Imprint | Privacy Policy | Site Map
©2003 - 2008 Fullspectrum

Publications/Abstracts MJC

Abstracts of MJC Publications

Abstract: This work presents the results of optimizing the microstructure of tunnel diodes in metamorphic triplejunction solar cells regarding the generation of stacking faults due to lattice-mismatch. Therefore it reports on the MOVPE growth and the morphology of highly doped AlGaInAs layers used for tunnel diodes implemented into multi-junction solar cells. Furthermore, investigations about reproducible growth and reactor homogeneity are described. Temperature variations, gas flow symmetry and pyramidal defects in GaInP have been studied.

Abstract: The present work presents some lines of research aimed to contribute to a better performance of multi-junction solar cells at very high concentrations ( 1000 suns) by minimising of the series resistance of these devices. In the first section, a set of results is presented to as certain the potential of tellurium as a possible n-type dopant to improve the performance of tunnel junctions and/or the top cell emitter. Some anomalies in the incorporation of Te into GaInP are described and their impact on solar cell growth and operation is discussed. In the second section, the contribution of the bottom cell BSF layer to the series resistance is analyzed by comparing three different alternatives, namely p2+ GaAs; p GaInP; and p+ Al0.2Ga0.8As. BSFs made of moderately doped GaInP are demonstrated to contribute significantly to the series resistance of the device, whilst p2+GaAs layers are shown to produce lower photocurrents. On the other hand, p2+Al0.2Ga0.8As layers are shown to unite both high photocurrents and low series resistance, being thus the optimum option.

Abstract: III-V high concentrator solar cells are promising candidates for reducing the cost of photovoltaic electricity in terrestrial applications. Nevertheless, before their commercialization a complete assessment of their reliability must be carried out. For this purpose, a full strategy considering real time tests and homologated accelerated tests has been designed. First results of accelerated tests are now available and presented in this paper. Accelerated step-stress temperature tests have been carried out by a laboratory certified according to ISO standards and taking into account optoelectronics standards. For simulating real operation conditions, all devices subjected to step-stress temperature, have been biased at the same current level as they would handle working at 1,000 suns. The results show that, regarding this particular test, these solar cells present MTTF values up to 113 years.

Abstract: The IES-UPM has a large trajectory in the III-V concentration solar cells research. Nowadays, the long term aim is to achieve efficiencies around 35% at 1000 X by multijunction solar cells and to transfer the technology to industry. For this purpose three complementary lines are ongoing: the growth of III-V and solar cells structures by MOVPE, the reliability study of concentrator solar cells and the modelling of the devices by means of distributed models.

Abstract: An efficient germanium cell is a key element for attaining high efficiency in state-of-the-art triple junction solar cells. This work summarizes our efforts in the field of modeling of the quantum efficiency of germanium p/n junctions for photovoltaic applications. An analytic tool is presented and the most relevant parameters are discussed and modeled. Finally, some hints for the optimum design of germanium solar cells are deduced form the simulations.

Abstract: III-V Multi Junction (MJ) solar cells based on Light Emitting Diode (LED) technology have been proposed and developed in recent years as a way of producing cost-competitive photovoltaic electricity. As LEDs are similar to solar cells in terms of material, size and power, it is possible to take advantage of the huge technological experience accumulated in the former and apply it to the latter. This paper analyses the most important parameters that affect the operational lifetime of the device (crystalline quality, temperature, current density, humidity and photodegradation), taking into account experience on the reliability of LEDs. Most of these parameters are less stressed for a III-V MJ solar cell working at 1000 suns than for a high-power LED. From this analysis, some recommendations are extracted for improving the long-term reliability of the solar cells. Compared to high-power LEDs based on compound semiconductors, it is possible to achieve operational lifetimes higher than 105 hours (34 years of real-time operation) for III-V high-concentration solar cells.

Abstract: In this work described are various approaches to IV curves measurements of monolithic dual- and triple-junction concentrator GaInP/GaAs/Ge cells under illumination with the aim of a comprehensive indoor characterization. The measurements were performed using a newly developed four-lamp flash instrument. Tailoring the illumination spectrum is realized by filtering individual lamps and by varying of their intensities to fit different standard sun spectra on the level of several hundreds suns (concentration as high as 20000X is realized without filtering). Also, spectra of the definite configurations are formed for IV curve measurements under special illumination conditions. TJ cell samples were measured to reveal their behavior in the conditions of the balance, or strong disbalance, of photogenerated current densities in each subcell.

