Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

All-inorganic perovskites (CsPbI3 and CsPbI2Br), owing to their greater thermal stability compared to organic–inorganic hybrid perovskites, are becoming popular in perovskite photovoltaics, but the problem that remains with CsPbI2Br (or CsPbI3) is the humidity-assisted phase transformation. Herein, we report on the formation of CsPbI2Br α-phase and improvement of its phase stability under ambient atmosphere (20–30% relative humidity) by Pb­(II) propionate additive in the CsPbI2Br precursor. Solar cells employing a CsPbI2Br film with an optimum concentration of the additive (1 mol %) and a donor–acceptor type polymer (synthesized by us) as dopant-free hole transport material that has a better energy level matching with CsPbI2Br (compared to other polymers like P3HT, PTAA, and asy-PBTBDT) work with a champion power conversion cell efficiency of 14.58%. A continuous increase in the open-circuit voltage, reaching 1.36 V for 5 mol % Pb­(II) propionate, indicates a remarkable defect-passivation effect by the additive.

Silver bismuth iodide (SBI) materials have recently gained attention as nontoxic alternatives to lead perovskites. Although most of the studies have been focusing on photovoltaic performance, the inherent ionic nature of SBI materials, their diffusive behavior, and influence on material/device stability is underexplored. Herein, AgBi2I7, Ag2BiI5, and Ag3BiI6 thin films are developed in controlled ambient humidity conditions with a decent efficiency up to 2.32%. While exploring the device stability, it is found that Ag3BiI6 exhibits a unique ion‐migration behavior where Ag+, Bi3+, and I− ions migrate and diffuse through the dopant‐free hole transport layer (HTL) leading to degradation. Interestingly, this ion‐migration behavior is relatively fast for the case of antisolvent‐processed Ag3BiI6 thin‐film‐based devices contrasting the case of without antisolvent and is not observed for other SBI material‐based devices. Theoretical calculations suggest that low decomposition enthalpy favors the decomposition of Ag3BiI6 to AgI and BiI3 causing migration of ions to the electrode which is protected by using a thick HTL . The new mechanism reported herein underlines the importance of SBI material composition and fundamental mechanism understanding on the stability of Ag3BiI6 material for better solar cell design and also in extending the applications of unique ion‐migration behavior in various optoelectronics.

Organic–inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a “work in progress” due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs.

Metal halide perovskite materials have demonstrated unique properties for applications in optoelectrical devices such as solar cells, light emitting diodes and photodetectors etc. The unprecedented world-record energy conversion efficiency over...