XPS surface analysis of monocrystalline silicon solar cells for manufacturing control

X-ray photoelectron spectroscopy (XPS) has been applied to surfaces of silicon wafers in the different stages of the assembly line for large-scale monocrystalline silicon solar cell manufacturing (ISOFOTON, Malaga, Spain). XPS results have shown that a considerable amount of carbon is present on the pyramidal-textured monocrystalline silicon surface. This amount decreases slightly but is still present after the process of phosphor diffusion (p-n junction), as well as after subsequent calcination in humid air for SiO2 film formation (passivation). This amount of carbon may be buried during the process of CVD coating an anti-reflection TiO2 film. After calcination of the film in order to obtain the TiO2 rutile phase, an even higher amount of carbon is detected on the TiO2 anti-reflection coating surface. This indicates that not all organics from the tetra-isopropile ortho-titanate (TPT) precursor were released from the film. Furthermore, in this case phosphor is found in excess on the SiO2 wafer surface (dead layer) and also on the rutile TiO2 surface, indicating that an extra phosphor diffusion from the bulk silicon through the TiO2 film has taken place during calcination. These results demonstrate how thermal treatments applied in the solar cell manufacturing assembly line can influence and may change the intended compositional distribution. These treatments may also introduce defects that act as recombination centres for charge carriers in the solar cell device.