PHOTOELECTROCHEMICAL PROPERTIES OF ANODIC CADMIUM-SULFIDE FILMS

Photoelectrochemical and semiconductor properties of cadmium sulphide films grown on cadmium in alkaline sodium sulphide solutions were characterized by measurements of capacitances, photopotentials and photocurrents as functions of film thickness, wavelength and electrode potential. These properties were similar to those of CdS single crystals apart from effects of the disordered lattice. Donor densities were of the order 10(18) cm -3-10(19) cm -3 decreasing with film thickness. The photocurrents grew linearly with (hv - e(o)E(g))2 for wavelengths < 511(+/- 5) nm corresponding to a band gap energy of about e(o)E(g) = 2.42 eV in agreement with measurements of photopotentials. At > 511 nm an Urbach tail of the photocurrents was observed. The dependence of the photocurrents on the electrode potential was characteristic of the Poole-Frenkel effect. Illumination increased the current efficiencies for the dissolution of cadmium ions and to a minor degree of the oxidation of sulphide to polysulphide. Accordingly, the current efficiency for film growth was decreased. Photoeffects were not observed in the first 1.3 nm of the film adjacent to the metal due to a negative space charge layer.