Alternative Window Schemes for CuInSe2-Based Solar Cells: Annual Report, 1 November 1995 - 31 October 1996

The objectives of this program are to develop alternate heterojunction partner layers (buffer layers) for high-efficiency CuInSe2-based thin-film solar cells, and improve the understanding of how these layers and the details of processing affect cell performance. Investigations have primarily involved three tasks: (1) metal-organic chemical vapor deposition (MOCVD) growth of noncadmium-containingbuffer layers, (2) optimized processing of buffer layers for high-efficiency solar cells, and (3) electrical and physical characterization of layers and devices. Investigations of alternate buffer layers emphasized studies of ZnO grown by MOCVD. Using CIS substrates obtained from Siemens for process development, researchers determined that growth procedures yielding good results with Siemens CIS(nonsulfur-containing material) substrates also worked well with NREL CIGS material. A two-step process was developed for growing highly resistive ZnO buffer layers (i-ZnO). In particular, after growing 100 to 150 of ZnO at 250 C, an additional 600 to 800 were grown at 100 C. Collaboration with NREL resulted in a n-ZnO/i-ZnO/CIGS cell with a total-area efficiency of 12.7%, and an active-areaefficiency greater than 13%. After growing i-ZnO with the two-step process on NREL CIGS material, the i-ZnO/CIGS film structure was sent to NREL for deposition of a transparent conducting oxide (TCO), namely, conducting ZnO (n-ZnO). Collector grids and a MgF2 antireflective coating were also deposited at NREL. Low-level efforts were devoted to studies of ZnSe and InSe buffer layers. A total-areaefficiency of 9.5% was achieved for a completed ZnSe/CIS cell making use of a RF-sputtered ZnO for a TCO. Investigations of InxSey (InSe) buffer layers also began this past year. InSe films were grown by reacting H2Se and ethyldimethylindium with substrate temperatures in the range of 300 to 400 C. Test cells showed promise, suggesting further work is warranted if time permits. Spectroscopicellipsometry was used to investigate the surface layer of CIS substrates and ZnO buffer layers. Measurements and analyses indicated that a surface layer about 250 thick and characterized by interband transitions of 1.15 eV and 1.26 eV exists on Siemens surfaces. Finally, simulation studies carried out for ZnO/CIS cells with PC-1D indicate that the low efficiencies experienced for structures withlow-resistivity ZnO buffer layers may be a result of enhanced depletion-region recombination caused by the ZnO deposition process.