High temperature processing of TiO2 thin films for application in silicon solar cells

Low temperature deposited coatings have the potential to reduce the production costs of silicon (Si) photovoltaic devices, such as the buried-contact (BC) solar cell. This work investigates the potential of titanium dioxide (TiO2) films to replace silicon dioxide (SiO2) or nitride coatings currently implemented in the BC fabrication sequence, which requires films to be stable at temperatures up to 1000 °C in a variety of gas atmospheres. The authors demonstrate that: (i) TiO2 films do not reduce the bulk minority carrier lifetime of the silicon wafer after lengthy high temperature processing, however chemical reduction of the TiO2 film can occur if samples are not loaded in an oxygen-containing ambient; (ii) a thin SiO2 passivation layer can be formed at the TiO2:Si interface by performing a brief oxidation after TiO2 film deposition; (iii) while TiO2 coatings function as a phosphorus diffusion barrier, reactions between TiO2 and the phosphorus source result in irreversible damage to the TiO2 film; (iv) phosphorus-doped TiO2 films can act as an n-type dopant source, however further reactions with phosphorus limit the usefulness of the film. Thus, TiO2 films are compatible with the high temperature processing required for BC solar cells, but not with any phosphorus diffusion steps.