Abstract: A phenomenological model is suggested to describe nonradiative recombination of optical excitations in disordered semiconductor heterostructures. The general property of disordered materials is a strong decay of the photoluminescence intensity with rising temperature. We show that this temperature dependence is a consequence of the interplay between radiative and nonradiative recombination and hopping dynamics of excitations in the manifold of localized states created by disorder potential. The dynamics is studied by kinetic Monte Carlo simulations. Experimental data on the thermal quenching of the photoluminescence intensity in GaInNAs/GaAs and GaNAsP /GaP quantum wells are presented, which are in good agreement with the theoretical results.

Abstract: The nearest-neighbour bond configuration affects decisively many important characteristics of semiconductor metastable alloys, in particular the width of the band gap. We study effects of formation of a short-range order in GaInNP/GaP heterostructures. Annealing experiments on GaInNP /GaP quantum wells reveal a blueshift of the photoluminescence spectra and an enhancement of the photoluminescence quantum efficiency as compared to the as-grown material. We show that the replacement of Ga-N bonds by In-N bonds upon annealing can account for the observed phenomena, in particular for the enhancement of the band gap. Calculations carried out in the frame of the density-functional theory and also using an empirical energy functional show that the driving force for the rearrangement in the nitrogen local environment is reduction of the strain contribution to the alloy total energy functional. Analytical treatment of correlation effects on the configurational entropy contribution to the alloy free energy is suggested. We show that the entropy factor is not negligible and it plays a crucial role when determining the thermodynamically favourable atomic configuration.

Abstract: A method to visualize and investigate radiative recombination processes in compound semiconductor materials by utilizing the effect of optical coupling in III-V multilayer systems is presented. For this purpose, a semiconductor material of interest is grown on an activated germanium Ge substrate which then serves as a photodiode. By means of spectral response measurements of the Ge photodiode, a response signal from the upper layers can be detected. It is proven both by experiment and by modeling that the signals from these layers can only be explained by optical transport mechanisms, i.e., radiative recombination.

Abstract: A method to visualize and investigate radiative recombination processes in compound semiconductor materials by utilizing the effect of optical coupling in III-V multilayer systems is presented. For this purpose, a semiconductor material of interest is grown on an activated germanium Ge substrate which then serves as a photodiode. By means of spectral response measurements of the Ge photodiode, a response signal from the upper layers can be detected. It is proven both by experiment and by modeling that the signals from these layers can only be explained by optical transport mechanisms, i.e., radiative recombination.

Abstract: An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.

Abstract: In this work described are various approaches to IV curves measurements of monolithic dual- and triple-junction concentrator GaInP/GaAs/Ge cells under illumination with the aim of a comprehensive indoor characterization. The measurements were performed using a newly developed four-lamp flash instrument. Tailoring the illumination spectrum is realized by filtering individual lamps and by varying of their intensities to fit different standard sun spectra on the level of several hundreds suns (concentration as high as 20000X is realized without filtering). Also, spectra of the definite configurations are formed for IV curve measurements under special illumination conditions. TJ cell samples were measured to reveal their behavior in the conditions of the balance, or strong disbalance, of photogenerated current densities in each subcell.

Abstract: A promising way for solar concentrator module design is the use of the highly-efficient multijunction III-V cells together with small-aperture area Fresnel-type solar concentrators. In the developed modules, the 50x 50 cm2 panels of integrated Fresnel lenses (each lens is 4x4 cm2 in aperture area) have a composite structure: microprisms are formed from transparent silicone contacting with glass sheet. A comprehensive analysis has been conducted concerning concentration properties of the lenses. Such a lens material parameter as refraction index and its dependence on wavelength was involved in computer modeling and measurement procedure at optical efficiency evaluation. As a result, lens profile was under optimization bearing in view different aspects, such as focal distance, receiver diameter, sun illumination spectrum, sensitivity spectra of the sub-cells in a triple-junction cell and others.
Overall conversion efficiency in a test module of described design as high as 26.5% has been measured.

