Tunneling currents in the copper sulfide/cadmium sulfide heterojunction

The parameters controlling the photovoltaic properties of the Cu
2
S/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 Cu
2
S is the mechanism controlling the current flow, the
J_{SC}-V_{OC}
characteristics of the device are described by the equation
J_{SC} = J_{00} \exp (a_{i}(E_{I} - \Phi_{T}_{0})) {\exp (a_{i}q V_{OC})-1}
where J
SC
is the short-circuit current, J
00
is the current preexponential factor,
\Phi _{T}_{0}
is the zero-bias barrier height in the CdS, E
I
is the ionization energy of the deep donor, V
OC
is the open-circuit voltage, and a
i
is 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 Cu
2
S 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, the
J_{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 10
¹⁹
cm
^-3
near the interface. The donor level acts as a recombination center, reducing J
SC
, and as a tunneling center, reducing V
OC
. 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.