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 are multilayer structures consisting of an absorber, a buffer and a window layer. Modelling 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 lm 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 build in voltage of the heterostructure is 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, comparable to that of the reference cells with CdS 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 this can lead to a direct correlation between the electronic structure and the solar cell performanc