Simulation study of open circuit voltage loss at Schottky top contact in ultra-shallow junction silicon solar cells

Efficient photovoltaic (PV) energy conversion depends on efficient light absorption and photo-carrier collection. In a PV device, photo-carrier concentration exponentially decreases from surface to bulk due to light absorption. Meanwhile, since a conventional PV device relies on the built-in electric field in the p-n junction to separate photo-carriers, naturally shallow p-n junction should benefit efficient carrier collection. However, shallow junction, especially ultra-shallow junction photovoltaic devices have to be well designed otherwise the expected performance cannot be achieved. Here we investigated open circuit voltage loss at the Schottky top contact of ultra-shallow junction PV devices using Silvaco semiconductor device simulation package. It was found that the property of top contact has significant effect on Voc especially for ultra-shallow junction devices. Particularly, diode I-V characteristics of ultra-shallow junction photovoltaic devices with flat band and Schottky top contacts in dark condition were simulated. And the latter case showed larger reverse saturation current resulting in Voc loss. This behavior is attributed to increased hole tunneling to top contact through n-type region for small junction depth, with existence of Schottky contact barrier for electrons. Furthermore, the simulation results clearly showed there is a critical junction depth for shallow p-n junction when it is applied for ultra-shallow junction PV solar cell, which may be generalized to provide a guideline for design of shallow junction photovoltaic device made of other materials.