Effects of Na on the properties of Cu(In,Ga)S2 solar cells

We have prepared Cu(In,Ga)S2 solar cells with changing Na concentration using several preparation methods. We have investigated the hole density and solar cell properties. The hole density increases with increasing Na concentration up to 1017 cm− 3, and stays almost constant for higher concentrations. It can be considered that Na might passivate a single type of donor up to 1017 cm− 3. For all preparation methods, the efficiencies of solar cells prepared with the Cu(In,Ga)S2 thin films increase with increasing Na concentration. We propose a Na diffusion model derived from depth profiles obtained for each of the preparation methods.