Intra-band Gap Research Articles

Luminescent photoelectrochemical cells. 5. Multiple emission from tellurium-doped cadmium sulfide photoelectrodes and implications regarding excited-state communication

Samples of single-crystal, n-type, 100 ppm Te-doped CdS (CdS:Te) exhibit two emission bands when excited at several ultraband gap wavelengths. One emission band with lambda sub max approximates 510 nm is near the band gap of CdS:Te (approximates 2.4 eV) and is likely edge emission. Its decay time, measured for samples immersed in sulfide solution between 295 and 333 K, is faster than the pulse from the N2-pumped dye laser (approximates 7 nsec) used for 458-nm excitation. The other emissive transition, lambda sub max approximates 600 nm, involves intraband gap states introduced by Te. Intensity-time curves for this band have rise times of less than 10 nsec and generally exhibit nonexponential decay; values of tau sub 1/e are typically 120 + or - 40 and 80 + or - 30 nsec at 295 and 333 K, respectively. When CdS:Te is used as the photoanode in a photoelectrochemical cell employing sulfide electrolyte, both emission bands are quenched in parallel during passage of photocurrent resulting from 457.9- or 476.5-nm Ar ion laser excitation. Parallel quenching is observed at several temperatures for which the excited-state kinetic schemes are demonstrably different: open-circuit emission spectra reveal that both the absolute and relative intensities of themore » two emission bands are affected by temperature. With increasing temperature (295-333 K) the absolute intensities of both bands decline; a semilog plot of the ratio of 510- to 600-nm intensity vs. T to the -1 is linear with a slope of approximates 0.2 eV, corresponding to the energy gap between the two excited states. The existence of excited-state communication is inferred from the effects of temperature and electrode potential on multiple emission.« less

ChemInform Abstract: LUMINESCENT PHOTOELECTROCHEMICAL CELLS. 4. ELECTROLUMINESCENT PROPERTIES OF UNDOPED AND TELLURIUM‐DOPED CADMIUM SULFIDE ELECTRODES

AbstractEinkristall‐Elektroden aus undotiertem und aus Te‐dotiertem (100 und 1000 ppm) CdS zeigen in wäßrigen alkalischen S2Oä′‐Lösungen Elektrolumineszenz.

Luminescent photoelectrochemical cells. 4. Electroluminescent properties of undoped and tellurium-doped cadmium sulfide electrodes

Single-crystal, n-type, undoped CdS and Te-doped CdS (100 and 1000 ppM CdS:Te) electrodes exhibit electroluminescence (EL) in aqueous alkaline peroxydisulfate electrolyte. Addition of Te to CdS introduces intraband gap states which drastically alter the EL spectral distribution. The EL spectrum of undoped CdS consists of a sharp band near the band gap energy (approx. 2.4 eV) with lambda/sub max/ approx. 510 nm and a weaker, broader band at lower energy; the EL spectra of 100 and 1000 ppM CdS:Te also exhibit a sharp band at approx. 510 nm but are dominated by broad bands with uncorrected lambda/sub max/ approx. 600 and approx. 620 nm, respectively. Photoluminescence (PL) spectra, obtained with 457.9-nm excitation, aid in the assignments of the emissive transitions; PL spectra are generally similar to their EL counterparts. The potential dependence of the EL spectra was examined between approx. -1.2 V (onset) and -2.0 V vs. SCE. Changes in potential can affect both the relative and absolute intensities of the bands present in the EL spectra. Lower-limit, order-of-magnitude estimates of instantaneous EL efficiency, phi/sub EL/, have been made. Under steady-state conditions phi/sub EL/ is greater than or equal to 10/sup -6/ for undoped CdS and greater than or equalmore » to 10/sup -5/ for CdS:Te. Results are discussed in terms of interfacial charge-transfer processes and the excited-state manifolds of CdS and CdS:Te.« less