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The understanding of lattice instabilities is of vast importance in material science. The famous example is the Peierls instability of one-dimensional metals and for strongly-nested Fermi surfaces in two and three dimensions. Through an analysis of H and Li chains in band theory, we find that the Bloch wave nature of the wavefunctions, if involving strong k-dependent hybridization of oppositeparity atomic states, strongly suppresses susceptibility peaks and associated instabilities and is thus essential to consider in searching for materials with strong responses to external perturbations., Comment: Corrected acknowledgements

The photocorrosion of n‐Cd(Se,Te) electrodes, in potassium and cesium polysulfide solutions, is investigated by x‐ray photoelectron spectroscopy and Auger electron spectrometry. It is shown that Se and Te are exchanged with sulfur from the solution during photocorrosion, possibly via two separate mechanisms: (a) preferential bleaching of tellurium from the crystal matrix, and (b) uniform photocorrosion of the crystal as a whole. Photocurrent spectrum measurements which are presented suggest that a graded band gap is formed when tellurium is preferentially bleached out of the original Cd(Se,Te) crystal. It is shown that cesium is present on the electrode surface which may explain the negative shift of the flat band potential, i.e., increased open circuit voltage of the photoelectrochemical cell. This suggests that the cesium treatment could be used to increase the open circuit voltage of photovoltaic cells.

The photoelectrochemistry of the spinel phase CuIn5S8 in polysulfide electrolyte was studied. A sequence of annealing in S and in vacuum was found to give optimal material for this purpose. The performance of such material could be improved by acid etching and photoelectrochemical etching; both of these treatments deplete the top layer of Cu. Notwithstanding obvious kinetic limitations in polysulfide, the title system shows a negative temperature dependence. The output stability which is inferior to that of n-CuInS2 systems in polysulfide, is limited by increasing dark currents. These may be due to the formation of CuS on the surface. While, for all examples studied, the indirect bandgap around 1.3 eV was observed, the direct gap around 1.5 eV was seen clearly only for photoelectrochemically etched electrodes. It is suggested that the poor photovoltaic performance and mediocre output stability behaviour of CuIn5S8 photoanodes is related to the inability of the material to tolerate significant Cu depletion and to the absence of a homogeneous indium oxide surface layer. This latter finding may be connected with the fact that most of the indium is octahedrally, rather than tetrahedrally coordinated, in contrast to what is the case for the chalcopyrite type disulfide.

Etude des proprietes photoelectrochimiques de CdIn 2 Se 4 . Etude de l'augmentation du photocourant dans differents electrolytes par attaque photoelectrochimique. Etude de la degradation du photocourant dans les polysulfures due a la photocorrosion

Analyse de surface par spectrometrie photoelectronique RX et par spectrometrie electronique Auger de couches minces de n-CuInS 2 et n-CuInSe 2 apres leur utilisation comme photoanodes dans des solutions de polysulfure