Infrared optical characterization of epitaxial layer – substrate systems (II). Experimental results for AlxGa1−xAs/GaAS

Infrared reflectance and transmittance spectra and Raman scattering spectra of the epitaxial layer-substrate system Al_xGa_1−xAs/GaAs with compositions in the range x = = 0.08–0.49 are measured in the wavenumber range from 40 to 4000 cm⁻¹. In analysing the spectra in terms of the respective theoretical relations for an optical two-layer system the thickness of the layers, the optical mode characteristics and the free carrier parameters are determined. From a comparison with existing literature data for Al_xGa_1−xAs it is concluded that infrared optical measurements on epitaxial layer-substrate systems can be successfully employed to evaluate the material parameters of epitaxial layers with thicknesses down to a few micrometers.     

Photoconductivity as loss mechanism in organic solar cells

We observe that the reverse current under illumination in solar cells containing C₆₀ and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current–voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C₆₀ layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C₆₀ absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power‐generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Correlation of π-conjugated oligomer structure with film morphology and organic solar cell performance

The novel methyl-substituted dicyanovinyl-capped quinquethiophenes 1-3 led to highly efficient organic solar cells with power conversion efficiencies of 4.8-6.9%. X-ray analysis of single crystals and evaporated neat and blend films gave insights into the packing and morphological behavior of the novel compounds that rationalized their improved photovoltaic performance.     

Coupling Considerations for Fabrication of Acoustooptic Tunable Filter

Induced acoustic wave to bare fiber through various types of horn are examined. The center wavelength and extinction ratio of the notch filter are dynamically tunable and dependent on the RF signals.     

Effect of viscous dissipation on the mixed convection heat transfer from an exponentially stretching surface

The mixed convection flow and heat transfer from an exponentially stretching vertical surface in a quiescent fluid is analyzed using similarity solution technique. Wall temperature and stretching velocity are assumed to have specific exponential function forms. The influence of buoyancy along with viscous dissipation on the convective transport in the boundary layer region is analyzed in both aiding and opposing flow situations. The flow is governed by the mixed convection parameter Gr/Re². The velocity and temperature inside the boundary layer are observed to be influenced by the parameters like Prandtl number Pr, Gebhart number Gb. Significant changes are observed in non-dimensional skin friction and heat transfer coefficients due to viscous dissipation in the medium. The flow and temperature distributions inside the boundary layer are analyzed and the results for non-dimensional skin friction and heat transfer coefficients are discussed through computer generated plots.     

Calculation of long-wavelength optical phonons and lattice reflection spectra of Al1−xGaxP mixed crystals

The two-mode behaviour of the infrared lattice vibrations of Al_1–x Ga _x P mixed crystals is described by the random-element-isodisplacement model with an effective electric field and phenomenological damping (EREI). The calculated long-wavelength transverse and longitudinal optical frequencies in dependence on the compositionx are in agreement with experimental measurements. The infrared lattice reflection spectra are calculated and fitted to the measured one by variation of damping parameters. The final values of the damping parameters are represented in dependence on the compositionx.This work has been done in the Arbeitsgemeinschaft AIII-BV-Halbleiter of the Karl-Marx-Universität Leipzig and was supported by the VEB Halbleiterwerk Frankfurt/Oder.