Investigation on Structurally Engineered Cs₂AgInCl₆ via Alloying (Na⁺)/Doping (Cr³⁺, Mn²⁺) for Flexible Phosphor-Based White Light Devices and Latent Fingerprint Detection Application

Recently, solid-state lighting (SSL) technology has been a major breakthrough in lighting systems, contributing notably to both economic and environmental sustainability. Halide double perovskite (HDP) based single component white light emitting semiconductors are currently a hot topic among researchers in the field of SSL technology, as they provide greater structural stability, flexibility, nontoxicity, etc. Herein, we chose Cs2AgInCl6 (CAIC) from the HDP family as our white light emitter owing to its direct bandgap nature and ability to produce broad emission arising from the self-trapped excitons (STE) which are produced by the strong-electron phonon coupling. However, it suffers from low photoluminescence intensity due to its parity-forbidden transition arising from the same even-parity of valence band maximum (VBM) and conduction band minimum (CBM). Thus, Na alloying in Cs2AgInCl6 has been implemented to break the parity via isolation of the [AgCl6]5– octahedron that restricts the contributions of the Ag-4d orbital in the VBM that induces the odd–even parity transition. To further enhance the luminescence properties of the material, the optimum composition Cs2Ag0.4Na0.6InCl6 (A4N6) was chosen for transition ion doping (Mn2+ and Cr3+) to increase photoluminescence properties. Eventually, Cs2Ag0.4Na0.6InCl6:Mn6% (A4N6-Mn6) showed a photoluminescence intensity 30 times more than pristine CAIC with a photoluminescence quantum yield (PQLY) of 37.7%, CIE color coordinates (0.36, 0.44), and correlated color temperature of 4757 K. With best-performing materials, we have successfully fabricated a flexible phosphor-based white light-emitting device. It seems that this structural engineering led to the tuning of light from a cold white light region toward a warm white light region that can be efficiently used in household applications. Furthermore, Cs2Ag0.4Na0.6InCl6:Mn6% also exhibits the capability of visualizing latent fingerprint (LFP) detection, which will be used for forensic analysis in the future.