Passivation properties of tunnel oxide layer in passivated contact silicon solar cells

Abstract

Passivated contact in advanced high-efficiency silicon solar cells based on the full back surface field (BSF) is reported here in based on the application of a tunnel oxide layer that is less than 2nm thick. The open-circuit voltage (Voc) was significantly improved via interface passivation due to insertion of the tunnel oxide layer. During oxide layer growth, a transition region, such as a sub-oxide, was observed at a depth of about 0.75nm in the growth interface between the silicon oxide layer and silicon substrate. The properties of the less than 2nm thick tunnel oxide layer were primarily affected by the characteristics of the transition region. The passivation characteristics of tunnel oxide layer should depend on the physical properties of the oxide. The interface trap density Dit is an important parameter in passivation and is influenced by the stoichiometry of the oxide which in turn strongly affected by the fabrication and the post annealing conditions. During heat treatment of a-Si:H thin films (for the purpose of crystallization to form doped layers), thin film blistering occurs due to hydrogen effusion on flat substrate surfaces. To minimize this behavior, we seek to control the surface morphology and annealing profile. Also, the passivation quality of passivataed contact structure declined for the sample annealed above 900°C. This decline was attributed not only to local disruption of the tunnel oxide layer, but also to phosphorus diffusion. The resistivity of the tunnel oxide layer declined precipitously for the sample annealed above 900°C. On the basis of these, implied Voc over 740mV was achieved in n-type Si wafer through the control of the oxide stoichiometry via optimizing the annealing conditions.