Photoluminescence and electroluminescence in graded cadmium sulfoselenide electrodes: Applications to photoelectrochemical cells

Abstract

Inhomogeneous samples of n-type CdS x Se 1− x (0 ≤ x < 1) were prepared by vapor-phase diffusion of S into a single-crystal CdSe substrate. Characterization of the samples by Auger electron spectroscopy (AES) Ar ion sputter etching reveals that S has substituted for Se in the lattice to produce a graded region: The depth profile analysis indicates that from a composition with x nearly unity at the surface, x monotonically declines to zero over a distance of ∼1 μm. Correspondingly, the band gap energy diminishes from ∼2.4 eV for the CdS-like composition to ∼1.7 eV for CdSe. Photoluminescence (PL) and electroluminescence (EL) from the graded material appear to derive from the luminescence of the CdS x Se 1− x compositions which make up the graded region: Emission from ∼500–750 nm matches the spectral region spanned by PL and EL from homogeneous, single-crystal CdS x Se 1− x samples which emit near their band gap energies. A previously established linear correlation between emission maxima (nm) and composition in homogeneous CdS x Se 1− x samples provides a spatial probe of electronhole ( e −- h +) pair recombination in the inhomogeneous material: Regions from which PL and EL originate can be inferred from their spectral distribution in combination with the AES/depth profile data. PL spectra are thus shown to be dependent on excitation wavelength in a manner consistent with relative optical penetration depth. EL spectra are potential dependent and provide evidence that increasingly cathodic potentials shift the spatial origin of EL, on average, nearer to the semiconductor surface. The inhomogeneous samples can be used as photoanodes of photoelectrochemical cells employing aqueous (poly)sulfide electrolyte. Photoaction spectra exhibit their principal onset at ∼560 nm, indicating that the S-rich, near-surface region is primarily responsible for photocurrent. This spatial origin of photocurrent is also reflected in the nonuniform quenching of PL accompanying passage of photocurrent from 457.9-nm excitation. With certain assumptions, these quenching properties provide a crude map relating the efficiency of e −- h + pair separation to distance from the semiconductor-electrolyte interface; the correlation indicates that negligible separation occurs beyond ∼0.2 μm. Comparisons of these PL and EL properties with those of related graded materials and with homogeneous CdS x Se 1− x samples are discussed.