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We have demonstrated the fabrication and characterization of hybrid CdSe/ZnS quantum dot (QD)-InGaN blue LEDs. The chemically synthesized red light (lambda = 623 nm) QD solutions with different concentrations were dropped onto the blue InGaN LEDs with an emission peak of 453 nm and the turn-on voltage of 2.6 V. In this configuration, the CdSe/ZnS core/shell QDs played the role of a color-conversion center. It was clearly observed that the emission intensity from QDs was increased with increasing QD concentration. With a QD concentration of 10 mg/ml in toluene was incorporated, the ratio of emission intensity of QDs to that of InGaN quantum wells reached 0.17, whereas the Commission Internationale de lpsilaEclairage (CIE) chromaticity coordinates greatly shifted to (0.29, 0.14). From the spatial mapping of electroluminescence spectra, the decrease of the intensity of E QW seems to be faster than that of E QD, which suggests that the QD film thickness may be thicker in the edge of the surface of InGaN chip. There will, therefore, convert higher proportion of blue light to red light. Also, the resin-encapsulated hybrid LEDs have a divergence angle (the full angle at 1/e 2 intensity) of about 20deg as the device is operated at 10 mA. Furthermore, under the injection current of 20 mA and room temperature, this device can be operated for more than 1000 h without any obvious degradation. From our results, it can be proven that the synthesized QDs are promising nanophosphors for color-conversion applications of solid-state LEDs. However, to more efficiently convert the blue light to red light, a denser QD solution with higher quantum yield must be utilized.
Date of Publication: July 2008