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Development of a High Volume Capable Process to Manufacture High Performance Photovoltaic Cells: Cooperative Research and Development Final Report, CRADA Number CRD-08-322

Description: The intent of the work is for RFMD and NREL to cooperate in the development of a commercially viable and high volume capable process to manufacture high performance photovoltaic cells, based on inverted metamorphic (IMM) GaAs technology. The successful execution of the agreement will result in the production of a PV cell using technology that is capable of conversion efficiency at par with the market at the time of release (reference 2009: 37-38%), using RFMD's production facilities. The CRADA work has been divided into three phases: (1) a foundation phase where the teams will demonstrate the manufacturing of a basic PV cell at RFMD's production facilities; (2) a technology demonstration phase where the teams will demonstrate the manufacturing of prototype PV cells using IMM technology at RFMD's production facilities, and; (3) a production readiness phase where the teams will demonstrate the capability to manufacture PV cells using IMM technology with high yields, high reliability, high reproducibility and low cost.
Date: November 1, 2012
Creator: Geisz, J. F.
Partner: UNT Libraries Government Documents Department

Direct Comparison of Inverted and Non-Inverted Growths of GaInP Solar Cells: Preprint

Description: The inverted growth of III-V solar cells presents some specific challenges that are not present in regular, non-inverted growths. Because the highly doped top contact layer is grown first, followed by the lengthy high-temperature growth of the remainder of the structure, there is ample time for the dopants in the contact layer to diffuse away. This leads to an increase in the contact resistance to the top layer, and a corresponding drop in voltage. The diffusion of dopants in other layers is similarly altered with respect to the non-inverted configuration because of the change in growth sequence. We compare the dopant profiles of inverted and non-inverted structures by using secondary ion mass spectroscopy and correlate the results with the observed performance of the devices. We also describe a technique for growing a GaInAsN contact layer in the inverted configuration and show that it achieves a specific contact resistance comparable to what is normally observed in non-inverted cells.
Date: May 1, 2008
Creator: Steiner, M. A.; Geisz, J. F.; Reedy Jr, R.C. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

Direct Comparison of Inverted and Non-inverted Growths of GaInP Solar Cells (Presentation)

Description: The motivation for this presentation is that growing inverted cells may enable technological advances in solar cell fabrication, leading to higher efficiencies. Differences in dopant diffusion during inverted vs. upright growths may lead to differences in atomic depth profiles; changes in carrier concentrations; higher contact resistance and lower overall performance. This presentation summarizes that excellent performance is achievable in both upright and inverted configurations with proper consideration; subtle differences in depth profile QE and JV between upright and inverted growths due to dopant diffusion; and GaInAsN contact layer is resilient to length annealing and more work is necessary to determine why.
Date: May 1, 2008
Creator: Steiner, M. A.; Geisz, J. F.; Reedy,Jr. R. C. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

Impurity-Band Model for GaP1-xNx

Description: Low-temperature absorption studies on free-standing GaP1-xNx films provide direct experimental evidence that the host conduction-band minimum (CBM) near X1C does not plunge downward with increased nitrogen doping, contrary to what has been suggested recently; rather, it remains stationary for x up to 0.1%. This fact, combined with the results of earlier studies of the CBM at ..GAMMA.. and conduction-band edge near L, confirms that the giant bandgap lowering observed in GaP1-xNx results from a CBM that evolves purely from nitrogen impurity bands.
Date: November 1, 2005
Creator: Fluegel, B.; Zhang, Y.; Geisz, J. F. & Mascarenhas, A.
Partner: UNT Libraries Government Documents Department

GaNPAs Solar Cells Lattice-Matched To GaP: Preprint

Description: This conference paper describes the III-V semiconductors grown on silicon substrates are very attractive for lower-cost, high-efficiency multijunction solar cells, but lattice-mismatched alloys that result in high dislocation densities have been unable to achieve satisfactory performance. GaNxP1-x-yAsy is a direct-gap III-V alloy that can be grown lattice-matched to Si when y= 4.7x - 0.1. We propose the use of lattice-matched GaNPAs on silicon for high-efficiency multijunction solar cells. We have grown GaNxP1-x-yAsy on GaP (with a similar lattice constant to silicon) by metal-organic chemical vapor phase epitaxy with direct band-gaps in the range of 1.5 to 2.0 eV. We demonstrate the performance of single-junction GaNxP1-x-yAsy solar cells grown on GaP substrates and discuss the prospects for the development of monolithic high-efficiency multijunction solar cells based on silicon substrates.
Date: May 1, 2002
Creator: Geisz, J. F.; Friedman, D. J. & Kurtz, S.
Partner: UNT Libraries Government Documents Department

