Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb-free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb-free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb-free perovskites and perovskite derivatives do not.

From Lead Halide Perovskites to Lead-Free Metal Halide Perovskites and Perovskite Derivatives

Progress of Lead-Free Halide Double Perovskites

Rational Design of Halide Double Perovskites for Optoelectronic Applications

X-rays are widely used in probing inside information nondestructively, enabling broad applications in the medical radiography and electronic industries. X-ray imaging based on emerging lead halide perovskite scintillators has received extensive attention recently. However, the strong self-absorption, relatively low light yield and lead toxicity of these perovskites restrict their practical applications. Here, we report a series of nontoxic double-perovskite scintillators of Cs2Ag0.6Na0.4In1-yBiyCl6. By controlling the content of the heavy atom Bi3+, the X-ray absorption coefficient, radiative emission efficiency, light yield and light decay were manipulated to maximise the scintillator performance. A light yield of up to 39,000 ± 7000 photons/MeV for Cs2Ag0.6Na0.4In0.85Bi0.15Cl6 was obtained, which is much higher than that for the previously reported lead halide perovskite colloidal CsPbBr3 (21,000 photons/MeV). The large Stokes shift between the radioluminescence (RL) and absorption spectra benefiting from self-trapped excitons (STEs) led to a negligible self-absorption effect. Given the high light output and fast light decay of this scintillator, static X-ray imaging was attained under an extremely low dose of ∼1 μGyair, and dynamic X-ray imaging of finger bending without a ghosting effect was demonstrated under a low-dose rate of 47.2 μGyair s-1. After thermal treatment at 85 °C for 50 h followed by X-ray irradiation for 50 h in ambient air, the scintillator performance in terms of the RL intensity and X-ray image quality remained almost unchanged. Our results shed light on exploring highly competitive scintillators beyond the scope of lead halide perovskites, not only for avoiding toxicity but also for better performance.

Low-dose real-time X-ray imaging with nontoxic double perovskite scintillators.

Highly efficient blue-emitting three-dimensional (3D) lead-free halide perovskites with excellent stability have attracted worldwide attention. Herein, a doping route was adopted to incorporate Sb3+ ions into the Cs2NaInCl6 for decorating the electronic band structure. Due to the moderate electron-phonon coupling, the Sb3+-doped Cs2NaInCl6 double perovskites showed a narrow and relatively unusual blue emission of self-trapped excitons (STEs). Density functional theory (DFT) calculation indicated that the doped Sb3+ ions could break the parity-forbidden transition rule and modulate the density of state (DOS) population effectively to boost the PLQY of STEs drastically. The optimized Sb3+:Cs2NaInCl6 exhibited a PLQY of up to 75.89% and excellent stability under the consecutive illumination of 365 nm UV light for 1000 h. This kind of highly efficient lead-free Sb3+-doped Cs2NaInCl6 double perovskites may overcome the bottlenecks of severe toxicity and insufficient stability and therefore have an extensive application in the scarce blue photonic and optoelectronic fields.

Highly Efficient Blue Emission from Self-Trapped Excitons in Stable Sb3+-Doped Cs2NaInCl6 Double Perovskites.

The incorporation of impurity ions or doping is a promising method for controlling the electronic and optical properties and the structural stability of halide perovskite nanocrystals (NCs). Herein, we establish relationships between rare-earth ions doping and intrinsic emission of lead-free double perovskite Cs2 AgInCl6 NCs to impart and tune the optical performances in the visible light region. Tb3+ ions were incorporated into Cs2 AgInCl6 NCs and occupied In3+ sites as verified by both crystallographic analyses and first-principles calculations. Trace amounts of Bi doping endowed the characteristic emission (5 D4 →7 F6-3 ) of Tb3+ ions with a new excitation peak at 368 nm rather than the single characteristic excitation at 290 nm of Tb3+ . By controlling Tb3+ ions concentration, the emission colors of Bi-doped Cs2 Ag(In1-x Tbx )Cl6 NCs could be continuously tuned from green to orange, through the efficient energy-transfer channel from self-trapped excitons to Tb3+ ions. Our study provides the salient features of the material design of lead-free perovskite NCs and to expand their luminescence applications.

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Incorporating Rare‐Earth Terbium(III) Ions into Cs 2 AgInCl 6 :Bi Nanocrystals toward Tunable Photoluminescence

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Manipulation of Bi3+/In3+ Transmutation and Mn2+-Doping Effect on the Structure and Optical Properties of Double Perovskite Cs2NaBi1-xInxCl6

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Lead-Free Perovskite Derivative Cs2SnCl6−xBrx Single Crystals for Narrowband Photodetectors

The electronic and optical properties of perovskites are related to the local structures of the compounds and define their functional applications. Herein we have prepared a double perovskite Cs2AgSbCl6, which crystallized in the cubic structure with the space group Fm-3m and the material is found to have a varied band gap associated with different body colors. The anti-site defect model was established to investigate transposition influence on the optical and electronic properties of the double-perovskite Cs2AgSbCl6, and the proposed model clearly explained the as-observed variable body color. Cs2AgSbCl6 perovskite has a high decomposition temperature and is stable upon prolonged exposure to air and moisture, which emphasize its potential in the field of photovoltaic absorbers and optoelectronic applications.

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Exploring the transposition effects on the electronic and optical properties of Cs2AgSbCl6via a combined computational-experimental approach

A supersaturation of nitrogen atoms is found in the surface layer of microstructured silicon after femtosecond (fs) laser irradiation in NF3. The average nitrogen concentration in the uppermost 50 nm is about 0.5 ± 0.2 at. %, several orders of magnitude higher than the solid solubility of nitrogen atoms in silicon. The nitrogen-hyperdoped silicon shows high crystallinity in the doped layer, which is due to the repairing effect of nitrogen on defects in silicon lattices. Nitrogen atoms and vacancies can be combined into thermal stable complexes after fs laser irradiation, which makes the nitrogen-hyperdoped silicon exhibit good thermal stability of optical properties.

Ximing Rong

Papers

Incorporating Rare‐Earth Terbium(III) Ions into Cs 2 AgInCl 6 :Bi Nanocrystals toward Tunable Photoluminescence

Manipulation of Bi3+/In3+ Transmutation and Mn2+-Doping Effect on the Structure and Optical Properties of Double Perovskite Cs2NaBi1-xInxCl6

Lead-Free Perovskite Derivative Cs2SnCl6−xBrx Single Crystals for Narrowband Photodetectors

Exploring the transposition effects on the electronic and optical properties of Cs2AgSbCl6via a combined computational-experimental approach

A nitrogen-hyperdoped silicon material formed by femtosecond laser irradiation