Efficiency improvements of white-light CdSe nanocrystal-based LEDs

We have recently demonstrated white-light devices based on single size, ultra-small cadmium selenide (CdSe) nanocrystals that,possess broadband emission (420-710 nm). When encapsulated in a polymer and coated on commercial UV LEDs, these nanocrystals act as frequency downconverters to produce high quality white light. These devices emit very pure white light with excellent CIE coordinates (0.35, 0.31) and a very high color rendering index of 93 when excited with a 365 nm LED. The relatively low luminous efficiency of preliminary devices, up to 1 lm/W, is a result of unoptimized device geometry, low light extraction efficiency, low UV LED efficiency, and low nanocrystal quantum yield (similar to 10%). In order to address the issue of efficiency, we have pursued two studies. First, we investigate the use of photonic crystal mirrors, chirped mirrors, and thin film filters to increase the extraction efficiency and decrease UV light leakage from the device. Simulation results of chirped mirrors consisting of alternating layers of silicon dioxide (SiO2) and hafnium dioxide (HfO2) have demonstrated high reflectance throughout the visible region, thus increasing extraction efficiency by reflecting emitted white light out of the device that would normally be lost. Second, we examine the tradeoff of color quality and efficiency that results from using blue LEDs for excitation: unabsorbed blue light will imbalance the white-light spectrum, but the conversion of blue to white light yields a lower Stokes loss than UV excitation light.