UC Irvine

Most of white light emitting diode emitters (WLEDs) for lighting and backlighting application are now produced by combining a blue wavelength emitting LED chip with wavelength-conversion materials, such as phosphor and quantum dots. Those WLED emitters are fully encapsulated by optical grade silicone with an intention of photon extraction improvement, optically reflecting surfaces protection, and isolate the LED package from the attack of chemical incompatible factors, such as humidity and halide chemicals. A relentless need for opto-electronic devices with reduced manufacturing cost calls for thinner back-lighting modules and thus thinner profile LED emitters. My thesis research is directed to investigate the optimization of materials and packaging processes for phosphor or quantum dot based white LEDs. Specific projects are focusing on the investigation on the feasibility of low profile lead frame based phosphor WLED emitters free of bulk encapsulant. As well as developing a cost-effective wavelength conversion films for chip scale WLED and LCD BLU. the investigation on the role of novel nano-particles, porous and asymmetric diffusing agents in enhancing the lighting extraction efficiency and reducing the phosphor or QD amount needed as a thin wavelength conversion film.The first part of this work, The junction temperature of a phosphor based white LED (WLED) is investigated as a function of the thickness of phosphor-silicone mixture coated on the chip. It is demonstrated that, to the contrary of a common believe, for a given correlated color temperature (CCT) with a fixed amount of phosphor in the mixture, the WLED junction temperature increases with decreasing thickness of the silicone phosphor mixture coating thickness. This observation can be explained in terms of the different coating thickness dependence of thermal conductivity and the amount of backward scattering of photons into the chip respectively. Although, as expected, the thermal resistance of the coating layer will decrease as the thickness decrease which will contribute to an enhanced thermal dissipation from the chip and thus a reduced junction temperature, but a much larger decrease is expected by the amount of backward scattering of photons into the chip, which results in an overall increase in the junction temperature as the coating thickness is reduced. The second part of this work ,A packaging approach for cost effective, and ultrathin high power phosphor-based white light-emitting-diode (WLED) emitters with outstanding reliability is presented. By removing the encapsulant in the conventional method, the new approach employs a single silicone-phosphor thin layer instead of multiple phosphor layers in chip scale package (CSP) WLED for both encapsulation as well as wavelength conversion. It is demonstrated that the presented WLEDs exhibit superior optical and thermal performance, as well as life-time durability. Accordingly, the present work offers an high reliability, cost-effective alternative approach for ultrathin high power WLED emitters with significant advantages over the conventional packaging structure and chip scale packaging methods. In the third part of this work, the role of nano-sized porous TiO2 diffuser in reducing required amount of wavelength conversion agents, such as phosphor and quantum dots, in the wavelength conversion layer is reported for the first time. It is experimentally demonstrated that for a given specific correlated color temperature (CCT), the existence of nano-sized porous TiO2 diffuser can greatly reduce the required phosphor amount, compared to the non-porous nanosized TiO2. Specifically, in a wide range of CCTs, a 20% or so reduction is observed in the required phosphor amount. This great reduction is demonstrated by a significant increase in the interaction between input photon with phosphor within wavelength conversion file due to the existence of nano-pores in the nano-size porous TiO2 diffuser. In the last part of this work, we proposes a novel light-scattering film with TiO2 nanocylinder diffuser particle loaded. It is designed to improve the phosphor and quantum dot (QD) usage efficiency in cost-effective light wavelength conversion film for various optical performance in the lighting industry. The TiO2 nano cylinder diffusing agents enhances the light scattering and enables a decrease of light wavelength conversion materials necessary to obtain specific CCT. Based on the experimental result of TiO2 diffusing film diffuse properties, the phosphor usage efficiency is significantly increased, as well as the light output power enhancement. A 10% - 20% improvement was found at a driving current of 350mA in TiO2 diffusing agent loaded light wavelength conversion film. It is experimentally confirmed that the dispersion of TiO2 nanocylinder diffusing particles in light wavelength conversion film can exhibit the same level of luminous output due to the reflection reduction, scattering enhancement and Rayleigh scattering effects, even when the amount of phosphor materials used is reduced by 20%. Accordingly, with the low cost benefits, easy fabrication and superior optical characteristics, the TiO2 nano cylinder particle can be an effective performance enhancer for further phosphor and QD optical device applications.