a-Si is commonly considered as a primary candidate for the formation of passivation layer in heterojunction (HIT) solar cells. However, there are some problems when using this material such as significant losses due to recombination and parasitic absorption. To reduce these problems, a wide bandgap material is needed. A wide bandgap has a positive influence on effective transmittance, reduction of the parasitic absorption, and prevention of unnecessary epitaxial growth. In this paper, the adoption of a-SiO_x:H as the intrinsic layer was discussed. To increase lifetime and conductivity, oxygen concentration control is crucial because it is correlated with the thickness, bonding defect, interface density (D_it), and band offset. A thick oxygenrich layer causes the lifetime and the implied open-circuit voltage to drop. Furthermore the thicker the layer gets, the more free hydrogen atoms are etched in thin films, which worsens the passivation quality and the efficiency of solar cells. Previous studies revealed that the lifetime and the implied voltage decreased when the a-SiOx thickness went beyond around 9 nm. In addition to this, oxygen acted as a defect in the intrinsic layer. The D_it increased up to an oxygen rate on the order of 8%. Beyond 8%, the D_it was constant. By controlling the oxygen concentration properly and achieving a thin layer, high-efficiency HIT solar cells can be fabricated.

Highly photosensitive and wide bandgap amorphous silicon oxide (a-SiO_x:H) films were developed at low temperature ranges (100~150℃) with employing plasma-enhanced chemical vapor deposition by optimizing H₂/SiH₄ gas ratio and CO₂ flow. Photosensitivity more than 10⁵ and wide bandgap (1.81~1.85 eV) properties were used for making the a-SiO_x:H thin film solar cells, which exhibited a high open circuit voltage of 0.987 V at the substrate temperature of 100℃. In addition, a power conversion efficiency of 6.87% for the cell could be improved up to 7.77% by employing a new n-type nc-SiO_x:H/ZnO:Al/Ag triple back-reflector that offers better short circuit currents in the thin film photovoltaic devices.