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.

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.

The efficiency of organic-inorganic hybrid lead halide perovskite solar cells (PSCs) has increased over 25% within a frame of ten years, which is phenomenal and indicative of the promising potential of perovskite materials in impacting the next generation solar cells. Despite high technology readiness of PSCs, the presence of lead has raised concerns about the adverse effect of lead on human health and the environment that may slow down or inhibit the commercialization of PSCs. Thus, there is a dire need to identify materials with lower toxicity profile and comparable optoelectronic properties in regard to lead-halide perovskites. In comparison to tin-, germanium-, and copper-based PSCs, which suffer from stability issues under ambient operation, bismuth-based perovskite and perovskite-inspired materials have gained attention because of their enhanced stability in ambient atmospheric conditions. In this topical review, we initially discuss the background of lead and various lead-free perovskite materials and further discuss the fundamental aspects of various bismuth-based perovskite and perovskite-inspired materials having a chemical formula of A₃Bi₂X₉, A₂B′BiX₆, B′_aBi_bX_a+3b (A = Cs⁺, MA⁺ and bulky organic ligands; B′ = Ag⁺, Cu⁺; X = I⁻, Cl⁻, Br⁻) and bismuth triiodide (BiI₃) semiconducting material particularly focusing on their structure, optoelectronic properties and the influence of compositional variation on the photovoltaic device performance and stability

Halide perovskite solar cells are approaching commercialization, with solution processing emerging as a key method for large‐scale production. This study introduces a significant advancement: using non‐toxic solvents like water and alcohol in perovskite precursor inks facilitated by the protic ionic liquid methylammonium propionate (MAP). MAP effectively dissolves perovskite precursors such as lead acetate and methylammonium iodide, enabling the first stable water‐based perovskite precursor ink suitable for one‐step slot‐die coating. This new ink formulation contrasts with conventional dimethylformamide (DMF) and dimethylsulfoxide (DMSO)‐based inks, as evidenced by in‐situ grazing incidence wide‐angle X‐ray scattering (GIWAXS), which revealed an intermediate‐free liquid‐to‐solid transition. In‐situ mass spectrometry also showed that organic molecules evaporate during annealing, resulting in a crystalline perovskite phase. Optimization of the solvent mixture to H₂O/IPA/MAP enabled successful slot‐die coating, yielding perovskite solar cells with an efficiency of up to 10%. This eco‐friendly ink reduces toxicity and environmental impact compared to DMF‐based inks, offering a longer shelf life and the possibility of using the ink in ambient conditions. This pioneering work represents the first report of a water‐based green ink formulation for one‐step thin film coating at room‐temperature conditions by slot‐die coating, highlighting its potential for sustainable commercial applications.

Electron transporting layers facilitating electron extraction and suppressing hole recombination at the cathode are crucial components in any thin-film solar cell geometry, including that of metal–halide perovskite solar cells. Amorphous tantalum oxide (Ta₂O₅) deposited by spin coating was explored as an electron transport material for perovskite solar cells, achieving power conversion efficiency (PCE) up to ~14%. Ultraviolet photoelectron spectroscopy (UPS) measurements revealed that the extraction of photogenerated electrons is facilitated due to proper alignment of bandgap energies. Steady-state photoluminescence spectroscopy (PL) verified efficient charge transport from perovskite absorber film to thin Ta₂O₅ layer. Our findings suggest that tantalum oxide as an n-type semiconductor with a calculated carrier density of ~7 × 10¹⁸/cm³ in amorphous Ta₂O₅ films, is a potentially competitive candidate for an electron transport material in perovskite solar cells.

Thin‐film perovskite light‐emitting diodes have gained increasing attention in the last 6 years. With the possibility to process the emitting layer from solution, the way for 1D morphology of the semiconductor for flexible devices is paved. Herein, for the first time single‐step fabrication of CsPbBr₃@PVP nanofibers in a customized electrospinning process performed under ambient conditions from a water‐based precursor solution is reported. The water‐based approach allows the incorporation of a conductive polymer into the compound fiber by blending the perovskite precursor ink with commercially available PEDOT:PSS dispersion. The results demonstrate electrospun fiber mats which are stable at ambient conditions for at least 5 months and can be utilized in electroluminescence devices. Photoluminescence studies on the perovskite fibers reveal a blueshift of the emission peak compared to thin films possibly due to the generation of nanocrystals of ≈12 nm by in situ nanocrystal pinning as confirmed by transmission electron microscopy. A proof‐of‐concept electrically pumped light‐emitting device is built with the obtained fiber mat. The perovskite nanofibers offer promising applications in flexible and stretchable optoelectronics.