Investigation of polycrystalline thin-film CuInSe(sub 2) solar cells based on ZnSe and ZnO buffer layers. Final report, February 16, 1992--November 15, 1995

摘要

The major objective of this program was to determine the potential of ZnSe and ZnO buffer layers in solar cells based on CuInSe(sub 2) and related alloys. Experimental studies were carried out with CIS and CIGSS substrates. ZnSe films were deposited by a CVD process which involved the reaction of a zinc adduct and H(sub 2)Se. Al/ZnSe/CIS test cells were used for process development. Test cell performance aided in determining the optimum thickness for ZnSe buffer layers to be in the range of 150 (angstrom) to 200 (angstrom) for Siemens CIS material, and between 80 (angstrom) and 120 (angstrom) for the graded absorber material. If the buffer layers exceeded these values significantly, the short-circuit current would be reduced to zero. The best efficiency achieved for a ZnSe/CIS cell was an active area value of 9.2%. In general, deposition of a conductive ZnO film on top of a ZnSe/CIS structure resulted in either shunted or inflected I-V characteristics. Two approaches were investigated for depositing ZnO buffer layers, namely, chemical bath deposition and CVD. CVD ZnO buffer layers are grown by reacting a zinc adduct with tetrahydrofuran. Best results were obtained for ZnO buffer layers grown with a substrate temperature ca. 225--250 C. These studies concentrated on Siemens graded absorber material (CIGSS). ZnO/CIS solar cells have been fabricated by first depositing a ZnO buffer layer, followed by deposition of a low resistivity ZnO top contact layer and an Al/Ag collector grid. Several cells were fabricated with an area of 0.44 cm(sup 2) that have total area efficiencies greater than 11%. To date, the best performing ZnO/CIS cell had a total area efficiency of 11.3%. In general, the authors find that ZnO buffer layers should have a resistivity > 1,000 ohm-cm and have a thickness from 200 (angstrom) to 600 (angstrom). CIS cells studies with ZnO buffer layers grown by CBD also show promise. Finally, simulation studies were carried out using the 1-D code, PC-1D.