Potential distribution of Cu(In,Ga)(S,Se)2-solar cell cross-sections measured by Kelvin probe force microscopy

Current high-efficiency chalcopyrite thin-film solar cells contain multilayer structures consisting of an absorber, a buffer and a window layer. The modeling and optimization of the structures have been hampered by the lack of characterization methods to assess the electrical potential and conduction band alignment in actual devices. In this work, Kelvin probe force microscopy (KPFM) under ultrahigh vacuum (UHV) conditions is used to directly image the electronic structure of a complete thin film solar cell based on Cu(In,Ga)(S,Se)(2) absorber material. The potential distribution along different solar cells is directly measured by KPFM on polished and UHV-cleaned cross-sections. Due to the high-energy sensitivity together with a lateral resolution in the nanometer range, detailed information about the various interfaces within the heterostructure is obtained. In combination with simulations of the tip-sample interaction, the work function of the different layers and the built-in voltage of the heterostructure are deduced. In our previous work, we have demonstrated that the use of a Zn1-xMgxO alloy instead of the i-ZnO layer influences the conduction band offset between chalcopyrite absorber and window layer. This substitution enabled us to improve the solar cell performance from eta=6.3% for the CdS-free solar cell with pure i-ZnO to eta=12.5% for the cell with Zn1-xMgxO, which is comparable to that of the reference cells with a US buffer (eta=13.2%). We present KPFM studies of comparable devices to illustrate the possibilities of our novel characterization method. The studies demonstrate that KPFM is an excellent tool for the characterization of heterostructures on a nanometer length scale. In chalcopyrite solar cells, the KPFM technique can lead to a direct correlation between the electronic structure and the solar cell performance. (c) 2004 Elsevier B.V. All rights reserved.