0.7-eV GaInAs Junction for a GaInP/GaAs/GaInAs(1-eV)/GaInAs(0.7-eV) Four-Junction Solar Cell: Preprint

Description: We discuss recent developments in III-V multijunction solar cells, focusing on adding a fourth junction to the Ga0.5In0.5P/GaAs/Ga0.75In0.25As inverted three-junction cell. This cell, grown inverted on GaAs so that the lattice-mismatched Ga0.75In0.25As third junction is the last one grown, has demonstrated 38% efficiency, and 40% is likely in the near future. To achieve still further gains, a lower-bandgap GaxIn1-xAs fourth junction could be added to the three-junction structure for a four-junction cell whose efficiency could exceed 45% under concentration. Here, we present the initial development of the GaxIn1-xAs fourth junction. Junctions of various bandgaps ranging from 0.88 to 0.73 eV were grown, in order to study the effect of the different amounts of lattice mismatch. At a bandgap of 0.88 eV, junctions were obtained with very encouraging {approx}80% quantum efficiency, 57% fill factor, and 0.36 eV open-circuit voltage. The device performance degrades with decreasing bandgap (i.e., increasing lattice mismatch). We model the four-junction device efficiency vs. fourth junction bandgap to show that an 0.7-eV fourth-junction bandgap, while optimal if it could be achieved in practice, is not necessary; an 0.9-eV bandgap would still permit significant gains in multijunction cell efficiency while being easier to achieve than the lower-bandgap junction.
Date: May 1, 2006
Creator: Friedman, D. J.; Geisz, J. F.; Norman, A. G.; Wanlass, M. W. & Kurtz, S. R.
Partner: UNT Libraries Government Documents Department

Examination of 1D Solar Cell Model Limitations Using 3D SPICE Modeling: Preprint

Description: To examine the limitations of one-dimensional (1D) solar cell modeling, 3D SPICE-based modeling is used to examine in detail the validity of the 1D assumptions as a function of sheet resistance for a model cell. The internal voltages and current densities produced by this modeling give additional insight into the differences between the 1D and 3D models.
Date: June 1, 2012
Creator: McMahon, W. E.; Olson, J. M.; Geisz, J. F. & Friedman, D. J.
Partner: UNT Libraries Government Documents Department

New Materials for Future Generations of III-V Solar Cells

Description: Three- and four-junction III-V devices are proposed for ultrahigh-efficiency solar cells using a new 1-eV material lattice-matched to GaAs, namely, GaInNAs. We demonstrate working prototypes of a GaInNAs-based solar cell lattice-matched to GaAs with photoresponse down to 1 eV. Under the AM1.5 direct spectrum with all the light higher in energy than the GaAs band gap filtered out, the prototypes grown with base doping of about 10{sup 17} cm-3 have open-circuit voltages ranging from 0.35 to 0.44 V, short-circuit current densities of 1.8 mA/cm2, and fill factors from 61% to 66%. To improve on the current record-efficiency tandem GaInP/GaAs solar cell by adding a GaInNAs junction, the short-circuit current density of this 1-eV cell must be significantly increased. Because these low short-circuit current densities are due to short diffusion lengths, we have demonstrated a depletion-width-enhanced variation of one of the prototype devices that trades off decreased voltage for increased photocurrent, with a short-circuit current density of 7.4 mA/cm2 and an open-circuit voltage of 0.28 V.
Date: October 6, 1998
Creator: Geisz, J. F.; Friedman, D. J.; Olson, J. M.; Kramer, C.; Kibbler, A. & Kurtz, S. R.
Partner: UNT Libraries Government Documents Department

Electron Microscopy Studies of GaP(N,As) Grown on Si

Description: The objective of this work is to perform transmission electron microscopy (TEM) studies of GaP(N,As) alloys grown by metal-organic chemical vapor deposition (MOCVD) on Si substrates. These alloys are of interest for the fabrication of high-efficiency tandem solar cells based on Si. The results indicated that the nucleation and growth conditions used are critical for obtaining planar epitaxial layers with a low defect density. In particular, antiphase domains are eliminated using a low growth temperature. TEM studies of these alloy layers, which contain only a few percent N, revealed no phase separation. However, electron diffraction studies revealed the first evidence of CuPt-type atomic ordering in these P-rich, dilute nitride alloy layers.
Date: January 1, 2005
Creator: Norman, A. G.; Geisz, J. F.; Olson, J. M.; Jones, K. M. & Al-Jassim, M. M.
Partner: UNT Libraries Government Documents Department

Analysis of Nitrogen Incorporation in Group III-Nitride-Arsenide Materials Using a Magnetic Sector Secondary-Ion Mass Spectrometry (SIMS) Instrument: Preprint

