Luminescent photoelectrochemical cells. 6. Spatial aspects of the photoluminescence and electroluminescence of cadmium selenide electrodes

Abstract

Samples of single-crystal, n-type CdSe emit when excited with ultraband gap excitation. The emission band (lambda/sub max/ approx. = 720 nm) is near the band gap of CdSe (approx. 1.7 eV); its energy, decay time, and temperature dependence are consistent with its description as edge emission. Photoluminescence (PL) spectra can be dependent on excitation wavelength and show evidence of self-absorption effects: PL spectra obtained with 457.9-nm excitation are broadened in the high-energy portion of the band relative to spectra obtained with more deeply penetrating 632.9-nm excitation. Measured PL efficiencies, theta/sub r/, are approx.10/sup -4/ in air at 295 K. When CdSe is used as the photoanode of photoelectrochemical cells employing aqueous polychalcogenide electrolytes, emission is quenched by the passage of photocurrent resulting from ultraband gap excitation. Electroluminescence (EL) can be observed from CdSe when the semiconductor is used as a dark cathode in aqueous, alkaline peroxydisulfate electrolyte. The EL spectrum is similar to the PL spectrum, suggesting the involvement of a common emissive excited state. Differences in the breadths of the spectra, however, indicate that, on average, EL is produced nearer to the semiconductor-electrolyte interface than PL under comparable experimental conditions. Measured EL efficiencies, theta/sub EL/, approaching 10/sup -3/ atmore » -1.50 V vs SCE are comparable to PL efficiencies measured at this potential in hydroxide solution and provide evidence that the emissive excited state can be efficiently populated in an EL experiment; these lower-limit estimates of theta/sub EL/ and theta/sub r/ decline in passing to potentials near the onset of EL, approx.-0.9 V vs SCE. Spatial features of these comparisons are discussed.« less