Energy Transfer Mechanisms Driving Dual Visible–NIR Emission in Rare‐Earth Doped Double Perovskites for Multifunctional Applications

ABSTRACT Lead‐free double perovskites (LFDPs), as environmentally sustainable alternatives to lead‐based perovskites, have attracted growing attention for a wide range of optoelectronic applications. Among various strategies, doping has emerged as a powerful approach to tune and enhance their optical properties. However, realizing their practical potential requires a comprehensive understanding of their photophysical behavior. Here, we investigate Bi 3+ /Ho 3+ co‐doped Cs 2 AgInCl 6 nanocrystals, comprising 1% Bi 3+ and variable Ho 3+ content (referred to as Ho 3+ ‐doped CABIC), which are direct‐bandgap double perovskite nanocrystals (DPNCs). These nanocrystals exhibit dual visible and near‐infrared (NIR) emission, combined with excellent thermal and moisture stability. Efficient energy transfer from self‐trapped excitons (STEs) in the host lattice to Ho 3+ ions plays a central role in enabling strong dual visible–NIR emission. Femtosecond transient absorption spectroscopy revealed a transfer time of 95 ps, highlighting a fast and effective sensitization pathway. Notably, in this efficiently luminescent sample, THz spectroscopy revealed reduced photoconductivity and carrier mobility, attributed to a higher effective mass and the presence of Ho 4f states near the conduction band minimum, as supported by DFT calculations. Beyond fundamental insights, the multifunctionality of Ho 3+ ‐doped CABIC is demonstrated through its application in optical thermometry and phosphor‐converted light‐emitting diodes (pc‐LEDs). A maximum relative temperature sensitivity of 0.76% K −1 is achieved, outperforming many conventional thermometric materials. Furthermore, the fabricated pc‐LED exhibited warm orange‐yellow light with CIE coordinates of (0.48, 0.46) together with efficient NIR emission. These findings position Ho 3+ ‐doped CABIC nanocrystals as promising candidates for next‐generation eco‐friendly lighting and photonic technologies.