Description: Presented at the 2001 NCPV Program Review Meeting: Group III-nitride-arsenide materials were studied by SIMS, XRD, and Profiler to determine small amounts of nitrogen that can lower the alloys bandgap significantly.
Date: October 1, 2001
Creator: Reedy, R. C.; Geisz, J. F.; Kurtz, S. R.; Adams, R. O. & Perkins, C. L.
Partner: UNT Libraries Government Documents Department

Excitons and Recombination in Photovoltaic Materials

Description: High spatial resolution ({approx} 0.7{micro}m) scanning confocal microscopy, combined with low-temperature (5K) photoluminescence (PL) spectroscopy, can be used to probe the spatial variations in the spectral properties of photovoltaic materials with sub- micron spatial resolution ( {approx} 0.7{micro}m). We report on the successful demonstration of this technique applied to two particular photovoltaic systems: a partially ordered GaInP{sub 2} epilayer, and a released (exposing the CdTe/CdS interface) polycrystalline CdTe film.
Date: October 16, 1998
Creator: Smith, S.; Cheong, H. M.; Fluegel, B. D.; Geisz, J. F.; Olson, J. V.; Dhere, R. et al.
Partner: UNT Libraries Government Documents Department

Phase Separation and Facet Formation during the Growth of (GaAs)1-x(Ge2)x Alloy Layers by Metal Organic Vapour Phase Epitaxy

Description: Metal organic vapour phase epitaxy (GaAs)1-x(Ge2)x alloy layers, 0<x<0.22, were grown at temperatures between 640 and 690 C, on vicinal (001) GaAs substrates. Phase separation occurred in all the layers. The phase-separated microstructure changed with alloy composition, growth temperature, and substrate orientation. In x {approx} 0.1 layers grown at 640 C, Ge segregation occurred on {l_brace}115{r_brace}B planes associated with a {l_brace}115{r_brace}B surface faceting. Increase in growth temperature led to the formation of large, (001)-oriented, irregular-shaped platelets of Ge-rich material. Growth on {l_brace}115{r_brace}B substrates resulted in a ''natural superlattice'' of GaAs/Ge along the growth direction.
Date: September 13, 1999
Creator: Norman, A. G.; Olson, J. M.; Geisz, J. F.; Moutinho, H. R.; Mason, A.; Al-Jassim, M. M. et al.
Partner: UNT Libraries Government Documents Department

Optical Investigation of GaNAs

Description: A systematic study of the energy and time-resolved photoluminescence of GaInP/GaNxAs1-x double heterostructures has been performed for 0=x=1.3%. A large temperature-dependent optical-bowing coefficient (about 20-25 eV) is observed and the bandgap variation with temperature is found to depend on the nitrogen content. Finally, the minority-carrier lifetime is not simply related to the nitrogen content. Instead, the recombination rate is proportional to the majority-carrier concentration for x=0.3% and the carbon concentration for x=0.3%.
Date: September 28, 1998
Creator: Keyes, B. M.; Geisz, J. F.; Dippo, P. C.; Reedy, R.; Kramer, C.; Friedman, D. J. et al.
Partner: UNT Libraries Government Documents Department

Mutual Passivation in Dilulte GaNxAs1-x Alloys

Description: The dilute GaN{sub x}As{sub 1-x} alloys (with x up to 0.05) have exhibited many unusual properties as compared to the conventional binary and ternary semiconductor alloys. We report on a new effect in the GaN{sub x}As{sub 1-x} alloy system in which electrically active substitutional group IV donors and isoelectronic N atoms passivate each other's activity. This mutual passivation occurs in dilute GaN{sub x}As{sub 1-x} doped with group IV donors through the formation of nearest neighbor IV{sub Ga-}N{sub As} pairs when the samples are annealed under conditions such that the diffusion length of the donors is greater than or equal to the average distance between donor and N atoms. The passivation of the shallow donors and the N{sub As} atoms is manifested in a drastic reduction in the free electron concentration and, simultaneously, an increase in the fundamental band gap. This mutual passivation effect is demonstrated in both Si and Ge doped GaN{sub x}As{sub 1-x} alloys. Analytical calculations of the passivation process based on Ga vacancies mediated diffusion show good agreement with the experimental results.
Date: March 21, 2005
Creator: Yu, K.M.; Walukiewicz, W.; Wu, J.; Mars, D.E.; Scarpulla, M.A.; Dubon, O.D. et al.
Partner: UNT Libraries Government Documents Department

