Two-dimensional (2D) Ruddlesden–Popper (RP) perovskites are emerging materials for light-emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions, which limits the color quality and performance of the light source. However, the origin of such broadband emission in ⟨100⟩-oriented perovskites is still under debate. Here, we experimentally and theoretically demonstrate that unlike ⟨110⟩-oriented RP perovskites, the broadband emission of the 2D ⟨100⟩-oriented RP (PEA)2PbI4 (PEA = C6H5C2H4NH3 +) perovskites originates from defect-related luminescence centers. We find that the broadband emission of this prototype 2D structure can be largely suppressed by using excess PEAI treatment. Density functional theory (DFT) calculations indicate that iodine (I) vacancies both in the bulk and on the surface are responsible for the broadband emission. We attribute the decreased broadband emission after PEAI treatment to the passivation of both undercoordinated Pb2+ ions on the surface and I vacancies in the bulk through I– ion migration.

Lead halide compounds, including lead halide perovskite nanocrystals (NCs), have attracted the interest of researchers in optoelectronics and photonics because of their high photoluminescence quantum yields (PLQYs) coupled with relatively short PL lifetimes (on the order of a few nanoseconds). However, lead-free metal halides of high PLQY, including double perovskites and their doped NCs, typically possess long PL lifetimes (up to microseconds) that limit their application space. Here, we introduce CsMnBr3 NCs, which are lead-free and red-emitting, that combine a high PLQY with an exceptionally short radiative lifetime (on the order of picoseconds). We find that the octahedral coordination of Mn2+ in CsMnBr3 induces a red emission centered at ∼643 nm with a PLQY of ∼54% and a fast radiative decay rate. Femtosecond transient absorption and transient PL spectroscopies reveal the existence of a low-lying excited state of Mn2+ that relaxes to the ground state within around 605 ps by emitting light at around 643 nm. At greater excitation energies, higher excited states of Mn2+ relax in the sub-nanosecond time scale to this low-lying excited state. A similarly positioned PL peak with a short picosecond scale PL lifetime and a PLQY of ∼6.7% was also detected in bulk CsMnBr3 single crystals reported in this studya relatively high quantum yield for a bulk material. Our experimental results and density functional theory modelling show that the crystal structure and the strong coupling among Mn2+ ions govern those luminescence properties of CsMnBr3 NCs and single crystals. These findings pave the way for new lead-free materials that combine high PLQY and ultrafast luminescence.

Flexible copper halide films of 400 cm2 area were fabricated with outstanding mechanical stability, excellent film uniformity, nearly 100% photoluminescence quantum yields, and resistance to water and heat. The re-absorption-free X-ray imaging scintillators engineered based on these films exhibit superior scintillation performance with a detection limit as low as 48.6 nGy/s and 17 lp/mm X-ray imaging resolution, representing the highest imaging resolution for powder-based screens.

The architectural design and fabrication of low-cost and reliable organic X-ray imaging scintillators with high light yield, ultralow detection limits, and excellent imaging resolution is becoming one of the most attractive research directions for chemists, materials scientists, physicists, and engineers due to the devices' promising scientific and applied technological implications. However, the optimal balance between the X-ray absorption capability, exciton utilization efficiency, and photoluminescence quantum yield (PLQY) of organic scintillation materials is extremely difficult to achieve because of several competitive nonradiative processes, including intersystem crossing and internal conversion. Here, we introduced heavy atoms (Cl, Br, I) into thermally activated delayed fluorescence (TADF) chromophores to significantly increase their X-ray absorption cross-section while maintaining their unique TADF properties and high PLQY. Most importantly, the X-ray imaging screens fabricated using TADF-Br chromophores exhibited a relative light yield of approximately 20,000 photons/MeV, which is comparable with some inorganic scintillators. In addition, the detection limit of 64.5 nGy s -1 is several times lower than the standard dosage for X-ray diagnostics, demonstrating its high potential in medical radiography. Moreover, a high X-ray imaging resolution of 18.3 line pairs (lp) mm -1 was successfully achieved, exceeding the resolution of all the reported organic scintillators and most conventional inorganic scintillators. This study could help revive research on organic X-ray imaging scintillators and pave the way toward exciting applications for radiology and security screening.

Scintillators are critical for high-energy radiation detection across a wide array of potential applications, from medical radiography and safety inspections all the way to space exploration. However, constrained by their current shortcomings, including high-temperature and complex fabrication as well as inherent brittleness and fragility among thick films and bulk crystals, traditional scintillators are finding it difficult to meet the rising demand for cost-effective, ecofriendly, and flexible X-ray detection. Here, we describe the development of high-performance and flexible X-ray scintillators based on films of Cu-doped Cs2AgI3 that exhibit ultrahigh X-ray sensitivity. The materials exhibit a high scintillation light yield of up to 82 900 photons/MeV and a low detection limit of 77.8 nGy/s, which is approximately 70 times lower than the dosage for a standard medical examination. Moreover, richly detailed X-ray images of biological tissue and electronic components with a high spatial resolution of 16.2 lp/mm were obtained using flexible, large-area, solution-processed scintillation screens.