Effect of plasma exposure on the performance of indium tin oxide/InP solar cells prepared by radio-frequency and ion-beam sputtering

Abstract

Previous work has shown that the performance of p-type single crystal/n-type thin film indium tin oxide (ITO) solar cells, can be enhanced by exposing the substrates (InP) to a very low power rf plasma prior to deposition of the ITO and by depositing the ITO very slowly. The mechanism underlying this enhancement is the subject of this experimental investigation. The principal variables studied were the duration of the low power plasma exposure (LPPE), the doping density of the substrate, and the methods of depositing the ITO. Comparison of the cells has been made on the basis of their current/voltage characteristics and their quantum efficiency. The analysis is consistent with there being a thin n-type layer at the interface between the InP and the ITO. For the ion-beam deposited ITO, the n-type layer is vanishingly small but for the rf deposited cells it is 70–100 Å thick; the thickness reducing with longer LPPE. The thickness of this layer appears to control the device performance; the cells with thicker layers have much larger open-circuit voltage and somewhat lower currents, whereas the reverse is true for thinner layers.