Electronic structure of SrPt_4Ge_{12}: a combined photoelectron spectroscopy and band structure study

We present a combined study of the electronic structure of the superconducting skutterudite derivative SrPt4Ge12 by means of X-ray photoelectron spectroscopy and full potential band structure calculations including an analysis of the chemical bonding. We establish that the states at the Fermi level originate predominantly from the Ge 4p electrons and that the Pt 5d shell is effectively full. We find excellent agreement between the measured and the calculated valence band spectra, thereby validating that band structure calculations in combination with photoelectron spectroscopy can provide a solid basis for the modeling of superconductivity in the compounds MPt4Ge12 (M = Sr, Ba, La, Pr) series

Single Second Laser Annealed CuInSe2 Semiconductors from Electrodeposited Precursors as Absorber Layers for Solar Cells

Cu(In,Ga)Se2 (CIGSe) is a polycrystalline absorber layer in thin film solar cells with solar conversion efficiencies exceeding 20%. High temperature annealing for periods of minutes to hours is currently required to convert amorphous or nanocrystalline precursor material into high quality Cu(In,Ga)Se2 absorber layers. In this work, we perform the critical annealing step, using a 1064 nm laser, on electrodeposited precursor layers containing Cu, In, and Se, for times of 0.3-60 s thus synthesizing CuInSe 2 absorber layers. An annealing time of 1 s is found to be sufficient to remove elemental concentration gradients in the bulk of the layer and to increase the average implied crystallite size (crystal coherence length, as determined by X-ray diffraction, XRD). Therefore the rate-determining step in producing higher quality layers with short annealing times is the rate of grain growth and not atomic diffusion. Optoelectronic analysis of the absorber layers revealed p-type doping with improved radiative recombination compared to the precursors. Laser annealed CuInSe2 layers did not produce working photovoltaic devices. This is first attributed to a loss of Se that occurs during laser annealing, resulting in detrimental substoichiometric quantities of Se in the absorber. Second, the likely presence of a thick surface layer of the CuIn3Se5 phase is expected to detrimentally impact device performance. These findings must be addressed if annealing times of the CuInSe2 absorber layer are to be reduced to seconds. \ua9 2013 American Chemical Society

Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells