Relative carrier densities and trap effects on the properties of CdS/CdTe

Abstract

It is shown that the open-circuit voltage of CdS/CdTe heterojunction solar cells is a maximum when the carrier densities in the CdS and CdTe are approximately equal, because of the effects of a level lying 0.45 eV below the conduction band in CdTe on the junction transport. Cells investigated include those prepared by vacuum evaporation of CdS on thin-film CdTe, deposited by hot-wall vacuum evaporation on graphite substrates and on single-crystal p-CdTe:P substrates, and on single-crystal p-CdTe:P substrates. The existence of a level in the CdTe lying 0.45 eV below the conduction band is identified by measurements of the extrinsic photoresponse of CdS/CdTe junctions, and a model using this level defining junction transport is consistent with the dark and light current versus voltage relationships observed. This model involves two recombination paths: (1) depletion-layer recombination in the CdTe through the 0.45-eV level, which dominates when the electron density in the CdS in much larger than the hole density in the CdTe, and (2) tunneling of electrons from the CdS conduction band to the 0.45-eV level in the CdTe, followed by recombination, which dominates in other cases.