Enhancement of NIR Luminescence Properties in Li ₃ Cs ₂ Sr ₂ B ₃ P ₆ O ₂₄ : Cr ³⁺ Phosphor via Yb ³⁺ Co‐Doping for Spectroscopy Applications

The extension of near-infrared (NIR) emission beyond the conventional 700- to 900-nm range is crucial for advancing next-generation NIR phosphor-converted light-emitting diodes (NIR pc-LEDs). In this regard, co-doping strategies involving Yb³⁺ have emerged as a promising approach to achieve broader spectral coverage with improved performance. Herein, a series of Li₃Cs₂Sr₂B₃P₆O₂₄: Cr³⁺, Yb³⁺ co-doped phosphors are developed for the first time. Upon 450-nm excitation, the optimal LCSBPO: 0.1Cr³⁺, 0.02Yb³⁺ phosphor delivers intense emission covering 900-1150 nm, which originated from the Stark-split ²F_5/2 → ²F_7/2 transitions of Yb³⁺. Co-doping results in a remarkable 61-fold increase in Yb³⁺ emission compared to mono-doped Yb³⁺ and a three-fold enhancement in overall luminescence relative to LCSBPO: 0.1Cr³⁺. Specifically, the internal quantum efficiency (IQE) improves from 33.7% to 50.6%. Despite the poor luminous thermal stability at 120°C (39.5%), the distinct thermal responses of Cr³⁺ and Yb³⁺ emissions enable ratiometric temperature sensing with a high relative sensitivity (S_r) of 1.93% K^-1 at 30°C. Additionally, an NIR pc-LED fabricated by integrating LCSBPO: 0.1Cr³⁺, 0.02Yb³⁺ with a 450-nm blue LED demonstrates promising practical applications in night vision imaging and non-destructive analysis.