Efficiency Color Down-Conversion Of WLEDs By Localized Surface Plasmons (LSPs) Of Metal Nanoparticles

Recently the surface plasmon polaritons(SPPs) have drawn much attention with the rapid developments of metal nano-technology and the fabrication process. The properties of SPP effect can be manipulated by designing various structures of metal nanoparticles(NPs). The SPP effect can be applied to improve the performance of light-emitting diodes(LEDs). This thesis aims to achieve efficient color down-conversion through facile localized surface plasmons(LSPs)-mediated energy transfer in a hybrid structure comprising blue-emitting InGaN/GaN multiple quantum wells(MQWs), Ag NPs, and a light-emitting polymer, and explore its applicability for high-efficiency white light emission. Self-assembled Ag NPs will be prepared on top of a typical blue InGaN/GaN MQW LEDs structure and covered by a homogenous film of yellow-emitting poly(9,9-dioctyfluorene-alt-benzothiazole)(F8BT). Effective nonradiative energy transfer from the In GaN/GaN MQW LEDs to LSPs on the Ag NPs creates a fast path for electron-hole recombination in the MQWs, and thus greatly increases the spontaneous emission efficiency. The work in this thesis is summarized as follows.Firstly the scattering and absorption properties of metal nanoparticles are calculated based on Mie theory. Changing the materials and size of metal NPs can adjust the peak position and intensity of SPPs. It is shown that Ag NPs with the radius of 100 nm has the greatest scattering efficiency and relatively smaller absorption loss.Secondly the scattering and extinction effects of Ag nanoparticles can be solved based on Mie theory when they are placed in GaN medium. The variation of the particle size changes the resonance peak position. The results exhibit that the Ag nanoparticles with the diameter of 13 nm corresponds to SPP resonance peak at 470 nm. So the energy transfer and strong coupling between the blue light and the quantum wells can be achieved.Thirdly the plasmonic effects in simplified GaN-Ag-F8 BT model are studied by the use of numerical simulations. The simulation results show that the near field of the F8 BT layer is enhanced obviously due to the SPP resonance of Ag nanoparticles. The comparation is conducted among the three models including no nanoparticle, only a single particle and a period structure. The results show that owing to Ag nanoparticles most of the incident light can be scattered into F8 BT layer wtih a lower permittivity. When the distance of two nanoparticles is 8nm, the light is scaterred mostly into F8 BT layer and the ratio is 68%. It is possible to greatly enhance the light absorption of F8 BT layer and thus improve the efficiency of the device.Finally since it is so difficult to obtain the Ag nanoparticle with the diameter of 13 nm, a suitable Ag-Au alloy particle is fabricated to replace it. Subsequently the white LED with high quantity is successfully fabricated on the blue LED chip. The down conversion efficiency is measured and the value is 18%.