Alternative Heterojunction Partners for CIS-Based Solar Cells: Annual Subcontract Report, 29 December 1997 - 28 December 1998

The focus of the Phase I effort concerned further development of ZnO buffer layers. This work included further optimization of the metal-organic chemical vapor deposition (MOCVD) growth process and investigations of the interaction of zinc and oxygen with the absorber layers. Although much of our work had been done with Siemens' CIS material prior to this reporting period, a process for growingZnO buffer layers on Siemens' CIGSS absorber had not been developed. We determined that a two-step procedure involving raising the substrate temperature to 250?C in nitrogen and then growing the buffer layer at 100?C works well with CIGSS material. Through collaboration with the Institute of Energy Conversion (IEC), completed cells with efficiencies in the 11% to 12% range were fabricated withthe following structure: RF n-ZnO/i-ZnO/CIGSS. Cells with this structure were included as part of the Transient team studies. Cells were subjected to dark storage at 80?C, followed by a light soak at 40?C at IEC. Illuminated I-V curves taken at each stage of the study determined that these cells do not degrade under dark-storage conditions, which had been observed for Siemens cells with CdSbuffer layers. To understand the reaction of zinc and oxygen with the absorber layers, secondary ion mass spectroscopy (SIMS) depth concentration profiles were obtained for i-ZnO/CIS structures through collaboration with Angus Rockett at the University of Illinois. SIMS profiles were obtained for ZnO films grown on polycrystalline CIS and epitaxial CIS films grown on GaAs. Comparison of theprofiles strongly suggests that zinc and oxygen diffuse into the CIS along grain boundaries during the MOCVD growth process. It is also proposed that excess zinc along grain boundaries may result in the grain boundaries being n-type, which can result in enhanced loss currents. This model is consistent with the apparent requirement that cell structures with MOCVD buffer layers must undergo anaging process in air before efficient cells can be obtained. Future studies will investigate processes that allow the aging step to be eliminated.