We investigated lateral charge carrier transport in indium gallium nitride InGaN/GaN multi-quantum wells for two different samples, one sample emitting green light at about 510?nm and the other emitting cyan light at about 470?nm. For the cyan light emitting sample we found a diffusion constant of 1.2?cm2/s and for the green light emitting sample 0.25?cm2/s. The large difference in diffusion constant is due to a higher point defect density in the green light emitting quantum wells (QWs) as high indium incorporation tends to reduce material quality.

In order to investigate a further aspect of the charge carrier dynamics in InGaN based LED structures, Julia Danhof et al. (pp. 480–484) measured the lateral charge carrier diffusion in light emitting quantum well structures. They found that there is indeed lateral movement of charge carriers on a micrometer length scale in InGaN/GaN quantum wells which was not clear a priori. It was demonstrated that the main mechanism behind the lateral charge carrier motion is diffusion, while the influence of drift due to in-plane internal electrical fields is negligible. In this work the charge carrier diffusion constant was determined for two samples emitting light at different wavelengths. The difference of a factor five in the diffusion constant is due to variation of the material quality.

Excitation density-dependent microphotoluminescence measurements were performed on a green light (515 nm) emitting InGaN/GaN multiquantum well sample of low threading dislocation density (5 x 10(7)cm(-2)). For the observed structure we find a different shape of the local internal quantum efficiency (IQE) curve at spots of different efficiency. Evaluation of the measured local IQE curves in terms of the standard model including ShockleyReadHall (SRH), radiative recombination, and a third order loss term (droop) leads to the conclusion that SRH nonradiative recombination varies spatially on a sub-micrometer length scale while both other terms remain constant.

For an InGaN/GaN multiquantum well sample with low threading dislocation density (TDD), temperature-dependent microphotoluminescence measurements were performed. Evaluation of the behavior of the photoluminescence peak energy leads to the conclusion that even at a length scale of about 500 nm we still see the same S-shape of the temperature-dependent energy shift that was observed on a far larger scale. The behavior of the temperature-dependent full width at half maximum of the spectra and the integrated intensity leads to similar conclusions. We therefore demonstrate that the phenomenon responsible for the S-shape acts on a length scale that is far smaller than 500 nm. In addition, the behavior of the integrated intensity shows a fluctuation of local point defect density in the analyzed sample with its low TDD. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We investigate the possibility to obtain efficient green light sources for high-power applications by full luminescence conversion via InGaN/GaN multi-quantum-well (MQW) structures. Using an ultraviolet (UV) light-emitting diode (LED) for optical pumping of a forty-period (40x) MQW structure emitting in the green spectral range, we show that a luminous flux comparable to the best reported values for direct green LEDs can be achieved. The position of the efficiency maximum on the excitation density axis is shifted by an order of magnitude to an equivalent of 28 Acm–2 while maintaining comparable absolute values. We propose the concept of processing suitable platelets placed directly on a pump LED as a general experimental tool to determine absolute efficiency values of structures that cannot be pumped by electrical means (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The lateral mode profile of pulsed broad ridge 405nm (Al, In)GaN laser diodes grown on GaN and SiC substrates, respectively, is investigated by temporal and spectral resolved scanning near-field optical microscopy. During the first microsecond of the pulse, we observe changes both in the spatial mode profile and in the spectral regime caused by thermal and carrier induced modification of the waveguide refractive index, before stable filaments build up. In quasi-cw operation, a correlation between the lateral mode profile and the corresponding spatial resolved longitudinal mode pattern can be found. The results show that different filaments have different effective refractive indices and thus build up separate longitudinal mode combs.

The charge transport via motion of charge density-waves (CDW) requires a conversion of the charge carriers provided by the contacts (called single charge carriers or quasiparticles) into collective ones which are part of the CDW and vice versa. The conversion occurs via a so-called phase slip, i.e. phase fronts are added to or removed from the CDW, leading to a stress or strain in the CDW, respectively, which relaxes by a motion of the CDW. The dynamics of the conversion process has a formative influence on the time-resolved signals measured at the contacts. We introduce a phase-slip model which comprises the shape, the location, and the statistics of the intervals between successive phase-slip events and is able to describe noise phenomena often observed in CDW systems. The Fourier transform of the signal produced by a single phase-slip event deduced from the well-established Fukujama–Lee–Rice model can be identified with the broad-band noise spectrum measured in experiment. The statistics which describes the sequence of these single events determines the narrow-band noise. We compare the noise spectra gained from the model with the ones obtained from our noise measurements on o-TaS3 samples.

