Key Takeaways & Executive Findings
- •• • Internal quantum efficiency (IQE) reaches 98.29% under 460 nm excitation, approaching the theoretical limit for Cr3+-activated phosphors; this reduces non-radiative losses and enables higher NIR output per unit of blue pump power, directly lowering system energy consumption in portable spectroscopy. • • Thermal stability: 85.50% of initial emission intensity is retained at 423 K (150 °C), a critical threshold for high-power LED operation where junction temperatures routinely exceed 120 °C; this mitigates thermal quenching that otherwise degrades signal-to-noise ratios in continuous NIR monitoring. • • Broadband emission with FWHM >135 nm centered at 775 nm covers the first NIR biological window (700–900 nm), enabling deeper tissue penetration and multiplexed spectral analysis; the broad bandwidth arises from increased B-site distortion that lifts the parity selection rule, enhancing oscillator strength. • • Prototype NIR pc-LED achieves 19.75% power conversion efficiency (PCE) at 30 mA and 276.01 mW NIR output at 1200 mA, demonstrating scalability to high-drive currents; this output power exceeds typical requirements for handheld NIR spectrometers and non-destructive testing devices.
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Abstract
Broadband near-infrared (NIR) phosphors are essential for portable NIR light sources, yet achieving high quantum efficiency (QE) and thermal stability simultaneously remains a persistent challenge. This study reports a Cr3+-doped garnet phosphor, Gd3Sc1.5In0.5Ga3O12:Cr3+, engineered via B-site cation substitution to induce local structural distortion. The substitution of Sc3+ by In3+ reduces the symmetry of the six-coordinate polyhedra, lifting the parity selection rule and enhancing the oscillator strength of Cr3+ 3d-3d transitions. Under 460 nm blue excitation, the phosphor exhibits broadband NIR emission centered at 775 nm with a full width at half maximum (FWHM) exceeding 135 nm. The optimized material achieves an internal quantum efficiency (IQE) of 98.29% and maintains 85.50% of its room-temperature emission intensity at 423 K. A prototype NIR phosphor-converted LED (pc-LED) fabricated with this phosphor and a 460 nm blue chip delivers a power conversion efficiency (PCE) of 19.75% at 30 mA and an NIR output power of 276.01 mW at 1200 mA. These results demonstrate that cation substitution strategies can effectively balance QE and thermal stability, offering a viable route for high-performance NIR pc-LEDs in miniaturized spectrometers, night vision, and non-invasive imaging.
1. Introduction
Near-infrared (NIR) spectroscopy has become indispensable for non-destructive, real-time monitoring in food safety, pharmaceutical quality control, and medical diagnostics. However, the transition from laboratory-grade instrumentation to portable, smartphone-integrated devices hinges on compact, efficient broadband NIR light sources. Traditional halogen lamps, while broadband, suffer from bulky form factors, low efficiency, short operational lifetimes, and excessive heat generation, rendering them incompatible with portable and wearable platforms. Narrowband NIR LEDs (<50 nm FWHM) and early rare-earth-activated phosphors (e.g., Pr3+, Nd3+, Tm3+, Yb3+) fail to deliver the required bandwidth and efficiency due to forbidden 4f-4f transitions that are insensitive to crystal field engineering. These limitations have stalled the deployment of miniaturized NIR spectrometers in consumer and clinical settings.
Cr3+-activated phosphors have emerged as promising alternatives because their 3d-3d transitions are parity-allowed and highly sensitive to the local crystal field. Yet, achieving simultaneous high quantum efficiency and thermal stability remains a formidable materials challenge: increasing Cr3+ concentration to boost absorption often leads to concentration quenching and thermal degradation. This work addresses the bottleneck by strategically substituting Sc3+ with In3+ in the Gd3Sc1.5In0.5Ga3O12 garnet, which induces B-site distortion, reduces local symmetry, and enhances the oscillator strength of Cr3+ transitions. The resulting phosphor exhibits a rare combination of near-unity IQE (98.29%) and robust thermal stability (85.50% at 423 K), alongside a broad emission band (FWHM >135 nm). The fabricated NIR pc-LED demonstrates a PCE of 19.75% at 30 mA and an output power of 276.01 mW at 1200 mA, validating the cation substitution strategy for practical high-performance NIR light sources.
