Ion implantation for silicon solar cells

Abstract

Ion implantation is investigated as a method for forming the heavily doped regions of silicon solar cells. Research is conducted with the aim of implementing ion implantation into the fabrication process of the Australian National University's interdigitated back contact and Sliver solar cell designs. High temperature annealing of boron and phosphorus implanted silicon is investigated, with an emphasis on thermal oxidation. The effect of annealing ambient on B implanted Si is studied, with an oxidising ambient shown to result in higher carrier recombination and lower dopant activation compared with annealing in an inert ambient. The effect of process temperature during inert annealing of B implanted Si is investigated. The impact of implantation damage for B implanted Si after high temperature annealing is quantified and used to identify the regimes of implantation damage by their impact on electron-hole recombination as a function of implantation fluence and sheet resistance. The defects present after annealing B implanted Si are studied and related to the observed trends in recombination. An oxidation recipe optimised to minimise recombination after B implantation is developed by minimising oxidation time and including a high temperature inert anneal prior to oxidation. The temperature dependence of P implanted Si during thermal oxidation is investigated for a range of implantation fluences. It is found samples that provided the samples is completely amorphised during implantation, annealing at 900 degrees Celsius is as effective as 1050 degrees Celsius. Studies of recombination and contact resistivity are used to design a P implanted point contact for an interdigitated back contact solar cell. Solid phase epitaxial regrowth is used to anneal P implanted Si at 600 degrees Celsius for 10 minutes. The enhancement in dielectric etch rate after implantation is used as a method to form self-aligned, localised doping and electrical contacts. Implanting dopant atoms through a dielectric layer locally enhances the etch rate relative to non-implanted regions of the same dielectric. Chemical etching selectively exposes on the regions doped by implantation while passivation is preserved in the surround dielectric. Laser processing is investigated as a low thermal budget technique to anneal implanted Si. Laser doping from a dielectric layer implanted with dopant atoms is presented as a method for forming self-aligned doping and contacts. This method was demonstrated using a dielectric stack of silicon oxide and silicon nitride implanted with P and an a-Si passivation layer implanted with B. Ion implantation is used to fabricate interdigitated back contact and Sliver solar cells, achieving significant process simplification compared with reference fabrication processes. The fabrication process for each cell type is reduced to a single high temperature step only. For Sliver cells, conversion efficiency of 16% is achieved and efficiency greater than 22% is demonstrated for interdigitated back contact cells, where implantation does not limit the cell performance.