Influence of substrates on the electrical properties of Cu(In,Ga)Se-2 thin films

Thin films based on Cu(In,Ga)Se-2 prepared on alkali free substrates are compared to films prepared on soda lime glass. On the latter, the presence of sodium species as detected with X-ray photoelectron spectroscopy is correlated with an enhanced formation of Se-O, In-O and Ga-O bonds at the surface after several days air exposure. The electrical conductivity is also one order of magnitude higher for films on soda lime glass. Solar cells prepared on these substrates exhibit increased open circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge width. The observations can be explained in terms of an increased net acceptor density in polycrystalline Cu(In,Ga)Se-2 when prepared on soda lime glass substrates. An alkali-metal-promoted oxidation of the surface is discussed.