Theoretical Design And Fabrication Of Spectrumsplitting And Beam-Concentrating Optical Element For Photovoltaics Application

As the energy consumption grows up rapidly in recent years,the efficient utilization of solar energy becomes one of the major problems in the world.Solar cells attract worldwide interests of researchers due to their clean and feasible properties.Single-junction solar cell can only convert part of sun-light energy above its band-gap into electrical energy,and the photoelectric conversion efficiency suffers from the Shockley-Queisser limit of 33.7%.To break out the limit,multi-junction solar cells are proposed,including the tandem and lateral multi-junction solar cells.The tandem multijunction solar cell achieves a high efficiency of 38.8% under 1 sun,which is fabricated by stacking different material layers of different band-gaps to utilize more energy.However,the interface transport and the lattice matching challenge the further improvement of tandem multi-junction solar cells’ performance.To achieve higher photoelectric conversion efficiency and overcome above problems of tandem solar cells,the lateral multi-junction solar cell which assisted with spectral-splitting and beamconcentrating(SSBC)system is another attractive choice.In this dissertation,SSBC Diffractive Optical Elements(DOEs)are designed and fabricated for photovoltaics application.Firstly,high performance SSBC DOEs,whose working wavelength ranges are between 400 nm and 700 nm,are designed by Gratings-Fresnel algorithm and Yang-Gu algorithm respectively.The designed wavelengths are 450 nm,550nm and 650 nm,while the pixels in X axes is 2048.The optical efficiencies of SSBC DOEs among the three designed wavelengths are 81.99% and 82.20%.When the pixels in X axes increases to 4096,the optical efficiencies of SSBC DOEs among the three designed wavelengths are 86.68% and 86.74%.Furthermore,another SSBC DOE with wider working wavelength range(400-900nm)is designed for actual solar cell application.The designed wavelengths are 450 nm,570nm,730 nm and 850 nm while the pixels in X axes is 2048.The average optical efficiency of the SSBC DOE among the working wavelength range are 73.68%.The three DOEs all perform well in the sunlight spectralsplitting and beam-concentrating.Then,a micro-fabrication technology of silicon optical elements,including UV lithography and plasma etching,are improved and SSBC optical elements with high quality are fabricated.The minimum lithography precision of fabricated elements is 10 um and the maximum etch depth is 8.213 um.Meanwhile,an automatic optical detection system with high precision are set up to investigate the optical performance of SSBC DOE.The output-plane intensity distributions and the optical efficiencies are measured in this detection system.The optical efficiency of DOE whose working wavelength range is 400-700 nm is measured as 60.07% while the optical efficiency of DOE whose working wavelength range is 400-900 nm is measured as 58.0%.The experimental results match the theoretical results well.Finally,a photoelectric detection system under simulated sunlight is improved to character the photoelectric performance of SSBC photovoltaics.The photovoltaics system contains a dye-sensitized solar cell and a GaAs solar cell.With the assistance of fabricated SSBC DOE,the photoelectric conversion efficiency of system increases from 12.86% to 15.64%.The photovoltaics system assisted with SSBC DOE provides a feasible approach towards high photoelectric conversion of solar energy.Moreover,a binary detection system is improved to study the spectrum splitting property evolution and crack evolution of the photonic crystal element.This detection technique helps fabricate photonic crystal element with better quality.