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Optically active defects often play a crucial role in governing the light emission as well as the electronic properties of materials. Moreover, defect-mediated states in the midgap region can trap electrons, thus opening a path for the recombination of electrons and holes in lower energy states that may require phonons in the process. Considering this, we have probed electron–phonon interaction in halide perovskite systems with the introduction of defects and investigated the thermal effect on this interaction. Here, we report Raman spectroscopic study of the thermal evolution of electron–phonon coupling, which is tunable with the crystal growth conditions, in the halide perovskite systems Cs2AgInCl6 and Cs2NaInCl6. The signature of electron–phonon coupling is observed as a Fano anomaly in the lowest frequency phonon mode (51 cm−1), which evolves with temperature. In addition, we observe a broad band in the photoluminescence (PL) measurements for the defect-mediated systems, which is otherwise absent in defect-free halide perovskite. The simultaneous observation of the Fano anomaly in the Raman spectrum and the emergence of the PL band suggests the defect-mediated midgap states and the consequent existence of electron–phonon coupling in the double perovskite. [ABSTRACT FROM AUTHOR]

Crown ether anchored organic‐inorganic hybrid halides have been recently reported as interesting luminescent materials in the visible region of electromagnetic spectrum. Is it possible to develop such crown ether anchored hybrid materials for near infrared emission? Motivated by this question, we designed a new hybrid material, namely, [(18‐Crown‐6)K][MoOCl4(H2O)]. 18‐Crown‐6 ether bound with K+ form the cationic part [(18‐Crown‐6)K]+. The K+ of [(18‐Crown‐6)K]+ electrostatically interacts with Cl− of the anionic part [MoOCl4(H2O)]−, forming the hybrid crystal [(18‐Crown‐6)K][MoOCl4(H2O)]. It crystallizes in orthorhombic crystal system with Pnma space group. The Mo(V) possesses one d‐electron (d1) in C4v ${{C}_{4v}}$ point group symmetry in the [MoOCl4(H2O)]− polyhedra. This electronic configuration leads to multiple spin‐allowed d-d ${d-d}$ transitions along with a ligand to metal charge transfer (LMCT) resulting into multiple optical absorption bands in the near UV‐visible‐near infrared (NIR) region. The lowest energy d-d ${d-d}$ transition via 2E (dxz1 ${{d}_{xz}^{1}}$ , dyz ${{d}_{yz}}$)/2E (dxz,dyz1 ${{d}_{xz},{d}_{yz}^{1}}$)→ ${\to }$ 2B2 (dxy1 ${{d}_{xy}^{1}}$) leads to NIR PL with peak at 952 nm, but with a poor intensity at room temperature. [ABSTRACT FROM AUTHOR]

Phosphor converted light emitting diodes (pc‐LEDs) have revolutionized solid‐state white lighting by replacing energy‐inefficient filament‐based incandescent lamps. However, such a pc‐LED emitting ultrabroad near‐infrared (NIR) radiations still remains a challenge, primarily because of the lack of ultrabroad NIR emitting phosphors. To address this issue, we have prepared 2.5 % W4+‐doped and 2.8 % Mo4+‐doped Cs2Na0.95Ag0.05BiCl6 perovskites emitting ultrabroad NIR radiation with unprecedented spectral widths of 434 and 468 nm, respectively. Upon band‐edge excitation, the soft lattice of the host exhibits broad self‐trapped exciton (STE) emission covering NIR‐I (700 nm), which then nonradiatively excites the dopants. The π ${\pi }$ –donor ligand Cl− reduces the energy of dopant d–d transitions emitting NIR‐II with a peak at ~950 nm. Vibronic coupling broadens the dopant emission. The large spin‐orbit coupling and local structural distortion might possibly enhance the dopant emission intensity, leading to an overall NIR photoluminescence quantum yield ~40 %. The composite of our ultrabroad NIR phosphors with biodegradable polymer polylactic acid could be processed into free‐standing films and 3D printed structures. Large (170 × ${\times }$ 170  mm2 ${m{m}^{2}}$ ), robust, and thermally stable 3D printed pc‐LED panels emit ultrabroad NIR radiation, demonstrating NIR imaging applications. [ABSTRACT FROM AUTHOR]

A phosphor emitting both white light and broad near‐infrared (NIR) radiation can simultaneously provide visual inspection and early signs of rotting of food products. The broad NIR emission is absorbed by the vibrational overtones of water molecules present in food items, providing the non‐invasive image contrast to assess the food freshness. Here we design a phosphor, namely, Cr3+‐Bi3+‐codoped Cs2Ag0.6Na0.4InCl6, that simultaneously emit warm white light and broad NIR (1000 nm) radiation with quantum yield 27 %. This dual emitter is designed by combining the features of s2‐electron (Bi3+) and d3‐electron (Cr3+) doping in a weak crystal field of the halide perovskite host. 6s2→6s16p1 ${6{s}^{2}\to 6{s}^{1}6{p}^{1}}$ excitation of Bi3+, using a commercial 370 nm ultraviolet light‐emitting‐diodes (UV‐LED), yields both the emissions. A fraction of the excited Bi3+ dopants emit the warm white light, and the other fraction transfers its energy non‐radiatively to Cr3+. Then the Cr3+ de‐excites emitting broad NIR emission. Temperature dependent (6.4–300 K) photoluminescence in combination with Tanabe‐Sugano diagram show that the Cr3+ experiences a weak crystal field (Dq/B ${{D}_{q}/B}$ =2.2), yielding the 4T2→4A2 ${{{\rm \ }}^{4}{{\rm T}}_{2}\to {{\rm \ }}^{4}{{\rm A}}_{2}}$ NIR emission. As a proof of concept, we fabricated a panel containing 122 phosphor‐converted LEDs, demonstrating its capability to inspect food products. [ABSTRACT FROM AUTHOR]

Cs2NaInCl6 double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short‐wave infrared (SWIR) region. Here we introduce such functionalities in Cs2NaInCl6 by codoping Sb3+ (s‐electron doping) and Er3+ (f‐electron doping) ions. Sb3+ doping introduces optically allowed 5s2→ 5s15p1 electronic absorption at the sub‐band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f‐f electronic absorption of Er3+ is parity forbidden. Codoping Sb3+–Er3+, leads to transfer of excitation energy from Sb3+ to Er3+, yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc‐LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V. [ABSTRACT FROM AUTHOR]