Glass-Crystallized Luminescence Translucent Ceramics toward High-Performance Broadband NIR LEDs

Abstract

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are newly emergent broadband light sources for miniaturizing optical systems like spectrometers. However, traditional converters with NIR phosphors encapsulated by organic resins suffer from low external quantum efficiency (EQE), strong thermal quenching as well as low thermal conductivity, thus limiting the device efficiency and output power. Through pressureless crystallization from the designed aluminosilicate glasses, here broadband Near-infrared (NIR) emitting translucent ceramics are developed with high EQE (59.5%) and excellent thermal stability (<10% intensity loss and negligible variation of emission profile at 150 °C) to serve as all-inorganic visible-to-NIR converters. A high-performance NIR phosphor-converted light emitting diodes is further demonstrated with a record NIR photoelectric efficiency (output power) of 21.2% (62.6 mW) at 100 mA and a luminescence saturation threshold up to 184 W cm^-2 . The results can substantially expand the applications of pc-LEDs, and may open up new opportunity to design efficient broadband emitting materials.

Keywords: Cr3+; broadband light sources; garnets; glass crystallization; light conversion.

Figure 1

Glass crystallization and structural characterization. (a) The transmission spectra of YCAS:0.04Cr³⁺ glass and ceramics annealed at different temperatures. The insets on the top are the photographs of the as‐prepared samples (0.4 mm in thickness and 4–5 mm in diameter). The absorption band from 700 to 1200 nm is indicative of the unreduced Cr⁴⁺. b,c) The HRTEM images of the samples annealed at 1050 °C (b) and 1250 °C (c). The insets are the fast Fourier transform (FFT) patterns. d) The TEM image of YCAS:0.04Cr³⁺ ceramics annealed at 1050 °C. The SEM images of YCAS:0.04Cr³⁺ ceramics annealed at 1150 °C (e) and 1250 °C (f). The insets of d–f) are the size distributions. Scale bars: 10 nm (b), 5 nm (c), 50 nm (d), 200 nm (e), and 500 nm (f).

Figure 2

Room‐temperature luminescence properties. a) The normalized emission spectra of Y_{3− x} Ca _x Al_{4.96− x} Si _x O₁₂:0.04Cr³⁺ (x = 0.0, 0.2, 0.6, 1.0) phosphor (x = 0.0) or ceramics (x = 0.2, 0.6, 1.0). Note that the composition of Y₃Al₅O₁₂:0.04Cr³⁺ cannot be melted and quenched into glass. b) The excitation and emission spectra of YCAS:0.04Cr³⁺ ceramic (λ _ex = 440 nm, λ _em = 760 nm); c) The IQE values of YCAS:0.008Cr³⁺ ceramics annealed at different temperatures. d) The EQE, IQE and AE of YCAS:yCr³⁺ (y = 0.004–0.08) ceramics annealed at 1250 °C. The EQE of YCAS:0.04Cr³⁺ phosphor powder is shown for comparison.

Figure 3

High‐temperature luminescence properties. a) A contour plot of the emission spectra (y‐axis) and temperature (x‐axis) showing the color stability with increasing temperature for YCAS:0.04Cr³⁺ (λ _ex = 440 nm) annealed at 1250 °C. b) Integrated emission intensities of YCAS:yCr³⁺ ceramics (y = 0.016, 0.04, and 0.08) annealed at 1250 °C as a function of the temperature. Note that the values are normalized by that at 30 °C. c) Normalized emission spectra of YCAS:0.04Cr³⁺ ceramics annealed at 1250 °C at different temperatures.

Figure 4

Demonstration of all‐inorganic NIR pc‐LEDs. a) The illustration and photograph of the as‐prepared NIR pc‐LED device by combining translucent YCAS:Cr³⁺ ceramics (0.4 mm in thickness) with a 450 nm LED chip (3 × 3 mm, 1 W). Silicone is used to just seal the chink. b) NIR output power and photoelectric efficiency of the pc‐LED as a function of driving current. The NIR photoelectric efficiency is defined as the percentage of the output power of NIR light (650–1100 nm) to the input electric power of pc‐LED. c) NIR light output power of YCAS:Cr³⁺ ceramics with different Cr³⁺ concentrations (y) as a function of the power density of the incident 450 nm laser. d) Photo of the clover lighted by the as‐fabricated NIR pc‐LED in the darkness, which is taken by a NIR camera. The inset is the photo of the clover under day light. e,f) NIR transilluminated photos of the fingers and the palm to clearly show the veins therein. The inset is the photo of the corresponding hand under day light.