Influence of Na and S incorporation on the electronic transport properties of Cu(In,Ga)Se/sub 2/ solar cells

The controlled incorporation of Na as well as of S into the absorber material of the Cu(In,Ga)Se/sub 2/ solar cells leads to an increase of conversion efficiency. We investigate the influence of both ingredients on the electronic transport properties of the cells by means of quantum efficiency and temperature dependent current-voltage measurements, as well as by admittance spectroscopy. We find that the incorporation of Na leads to a shallow acceptor state at about 75 meV above the valence band, thus increasing the free carrier concentration in the absorber material. The benefit of S seems to originate from a passivation of deep trap states rather than from a widening of the band gap due to the formation of a Cu(In,Ga)(Se,S)/sub 2/ compound.