Bulk p-CuInSe2 photo-electrochemical solar cells

Dense CuInSe₂ of high quality, prepared by the fusion technique in evacuated quartz ampoule from stoichiometric melt, crystallizes in the chalcopyrite structure. Compositional analysis carried out by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy (EDS) indicates a uniform distribution of elements through the depth and a composition close to the stoichiometry. The diffuse reflectance spectrum gives a band gap at 0.94 eV. The electrical conductivity follows an Arrhenius-type law with activation energy of 23 meV in conformity with polarons hopping. Above 320 °C, CuInSe₂ undergoes an irreversible oxidation. The thermal variation of the thermopower indicates p-type behavior attributed to copper deficiency and a hole mobility μ_300 K of 0.133 cm² V⁻¹ s⁻¹, thermally activated. In KCl media, the compound exhibits an excellent chemical stability with a corrosion rate of 8 μmol cm⁻² month⁻¹. The photo-electrochemical properties, investigated for the first time on the ingots, confirm the p-type conductivity. From the capacitance measurements, the flat band potential (V_fb=−0.62V_SCE) and the holes density (N_A=4×10¹⁷ cm⁻³) were determined. The valence band, located at 4.43 eV below vacuum, is made up of mainly Se orbital with little admixture of Cu character. The change of the electrolyte causes a variation in the potential V_fb (dV_fb/dpH=−0.058 V pH⁻¹) indicating strong OH⁻ adsorption. The fill factor in S²⁻ media was found to be 0.54; such result was corroborated by semi-logarithmic plots.     

When the Locus Coeruleus Speaks Up in Sleep: Recent Insights,Emerging Perspectives

For decades, numerous seminal studies have built our understanding of the locus coeruleus (LC), the vertebrate brain’s principal noradrenergic system. Containing a numerically small but broadly efferent cell population, the LC provides brain-wide noradrenergic modulation that optimizes network function in the context of attentive and flexible interaction with the sensory environment. This review turns attention to the LC’s roles during sleep. We show that these roles go beyond down-scaled versions of the ones in wakefulness. Novel dynamic assessments of noradrenaline signaling and LC activity uncover a rich diversity of activity patterns that establish the LC as an integral portion of sleep regulation and function. The LC could be involved in beneficial functions for the sleeping brain, and even minute alterations in its functionality may prove quintessential in sleep disorders.     

First principles calculations on elasticity, electronic structure and bonding properties of antiperovskites ANTi3 (A = Al, In and Tl)

We use an ab initio pseudopotential plane wave (PP-PW) method within the generalized gradient approximation (GGA) and the local density approximation (LDA) to study the structural, elastic and electronic properties of the unexplored antiperovskite ANTi₃ compounds. The elastic constants C₁₁, C₁₂, C₄₄ and their pressure dependence are calculated. We derived the bulk, shear and Young's moduli for ideal monocrystalline and for polycrystalline ANTi₃ aggregates which we have classified as ductile in nature. Band structures reveal that these compounds are conductors. The covalent ionic bands nature is due to the strong hybridization between Ti 3d and N 2p states. The Ti 3d states play dominant roles near the Fermi levels for all these compounds. The energy difference between spin polarized calculations and the nonspin polarized calculations indicate that ANTi₃ compounds exhibit magnetism at their equilibrium lattice constants.     

Physical,photoelectrochemical properties of CuIn3Se5 and relevance for hydrogen production

CuIn₃Se₅, prepared by the fusion technique crystallizes in the P-chalcopyrite structure and exhibits n-type conduction ascribed to indium excess. The electrical conductivity follows an Arrhenius-type law with activation energy of 0.35 eV and an electron mobility of 10⁻⁴ cm² V⁻¹ s⁻¹ in conformity with small polaron hopping. The optical gap (1.19 eV), determined from the diffuse reflectance spectrum, is properly matched to the sun spectrum. CuIn₃Se₅ is chemically stable and a corrosion rate of only 1.2 μmol year⁻¹ is found at neutral pH. The slope and the intercept to C⁻² = 0 of the Mott Schottky plot gives respectively an electron density of 3.75 × 10¹⁶ cm⁻³ and a flat band potential of −0.22 V_SCE. The conduction band (−0.74 V_SCE) therefore lies below the potential of H₂O/H₂ couple and as application, H₂ photo-production is successfully achieved over CuIn₃Se₅. The best performance is obtained in S₂O₃²⁻ solution (10⁻² M, pH ∼ 7) with an evolution rate of 0.54 mL g⁻¹ min⁻¹. The conversion efficiency (0.13%) is due to the formation of small depletion width (230 nm) and a large diffusion length compared to a very large penetration depth (∼1 μm). Attempts have been made to improve the photoactivity and the hetero-system CuIn₃Se₅/WO₃ is compared favorably with respect to CuIn₃Se₅. The photoactivity is ascribed to electrons transfer from the sensitizer CuIn₃Se₅-conduction band (CB), acting as electrons pump, to WO₃-CB (−0.4 V_SCE) resulting in the enhanced water reduction.