INFLUENCE OF SUBSTRATES ON THE ELECTRICAL-PROPERTIES OF CU(IN,GA)SE-2THIN-FILMS

Thin films based on Cu(In,Ga)Se-2 prepared on alkali free substrates a re compared to films prepared on soda lime glass. On the latter, the p resence of sodium species as detected with X-ray photoelectron spectro scopy 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 c onductivity is also one order of magnitude higher for films on soda li me glass. Solar cells prepared on these substrates exhibit increased o pen circuit voltages. Capacitance-voltage characteristics on junctions prepared on alkali free substrates show an increased space charge wid th. The observations can be explained in terms of an increased net acc eptor density in polycrystalline Cu(In,Ga)Se-2 when prepared on soda l ime glass substrates. An alkali-metal-promoted oxidation of the surfac e is discussed.