This article discusses the fundamental limitations imposed on Cu(In,Ga)Se2 based heterostructure solar cells by recombination in the bulk of the absorber and at the interface between the absorber and the CdS buffer layer. Bulk recombination to a certain extent can be minimized by increasing the doping density up to a limit where tunneling currents significantly enhance recombination. We propose simple schemes for the analysis of experimentally gained data. By comparison of theoretical models with experimental data we show that tunneling plays a role for polycrystalline Cu(In,Ga)Se2 in some cases and for CuGaSe2 in general.

This article investigates the electronic transport properties of ZnO/CdS/Cu(In,Ga)Se-2 heterojunction solar cells during and after illumination or forward bias in the dark. We observe a relaxation of the open circuit voltage under constant illumination as well as a relaxation current in the dark. Both phenomena are accompanied by an increase of the sample capacitance. We introduce a general quantitative model concept for the open circuit voltage relaxation and related effects in heterojunction devices that explains the phenomena as a consequence of the persistent capture of charge carriers within the space charge region. We apply our concept to develop a specific quantitative model for the observed metastablity in Cu(In,Ga)Sea heterojunction solar cells.

We investigate Cu(In,Ga)Se-2-based solar cells with a new ZnSe buffer layer deposited by metal-organic vapour deposition and compare their electronic properties to reference cells using a standard CdS buffer layer. The best solar cell with a ZnSe buffer layer achieves an efficiency of 11.6%. We further investigate a large series of solar cells with varied thickness of both types of buffer layers by means of quantum efficiency measurements in equilibrium and under light and voltage bias, The characterization of the devices concentrates on the analysis of the collection of photogenerated holes from the buffer layer. We introduce a nero method to determine the recombination probability of holes at the buffer/absorber ber interface, We find a similar interface recombination probability of about 40% for both devices, those with a ZnSe buffer layer and those with a CdS buffer layer. An anomalous enhancement of the quantum efficiency measured under current bias is ascribed to a barrier modulation effect which is caused by light absorbed in the buffer layer. Copyright (C) 1999 John Wiley & Sons, Ltd.

We investigate the optical and electronic properties of thin-film silicon solar cells by means of numerical simulations. The optical design under investigation is the encapsulated-V texture which is capable of absorbing sunlight corresponding to a maximum short circuit current density of 35 mA/cm/sup 2/. Since the layer thickness can be restricted to only 4 /spl mu/m, the encapsulated-V structure provides also a good collection efficiency for photogenerated charge carriers. Practical efficiencies around 12% can be expected for Si material with a minority carrier lifetime as low as 10 ns. Increased lifetimes of 100 ns allow for about 14% efficiency. The benefit of multiple junctions strongly depends on surface recombination. The efficiency of a single junction cell can be improved from 10% to 13% by a three junction device if the surface recombination velocity is as high as 10/sup 5/ cm/s. For moderate surface recombination the gain is only 1%.

We ascribe the relaxation of the open circuit voltage in Cu(In,Ga)Se2 heterojunction solar cells to persistent photoconductivity in the absorber material. The experimentally observed increase of the open circuit voltage during illumination is accompanied by an increase of the junction capacitance resulting from an effective increase of the space charge density by persistent trapping of photogenerated electrons.

We report on a detailed investigation on relaxations of the open circuit voltage V-OC of ZnO/CdS/Cu(In,Ga)Se-2 solar cells. Considering that persistent photoconductivity can be observed on Cu(In,Ga)Se-2 polycrystalline absorber layers as well as on Cu(In,Ga)Se-2 single crystals, we propose a model that describes the V-OC relaxation as a property of the Cu(In,Ga)Se-2 bulk material and not as a property of a specific interface of the heterojunction.

We investigate the electrical transport properties of Cu(In,Ga)Se-2 single crystals, thin films and solar cells by the analysis of the temperature dependent DC-and AC-conductance. Persistent Photoconductivity was found up to room temperature in single crystals as well as in polycrystalline thin films. Solar cells based on Cu(In,Ga)Se-2 absorber material display also a metastability in their admittance spectra. Despite the different preparation techniques for the single crystals on the one hand and the thin films and solar cells on the other hand we find a related metastability in their electrical properties.

