitating interfacial hole transfer. The combination of the above raises the solar cell efficiency from 19.5 to 20.8 % under 100 mW cm À2 illumination, the highest efficiency so far for inorganic metal halide perovskite solar cells (PSCs).

Inorganic perovskite based solar cells (PSCs) have been receiving unprecedented attention worldwide in the past several years due to their higher intrinsic stability towards high temperature and high theoretical power...