Wet-Chemical Conditioning of Silicon Substrates for a-Si:H/c-Si Heterojunctions

Abstract

The influence of wet-chemical silicon (Si) substrate pre-treatments on surface morphology and electronic interface properties is discussed for various hetero interfaces of crystalline Si (c-Si) and Si oxides (SiOx), or amorphous materials such as Si (a-Si:H), Si nitride (a-SiNx:H) and Si carbide (a-SiC:H), which are typically applied in Si heterostructure solar cells. Combined application of surface sensitive techniques, the field-modulated surface photovoltage (SPV), ex-situ and in-situ photoluminescence (PL) measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM), spectroscopic ellipsometry in the ultra-violet and visible region (UV-VIS-SE) and Fourier-Transform infrared ellipsometry (FTIR-SE), total hemispherical UV-NIR-reflectance measurements, microwave detected photo-conductance decay (μW-PCD) and quasi-steady-state photo conductance (QSSPC) provides detailed information about the influence of wet-chemical treatments on preparation induced micro-roughness, surface charge, energetic distribution of interface states D it(E) and the resulting interface recombination behaviour of wet-chemically passivated Si substrates with special surface morphology. The stability of wet-chemical surface passivation during storage in ambient air is found to be strongly influenced by the preparation-induced surface morphology. As shown for various heterojunction structures, the effect of optimized wet-chemical pre-treatments can be preserved during the subsequent soft plasma enhanced chemical vapour deposition of a-Si:H, a-SiNx:H or a-SiC:H. As demonstrated for selected examples, the results of these investigations could be successfully used to enhance the energy conversion efficiency of heterojunction solar cells prepared on flat, saw damage etched and textured Si substrates. Implementation of optimised wet-chemical surface pre-treatments prior to a-Si:H deposition in (ZnO/a-Si:H(n)/c-Si(p)/Al) heterojunction solar cells with pyramidal texturisation increased significantly the solar cell parameters I sc, V oc, fill factor and enhanced the solar cell efficiency from 17.4% (confirmed) to 18.4%.