Quasi-one-dimensional metals show a metal-semiconductor transition at a certain temperature. Below the so-called Peierls temperature, charge-density waves are developing within these metals. The resulting collective electronic state is responsible for unusual transport properties, such as nonlinear current-voltage characteristics with a threshold and current-voltage oscillations above threshold accompanied by broad-band noise. Current-voltage oscillations are thought to originate from charge-density waves when they begin to contribute to charge transport. The current-voltage characteristic is explained by the transition of electrons from the single-particle state to the collective electron state and vice versa (phase-slip acts). The current-voltage oscillations, however, are supposed to be generated in the bulk due to interaction with impurities. The purpose of the present Letter is to discuss a simple statistical model which describes the time-dependent process at the interface between charge-density waves and single-particle electrons. Our model can explain the transport phenomena that are mentioned above. We suppose the transport in the bulk to be diffusive.

Abstract We establish a straightforward connection between spatially extended systems, the dynamics of which are modeled with the help of partial differential equations and time-delay systems. To this end, we give a linear partial differential equation with a nonlinear boundary condition whose solutions are equivalent to the solutions of a time-delay differential equation. We observe that the phase space of these systems exhibits a pronounced structure. In this paper, we express the structure of the phase space of time-delay systems and the corresponding spatially extended system by distinguishing between a ‘linear subsystem’ and a ‘localized nonlinearity’. We find that the high-dimensional chaotic dynamics observed in time-delay systems is fundamentally different from the spatio-temporal chaos observed in homogeneous spatially extended systems, the dynamics of which is modeled with the help of nonlinear partial differential equations. To this end, we investigate the space-time correlation function and the ‘thermodynamic limit’.

The authors investigate the electrical transport properties of Cu(In,Ga)Se/sub 2/ polycrystalline thin films and solar cells during and after illumination or forward bias in the dark (the latter only for the cells). For the thin films, they find persistent photoconductivity. In the solar cells, they observed relaxation of the open circuit voltage and a metastable increase of the voltage under constant forward current in the dark. Both phenomena are accompanied by an increase in the sample capacitance. They present a model that explains all the observed metastable phenomena as a consequence of trapping of minority carriers in the bulk of the Cu(In,Ga)Se/sub 2/ material.

Introduced as a model for hyperchaos, the generalized R\"ossler system of dimension $N$ is obtained by linearly coupling $N\ensuremath{-}3$ additional degrees of freedom to the original R\"ossler equation. Under variation of a single control parameter, it is able to exhibit the chaotic hierarchy ranging from fixed points via limit cycles and tori to chaotic and, finally, hyperchaotic attractors. Through the help of a mode transformation, we reveal a structural symmetry of the generalized R\"ossler system. The latter will allow us to interpret the number, shape, and location in phase space of the observed coexisting attractors within a common scheme for arbitrary odd dimension $N.$ The appearance of hyperchaos is explained in terms of interacting coexisting attractors. In a second part, we investigate the Lyapunov spectra and related properties of the generalized R\"ossler system as a function of the dimension $N.$ We find scaling properties which are not similar to those found in homogeneous, spatially extended systems, indicating that the high-dimensional chaotic dynamics of the generalized R\"ossler system fundamentally differs from spatiotemporal chaos. If the time scale is chosen properly, though, a universal scaling function of the Lyapunov exponents is found, which is related to the real part of the eigenvalues of an unstable fixed point.

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3-5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se-2 regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current-voltage characteristics of Cu(In, Ga)Se-2-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves. (C) 1997 by John Wiley & Sons, Ltd.

We propose a method that is able to analyze chaotic time series, gained from exp erimental data. The method allows to identify scalar time-delay systems. If the dynamics of the system under investigation is governed by a scalar time-delay differential equation of the form $dy(t)/dt = h(y(t),y(t-\tau_0))$, the delay time $\tau_0$ and the functi on $h$ can be recovered. There are no restrictions to the dimensionality of the chaotic attractor. The method turns out to be insensitive to noise. We successfully apply the method to various time series taken from a computer experiment and two different electronic oscillators.