Abstract: This work presents efforts to simulate numerically the IV curve of an III-V based Esaki tunnel diode. Using a tunneling model, which takes into account the full nonlocality of the barrier, a good agreement between measured and simulated IV curves of a GaAs tunnel diode was achieved. The influence of a series resistance effect as well as the importance of trap assisted tunneling could be shown. In addition, we present a two-dimensional model of a III-V multi-junction solar cell including the numerical model of the investigated Esaki tunnel diode.

Abstract: (GaIn)(NAs)/GaAs multi-quantum-well heterostructures are grown by metal organic vapor phase epitaxy at low temperatures and are subsequently annealed in the reactor to optimize optoelectronic properties. Detailed optical and structural studies of the properties of the material, which change upon annealing under different As-stabilization as well as at different temperatures, reveal that there are two major effects of the anneal. The first one is the blue-shift of the fundamental band gap of the material, which can be attributed to a local change in the group-III environment of the nitrogen atoms. The second observation is a strong increase in photoluminescence (PL) intensity and decrease in PL linewidth upon anneal in H2-ambient, which can be attributed to the removal of non-radiative defects in the material. Chain-like N-ordering in growth direction, which induces strong inhomogeneous strain fields in the material and which can be dissolved upon anneal in H2-ambient might also act as one of the main non-radiative recombination centers in the (GaIn)(NAs) material system.

Abstract: In the last years high-concentration PV using III-V solar cells has become more attractive and several companies have been founded. The general goal of these companies is to introduce CPV technology into the market and to decrease the cost of PV-generated electricity. In our opinion, this creates a strong demand for industrial fabrication techniques. Therefore, in this paper we discuss the needs, challenges and actual status of CPV manufacturing processes. Some of the described processes relate closely to the needs of FLATCON®-type modules. Fraunhofer ISE developed this type of module, which is now commercialized by Concentrix-Solar.

Abstract: Photovoltaic Systems using high optical concentration offer the opportunity for considerable cost reduction in photovoltaic electricity production. The main short-term application field of such systems will be power plants in the area of 100 kW up to some 10 MW in regions having a high fraction of direct solar radiation. In order to activate the potential for cost reduction an optimized system design is essential: PV cell characteristics, optics, module design, mechanical tracking, power electronics, systems control and manufacturability have to be matched carefully. The paper will give an overview about high-concentration systems, their results and challenges.

Abstract: A seemingly hysteretic behavior in the IV-characteristic of multi-junction solar cells and other tunnel diode structures has been observed during the last two years of intensive research at Fraunhofer ISE. This paper provides the results of calculations and experiments to understand the reasons for this effect, appearing with all devices consisting of solar cells and tunnel diodes. Solutions to overcome the measurement difficulties and to derive parameters, such as the maximum tunneling current, are provided and the reason for the observed hysteretic behavior of multi-junction cells is explained.

Abstract: This paper discusses common difficulties in measuring tunnel diodes and sets a special focus on devices consisting of tunnel diodes and solar cells, such as multi-junction solar cells. The resulting theoretical IV-characteristics of tunnel diodes and solar cells when measured via 4-wire techniques are calculated and compared with experimentally measured IVcurves.
Solutions to overcome the measurement difficulties are provided and a method to infer the maximum tunneling current density of tunnel diodes in a device with solar cells is discussed. This work also describes how the elsewhere observed ostensible hysteresis with multi-junction solar cells is caused by the measurement setup or by large internal series resistances.

Abstract: Well-designed strategies are required in order to push III-V concentration to low cost scenarios that beat the prices of the already existing silicon-based concentration systems operating at 250-400 suns. Several laboratories and pilot production lines are starting to use in III-V solar cells similar approaches than those used for LEDs (Light Emitting Diodes). We firstly proposed the so-called “LEDlike approach” in 1997. From then, we have analysed theoretically its main aspects thus guiding our manufacture developments with high concentrator III-V cells. This paper goes a step forward in the description of the “LED-like approach” advantages and in the influence of such approach on final cost.

