White-lighting LEDs from InGaN blue LEDs and CdTe nanocrystals

GaN-based blue light-emitting diodes (LEDs) has been energy saving, commercial solid state lighting devices. But the research on GaN-based white-lighting LEDs has been expected to further study before commercialization. One kind of white LEDs consists of GaN-based LEDs and luminant phosphors, but conventional phosphors for GaN-based LEDs are undesirable, for example, phosphors matching GaN-based LEDs can hardly be found. Furthermore, losses caused by self-absorption or scattering of secondary photons in the phosphor medium itself contributes to a large part. In this study, CdTe nanocrystals as nanophosphor for GaN-based blue LEDs are discussed. CdTe NCs were prepared in aqueous solution employing thioglycolic acid (TGA) as stabilizer, the corresponding PL peak position of the NCs excited by 470 nm is 566 nm with full width at half maximum (FWHM) of 30 nm, indicating of monodisperse distribution. The large Stokes shift between its absorption and PL spectra would alleviate self-absorption loss. The InGaN/GaN multiple quantum wells (MQWs) epitaxial wafers were grown at (0001) plane sapphire substrate by the metalorganic chemical-vapor deposition systems (MOCVD), and flip-chip blue LEDs were fabricated with the standard procedure, blue light from MQWs was extracted from polished sapphire plane. A hybrid structure integrating flip-chip LEDs and nanocrystals was reported. CdTe NCs were deposited directly on transparent sapphire layer, which avoids the light emitted by CdTe NCs traped in high-index GaN cavity. And another advantage of the flip-chip structure for CdTe NCs reduces the losses from quenching of the luminescence, a behavior that has been well documented with NCs attaching to smooth metallic electrodes. Blue light part from InGaN LEDs in the hybrid structure provides blue element, and also acts as a pumping resource to excite CdTe nanocrystals to emit yellow light. From photoluminescence excitation (PLE) spectrum of CdTe NCs and electroluminescence (EL) of GaN-based LEDs, the absorption peak around 480 nm in the PLE spectrum match emission peak from InGaN blue LED, which indicates CdTe NCS can be efficiently excited by GaN blue light. The hybrid light of the structure was characterized by the room-temperature electroluminescence (EL) spectra, and CIE chromaticity coordinates corresponding to the EL spectra were obtained. With the hybrid structure, a hybrid white light has been achieved by combining blue component and photoluminescence of CdTe nanocrystals. Chromaticity coordinate of (x = 0.29, y = 0.30) was obtained at 10 mA driving current, and already lies in the white area. The experiment also conducted EL spectra with different forward driving current. It was demonstrated that the chromaticity coordinate of the hybrid device hardly varies with injection current, within 20 mA current level, the fluctuation of chromaticity coordinate lies about 5%. This is a favorable property of the hybrid LED as the color of the device is constant and independent of the injection current.