Value of thermal oxide for ion implant based c-Si solar cells

In this paper we present the improvements in cell performance for ion implanted C-Si solar cells by thermal oxidation. We have investigated the effect of oxidation on cell efficiency (CE) and systematically evaluated the effect of oxide thickness on emitter quality for the ion implanted emitters. It is shown that the CE of ion implanted cells increases by ~0.3% absolute with increase in oxide thickness and is attributed to lower J_0e. Additionally, the effect of oxidation on dopant profile of these emitters is also characterized using sheet rho (R_sheet) measurements and SIMS analysis, which demonstrate that an optimal profile can be achieved with a single step anneal/oxidation of the ion implanted emitters. The lower J_0e achieved for the ion implanted emitters with an oxide layer (10-30nm) is due to superior passivation quality of the thermal oxide and the optimized dopant profile. It is to be noted here that the thermal oxidation is part of the post implant anneal and does not lead to excessive drive in and allows controlling the dopant profile/junction depth (x_j) very precisely. This unique ion implant based process flow also offers a distinct advantage over two step diffusion process(es) (POCl₃ → PSG etch → high temperature thermal oxidation) which are required to passivate emitters fabricated using POCl₃ with a thermal oxide. Thus, the proposed approach of single step anneal/oxidation of ion implanted emitters offers an attractive solution for passivating surfaces and manufacturing high efficiency solar cells with advanced architectures.