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

摘要

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.