EBIC investigations of junction activity and the role of oxygen in CdS/CuInSe₂ devices

EBIC characterization of CdS/CuInSe₂ device cross sections has been used to investigate junction activity as a function of post-deposition oxidation treatments. It was determined that the deposition of CdS onto single-crystal p-CuInSe₂ results in type conversion and the formation of a CuInSe₂ homojunction, rather than the expected heterojunction. Homojunctions have also been observed in as-deposited CdS/CuInSe₂ thin film devices. Post-deposition oxygen treatments of thin film devices serve to move the device junction nearer, if not up to, the heteroface and to largely remove both large (mm) and small (μm) scale spatial variations in the I_sc, thereby contributing to device performance improvement. This appears to occur by type converting the unintended CuInSe₂ n-layer via the elimination of deep level donor states. These processes are reversible by chemical reduction. They are quite temperature sensitive and are active even at room temperatures. Both hydrazine and an electron beam act to remove "loosely bound" oxygen and thereby prepare the system for a more optimal and stable incorporation of oxygen. The CuInSe₂ film stoichiometry appears to determine the concentration and kinds of defects. The resulting defect chemistry, in concert with oxygen, determines the electrical properties of the material.