A New Low-Cost and Low-Temperature Chemical Passivation Process for Large Area Industrial Single Crystalline Silicon Wafers

In this paper, a unique low-cost, low-temperature chemical passivation process (named as NCPRE-oxide) is investigated. This process uses only a single chemical to grow an ultrathin oxide layer of thickness ~1.7 nm on the silicon surface. Being a single component chemical process, the solution is homogeneous which helps in uniform growth of silicon oxide layer. Our study establishes that the introduction of this chemical oxide layer in between silicon and silicon nitride improves passivation on both n-type and p-type surfaces. An enhancement in the effective minority carrier lifetime (τeff) is also observed on the pyramidal textured surface with chemical oxide/silicon nitride stack for both the p-base and phosphorous diffused silicon surface. The versatility of the NCPRE-oxide is verified by X-ray photoelectron spectroscopy (XPS), carrier lifetime measurements and photoluminescence (PL) imaging.

I. Introduction

Silicon solar cells suffer from different types of losses like electrical, optical and recombination losses [1]. Surface recombination is one of the major recombination losses in thin Si wafers [2]. To reduce the surface recombination loss, various passivation layers are used on the solar cell surfaces. Hydrogenated silicon nitride (SiNx:H) layer is most commonly used for the passivation of the n⁺ emitter layer in conventional Al-BSF technology. Additionally, it acts as an anti-reflection layer to reduce optical losses. SiNx:H has a high density of positive fixed charge that prevents it from being used as a surface passivation layer on p + layer [3]. This problem can be addressed by introducing a thin silicon oxide layer between the SiNx:H and crystalline silicon [3], [4].