Self-trapped excitons (STEs) in metal halide materials are attracting an increasing level of interest due to their unique light emission properties. Light emission from STEs in metal halides is usually associated with excited-state structural deformation, which lowers the symmetry of local structures, as seen for the STEs in a wide range of materials systems. Here, we reveal a prototypic STE-associated structural “distortion” that, however, enhances the symmetry of local structures, in a series of all-inorganic copper­(I)-based halides Cs3Cu2X5 (X = Cl, Br, or I). We further find that the emission peaks of Cs3Cu2X5 blue-shift when the halogen changes from Cl to Br to I, which is the opposite of the trends found in traditional halide perovskites. This phenomenon is attributed to a synergetic combination of the significant change in band gap associated with structural deformation and a strong excitonic effect. Due to the highly localized electron and hole upon photoexcitation, Cs3Cu2Cl5 shows an extremely long and temperature-sensitive photoluminescence (PL) lifetime among metal halide materials with STEs. Remarkably, strong green emission with a PL quantum yield exceeding 90% is found in Cs3Cu2Cl5, opening the way to designing light emission compounds based on local symmetry-enhancing STE mechanisms.

Radioluminescent materials (scintillators) are widely applied in medical imaging, nondestructive testing, security inspection, nuclear and radiation industries, and scientific research. Recently, all‐inorganic lead halide perovskite nanocrystal (NC) scintillators have attracted great attention due to their facile solution processability and ultrasensitive X‐ray detection, which allows for large area and flexible X‐ray imaging. However, the light yield of these perovskite NCs is relatively low because of the strong self‐absorption that reduces the light out‐coupling efficiency. Here, NCs with self‐trapped excitons emission are demonstrated to be sensitive, reabsorption‐free scintillators. Highly luminescent and stable Cs3Cu2I5 NCs with a photoluminescence quantum yields of 73.7%, which is a new record for blue emission lead‐free perovskite or perovskite‐like NCs, is produced with the assistance of InI3. The PL peak of the Cs3Cu2I5 NCs locates at 445 nm that matches with the response peak of a silicon photomultiplier. Thus, Cs3Cu2I5 NCs are demonstrated as efficient scintillators with zero self‐absorption and extremely high light yield (≈79 279 photons per MeV). Both Cs3Cu2I5 NC colloidal solution and film exhibit strong radioluminescence under X‐ray irradiation. The potential application of Cs3Cu2I5 NCs as reabsorption‐free, low cost, large area, and flexible scintillators is demonstrated by a prototype X‐ray imaging with a high spatial resolution.

The present work reports highly efficient flexible and reabsorption-free scintillators based on two zero-dimensional (0D) organic copper halides (TBA)­CuX2 (TBA = tetrabutylammonium cation; X = Cl, Br). The (TBA)­CuX2 exhibit highly luminescent green and sky-blue emissions peaked at 510 and 498 nm, with large Stokes shifts of 224 and 209 nm and high photoluminescence quantum yields (PLQYs) of 92.8% and 80.5% at room temperature for (TBA)­CuCl2 and (TBA)­CuBr2 single crystals (SCs), respectively. Interestingly, above room temperature, their PLQYs increase with temperature and reach near unity at 320 and 345 K for (TBA)­CuCl2 and (TBA)­CuBr2, respectively. The excellent properties originate from self-trapped excitons (STEs) in individual [CuX2]− quantum rods, which is demonstrated by the temperature-dependent PL, ultrafast transient absorption (TA) combined with density functional theory (DFT) calculations. The (TBA)­CuX2 scintillators show bright radioluminescence (RL), impressive linear response to dose rate in a broad range, and high light yields. Their potential application in X-ray imaging is demonstrated by using (TBA)­CuX2 composite scintillation screens. Importantly, flexible scintillators are demonstrated to be superior than flat ones for imaging nonplanar objects by conformally coating, which produce accurate images with negligible distortion.

Infrared solar cells that utilize low-bandgap colloidal quantum dots (QDs) are promising devices to enhance the utilization of solar energy by expanding the harvested photons of common photovoltaics into the infrared region. However, the present technology for synthesis of PbS QDs cannot produce highly efficient infrared solar cells. Here, we develop a general synthesis framework for low-bandgap PbS QDs (0.65-1 eV) via cation-exchange from ZnS nanorods (NRs). First, ZnS NRs are converted to superlattices with segregated PbS domains within each rod. Then, sulfur precursors are released via dissolution of the ZnS NRs during the cation-exchange, which promotes size-focusing of PbS QDs. PbS QDs synthesized through this new method have the advantages of high monodispersity, ease of size control, in-situ passivation of chloride, high stability, and "clean" surface. We fabricated infrared solar cells based on these PbS QDs with different bandgaps, using conventional ligand exchange and device structure. All of our devices produced in this manner show excellent performance, showcasing the high quality of our PbS QDs. The highest performance of infrared solar cells was achieved using ~0.95 eV PbS QDs, exhibiting an efficiency of 10.0% under AM 1.5 solar illumination, a perovskitefiltered efficiency of 4.2% and a silicon-filtered efficiency of 1.1%.