Light Trapping And Absorption Enhancement For High Performance Solar Cells And Infrared Photodetectors

This dissertation presents the work on light trapping and absorption enhancement in solar cells and infrared photodetectors. Hemispherical microstructure based omni-directional anti reflection (Omni-AR) coating has been designed and developed for solar cells, based on rigorous coupled-wave analysis (RCWA) and convective coating process. Omni-AR coating enhances light absorption in solar cells, due to the reduced reflection on the structured solar cell surfaces, and the increased optical path length (OPL) inside the solar cells. Omni-AR coating has been demonstrated as a cost-effective light trapping approach for different material systems, with very low reflection and high transmission over a large range of incident angles (0-60 degree) and over a wide spectral range (400-1000nm). Omni-AR coating has also been applied on commercial solar cells based on the cost-effective solution techniques. With omni-AR coating, we obtained increased short circuit current, and increased conversion efficiency, in both organic solar cells and amorphous silicon solar cells. Significant infrared absorption enhancement can also be achieved in photonic crystals (PC) cavities. In 1D PC structures, the ratio of absorption enhancement to suppression at the photonic bandedge can be as high as 40. The absolute infrared absorption can be tuned and enhanced by more than 90% in both the defect cavity and the resonance cavity 1D PC structures. In 2D photonic crystals slab (PCS) cavities, the absorption enhancement factor is more than 6000 under lateral light stimulation. With vertical light stimulation, the enhancement factor is about 100. Enhanced infrared absorption can also be obtained through Fano resonance based PC structures. The light is effectively coupled between the in-plane discrete resonance modes and the vertical continuum radiation modes. Based on this principle, 2D PC patterned silicon nanomembrane (SiNM) Fano filters are fabricated and transferred onto transparent substrates. This type of ultra compact SiNM Fano filters demonstrates strong angular and polarization dependent transmission properties. More significantly, enhanced absorption is achieved via the incorporation of the spectrally aligned colloidal quantum dots (CQDs) into the air holes of Fano filters. Finally, we propose two kinds of spectrally-selective photodetector structures, based on Fano resonance filters.