The chemical bath deposited CdS/Cu(In,Ga)Se2 interface as revealed by X-ray photoelectron spectroscopy

The highest efficiencies of Cu(In,Ga)Se-2 thin-film solar cells have been achieved when incorporating a CdS buffer layer grown by chemical bath deposition (CBD). To elucidate the specific properties of the CBD-CdS/Cu(In,Ga)Se-2 interface, device-quality Cu(In,Ga)Se-2/Mo/glass substrates were subjected to two different CBD baths [using different thiourea (Tu) concentrations] and examined by X-ray photoelectron spectroscopy (XPS). The untreated Cu(In,Ga)Se-2 surface was found to additionally contain a sodium compound (convincingly identified as Na2CO3) and native oxides. A short exposure to either of the CBD baths was sufficient to strongly reduce the amount of sodium carbonate and oxides as well as to deposit Cd at the interface. The two CBD baths differed in the efficiency of depositing sulfur and impurities at the interface: after a 0.5 min "standard" CBD bath, S and carbon-nitrogen (C=N) impurities were detected. In the "nonstandard" bath (using a much lower Tu concentration), S deposition was delayed and a C=N incorporation never occurred. The different properties of the two interfaces are discussed in relation to a recent model suggesting the importance of Cd doping to create a good Cu(In,Ga)Se-2 heterojunction. Furthermore, our XPS analysis could never establish any alloy formation [like Cd(In,Ga)(x)Se-y)] by ion exchange at the Cu(In,Ga)Se-2 surface. (C) 1999 The Electrochemical Society. S0013-4651(98)07-087-5. All rights reserved.