Metal halide perovskites have recently emerged as one of the most promising classes of semiconductors for various applications, especially in the field of optoelectronics. Lead-based halide perovskite materials, virtually unexploited for decades, have become prominent candidates due to their unique and intrinsic physicochemical and optical properties. Current challenges faced by the scientific community to capitalize on the properties of Pb-based perovskites are mainly associated with environmental concerns due to the toxicity of Pb and their poor stability. Under this context, over recent years, a number of new Pb-free halide perovskite (and perovskite-like) semiconductor classes have been introduced. This Perspective reviews recent developments in Pb-free halide perovskites, which specifically target their application in solar cells, light-emitting devices, and photocatalysts. Each type of Pb-free material is paired with a specific optoelectronic application, and the latest record performances are reported. Although these materials do not yet exhibit as attractive intrinsic optoelectronic properties as the Pb-based halide perovskites, their potential as alternatives for well-suited applications is discussed.

Among the CuIn1−xAlxSe2 alloys, that with x = 0.25 is the optimal one in terms of band gap and conversion efficiency. Its absorption coefficient is highly modified under biaxial strains that occur in thin films.

Two-dimensional MX2 (M = Mo, W; X = S, Se, Te) homo- and heterostructures have attracted extensive attention in electronics and optoelectronics due to their unique structures and properties. In this work, the layer-dependent electronic and optical properties have been studied by varying layer thickness and stacking order. Based on the quantum theory of atoms in molecules, topological analyses on interatomic interactions of layered MX2 and WX2/MoX2, including bond degree (BD), bond length (BL), and bond angle (BA), have been detailed to probe structure-property relationships. Results show that M-X and X-X bonds are strengthened and weakened in layered MX2 compared to the counterparts in bulks. X-X and M-Se/Te are weakened at compressive strain while strengthened at tensile strain and are more responsive to the former than the latter. Discordant BD variation of individual parts of WX2/MoX2 accounts for exclusively distributed electrons and holes, yielding type-II band offsets. X-X BL correlates positively to binding energy (Eb), while X-X BA correlates negatively to lattice mismatch (lm). The resulting interlayer distance limitation evidences constraint-free lattice of vdW structure. Finally, the connection between microscopic interatomic interaction and macroscopic electromagnetic behavior has been quantified firstly by a cubic equation relating to weighted BD summation and static dielectric constant.

Heterostructures based on a CuInSe2 absorber with an AlP buffer have a 12 meV conduction band offset and achieved 27.39% of conversion efficiency.