Local Structures and Interface Morphology of InGaAsN Thin Films Grown on GaAs

Description: The compound semiconductor system InGaAsN exhibits many intriguing properties which are particularly useful for the development of innovative high efficiency thin film solar cells and long wavelength lasers. The bandgap in these semiconductors can be varied by controlling the content of N and In and the thin films can yet be lattice-matched to GaAs. In the present work, x-ray absorption fine structure (XAFS) and grazing incidence x-ray scattering (GIXS) techniques have been employed to probe the local environment surrounding both N and In atoms as well as the interface morphology of InGaAsN thin films epitaxially grown on GaAs. The soft x-ray XAFS results around nitrogen K-edge reveal that N is in the sp{sup 3} hybridized bonding configuration in InGaAsN and GaAsN, suggesting that N impurities most likely substitute for As sites in these two compounds. The results of In K-edge XAFS suggest a possible trend of a slightly larger coordination number of As nearest neighbors around In atoms in InGaAsN samples with a narrower bandgap whereas the In-As interatomic distance remains practically the same as in InAs within the experimental uncertainties. These results combined suggest that N-substitution of the As sites plays an important role of bandgap-narrowing while in the meantime counteracting the compressive strain caused by In-doping. Grazing incidence x-ray scattering (GIXS) experiments verify that InGaAsN thin films can indeed form very smooth interfaces with GaAs yielding an average interfacial roughness of 5-20{angstrom}.
Date: February 23, 1999
Creator: Allerman, A.A.; Chen, J.G.; Geisz, J.F.; Huang, S.; Hulbert, S.L.; Jones, E.D. et al.
Partner: UNT Libraries Government Documents Department

Passivation of Interfaces in High-Efficiency Photovoltaic Devices

Description: Solar cells made from III-V materials have achieved efficiencies greater than 30%. Effectively ideal passivation plays an important role in achieving these high efficiencies. Standard modeling techniques are applied to Ga0.5In0.5P solar cells to show the effects of passivation. Accurate knowledge of the absorption coefficient is essential (see appendix). Although ultralow (<2 cm/s) interface recombination velocities have been reported, in practice, it is difficult to achieve such low recombination velocities in solar cells because the doping levels are high and because of accidental incorporation of impurities and dopant diffusion. Examples are given of how dopant diffusion can both help and hinder interface passivation, and of how incorporation of oxygen or hydrogen can cause problems.
Date: May 13, 1999
Creator: Kurtz, S. R.; Olson, J. M.; Friedman, D. J.; Geisz, J. F.; Kibbler, A. E. (National Renewable Energy Laboratory) & Bertness, K. A. (NIST, Boulder, Colorado)
Partner: UNT Libraries Government Documents Department

Nitrogen-Induced Modification of the Electronic Structure of Group III-N-V Alloys: Preprint

Description: Incorporation of nitrogen in conventional III-V compound semiconductors to form III-N-V alloys leads to a splitting of the conduction band into two nonparabolic sub-bands. The splitting can be described in terms of an anticrossing interaction between a narrow band of localized nitrogen states and the extended conduction-band states of the semiconductor matrix. The downward shift of the lower sub-band edge is responsible for the N-induced reduction of the fundamental band-gap energy. The modification of the conduction-band structure profoundly affects the optical and electrical properties of the III-N-V alloys.
Date: April 1, 1999
Creator: Walukiewicz, W.; Shan, W.; Ager III, J. W.; Chamberlin, D. R.; Haller, E. E. (Lawrence Berkeley National Laboratory); Geisz, J. F. et al.
Partner: UNT Libraries Government Documents Department

Mutual passivation of group IV donors and isovalent nitrogen in diluted GaN{sub x}As{sub 1-x} alloys

Description: We demonstrate the mutual passivation of electrically active group IV donors and isovalent N atoms in the GaN{sub x}As{sub 1-x} alloy system. This phenomenon occurs through the formation of a donor-nitrogen bond in the nearest neighbor IV{sub Ga}-N{sub As} pairs. In Si doped GaInN{sub 0.017}As{sub 0.983} the electron concentration starts to decrease rapidly at an annealing temperature of 700 C from {approx} 3 x 10{sup 19}cm{sup -3} in the as-grown state to less than 10{sup 16}cm{sup -3} after an annealing at 900 C for 10 s. At the same time annealing of this sample at 950 C increases the gap by about 35 meV, corresponding to a reduction of the concentration of the active N atoms by an amount very close to the total Si concentration. We also show that the formation of Si{sub Ga}-N{sub As} pairs is controlled by the diffusion of Si via Ga vacancies to the nearest N{sub As} site. The general nature of this mutual passivation effect is confirmed by our study of Ge doped GaN{sub x}As{sub 1-x} layers formed by N and Ge co-implantation in GaAs followed by pulsed laser melting.
Date: July 23, 2003
Creator: Yu, K.M.; Wu, J.; Walukiewicz, W.; Shan, W.; Beeman, J.; Mars, D.E. et al.
Partner: UNT Libraries Government Documents Department