Auger, ellipsometry, and environmental studies of thin films applied to schottky (MIS) solar cells

Schottky (MIS) solar cells typically consist of the structure 5Å Cr/50Å Cu/40Å Cr/20Å oxide/silicon. A study of metal diffusion and precise oxide thickness is important to predict the stability of such a device. Devices fabricated using the same process are shown to have almost identical electronic performance. Auger profiles show that the metal films do not penetrate through adjacent regions except when Ag is used in place of Cu. Ag does penetrate through the Cr and into the Si which alters the electronic properties. Ellipsometer studies show the insulator to range in thickness from 20Å to 28Å which may be controlled by variation of the heat-treatment cycle. Environmental studies of encapsulated solar cells show that polystyrene offers more protection than Sylgard l84. Failures due to dirt coatings, discoloration, thermal stresses, poor wire bonds, voltage degradation, and fill factor degradation have been observed. Properly fabricated and encapsulated cells have performed well for more than a year. A hermetic encapsulation may be necessary for future long term stability.