Holger H. Streckert

LUMINESCENT PHOTOELECTROCHEMICAL CELLS - 7. PHOTOLUMINESCENT AND ELECTROLUMINESCENT PROPERTIES OF CADMIUM SULFO-SELENIDE ELECTRODES.

Photoelectrochemical cells (PECs) are being widely studied as devices for optical energy conversion. The excited-state properties of the semiconductors around which PECs are constructed are crucial to efficient energy conversion. We have employed luminescence as a probe of these excited-state-properties, generally using materials such as n-type CdS:Te(Te-doped CdS) which exhibit subband gap emission. Recently we examined emission of band gap energy from n-type CdS and CdSe, two materials which have been used extensively in PEC studies. Since these two compounds form solid solutions over the entire composition range, the mixed compounds represent a natural extension of our emissive studies. We report herein that luminescence from samples of n-type, single-crystal CdSXSe1-X can be used to probe interfacial charge-transfer events relevant to PECs. Specifically, we demonstrate that photoluminescence (PL) can be perturbed and electroluminescence (EL) initiated by interfacial charge-transfer processes.

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

Inhomogeneous samples of n-type CdSxSe1−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 CdSxSe1−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 CdSxSe1−x samples which emit near their band gap energies. A previously established linear correlation between emission maxima (nm) and composition in homogeneous CdSxSe1−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 CdSxSe1−x samples are discussed.

Luminescent tellurium-doped cadmium sulphide electrodes as probes of semiconductor excited-state deactivation processes in photoelectrochemical cells

Correlations among quantum efficiencies for photocurrent (φx), emission (φr), and non-radiative recombination (φnr) are discussed with reference to data from single-crystal, n-type, 100 and 1000 p.p.m. CdS: Te-based photoelectrochemical cells (PECs) employing aqueous sulphide electrolyte. These materials emit while they serve as PEC electrodes. The assumption that the proportionality of φr to φnr is unaffected by potential, leads to a simple expression relating φx to φr for monochromatic excitation. Calculated and observed emission data are in reasonable agreement; sources of deviation are discussed. Polychromatic excitation is shown to yield photocurrents and emission intensities which are approximately a weighted average of the values obtained with the constituent monochromatic frequencies. Practical implications of the φx correlation with φr are described, as are related results from other PECs.