Potential cost reduction of buried-contact solar cells through the use of titanium dioxide thin films

Abstract

This paper explores the potential of applying titanium dioxide (TiO 2) thin films to the buried-contact (BC) solar cell. The aim is to develop a lower-cost BC technology that can be applied to multicrystalline silicon (mc-Si) wafers, the predominant substrate of the photovoltaics (PV) industry. The original BC solar cell used a thick, thermally grown, silicon dioxide (SiO 2) layer as the front surface dielectric coating. Upon commercialisation of the BC technology, BP Solar replaced this layer with silicon nitride (Si 3N 4), which exhibits improved optical properties. It is anticipated that production costs can be further reduced by using a low temperature deposited front surface dielectric coating, such as TiO 2, thereby reducing the number of lengthy high temperature processing steps, and developing a process such that it can be applied to mc-Si wafers. TiO 2 is chosen because of its optimal optical properties for glass-encapsulated silicon solar cells and familiarity of PV manufacturers with this material. The results presented resolve the issue of surface passivation with TiO 2 and demonstrate that TiO 2/SiO 2 stacks, achieved during a brief high-temperature oxidation process after TiO 2 thin film deposition, are compatible with high-efficiency solar cells. However, TiO 2 cannot perform all the necessary functions of the thick SiO 2 or Si 3N 4 layer, due to its inability to act as a phosphorus diffusion barrier. In light of these results, three alternate BC solar cell fabrication sequences are presented, and an initial conversion efficiency of 11.5% has been achieved from the first batch of solar cells in a non-optimised processes.