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A polycrystalline, thin-film CuxS/CdS solar cell was exposed to air saturated with water vapor at temperatures between 27 and 67 °C for up to 6 1/2 h. The short-circuit current decreased progressively from 11.7 to 1.02 mA/cm2. Subsequent heating in hydrogen at 150 °C for 680 h and in 170 °C air for 5 h restored the short-circuit current to 14.4 mA/cm2. Modeling of the measured quantum yield indicates that the degradation could be quantitatively explained by two effects: (1) the CuxS minority carrier electron diffusion length decreasing from 0.23 to 0.02 μm (±20%) and (2) the CuxS optical band gap increasing from 1.16 to 1.46 eV (±3%). The recovery was quantitatively modeled by the CuxS diffusion length increasing back to 0.24 μm and the CuxS band gap returning to 1.16 eV. A Burstein–Moss analysis shows the band-gap shift is due to the Fermi level penetration of the valence band as the measured hole concentration increased from 1.03(1020) cm−3 to 4.62(1021) cm−3 during degradation. A new band structure is proposed with six equivalent, indirect, valence band maxima located at 1.16 eV (±3%) below the conduction band edge followed by two direct maxima located at 1.28 and 1.8 eV (±3%) below the conduction band edge. A density-of-states effective mass ratio for holes of 2.0 (±30%) is found. The sharp decrease and recovery in diffusion length is explained by a transition between phonon-assisted and direct minority carrier recombination with changes in CuxS hole concentration.

The behaviour of subaerial particle-laden gravity currents (e.g. pyroclastic ows, lahars, debris¯ows, sediment-bearing¯oods and jo È kulhlaups)¯owing into the sea has been simulated with analogue experiments. Flows of either saline solution, simple suspensions of silicon carbide (SiC) in water or complex suspensions of SiC and plastic particles in methanol were released down a slope into a tank of water. The excess momentum between subaerial and subaqueous¯ow is dissipated by a surface wave. At relatively low density contrasts between the tank water and the saline or simple suspensions, the¯ow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water. The cloud then collapses gravitationally to form an underwater gravity current, which progresses along the tank¯oor. At higher density contrasts, the subaerial¯ow develops directly into a subaqueous¯ow. The¯ow slows and thickens in response to the reduced density contrast, which is driving motion, and then continues in the typical gravity current manner. Complex suspensions become dense¯ows along the tank¯oor or buoyant¯ows along the water surface, if the mixtures are suf®ciently denser or lighter than water respectively. Flows of initially intermediate density are strongly in¯uenced by the internal strati®cation of the subaerial¯ow. Material from the particulate-depleted upper sections of the subaerial¯ow becomes a buoyant gravity current along the water surface, whereas material from the particulateenriched lower sections forms a dense¯ow along the tank¯oor. Sedimentation from the dense¯ow results in a reduction in bulk density until the mixture attains buoyancy, lifts off and becomes a secondary buoyant¯ow along the water surface. Jo È kulhlaups, lahars and debris¯ows are typically much denser than seawater and, thus, will usually form dense¯ows along the seabed. After suf®cient sedimentation, the freshwater particulate mixture can lift off to form a buoyant¯ow at the sea surface, leading to a decoupling of the ®ne and coarse particles. Flood waters with low particulate concentrations (<2%) may form buoyant¯ows immediately upon entering the ocean. Subaerial pyroclastic ows develop a pronounced internal strati®cation during subaerial run-out and, thus, a¯ow-splitting behaviour is probable, which agrees with evidence for sea surface and underwater¯ows from historic eruptions of Krakatau and Mont Pele Âe. A pyroclastic¯ow with a bulk density closer to that of sea water may form a turbulent cloud, resulting in the deposition of much of the pyroclasts close to the shore. Dense subaqueous pyroclastic¯ows will Sedimentology (1999) 46, 523±536 Ó 1999 International Association of Sedimentologists 523 eventually lift off and form secondary buoyant¯ows, either before or after the transformation to a water-supported nature.

X-ray photoelectron spectroscopy spectra measured on copper sulfide (CuxS) films showed that a thin surface reaction product containing Cu in the +2 valence state was formed on CuxS films exposed to air for 46 h at 40 °C and 90% relative humidity. An entirely different CuxS surface reaction product layer was formed in dry air at 170 °C for 30 min and it contained sulfur in the +6 valence state. The copper (Cu) valence state in CuxS was not found to be +2 even when the x value was less than 1.9. When the argon sputter-cleaned surface of CuxS or CuxS/CdS films was exposed to room-temperature air for 10 min, cadmium (Cd) atoms appeared on the CuxS surface. X-ray powder diffraction patterns showed that CuO and CdS reacted at 500 °C in flowing nitrogen to form Cu2S and CdO. This cation exchange between CdS and copper oxide may explain the surface Cd on the CuxS films. The standard free energy of reaction between CuO and CdS is positive while that between Cu2O and CdS is negative. These results indicate a method for stabilizing CuxS/CdS solar cells against degradation.

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