Investigation of Polycrystalline Thin-Film CuInSe2 Solar Cells Based on ZnSe and ZnO Buffer Layers: Final Report, 16 February 1992 - 15 November 1995

The major objective was to determine the potential of ZnSe and ZnO buffer layers in solar cells based on CuInSe2 (CIS) and related alloys. Experimental studies were carried out with CIS and CuInGaSSe2 (CIGSS) substrates provided by Siemens Solar. ZnSe films were deposited by a chemical-vapor deposition (CVD) process that involved the reaction of a zinc adduct and H2SE. Al/ZnSe/CIS test cells wereused for process development. The optimum thickness for ZnSe buffer layers was found to be in the range of 150 to 200 .ANG. for Siemens CIS material, and between 80 and 120 .ANG. for the graded absorber material. If the buffer layers exceeded these values significantly, the short-circuit current would be reduced to zero. This effect is consistent with results reported in the literatureindicating a 0.9 eV band offset at the ZnSe-CIS interface. Completed cells were fabricated by using a low-resistance ZnO top-contact layer deposited by Siemens, and then depositing an Al/Ag collector grid at Washington State University. 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 structureresulted in either shunted or inflected I-V characteristics. Two approaches were investigated for depositing ZnO buffer layers: 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 .approxeq. 225-250 deg. C. These studies concentrated on Siemens gradedabsorber material (CIGSS). ZnO/CIS solar cells have been fabricated by first depositing a ZnO buffer layer, followed by a 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 cm2 that have total-area efficiencies > 11%. To date, the best performing ZnO/CIS cell was measured by NREL to have a total area, AM1.5Gefficiency of 11.3%. The active-area efficiency of the device was about 12%. In general, we find that ZnO buffer layers should have a resistivity > 1000 ohm-cm and have a thickness from 200-600 .ANG.. CIS cells studies with ZnO buffer layers grown by CBD also show promise. Finally, simulation studies were carried out using the one-dimensional code, PC-1D.