Abstract
A novel orange-yellow-emitting polymethyl methacrylate derivative grafted with cationic iridium(III) complex units was synthesized and used as down-conversion luminescent materials in light-emitting diodes (LEDs). The polymer had a thermal decomposition temperature (Td) of 275 °C. With the temperature increasing from 20 to 100 °C, its photoluminescent intensity decreased to 76.8% with thermal quenching activation energy (Ea) of 0.2775 eV. A series of LEDs was fabricated by 460 nm blue GaN chips and the polymer blended in silicone at different concentrations. At 4.0 wt %, a cold white LED was obtained, the correlated color temperature (CCT) was 10,050 K, color rendering index (CRI) was 71.2, luminous efficiency (ηL) was 5.3 lm·w−1, and Commission Internationale de L’Eclairage (CIE) chromaticity coordinates were (0.30, 0.24). At 5.0 wt.%, the LED emitted neutral white light, its CCT was 4938 K, CRI was 75, ηL was 13.8 lm·w−1, and the CIE value was (0.34, 0.27). At 5.5 wt %, 6.0 wt %, 7.0 wt %, and 8.0 wt %, the LEDs all emitted warm white light; their CCTs were 3446, 3093, 2557, and 2337 K, respectively; their CRIs were 73.6, 71.8, 63.8, and 59.0, respectively; their ηL were 18.1, 16.3, 14.8, and 13.7 lm·w−1, respectively; and their CIE values were (0.36, 0.30), (0.40, 0.35), (0.45, 0.38), and (0.50, 0.42), respectively. At 9.0 wt %, the blue light of GaN chip was completely absorbed by the polymer and only the orange-yellow light of the polymer emitted. The results suggested the polymer was a promising orange-yellow-emitting phosphor candidate for white LEDs, especially for warm white LEDs.
Highlights
As the new generation solid-state light sources, white light-emitting diodes (LEDs) have some fascinating advantages such as high efficiency, energy-saving, long lifetime, small size, and environmentally friendly properties, and have been widely used in general illumination, full-color displays, liquid crystal display backlights, automotive headlamps, and so on [1,2,3]
White LEDs are mainly fabricated by the combination of ultraviolet or blue chips and down-conversion luminescent materials [1,2,3,4,5,6]; relatively, the blue chips are more commonly used since the high brightness blue GaN chips were successfully contrived in the late 1980s and early 1990s [7,8]
Organic iridium(III) complexes have been widely used in many luminescence fields, including organic light-emitting diodes (OLEDs) [24,26,27], light-emitting electrochemical cells (LECs) [25,26,28], chemical sensors [29,30], bioimaging [30,31], and so forth
Summary
As the new generation solid-state light sources, white light-emitting diodes (LEDs) have some fascinating advantages such as high efficiency, energy-saving, long lifetime, small size, and environmentally friendly properties, and have been widely used in general illumination, full-color displays, liquid crystal display backlights, automotive headlamps, and so on [1,2,3]. Owing to high stability and high energy conversion efficiency, many inorganic luminescent materials have been used as phosphors in LEDs [5,6,12]; at the same time, because of some other advantages, such as broad emission spectra, good color regulatability via various organic groups, and hydrophobicity, all kinds of organic luminescent materials have been widely tried, including small-molecule fluorescent dyes [13,14], luminescent organic metal complexes [15,16,17,18,19], luminescent polymers [20,21], metal-organic framework materials [22,23], and so on. In order to explore more such luminescent materials, in this work, a novel polymethyl methacrylate derivative grafted with cationic iridium (III) complex units was synthesized and used in white LEDs
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