Tunneling currents in the copper sulfide/cadmium sulfide heterojunction

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Abstract
The parameters controlling the photovoltaic properties of the Cu2S/CdS heterojunction have been investigated. It is found that the behavior of the short-circuit current and the open-circuit voltage are describable in terms of a deep, donor-like level in the CdS region adjacent to the metallurgical interface. When tunneling from this level into the Cu2S is the mechanism controlling the current flow, theJ_{SC}-V_{OC}characteristics of the device are described by the equationJ_{SC} = J_{00} \exp (a_{i}(E_{I} - \Phi_{T}_{0})) {\exp (a_{i}q V_{OC})-1}where JSCis the short-circuit current, J00is the current preexponential factor,\Phi _{T}_{0}is the zero-bias barrier height in the CdS, EIis the ionization energy of the deep donor, VOCis the open-circuit voltage, and aiis a tunneling factor dependent on the net positive charge density in the CdS near the interface. The relative probability of tunneling from this level to the Cu2S is derived, as is the probability of tunneling from the level to the CdS conduction band. The photocapacitance effects observed in this junction are attributed to the joint action of this level and an acceptor state due to copper in the CdS. Combining the results from the tunneling calculation, theJ_{SC}-V_{OC}data, and the quenching spectra of the photocapacitance, the ionization energy of the donor level is determined to be 0.45 eV and the density of these levels exceeds 1019cm-3near the interface. The donor level acts as a recombination center, reducing JSC, and as a tunneling center, reducing VOC. Since these levels exist in junctions produced by the dipping method or by the dry method, they set fundamental limits to the efficiency of devices fabricated using these methods.

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