Design And Fabrication Of High-efficiency Light Trapping Structures For Silicon-based Thin-film Solar Cell

Silicon-based thin film solar cell has great potential on reducing costs due to its non-toxic,abundant materials,various substrate selection and other advantages,which has become the mainstream development of crystalline silicon solar cell.However,the reduction of the silicon absorptive layer results in significant reduction in the photoelectric conversion efficiency of the solar cell.In order to solve this problem,the most effective way is to introduce efficient light trapping structure to prolong optical path length of incident light in the solar cells.In this thesis,two-dimensional cosine texture and right-angled pyramid texture as light-trapping structures were proposed.The structures were sequentially designed and fabricated.The main contents are as follows:Firstly,we designed thin-film solar cells integrated with two-dimensional cosine and right-angled pyramid texture on a 10 ?m-thick silicon by using the rigorous coupled wave analysis(RCWA),respectively.By analyzing the characteristics of absorption spectrum,short circuit current density,angular response,electric field distribution etc,we obtained the optimal two-dimensional cosine texture with the period and height are 700 nm and 460 nm,respectively,and the optimal right-angled pyramid texture with the period and height are 750 nm and 440 nm,respectively.It is shown that the absorption enhancement of the optimal two-dimensional cosine texture performs similaly with the pyramid texture while the optimal right-angled pyramid texture better than the pyramid texture.The numerical simulations demonstrate that the absorptions in such two thin film solar cells are very close to the Yablonovitch limit for the entire solar spectrum and insensitive to the incident angle of the light.Then,we built two kinds of interference lithography system: Mach-Zehnder and Lloyd's-Mirror.Through optimization of thickness of photoresist,exposure and development time,a two-dimensional cosine structure(P=721nm,H=450nm)was fabricated by using Mach-Zehnder system,which was close to the optimized structure.The right-angled pyramid structure was made by using Lloyd's-Mirror system and the Fourier synthesis method.Finally,the etching process of reactive ion etching(RIE)for continuous structure was studied.The etching conditions of silicon(Si)and photoresist(AZ6112)with 1:1 etching ratio were optimized by “central combination method”.It is showed that the structures etched into the silicon layer have a uniform cosine texture by AFM scanning.By measuring the reflectance of the surface of the silicon wafer from 300 nm to 1120 nm,the average reflectivity was 7.07%,which proved that the light trapping structure had a good anti-reflection performance.