High open-circuit voltage (Voc) is the essential benefit of perovskite photovoltaic cells that has enabled high efficiency in power conversion. Voc is improved by successful passivation of defects at grain boundaries and hetero-junction interfaces. Various approaches of interfacial engineering have been attempted in the past decade by using modulator molecules and mixing 2D and 3D structures to perovskites, as overviewed by our group as a comprehensive review.¹ These efforts also lead to enhance the stability of the organic—inorganic hybrid perovskite devices. However, organic cations in hybrid perovskites and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120^oC). In this respect, combination of all-inorganic perovskite materials and dopant-free HTMs will be a main direction of perovskite photovoltaics.² Although CsPbX₃(X=Br, I) has been studied as a popular candidate of the all-inorganic perovskite, the stability of its photo-active black phase depends on the halide composition and CsPbI₃ lacks in high stability. We have focused our work on CsPbI₂Br as the stable visible light absorber. The CsPbI₂Br film was made into junction with a solution-processed dopant-free HTMs which are copolymer materials and thermally stable up to 300^oC. The quality of interfacial structure at the CsPbI₂Br and HTM junction was improved by inserting an ultra-thin layer (<3 nm) of amorphous SnOx. The all-inorganic and dopant-free perovskite device exhibits high Voc value beyond 1.4V with efficiencies >15%.³ The high level of Voc was maintained even under low light intensity as demonstrated by ideality factors as small as <1.4, indicating that defect-assisted recombination is suppressed well at the junctions of perovskite with HTM and SnOx. When the device performance was assessed under indoor illumination (200 lx luminance of LED), efficiency reached 35% or more, holding the value of Voc at 1.1V or more. It is very rare to find a photovoltaic device capable of Voc >1.1V under indoor illumination. Therefore, high Voc perovskite devices definitely work as the best power supply for IoT applications. For consumer electronics, however, a big challenge should also be directed to development of lead-free perovskite materials for environmental safety in practical applications.²