Wolfgang Ruppel

Upper limit for the conversion of solar energy

A semiconductor in the solar radiation field acts as a thermal electronic engine. It converts absorbed radiation heat into chemical energy of the excited electron-hole gas. In flow equilibrium, a homogeneous semiconductor gives off this chemical energy by radiative recombination to the surroundings. If provision is made, as by a p-n junction, to divert the excited electrons and holes, before they recombine, from their point of generation, their chemical energy may be converted into electrical energy. The ratio of this chemical energy current, which constitutes an upper limit for the obtainable electrical energy current, to the absorbed heat current is computed as a function of the value of the bandgap of the semiconductor. Under the assumption that the absorptivity of the electron-hole system of the semiconductor is unity for photon energies larger than the bandgap and zero for smaller photon energies, the conversion efficiency for unfocussed sunlight has a maximum of 30 percent for a bandgap of 1:3 eV.

Bulk photovoltaic effect in BaTiO3

Abstract In melt-grown BaTiO 3 single crystals steady-state photocurrents proportional to the light intensity have been observed parallel to the crystallographic c -axis. The open-circuit photovoltage exceeds the value of the band gap. Light polarized parallel and perpendicular to the c -axis respectively produces photovoltages with opposite sign. This photovoltaic effect is restricted to the ferroelectric phase.

The photovoltaic effect and the charge transport in LiNbO3

Abstract The charge transport giving rise to the photovoltaic effect is compared with the photoconductivity in the photon energy range from 2.5 eV to 5 eV. The photoconductivity at 2.5 eV is activated with about the same activation energy as the Hall mobility, which was measured for reduced samples between room temperature, where it is 1.1×10−3 cm2/Vs, and 200°C. The dependence on temperature, intensity, and time demonstrates different charge transport mechanisms for the short-circuit photocurrent and for the photoconductivity.

Optical investigations of the electrical anisotropy of α- and β-(BEDT-TTF)2I3

The anisotropy of the electrical transport properties at room temperature of the α- and β-phase of (BEDT-TTF)2I3 was determined from the polarized reflectance spectra.

Optical and electrical properties of black gold layers in the far infrared

Abstract Black gold layers, prepared by an evaporation process in an inert atmosphere, are an efficient absorber in the far infrared spectral region. Experimental results are presented for their reflectance in the wavelength range between λ=15 μm and 1000 μm, corresponding to a wave number k=1/λ between 10 cm−1 and 650 cm−1. The black gold layers may electrically be either metallic or semiconducting, the metallic ones exhibiting higher absorbance. The layers have a percolated structure whose dielectric properties are described by the Bergman formalism for an effective medium. On the basis of this model, the deviation of the optical properties of the layers from bulk gold are accounted for solely by the topology of the percolated structure. The values derived for the electron collision time in the gold particles and for the percolation factor agree with those derived from the electrical conductivity of the layers and its temperature dependence.

Anomalous photovoltage in BaTiO3

Abstract In ferroelectric melt-grown BaTiO3 single crystals steady-state short-circuit photocurrents have been observed parallel to the c-axis. At room temperature light polarized parallel and perpendicular to the c-axis produces short-circuit photocurrents with opposite sign. Along with the temperature dependence a physical model of this effect is outlined.

The origin of the photo-emf in ferroelectric and non-ferroelectric materials

A photo-emf arises from an optical excitation that is either asymmetric in k-space or occurs at asymmetries in real space, as is shown by a discussion of the Boltzmann transport equation. Excitation asymmetric in k-space leads to the photogalvanic effect. A photo-current is impressed with unpolarized light in the bulk of ferro- and pyroelectric crystals, and with polarized light even in non-pyroelectric piezoelectric crystals. Photon drag and optical rectification are outlined in connection with the photogalvanic effects. Excitation at asymmetries in real space leads to a barrier photo-emf, if both electrons and holes are mobile and if the electric and chemical potentials are not spatially constant in the unilluminated semiconductor. The Dember effect as being based on one mobile charged species only, does not represent a photo-emf. Anomalous photovoltages of a magnitude larger than the band gap, in particular in polytype zinc sulfide, that have been accounted for by a battery of barrier photo cells, are ...

Ferroelectric properties of thin NaNO2–layers

Abstract Thin layers of NaNO2 with a thickness of a few μm were grown from the melt on several substrate materials. They have single-crystalline areas of the order of cm2. They exhibit essentially the same dielectric and ferroelectric properties as bulk NaNO2-crystals. The spontaneous polarization is directed preferably perpendicular to the surface of the layer.

A 3 × 3 pyroelectric detector array with improved sensor technology

Abstract A 3 × 3 pyroelectric array with P(VDF/TrFE) copolymer films on integrated CMOS circuits is described. The maximum specific detectivity of the sensors is in the order of 107 cm √Hz/W. An improved technology for the poling process is discussed. In order to achieve maximal sensitivity of a thermal IR sensor, an efficient absorption of the incident radiation is necessary. Results of the technology and the properties of thin nickel films as absorbers are presented.

Spectral reflectance of the one-dimensional organic metals (perylene)2(PF6)1.1 × 0.8CH2Cl2 and (perylene)2(AsF6)1.1 × 0.7CH2Cl2

Abstract We report on the optical reflectance of (perylene) 2 (PF 6 ) 1.1 × 0.8CH 2 Cl 2 and (perylene) 2 (AsF 6 ) 1.1 × 0.7CH 2 Cl 2 single crystals in the energy range from 0.25 to 4 eV. For light polarized perpendicular to the molecular stacks the crystals exhibit the optical behaviour of an insulating molecular crystals, whereas for light polarized parallel to the stacks a pronounced plasma edge at h ω = 0.9 eV is observed. The experimental data were analyzed by a Lorentz-Drude model in order to determine the electrical conductivity, the carrier concentration, the effective mass, the collision time, the band width, and the Fermi vector of the crystals.