Abstract: This work presents some lines of research currently being investigated in our group that are aimed to contribute to a better performance of multi-junction solar cells at very high concentrations (~1000 suns) and, in addition, to achieve a better characterization of these devices also at high concentrations.
The improvement of the performance of the solar cell at high concentrations is addressed in two ways. First, a set of results is presented to ascertain the potential of tellurium as a possible n-type dopant to improve the performance of tunnel junctions. Then, the contribution of the bottom cell BSF layer to the series resistance is analysed.
On the other hand, the issue of the linearity of the short circuit current with the irradiance is investigated. This linearity is assessed by measuring the external quantum efficiency of solar cells with bias lights of different intensities.

Abstract: A novel process for the nucleation layer of GaAs on p-Ge wafers using MOVPE has been developed. It is based on a low temperature process with two steps: 1) a predeposition of a monolayer of Ga or As and 2) the subsequent of a GaAs buffer layer at low temperature. In this paper, a study of the characteristics of n-on-p GaAs solar cells grown on Ge wafers as a function of these nucleation conditions has been performed. In addition, SIMS and C-V-measurements have been used to analyze the diffusion processes taking place across the GaAs/Ge interface. From all of these measurements it can be concluded that a low temperature nucleation layer reduces the Ge out-diffusion. In addition, the predeposition of a Ga monolayer decreases the As diffusion into the Ge wafer as well as the Ge diffusion into the GaAs layer and results in improved solar cell characteristics (higher quantum efficiencies and fill factors) as compared to the predeposition of an As monolayer.

Abstract: Concentration based on III-V solar cells is one of the most promising technologies to reduce cost of PV electricity. To achieve high efficiency making a better use of the solar spectrum and under very high concentration, Multi Junction solar Cells are explored at the IES-UPM. To give a real understanding of all the phenomena occurring inside these devices, the development of a reliable theoretical model is essential.
In this paper we present the first results obtained in our laboratory simulating lattice-matched GaInP/GaAs dual junction solar cells. To achieve these results we numerically analyze the complete structure including the tunnel junction.

Abstract: This work reports on various approaches to illuminated I-V curve measurements in monolithic dual and triplejunction GaInP/GaAs/Ge cells with the aim of revealing the action of interconnecting tunnel diodes. The measurements were performed both for model samples using a combination of individual sub-cells, connected through an external tunnel diode and for practical concentrator cells with different peak current densities (Jp) of the tunnel diodes in a monolithic structure. Changing of load resistance, or changing of applied voltage from an external source, characterized by variable resistance, was used to obtain I-V curves under both continuous and flash illumination. Also, variation of cell internal resistance due to the current mismatch in sub-cells, caused by illumination spectrum variation, was proven by these measurements.

Abstract: III-V multijunction (MJ) solar cells consist of up to 20 layers containing different materials with variable doping concentrations and thicknesses. Due to these many parameters experimental investigations in MJ solar cells can be expensive and protracted. An optical and electrical simulation of such structures is an elegant option to highlight the importance of parameters for the performance of the cell. Using a good material database the cell structure can be optimized accurately. The individual subcells in a monolithic MJ cell are interconnected by heavily doped pn-junctions. Thus for a full simulation of a monolithic MJ cell it is essential to simulate a tunneling junction. This paper describes efforts to achieve this goal.

Abstract: The approach of mechanically stacking one cell over the other greatly expands the range of materials useful for tandem cell configurations and extends the range of converted solar spectrum.
To construct high-efficiency mechanically stacked multibandgap tandems the widely used MOCVD, LPE and diffusion process technologies for individual cells fabrication have been used.
This work presents the results of the concentrator InGaP/GaAs-GaSb triple- and InGaP/GaAs/Ge- InGaAsSb quadruple-junction monolithic/stacked tandem development.

Abstract: The MOVPE (Metal Organic Vapour Phase Epitaxy) growth of GaAs single-junction solar cells on layer transferred GaAs/Si-substrates is reported. This novel type of alternative substrates for III-V epitaxial growth was realised by performing the Smart CutTM technology.
HRXRD-measurements proved good material quality by a FWHM of about 30 arcsec and reveal a dislocation density below 4x105 cm-2 for a 5 μm thick GaAs layer. Due to the formation of cracks the performance of GaAs singlejunction solar cells has only been in the range of 12 %.

