Impact of surface roughness on the electrical parameters of industrial high efficiency NaOH–NaOCl textured multicrystalline silicon solar cell

Abstract

Sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) solution (1:1 ratio by volume) based texturization process at 80–82 °C is an easy, low cost and comparatively new and convenient option for fabrication of any multicrystalline silicon (mC-Si) solar cell. In the present study atomic force microscope is used to observe the intragrain surface in a miniscule area (3 μm × 3 μm) of NaOH–NaOCl textured surface by two and three dimensional analysis, roughness analysis and section analysis. The r.m.s value of the surface parameter of 7.0 nm ascertains the smoothness of the textured surface and further the surface reflectivity is minimized to 4–6% in the 500–1000 nm wavelength range by a proper silicon nitride anti-reflection coating. Comparing with the standard HF–HNO₃–CH₃COOH acid textured cell, the NaOH–NaOCl textured cell shows a comparatively lower value of series resistance of 7.17 mΩ, higher value of shunt resistance of 18.4 Ω to yield a fill factor of 0.766 leading to more than 15% cell efficiency in the industrial cell processing line. This AFM study yields different surface roughness parameters for the NaOH–NaOCl textured wafers which can be used as a reference standard for optimized texturing.

Introduction

Surface control of multicrystalline silicon (mC-Si) solar cells by texturing is one of the critical issues of high efficiency, large area and low cost industrial cells at mass production level. This process should create a damage-free Si-surface prior to diffusion to enhance cell open circuit voltage (V_oc). Also it should generate a minimum front surface reflection loss so that short circuit current (I_sc) of the solar cell is enhanced. There are several mC-Si chemical texturization processes available with their own advantages and disadvantages. The direction dependent anisotropic texturization by sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution is standard for monocrystalline silicon solar cells (Arndt et al., 1975). The anisotropic texturization by NaOH has high rate of etching for some orientation (i.e., 〈1 0 0〉) of the silicon, and for the other orientation (i.e., 〈1 1 1〉) of the silicon, the etch rate is very slow (Vazsonyi et al., 2007). This results in anisotropic texturing of mC-Si wafer over its whole surface and step heights are developed along the grain boundaries. Thus it generates substantial carrier leakage current resulting in low value of open circuit voltage (V_oc). The very common acidic etchant using hydrofluoric (HF), nitric (HNO₃) and acetic (CH₃COOH) acids give isotropic fast etching of silicon surface (Gandhi, 1968). After this texturing, silicon surface becomes smoother irrespective of grain orientations and hence the surface reflectivity enhances. However, the excessive controls required for this etching solution during etching process and the cost of these chemicals restrict its use only for specific purposes. An isotropic etching process containing HF–HNO₃-de-ionized (DI) water etching step followed by HF–HNO₃ etching step provides a good texturization option (Macdonald et al., 2004, Narayanan, 2002). It has two main advantages. Firstly, it forms rounded silicon surface having better anti-reflection property in place of flat silicon surface formed by acid texturing using HF–HNO₃–DI water combination. Secondly, it helps to form shallow front junction to enhance cell blue response (Nishimoto et al., 1999) by the reduction of dead layer (Lindmayer and Allison, 1973). However, the control parameters involved in this process again require costly equipments and costly chemicals and hence this process may be suitable only for photovoltaic plants of very large capacity. Thus, people often opt for only direction dependent silicon etching by high concentration sodium hydroxide (NaOH) or potassium hydroxide (KOH) solution (Arndt et al., 1975, Willeke et al., 1992) as a low cost and convenient option. A low cost chemical texturing for industrial mC-Si solar cell involving NaOH and sodium hypochlorite (NaOCl) offers an easy and low cost production alternative (Gangopadhyay et al., 2005, Basu et al., 2009, Dhasmana et al., 2009). This optimized etching solution does not have any effect on mC-Si grain boundaries (Basu et al., 2009, Vazsonyi et al., 2003, Gangopadhyay et al., 2007) and thus has an excellent isotropic etch characteristics to enhance parameters like V_oc and fill factor (FF). Additionally, the evolution of chlorine during silicon etching process removes the additional chlorine neutralization step (Gangopadhyay et al., 2005) which is generally must for a process involving NaOH.

Our paper reports the atomic force microscope (AFM) study for the mC-Si surfaces obtained by the NaOH–NaOCl texturization or polishing process for variation of surface roughness. The minute observations inside the grain of the wafers by AFM are done for an area of 3 μm × 3 μm to have a glimpse of intragrain roughness. The studies of surface morphology involve three dimensional surface analysis, two dimensional surface analysis, roughness analysis and surface profile analysis of textured surface. Finally solar cells have been fabricated by the conventional industrial fabrication process using the NaOH–NaOCl and conventional HF–HNO₃–CH₃COOH acid textured wafers. The dark current–voltage (DIV) and illuminated I–V (LIV) characteristics have been measured for the cells and all the cell electrical parameters are compared.