Detailed Micro Raman Spectroscopy Analysis of Doped Silicon Thin Film Layers and Its Feasibility for Heterojunction Silicon Wafer Solar Cells

Abstract

Hydrogenated doped silicon thin films deposited using RF (13.56 MHz) PECVD were studied in detail using micro Raman spectroscopy to investigate the impact of doping gas flow, film thickness, and substrate type on the film charac teristics. In particular, by deconvoluting the micro Raman spectra into amorphous and crystalline components, qualita tive and quantitative information such as bond angle disorder, bond length, film stress, and film crystallinity can be de termined. By selecting the optimum doped silicon thin film deposition conditions, and combining our p-doped and n-doped silicon thin films in different heterojunction structures, we demonstrate both (i ) an efficient field effect pas sivation and (ii ) further improvement to c-Si/a-Si:H(i) interface defect density with observed improvement in implied open-circuit voltage V OC and minority carrier lifetimes across all injections levels of interest. In particular, the hetero junction structure (a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(p)) demonstrates a minority carrier lifetime of 2.4 ms at an injection level of 10 15 cm - 3 , and a high implied open-circuit voltage of 725 mV. Simulation studies reveal a strong dependence of the interface defect density D it on the heterojunction silicon wafer solar cell performance, affected by the deposition conditions of the overlying doped silicon thin film layers. Using our films, and a fitted D it of 5 × 10 10 cm - 2 · eV - 1 , we demonstrate that a solar cell efficiency of ~22.5% can be potentially achievable.