Influence of Na on the Electro-Optical Properties of Cu(In,Ga)Se2

Polycrystalline Cu(In,Ga)Se2 (CIGS) thin films, grown by coevaporation of the constituent elements with different amounts of sodium (Na), were investigated. In some devices, an increased Na content was achieved through the incorporation of a thin layer of NaF deposited on the substrate prior to the growth of CIGS. The effect of Na on the electro-optical properties was addressed through characterization of the finished devices using photoluminescence (PL) and capacitance techniques. Results indicate the beneficial effect of Na as evidenced by increases in the device efficiency, open-circuit voltage, and PL intensity. Furthermore, these measurements provide evidence that Na 1) increases the net carrier concentration, and 2) reduces the number of gap states including those that act as minority-carrier traps.