Efficient white light generation using photonic crystal phosphors

Abstract

Generating a more efficient white light source is very important in various applications such as backlights of display panels, automobile lamps and lighting systems. In particular, white light-emitting diodes (LEDs) have gained great interest due to many unique advantages such as energy efficiency, environmental friendliness, long lifetime and compact size. The most common way to make a white light source is to combine a blue LED with one or more wavelength down-converting phosphors. In this context, phosphors are as important as blue LED chips as a component of white LEDs. Until now, however, most phosphor researches have focused on material aspects such as improvement of the performance of existing phosphor materials and development of new phosphors. As an alternative to material based approaches, our group have proposed and demonstrated a structural approach, that is, photonic crystal (PhC) phosphors. Group velocity of photon becomes zero at photonic band-edge (PBE) modes, which makes photons interaction with matter significantly stronger. Combining phosphors with PhC structure and tuning a PBE mode to the excitation photon energy, we obtained significantly stronger interaction between excitation photons and phosphor materials and thus much enhanced color conversion efficiency. In this thesis, I proposed an efficient white light generation method employing a multiple stack of the structurally engineered PhC phosphors on top of a blue LED chip. I designed and fabricated the one-dimensional (1D) PhC phosphors using red and green colloidal quantum dots (CQDs), with their band-edge resonance tuned at the excitation photon wavelength. The excitation resonance improves the interaction between the excitation photons from the blue LED chip and CQDs, resulting in enhanced absorption of excitation photons by CQDs and thus enhanced emission. White light was generated by stacking the red and green PhC phosphors on a blue LED chip. I observed 8% stronger white light out of 33% less CQD amounts in comparison with the structure-less reference phosphors, thanks to a higher color conversion efficiency enabled by PBE effect. In addition, I examined the optical properties of two-dimensional (2D) PhC phosphors. The symmetric 2D PhC phosphor exhibited enhancement factor of ~6 regardless of the polarization direction of excitation source. The asymmetric 2D PhC phosphor exhibited higher enhancement factor (~12) than other PhC phosphor structures due to the fact that both emission resonance and excitation resonance occurred. I expect that further optimizations in structural design and device fabrication will have an enormous impact on phosphors, white LEDs and other applications requiring efficient white light.