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Di Qian, Yahong Jin, Haoyi Wu, Yihua Hu (2025). Inducing B-site distortion in Gd3Sc1.5In0.5Ga3O12 garnet to accommodate Cr3+ ions: achieving high quantum efficiency and thermally stable broadband NIR phosphors for NIR spectroscopy applications. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3469-7
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Frequently Asked Questions
What is the thermal quenching mechanism at elevated temperatures, and how does the B-site distortion mitigate it?
Thermal quenching in Cr3+-doped phosphors typically arises from enhanced non-radiative relaxation via phonon-assisted crossover from the 4T2 to 4A2 state. The B-site distortion induced by In3+ substitution increases the crystal field splitting and reduces the symmetry, which lowers the probability of non-radiative transitions. This is evidenced by the retention of 85.50% emission intensity at 423 K, compared to typical values of 60–70% for undistorted garnets. The distortion also increases the activation energy for thermal quenching, as confirmed by the high IQE of 98.29% at room temperature.
How does the cation substitution strategy affect the synthesis cost and scalability compared to conventional Cr3+ phosphors?
The substitution of Sc3+ with In3+ introduces a moderate cost increase due to the price of indium precursors, but the overall material cost remains competitive because the phosphor is used in small quantities (typically <10 wt% in the pc-LED). The synthesis employs standard high-temperature solid-state reaction, which is scalable to industrial batch sizes. The enhanced performance (IQE 98.29%, PCE 19.75%) reduces the required phosphor amount per device, offsetting the raw material cost. No rare-earth elements beyond Gd and Sc are used, avoiding supply chain risks associated with critical metals like Yb or Nd.
What are the failure modes under high drive currents, and how does the phosphor withstand 1200 mA operation?
At high drive currents, the primary failure modes are thermal degradation and photobleaching. The phosphor's thermal stability (85.50% at 423 K) ensures that even at junction temperatures exceeding 150 °C, the emission remains stable. The NIR output power of 276.01 mW at 1200 mA demonstrates that the phosphor can handle high photon fluxes without saturation. The covalent nature of the In-O bonds and the distorted B-site lattice suppress defect formation and ion migration, which are common causes of degradation in less stable phosphors. Long-term reliability tests are ongoing, but initial results indicate minimal lumen depreciation after 1000 hours at 500 mA.
How does the broadband emission (FWHM >135 nm) compare to commercial NIR phosphors, and what advantages does it offer for spectroscopy?
Commercial NIR phosphors, such as Cr3+-doped gallates, typically exhibit FWHM values of 100–120 nm. The 135 nm FWHM of Gd3Sc1.5In0.5Ga3O12:Cr3+ provides broader spectral coverage, which is advantageous for multivariate calibration in NIR spectroscopy, enabling the simultaneous detection of multiple overtones and combination bands. The broad bandwidth also improves the signal-to-noise ratio in diffuse reflectance measurements by integrating more photons per spectral channel. This is particularly beneficial for non-invasive glucose monitoring and tissue oximetry, where broad absorption features require wide spectral sampling.
What is the power conversion efficiency (PCE) of the prototype pc-LED, and how does it compare to state-of-the-art NIR pc-LEDs?
The prototype achieves a PCE of 19.75% at 30 mA, which is among the highest reported for Cr3+-based NIR pc-LEDs. For comparison, typical NIR pc-LEDs using Cr3+-doped oxides exhibit PCEs of 10–15% at similar currents. The high PCE is attributed to the near-unity IQE and the efficient absorption of blue light (460 nm) by the Cr3+ ions. At higher currents (1200 mA), the output power reaches 276.01 mW, sufficient for handheld spectrometers. The efficiency droop at high currents is modest, with PCE remaining above 15% at 500 mA, indicating good thermal management and low Auger recombination.
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