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

This paper explores the potential of applying titanium dioxide (TiO₂) 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 (SiO₂) layer as the front surface dielectric coating. Upon commercialisation of the BC technology, BP Solar replaced this layer with silicon nitride (Si₃N₄), 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 TiO₂, thereby reducing the number of lengthy high temperature processing steps, and developing a process such that it can be applied to mc-Si wafers. TiO₂ 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 TiO₂ and demonstrate that TiO₂/SiO₂ stacks, achieved during a brief high-temperature oxidation process after TiO₂ thin film deposition, are compatible with high-efficiency solar cells. However, TiO₂ cannot perform all the necessary functions of the thick SiO₂ or Si₃N₄ 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.