Colloidal lead‐free perovskite nanocrystals have recently received extensive attention because of their facile synthesis, the outstanding size‐tunable optoelectronic properties, and less or no toxicity in their commercial applications. Tin (Sn) has so far led to the most efficient lead‐free solar cells, yet showing highly unstable characteristics in ambient conditions. Here, we propose the synthesis of all‐inorganic mixture Sn‐Ge perovskite nanocrystals, demonstrating the role of Ge²⁺ in stabilizing Sn²⁺ cation while enhancing the optical and photophysical properties. The partial replacement of Sn atoms by Ge atoms in the nanostructures effectively fills the high density of Sn vacancies, reducing the surface traps and leading to a longer excitonic lifetime and increased photoluminescence quantum yield. The resultant Sn‐Ge nanocrystals‐based devices show the highest efficiency of 4.9 %, enhanced by nearly 60 % compared to that of pure Sn nanocrystals‐based devices.

Photoelectrochemical water splitting and CO₂ reduction provide an attractive route to produce solar fuels while reducing the level of CO₂ emissions. Metal halide perovskites (MHPs) have been extensively studied for this purpose in recent years due to their suitable optoelectronic properties. In this review, we survey the recent achievements in the field. After a brief introduction to photoelectrochemical (PEC) processes, we discussed the properties, synthesis, and application of MHPs in this context. We also survey the state-of-the-art findings regarding significant achievements in performance, and developments in addressing the major challenges of toxicity and instability toward water. Efforts have been made to replace the toxic Pb with less toxic materials like Sn, Ge, Sb, and Bi. The stability toward water has been also improved by using various methods such as compositional engineering, 2D/3D perovskite structures, surface passivation, the use of protective layers, and encapsulation. In the last part, considering the experience gained in photovoltaic applications, we provided our perspective for the future challenges and opportunities. We place special emphasis on the improvement of stability as the major challenge and the potential contribution of machine learning to identify the most suitable formulation for halide perovskites with desired properties.

Quantitative assessment of photo-excitation intensity dependent electron and hole injections from CH₃NH₃PbI₃ perovskite to nanocrystalline TiO₂ and spiro-OMeTAD.

The ongoing Internet of Things revolution has led to strong demand for low‐cost, ubiquitous light sensing based on easy‐to‐fabricate, self‐powered photodetectors. While solution‐processable lead‐halide perovskites have raised significant hopes in this regard, toxicity concerns have prompted the search for safer, lead‐free perovskite‐inspired materials (PIMs) with similar optoelectronic potential. Antimony‐ and bismuth‐based PIMs are found particularly promising; however, their self‐powered photodetector performance to date has lagged behind the lead‐based counterparts. Aiming to realize the full potential of antimony‐based PIMs, this study examines, for the first time, the impact of their structural dimensionality on their self‐powered photodetection capabilities, with a focus on 2D Cs₃Sb₂I_9−xCl_x and Rb₃Sb₂I₉ and 0D Cs₃Sb₂I₉. The 2D absorbers deliver cutting‐edge self‐powered photodetector performance, with a more‐than‐tenfold increase in external quantum efficiency (up to 55%), speed of response (>5 kHz), and linear dynamic range (>four orders of magnitude) compared to prior self‐powered A₃M₂X₉ implementations (A⁺: monovalent cation; M³⁺: Sb³⁺/Bi³⁺; X⁻: halide anion). Detailed characterization reveals that such a performance boost originates from the superior carrier lifetimes and reduced exciton self‐trapping enabled by the 2D structure. By delivering cutting‐edge performance and mechanistic insight, this study represents an important step in lead‐free perovskite‐inspired optoelectronics toward self‐powered, ubiquitous light sensing.

Double perovskites are a promising family of lead-free materials that not only replace lead but also enable new optoelectronic applications beyond photovoltaics. Recently, a titanium (Ti)-based vacancy-ordered double perovskite, Cs₂TiBr₆, has been reported as an example of truly sustainable and earth-abundant perovskite with controversial results in terms of photoluminescence and environmental stability. Our work looks at this material from a new perspective, i.e., at the nanoscale. We demonstrate the first colloidal synthesis of Cs₂TiX₆ nanocrystals (X = Br, Cl) and observe tunable morphology and size of the nanocrystals according to the set reaction temperature. The Cs₂TiBr₆ nanocrystals synthesized at 185 °C show a bandgap of 1.9 eV and are relatively stable up to 8 weeks in suspensions. However, they do not display notable photoluminescence. The centrosymmetric crystal structure of Cs₂TiBr₆ suggests that this material could enable third-harmonic generation (THG) responses. Indeed, we provide a clear evidence of THG signals detected by the THG microscopy technique. As only a few THG-active halide perovskite materials are known to date and they are all lead-based, our findings promote future research on Cs₂TiBr₆ as well as on other lead-free double perovskites, with stronger focus on currently unexplored nonlinear optical applications.