Strategies to approach high performance in Cr³+-doped phosphors for high-power NIR-LED light sources

Near-infrared (NIR) spectroscopy's ability to penetrate organic matter makes it valuable for applications like food and medicine monitoring, bioimaging, and night vision. Smart NIR light sources, integrated with smartphones, promise convenient and rapid applications. Light-emitting diodes (LEDs), with their solid-state nature and small size, are ideal for these devices, unlike traditional incandescent lamps. However, NIR-LED chips produce narrow NIR emissions, limiting their applications. Phosphor-converted (pc) LEDs, similar to pc-white LEDs, offer a better solution by using a broadband NIR phosphor excited by a blue LED. The challenge lies in developing a broadband NIR phosphor efficiently excited by blue light. While various NIR phosphors exist, Cr³⁺-doped materials often exhibit high efficiency (IQE reaching 58–75%), but radiant power remains a limitation (14.7–54.29 mW at 100–130 mA). Achieving high radiance requires high-power LEDs operating at high currents, leading to significant heat generation. Therefore, high thermal stability to counteract thermal quenching is crucial alongside high quantum efficiency. Garnets, known for Cr³⁺'s high QE, are promising hosts. Ca₃Sc₂Si₃O₁₂ (CSSG) garnet, with its excellent thermal stability and high QE for Ce³⁺, is explored as a host for Cr³⁺. Previous studies showed low luminescence (IQE: 12.8%) and thermal stability in CSSG:Cr³⁺ due to impurities and Cr⁴⁺ oxidation during air sintering. This study proposes a strategy to optimize CSSG:Cr³⁺ by enhancing crystallinity, modifying micromorphology, and preserving the Cr³⁺ valence state.

Numerous NIR phosphors have been developed, with Cr³⁺-doped materials often showing high internal quantum efficiency (IQE) ranging from 58% to 75%. However, the radiant power of these phosphors has been limited, typically ranging from 14.7 mW to 54.29 mW when driven at currents between 100 mA and 130 mA. Recent advancements have improved the radiant power to 65.2 mW at 350 mA, highlighting the ongoing efforts to enhance the performance of these materials. The use of garnets as host materials for Cr³⁺ has shown promise due to their inherent properties, including high quantum efficiency and thermal stability. The Ca₃Sc₂Si₃O₁₂ (CSSG) garnet is a particularly interesting host material for Cr³⁺, but previous studies have reported low luminescence efficiency and thermal stability due to issues such as low crystallinity and oxidation of Cr³⁺ to Cr⁴⁺ during the synthesis process. These previous findings underscore the need for improved synthesis techniques that can address these challenges.

The study focused on optimizing CSSG:Cr³⁺ phosphor through controlled synthesis. The researchers investigated the impact of various fluxes (NH₄F, CaF₂, H₃BO₃, LiF, and Li₂CO₃) and sintering atmospheres (air and CO) on the phosphor's properties. The optimization process involved systematically varying the concentration of Cr³⁺ and the type and amount of flux added during synthesis. The samples were synthesized using a solid-state reaction method, and their properties were characterized using various techniques including X-ray diffraction (XRD) to analyze the crystal structure and phase purity, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) to examine the micromorphology and elemental distribution, X-ray photoelectron spectroscopy (XPS) to determine the valence state of Cr ions, diffuse reflectance spectroscopy (DRS) to measure the optical absorption, electron paramagnetic resonance (EPR) spectroscopy to identify the paramagnetic Cr³⁺ ions, and photoluminescence (PL) spectroscopy to measure the NIR emission intensity and quantum efficiency. Temperature-dependent PL measurements were also performed to assess thermal stability, and the electron-phonon coupling (EPC) mechanism was investigated using low-temperature (77 K) PL and PLE spectroscopy. Finally, the optimized phosphor was integrated into a high-power NIR-LED device, and its performance was evaluated by measuring its electroluminescence (EL) spectra, optical power, and conversion efficiency at various driving currents.

The addition of fluxes, particularly Li₂CO₃ (1 wt%), and sintering in a CO reducing atmosphere significantly improved the NIR emission intensity of CSSG:Cr³⁺. XRD analysis showed that the optimized synthesis conditions resulted in a more crystalline material with fewer impurity phases. SEM-EDS revealed a more homogeneous distribution of elements in the optimized samples. XPS results, while not showing a significant difference, were corroborated by DRS and EPR, which conclusively demonstrated the effective preservation of Cr³⁺ valence state and suppression of Cr⁴⁺ formation under reducing conditions. The optimized CSSG:6%Cr³⁺ phosphor exhibited a remarkably high internal quantum efficiency (IQE) of 92.3% and excellent thermal stability, retaining 97.4% of its room-temperature emission intensity at 150 °C. The activation energy for thermal quenching was determined to be 0.336 eV for the optimized sample, significantly higher than that of the initial sample (0.220 eV). Low-temperature PL and PLE spectroscopy revealed strong electron-phonon coupling, contributing to the broad emission band and large Stokes shift. The fabricated NIR-LED device, using the optimized CSSG:6%Cr³⁺ phosphor, achieved a high optical power of 109.9 mW at 520 mA, significantly exceeding previously reported values. Detailed analysis showed that the conversion efficiency from blue light to NIR light decreased at higher currents due to limitations in the blue LED chip's efficiency. The observed blueshift of the emission with increasing temperature was attributed to enhanced electronic transfer from the ⁴T₂g state to the ²E state assisted by electron-phonon coupling.

The results demonstrate a highly effective strategy for optimizing the performance of Cr³⁺-doped NIR phosphors. The combination of flux addition and reducing atmosphere sintering proved crucial in enhancing crystallinity, improving the homogeneity of the material, and maintaining the desired Cr³⁺ valence state. The high IQE and excellent thermal stability of the optimized CSSG:6%Cr³⁺ phosphor are directly linked to these improvements. The achieved high optical power of the NIR-LED device showcases its potential for practical applications. The slightly lower than expected conversion efficiency of the NIR-LED at high driving current is attributed to the performance limitations of the blue LED chip used in the device. Using a more efficient blue LED chip would significantly improve overall efficiency. The strong electron-phonon coupling in the CSSG host provides insight into the material's luminescence behavior and suggests opportunities for further improvements through material engineering.

This research presents a successful strategy for achieving high-performance Cr³⁺-doped NIR phosphors. The optimized CSSG:6%Cr³⁺ phosphor demonstrates exceptional IQE, thermal stability, and when integrated into an NIR-LED device, yields remarkably high optical power. Future research could explore alternative host materials, doping strategies, and synthesis methods to further enhance the properties of these NIR phosphors, potentially leading to even more efficient and powerful NIR light sources for various applications. Furthermore, investigating novel packaging techniques to better manage heat dissipation could also lead to improvements in device performance.

The study's measurements were limited to a wavelength range up to 850 nm, meaning the actual NIR emission intensity might be even higher than measured. The use of a specific blue LED chip in the device fabrication might limit the generalization of the findings on NIR-LED performance. Further optimization of the blue LED chip is needed for obtaining a higher NIR/input efficiency. The study focused on a specific garnet host material; exploring other host matrices could unveil alternative avenues for enhancing NIR phosphor performance.