SiO2 as barrier layer for Na out-diffusion from soda-lime glass

To reduce the manufacturing cost, soda-lime glass (SLG) is often used as the substrate for thin-film solar cells. It is known that SLG contains about 16 wt% Na in the form of Na₂O. During the growth of thin films, Na ions may diffuse out from SLG and diffuse into the grown thin films and affect the optoelectronic properties of the thin films. The widely used commercial F-doped Sn₂O (FTO) glass (Teck 15) has the structure SLG/FTO/SiO₂/FTO. The first FTO layer and the SiO₂ layer are very thin. The SiO₂ layer is often expected to be a barrier layer for Na out-diffusion from SLG. However, our transmission electron microscopy (TEM) study indicates that there is a significant amount of Na existing in the second FTO layer, further indicating that SiO₂ may not be a good barrier layer for Na out-diffusion. Therefore, we have investigated the possibility of a SiO₂ layer as a barrier layer for Na out-diffusion from SLG. We have deposited two types of samples, SLG/SiO₂/FTO and SLG/FTO, at 550°C. For comparison, these samples were also annealed at temperatures between 550° and 650°C. The as-deposited and annealed samples were examined by TEM, X-ray energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS). We found that SiO₂ is indeed an effective barrier layer for Na out-diffusion. Thermal annealing can enhance Na diffusion. It should be pointed out that our deposited SiO₂ layer may behave differently from the SiO₂ layer seen in the commercial FTO-coated glass.