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Ultra‐broadband near‐infrared (NIR) phosphor‐converted light‐emitting diodes (pc‐LEDs) have great application prospects in research and industrial fields. However, simultaneously achieving highly efficient and ultra‐broad NIR emission in phosphor material remains a challenge. Herein, an ultra‐broadband NIR emission in the range of 800–1200 nm with ultra‐high emission efficiency of up to 64.2% is first achieved by substituting the Sc3+ site in Cs2NaScCl6 perovskite single‐crystal with a Cr3+ ion. By combining the experimental results with first‐principles calculations, an efficient charge transfer sensitization process from host to Cr3+ is found to be the fundamental reason for the ultra‐high NIR emission efficiency. Benefiting from the excellent efficiency and superior chemical resistance to heat and UV radiation, an ultra‐broadband NIR pc‐LED made of this phosphor demonstrates great potential in biomedical imaging, nondestructive testing, and night‐vision devices. [ABSTRACT FROM AUTHOR]

Exciton localization is an approach for preparing highly luminescent semiconductors. However, realizing strongly localized excitonic recombination in low‐dimensional materials such as two‐dimensional (2D) perovskites remains challenging. Herein, we first propose a simple and efficient Sn2+ vacancy (VSn) tuning strategy to enhance excitonic localization in 2D (OA)2SnI4 (OA=octylammonium) perovskite nanosheets (PNSs), increasing their photoluminescence quantum yield (PLQY) to ≈64 %, which is among the highest values reported for tin iodide perovskites. Combining experimental with first‐principles calculation results, we confirm that the significantly increased PLQY of (OA)2SnI4 PNSs is primarily due to self‐trapped excitons with highly localized energy states induced by VSn. Moreover, this universal strategy can be applied for improving other 2D Sn‐based perovskites, thereby paving a new way to fabricate diverse 2D lead‐free perovskites with desirable PL properties. [ABSTRACT FROM AUTHOR]

Exciton localization is an approach for preparing highly luminescent semiconductors. However, realizing strongly localized excitonic recombination in low‐dimensional materials such as two‐dimensional (2D) perovskites remains challenging. Herein, we first propose a simple and efficient Sn2+ vacancy (VSn) tuning strategy to enhance excitonic localization in 2D (OA)2SnI4 (OA=octylammonium) perovskite nanosheets (PNSs), increasing their photoluminescence quantum yield (PLQY) to ≈64 %, which is among the highest values reported for tin iodide perovskites. Combining experimental with first‐principles calculation results, we confirm that the significantly increased PLQY of (OA)2SnI4 PNSs is primarily due to self‐trapped excitons with highly localized energy states induced by VSn. Moreover, this universal strategy can be applied for improving other 2D Sn‐based perovskites, thereby paving a new way to fabricate diverse 2D lead‐free perovskites with desirable PL properties. [ABSTRACT FROM AUTHOR]

Inherent "soft" ionic lattice nature of halide perovskite quantum dots (QDs), triggered by the weak Pb−X (X=Cl, Br, I) bond, is recognized as the primary culprit for their serious instability. A promising way is to construct exceedingly strong ionic interaction inside the QDs and increase their crystal cohesive energy by substituting the interior X− with highly electronegative F−, however, which is challenging and hitherto remains unreported. Here, a "whole‐body" fluorination strategy is proposed for strengthening the interior bonding architecture of QDs, wherein the F− are uniformly distributed throughout the whole nanocrystal encompassing both the interior lattice and surface, successfully stabilizing their "soft" crystal lattice and passivating surface defects. This approach effectively mitigates their intrinsic instability issues including light‐induced phase segregation. As a result, light‐emitting devices based on these QDs exhibit exceptional efficiency and remarkable stability. [ABSTRACT FROM AUTHOR]

Inherent "soft" ionic lattice nature of halide perovskite quantum dots (QDs), triggered by the weak Pb−X (X=Cl, Br, I) bond, is recognized as the primary culprit for their serious instability. A promising way is to construct exceedingly strong ionic interaction inside the QDs and increase their crystal cohesive energy by substituting the interior X− with highly electronegative F−, however, which is challenging and hitherto remains unreported. Here, a "whole‐body" fluorination strategy is proposed for strengthening the interior bonding architecture of QDs, wherein the F− are uniformly distributed throughout the whole nanocrystal encompassing both the interior lattice and surface, successfully stabilizing their "soft" crystal lattice and passivating surface defects. This approach effectively mitigates their intrinsic instability issues including light‐induced phase segregation. As a result, light‐emitting devices based on these QDs exhibit exceptional efficiency and remarkable stability. [ABSTRACT FROM AUTHOR]

All‐inorganic α‐phase CsPbI3 perovskite with a suitable bandgap and superb optoelectronic properties is transforming the landscape of perovskite photovoltaics, but its long‐term lability associated with "soft" ionic lattice still imposes a great challenge for practical applications. Herein, a unique solid‐solution fluorination strategy is proposed to deliver an "ideal" perovskite matrix of α‐phase CsPbI3 abundant with F ions through interlocking the soft lattice of CsPbI3 with cubic‐phase CsF·3/2HF. Such a sublattice interlocking can not only stabilize the soft lattice of α‐phase CsPbI3 perovskite nanocrystals but also passivate their notorious surface defects, thereby producing a "rigid" solid‐solution‐type perovskite/fluoride CsPbI3/CsF (termed as CsPbI3:F) nanocomposite with excellent long‐term stability and a near‐unity photoluminescence efficiency. Of particular note is that these CsPbI3:F nanocomposites can work well as effective grain boundary anchors to significantly improve the photovoltaic performance of perovskite solar cells because of their F‐rich perovskite lattice, achieving a T80 stability of 1500 h under continuous maximum power point tracking and AM 1.5G illumination without the need for encapsulation. This work paves a new way to deliver perovskite materials with desirable properties for photovoltaics. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials hold great promise as next‐generation efficient emitters due to their theoretical 100% internal quantum efficiency. However, such an intriguing photophysical mechanism is extremely rare in perovskites or their derivatives. Herein, the colloidal synthesis of two‐dimensional CsAgCl2 nanocrystals (NCs) with typical TADF feature is reported first. The TADF mechanism is investigated in detail by combining first‐principles calculations with experimental results, including temperature‐dependent spectral analysis and fs‐transient absorption spectra. A small singlet−triplet state splitting energy (0.082 eV) and a large spatial separation of charge density |Ψ|2 of valence band maximum and conduction band minimum are observed, which are prerequisites for TADF. Moreover, these CsAgCl2 NCs deliver an ultralow X‐ray detection limit of 170.5 nGyair s−1 and a high imaging resolution of 11.5 lp mm−1, much superior to the commercial scintillators. [ABSTRACT FROM AUTHOR]