Abstract: The dilute nitride (GaIn)(NAs) material system grown lattice matched to GaAs or Ge with a 1 eV bandgap is an interesting material for the use in multi-junction solar cells. By inserting a solar cell made of (GaIn)(NAs) as a fourth junction into the well developed 3-junction solar cell consisting of GaInP/GaInAs/Ge significant efficiency improvements can be expected. However, up to now the introduction of nitrogen into GaInAs tended to degrade the material quality resulting in low diffusion lengths. Still it is not clear whether this is due to extrinsic, growth related defects or an intrinsic property of the material. This study discusses up to which extent optimised growth and annealing conditions can be found to circumvent these properties of the material and hence improve device performance. First results of solar cell structures with optimised annealing conditions will be presented at the conference.

Abstract: We have studied systematically the nitrogen content in Ga_NAs_ /GaAs quantum wells by _002_dark-field transmission electron microscopy _TEM_. The nitrogen contents derived from this analysis, when assuming that all the atoms occupy their unperturbed positions in a virtual crystal, deviate significantly from the nitrogen contents we derive for the same samples by other methods; for example, high-resolution x-ray diffraction _XRD_ and dynamical simulation of those XRD patterns. The nitrogen causes a significant local strain in the crystal and can accordingly displace the neighboring atoms dramatically. We show that, if the structure factor of the crystals is recalculated, taking these static displacements of the Ga atoms into account, the composition derived from the TEM analysis with that from XRD is in perfect agreement. It is hence necessary for tetragonally distorted crystals that have mixed sublattices containing atoms with different covalent radii to take these static displacements into account when quantification of the composition from dark-field or high-resolution TEM images is aimed for.

Abstract: In order to get a cost competitive commercial product based in III-V solar cells, a complete assessment of their reliability must be done. A full strategy of tests has been designed to tackle this matter. Real time and homologated accelerated tests are ongoing, as well as a complete statistical analysis which will establish the main reliability parameters and the failure mechanisms.

Abstract: GaxIn1-xAs and GayIn1-yP layers were grown lattice mismatched to GaAs and Ge by lowpressure metal organic vapor phase epitaxy (LP-MOPVE). These materials are very promising for further increasing the efficiency of monolithic triple-junction solar cells. Different buffer layer structures were realized. Transmission electron microscopy and x-ray diffraction analysis were used to characterize the quality of the crystal. Both linear and step-graded buffers in GaxIn1-xAs were successfully used under an active solar cell structure. GayIn1-yP as buffer material showed a worse performance. Excellent solar cell performance was achieved for lattice mismatched single-, dual- and triple-junction solar cells.

Abstract: The development of III-V concentrator solar cells and thermophotovoltaic converters is at a critical point in which both sophisticated technology and an accurate modeling are required. This paper emphasizes the aspects relating to the modeling of multijunction solar cells for the concentration of applications and thermophotovoltaic converters. In the case of solar cells, the key aspects are
- Necessity of three-dimensional modeling,
- Consideration of real conditions of operation,
- Critical review of material parameters.
For TPV converters, the aforementioned aspects are also to be applied. Preliminarily, the material parameters of the less mature thermophotovoltaic semiconductors must be specified or even measured.

Abstract: The modelling of concentrator solar cells for real conditions of operation inside optical concentrators is a subject almost untreated. Consequently, this work highlights the main specific situations that should be included in a realistic modelling. The results of a 2-D modelling applied to the case of a 1000 sun GaAs concentrator solar cell inside a TIR-R concentrator are presented. The necessity of going towards a 3-D modelling is also stated. This task is being carried out now at IES-UPM.

Abstract: III-V concentrator solar cells have demonstrated a high degree of technological maturity. However, there are still important open questions about their long-term reliability. This paper aims to asses the transferability of the huge existing experience around the degradation and reliability of optoelectronic devices to terrestrial high concentrator III-V solar cells.

