H. J. Queisser

Solar cell efficiency and carrier multiplication in Si1−xGex alloys

Crystalline Si1−xGex compounds offer the possibility for tuning the electronic energy band structure with the chemical composition of the alloy in order to adapt the material for devices utilizing the energy of solar photons at an optimum. We concentrate on the efficiency enhancement due to carrier multiplication by impact ionization. We calculate the internal quantum efficiency and the possible solar cell efficiency for this material system. The number of impact-generated charge carriers is obtained by a simulation of the competing carrier–carrier and carrier–photon scattering processes. These calculations show that the wave vector dependence of the scattering processes is unimportant for good agreement between theoretical and experimental quantum efficiencies in Si and Ge. Finally, we calculate solar cell efficiencies under the ideal assumption of unity collection efficiency and radiative recombination only. Impact ionization enhances the theoretical conversion efficiency by 0.5 percentage point; this i...

High‐resolution direct‐display x‐ray topography

A new system for the direct video display of x‐ray diffraction topographs is described. The pattern is converted to an optical signal by a fine‐grain fluorescent screen, which is viewed by a video camera through a magnifying optical system. We have achieved a resolution of about 10 μm, suitable for detailed direct observations of individual dislocations in materials such as silicon. The system combines efficient usage of source intensity, large field of vision, and circumvention of an electronic suppression of Kα2 images.

Silicon epitaxial solar cell with 663‐mV open‐circuit voltage

Silicon films of 20 μm thickness have been grown epitaxially on silicon substrates by liquid‐phase epitaxy. Solar cells fabricated on such layers display open‐circuit voltages as high as 663 mV (AM1.5, 25 °C), a value which exceeds previous data by a large margin. High open‐circuit voltages are a prerequisite for thin‐film solar cells with high efficiencies. Our result has applications to both space cells and to low‐cost terrestrial cells.

Radiative efficiency limit of terrestrial solar cells with internal carrier multiplication

Charge multiplication by hot carriers in semiconductors opens a new perspective for improved photovoltaic energy conversion. Here, we estimate the theoretical limit for the efficiency of solar cells that make use of the best possible carrier multiplication and have the lowest possible recombination loss, i.e., radiative recombination only. For a terrestrial cell under 100 mW/cm2 air mass 1.5 global illumination, the uppermost efficiency amounts to 44.2%. Cells with internal carrier multiplication require a modified description of their saturation current density.

Density of states of grain boundaries in silicon

Abstract A new spectroscopic method for the evaluation of the density of states in semiconductor grain boundaries is presented. The method is based on a measurement of the intensity and wavelength dependence of the zero-bias photo capacitance of grain boundaries under sub-bandgap illumination. Experiments are performed on p-type silicon bicrystals. Bandtails and states close to midgap are observed.

Luminescence in slipped and dislocation‐free laser‐annealed silicon

Photoluminescence of cw laser‐annealed silicon shows a dramatic difference in electronic behavior of the reconstructed material depending upon either creation or suppression of dislocations. Beyond a critical exposure time slip appears, and the luminescence of these samples is dominated by dislocation‐related defect levels.

Calculated dispersion of surface excitons

Abstract We have calculated the dispersion of surface excitons from the frequency- and wavevector-dependent contribution to the dielectric function ϵ(ω, k ) by free excitons. The calculation is based on the surface impedance method by Fuchs and Kliewer. Numerical results are given for the case of CdS. We discuss possibilities for the experimental excitation of surface excitons.

Profiling of deep impurities by persistent photocurrent measurements

A method to obtain the volume density Z of deep impurities in semiconductors versus their spatial depth is described. Z is derived by measuring the increase of the photoinduced persistent sheet density of carriers Δ(nd) in a conductive layer as a function of incident photon dose Q. The slope of the Δ(nd) =  f(log Q) curve depends upon the concentration profile of the deep traps in the substrate beneath the conductive layer, as is demonstrated by experiments using GaAs.

Hot carrier cooling in gaas quantum wells

The cooling of a hot electron-hole plasma in undoped, p-doped, and n-doped GaAs/AlGaAs quantum wells of three different thicknesses (3, 9, and 20 nm) is investigated by picosecond luminescence spectroscopy. The energy loss of holes due to the Fröhlich interaction is at low excitation densities independent of well width and close to the value obtained by a simple theory. The rate strongly decreases with increasing excitation density. For electrons, the energy loss is even at low densities strongly reduced compared to the simple theory of the Fröhlich interaction. The reduction of the energy loss at high densities is independent of dimensionality and well width and not caused by screening or degeneracy effects. The energy loss due to acoustic deformation potential scattering depends on well width.

Optical determination of carrier mobility in GaAs

Abstract We analyze the radiative recombination of free electrons to acceptor-bond holes in GaAs at helium temperatures and obtain the energy distribution of conduction-band electrons in applied weak electric fields. We are able to determine electron temperatures, drift momenta, and mobilities as functions of field. Direct experimental proof is given for mobility enhancement through screening by a free-carrier plasma.