Space-charge-limited currents in CIS-based solar cells

Abstract

Non-linear shunts in Cu(In,Ga)Se2 solar cells have been well described mathematically using the model of a space-charge-limited current, but their physical origin remained unclear so far. We study space-charge-limited currents on Cu-rich CuInSe2 (CIS) devices, which represent a very suitable system: the devices always exhibit non-linear shunts with a very pronounced behavior. Here, we demonstrate a fundamental difference in the transport mechanism between the Cu-rich-based device and the conventional Cu-poor one. We discuss the location of a space-charge-limited current by comparing devices containing various component layers with Ohmic contacts. We confirm that Cu-rich CIS and cadmium sulfide layers alone do not create a non-linear shunt. Our experimental results demonstrate that the origin of the non-linear behavior is located at the interface between the absorber and buffer layers. Temperature dependent current-voltage measurements performed on Cu-rich-based CIS devices are discussed in agreement with a space-charge-limited current theory suggesting the model of an insulator with traps.