Optical and microstructural properties of TiO2 anti-reflection coatings deposited via in-line APCVD

Titanium oxide (TiO₂) was a commonly used anti-reflection coating (ARC) in the past, but has been displaced by silicon nitride (SiN_x) due to its ability to both passivate n+ emitters and provide the appropriate optical properties required for effective ARCs (refractive index, transparency, stability over time). However, with growing interest in n-type wafers, which feature p+ emitters, and the inability of SiN_x to provide effective field-effect passivation for p-type materials, other passivation materials are being actively investigated by the photovoltaics (PV) R&D community. Aluminum oxide (AlO_x) is one of such materials due to its ability to provide both low interface defect densities and sufficient negative fixed charge. Although AlO_x is transparent and stable, the refractive index isn´t ideally matched for a Si/air or Si/encapsulant interface. To circumvent the poor index matching, it has been shown that a multi-layer passivation/ARC stack is possible, allowing for both good passivation and ideal refractive index and sparking a renewed interest in TiO₂. The high quality passivation is obtained by using a thin passivation layer (<; 20 nm) like AlO_x, followed by a low cost optical material such as TiO₂ with a proper refractive index for an ARC. This paper addresses the optical and microstructural properties of TiO₂ films deposited at various temperatures using an in-line atmospheric chemical vapor deposition system (APCVD). The TiO₂ films are investigated by spectroscopic ellipsometry, reflectance spectroscopy and cross-sectional TEM. Additionally, experimental results for the ARC performance of multi-layer ARC structures featuring AlO_x/TiO₂ are presented.