Analysis of silicon wafer surface preparation for heterojunction solar cells using X-ray photoelectron spectroscopy and effective minority carrier lifetime
U.J. Nsofor, L. Zhang, A. Soman, C.M. Goodwin, H. Liu, K.D. Dobson, U.K. Das, T.P. Beebe Jr., S. Hegedus
We report a systematic study on the optimal conditions for silicon surface preparation to ensure excellent passivation at the crystalline-amorphous silicon (c-Si/a-Si:H) interface of silicon heterojunction solar cells for both untextured and chemically textured samples. X-ray photoelectron spectroscopy (XPS) was utilized to analyse the elemental composition of known silicon impurities on the wafer surface. Surface purity and passivation quality, characterized by effective minority carrier life time (τeff) and implied open circuit voltage (iVOC), were estimated using either quinhydrone-methanol solution or 10 nm intrinsic a-Si:H layers deposited using DC plasma process. This study confirms that surface damage etch (SDE), tetra-methyl ammonium hydroxide (TMAH) texturing and the subsequent TMAH residue removal are the most critical steps in the cleaning process, supporting a simplified wafer cleaning approach that is concise, repeatable and uses minimal volume of chemicals.