Impact of Na and S incorporation on the electronic transport mechanisms of Cu(In,Ga)Se2 solar cells

The controlled incorporation of trace amounts of Na as well as of S into the absorber material of Cu(In, Ga)Se-2 solar cells leads to an increase of conversion efficiency. We investigate the influence of both ingredients on the electronic loss mechanisms of the cells by means of temperature dependent d.c. current-voltage measurements and admittance spectroscopy. We find that incorporation of Na gives rise to a distinct dielectric loss peak with activation energy of about 75 meV and to an increase of free carrier concentration in the absorber material. The benefit of S seems to originate from a passivation of deep trap states leading to a dramatic change in the electronic transport properties and, finally, to an enhancement of the open circuit voltage. (C) 1998 Elsevier Science Ltd. All rights reserved.