Abstract: 3D distributed model for high concentrator solar cells has been implemented. No ohmic effect is neglected, making this simulation tool very versatile. The possibility of simulating the external connections and non-uniform illumination profiles makes this model very useful to optimize future structures and technological processes. In this paper, a high concentration single-junction GaAs solar cell has been studied under different sunlight concentrations. The voltage drop distribution in the complete surface of the solar cell working at the maximum power point, MPP, is presented showing the necessity of using a 3D model for solar cells under high concentration.

Abstract: Based on theoretical considerations bandgap combinations for multi-junction concentrator cells (one to six subcells) are calculated. The structures of these cells are adapted to either the AM1.5d reference spectrum or the new proposed reference spectrum with a lower aerosol optical depth. These bandgap combinations were used to calculate the energy production on one real day. Therefore outdoor spectra were measured. It is shown that the increase in efficiency can be cancelled out by losses due to higher spectral sensitivity when the number of junctions is increased over a certain value. Thus, this paper gives a contribution to the ongoing discussion concerning the optimal number of junctions under real working conditions with daily and yearly changing spectra.
(PDF version)

Abstract: 3D distributed model for high concentrator solar cells has been implemented. No ohmic effect is neglected, making this simulation tool very versatile. The possibility of simulating the external connections and non-uniform illumination profiles makes this model very useful to optimize future structures and technological processes. In this paper, a high concentration single-junction GaAs solar cell has been studied under different sunlight concentrations. The voltage drop distribution in the complete surface of the solar cell working at the maximum power point, MPP, is presented showing the necessity of using a 3D model for solar cells under high concentration.

Abstract: The following concepts connected with concentrator approach are under development in the PV Lab of the Ioffe Institute: small-aperture area and short focal length primary Fresnel lens concentrators; smooth-surface secondary minilenses; “all-glass” design for the modules; close-loop sun-tracking strategy for solar installations. Thermal properties of the sub-modules of different modifications have been experimentally examined; lens-cell alignment procedure including lens panel formation and cell mounting processes has been developed; specialized sun tracker for 1 kWp of installed capacity for the modules under development has been designed and built.
(PDF version)

Abstract: Key areas in the development of photovoltaic methods of solar energy conversion, which open up wide prospects for semiconductor solar energy conversion, are discussed. The article focuses mainly on photovoltaic cells based on III-V heterostructures, primarily on cascade solar cells, which provide the highest effi-ciency of solar energy conversion and are produced by high-tech methods such as MBE or MOCVD. It is shown that the use of intermediate sunlight concentration makes the area of solar cells smaller and, hence, lowers their cost proportionally to the sunlight concentration ratio.

Abstract: Key areas in the development of photovoltaic methods of solar energy conversion, which open up wide prospects for semiconductor solar energy conversion, are discussed. The article focuses mainly on photovoltaic cells based on III-V heterostructures, primarily on cascade solar cells, which provide the highest effi-ciency of solar energy conversion and are produced by high-tech methods such as MBE or MOCVD. It is shown that the use of intermediate sunlight concentration makes the area of solar cells smaller and, hence, lowers their cost proportionally to the sunlight concentration ratio.

Abstract: A theory is suggested for description of luminescence in semiconductor structures with essential role of localized states caused by disorder. The theory is based on the set of rate equations. On the contrary to most previous theoretical studies, electrons and holes are treated not in the form of excitons but rather as independent species. Theoretical results are compared with new experimental data for the time-resolved photoluminescence in GaInNAs/GaAs quantum wells.

Abstract: The dilute nitrides like (GaIn)(NAs) or Ga(NAsSb) are attractive materials for the next generation of photovoltaic cells with four to six active junctions. Unfortunately, these compounds suffer from a low minority carrier diffusion length. This can be partially compensated by choosing suitable device structures with a reduced prerequisite on the necessary current density. (GaIn)(NAs) solar cells with a bandgap between 1.0 and 1.2 eV have been grown by metal organic vapor-phase epitaxy on single- and 8x4-in multiwafer reactors. Short-circuit current densities up to 10.9 mA/cm2 (AM0) have been achieved for a (GaIn)(NAs) cell filtered with GaAs. This excellent value is sufficient for the application in five-or six-junction photovoltaic cells. Challenges are resulting from the transfer of growth conditions to a production size multi-wafer MOVPE reactor, which are discussed in this paper.