D. J. Friedman

National Renewable Energy Laboratory, Golden, Colorado, United States

Are you D. J. Friedman?

Claim your profile

Publications (224)383.26 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The inverted metamorphic solar cell has highly tunable bandgaps, in part due to the metamorphic subcells. Using phosphide-based compositionally graded buffers, we show a wide variety of GaInAs solar cells, ranging in bandgap from 1.2 to 0.7 eV. These metamorphic subcells are all high quality and can be used for a wide variety of multijunction designs. GaInAs solar cells with 0.70 eV bandgaps are developed using an InAsP buffer that extends beyond the InP lattice constant, allowing access to an additional 2 mA/cm2 of photocurrent at AM1.5D and 25 °C. This subcell is implemented into a four-junction inverted metamorphic solar cell combined with an appropriate antireflective coating, which increases the series-connected multijunction current by 0.5 mA/cm2 with respect to designs using 0.74-eV GaInAs. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. One-sun devices achieve 35.3 ± 1.2% efficiency under the AM0 spectra and 37.8 ± 1.2% efficiency under the global spectra at 25 °C. Concentrator devices designed for elevated operating temperature achieve 45.6 ± 2.3% peak efficiency under 690× the direct spectrum and 45.2 ± 2.3% efficiency at 1000× and 25 °C. Device optimization is performed for the direct spectrum on 1-sun devices with 2% shadowing, which achieve 39.8 ± 1.2% efficiency under the direct spectrum at 1 sun, highlighting the excellent performance and bandgap tunability of the four-junction inverted metamorphic solar cell.
    No preview · Article · Dec 2015 · IEEE Journal of Photovoltaics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Inverted metamorphic multijunction solar cells have been demonstrated to be a pathway to achieve the highest photovoltaic (PV) conversion efficiencies. Attaining high-quality lattice-mismatched (metamorphic) semiconductor devices is challenging. However, recent improvements to compositionally graded buffer epitaxy and junction structures have led to the achievement of high-quality metamorphic solar cells exhibiting internal luminescence efficiencies over 90%. For this high material quality, photon recycling is significant, and therefore, the optical environment of the solar cell becomes important. In this paper, we first present recent progress and performance results for 1- and 0.7-eV GaInAs solar cells grown on GaAs substrates. Then, an electrooptical model is used to assess the potential performance improvements in current metamorphic solar cells under different realizable design scenarios. The results show that the quality of 1-eV subcells is such that further improving its electronic quality does not produce significant Voc increases in the four-junction inverted metamorphic subcells, unless a back reflector is used to enhance photon recycling, which would significantly complicate the structure. Conversely, improving the electronic quality of the 0.7-eV subcell would lead to significant Voc boosts, driving the progress of four-junction inverted metamorphic solar cells.
    No preview · Article · Dec 2015 · IEEE Journal of Photovoltaics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Multijunction solar cells can be fabricated by mechanically bonding together component cells that are grown separately. Here, we present four-junction four-terminal mechanical stacks composed of GaInP/GaAs tandems grown on GaAs substrates and GaInAsP/GaInAs tandems grown on InP substrates. The component cells were bonded together with a low-index transparent epoxy that acts as an angularly selective reflector to the GaAs bandedge luminescence, while simultaneously transmitting nearly all of the subbandgap light. As determined by electroluminescence measurements and optical modeling, the GaAs subcell demonstrates a higher internal radiative limit and, thus, higher subcell voltage, compared with GaAs subcells without the epoxy reflector. The best cells demonstrate 38.8 ±1.0% efficiency under the global spectrum at 1000 W/m2 and ∼ 42% under the direct spectrum at ∼ 100 suns. Eliminating the series resistance is the key challenge for further improving the concentrator cells.
    No preview · Article · Nov 2015 · IEEE Journal of Photovoltaics
  • [Show abstract] [Hide abstract]
    ABSTRACT: The emission of light from each junction in a series-connected multijunction solar cell both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n ≠ 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs of the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements.
    No preview · Article · Oct 2015 · IEEE Journal of Photovoltaics
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report progress on the development of an advanced four-subcell IMM CPV solar cell that is designed for extremely high conversion efficiency under realistic concentrator operating conditions. Practical considerations allowing the design to mitigate problems related to Al-containing alloys, lattice mismatch, non-ideal short-wavelength response, and reflection losses are described. Performance modeling is used to guide the choice of optimal subcell band gaps for the new IMM cell. Early experimental efforts to develop and implement the new design are described and discussed. Copyright © 2015 John Wiley & Sons, Ltd.
    No preview · Article · Oct 2015 · Progress in Photovoltaics Research and Applications
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The performance of dual-junction solar cells with a Si bottom cell has been investigated both theoretically and experimentally. Simulations show that adding a top junction with an energy bandgap of 1.6 -1.9 eV to a standard silicon solar cell enables efficiencies over 38%. Currently, top junctions of GaInP (1.8 eV) are the most promising as they can achieve 1-sun efficiencies of 20.8%. We fabricated mechanically stacked, four terminal GaInP/Si tandem solar cells using a transparent adhesive between the subcells. These tandem devices achieved an efficiency of 27% under AM1.5g spectral conditions. Higher efficiencies can be achieved by using an improved Si-bottom cell and by optimizing the dual-junction device for long-wavelength light and luminescent coupling between the two junctions.
    Full-text · Article · Aug 2015 · Energy Procedia
  • Source

    Full-text · Dataset · Aug 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: Abstract The performance of tandem stacks of Group III-V multijunction solar cells continues to improve rapidly, both through improved performance of the individual cells in the stack and through increase in the number of stacked cells. As the radiative efficiency of these individual cells increases, radiative coupling between the stacked cells becomes an increasingly important factor not only in cell design, but also in accurate efficiency measurement and in determining performance of cells and systems under varying spectral conditions in the field. Past modeling has concentrated on electroluminescent coupling between the cells, although photoluminescent coupling is shown to be important for cells operating near their maximum power point voltage or below or when junction defect recombination is significant. Extension of earlier models is proposed to allow this non-negligible component of luminescent coupling to be included. The refined model is validated by measurement of the closely related external emission from both single and double junction cells.
    No preview · Article · Jul 2015 · Solar Energy Materials and Solar Cells
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Dual-junction solar cells consisting of rearheterojunction GaInP top cells and back-junction, backcontacted crystalline Si bottom cells were fabricated and characterized. Our calculations show that theoretical efficiencies up to 38.9% can be achieved with Si-based tandem devices. In our experiments, the two subcells were fabricated separately and stacked with an index matching fluid. In contrast to conventional mechanically stacked solar cells, that contain two metal grids at the interface, our concept includes a fully back contacted bottom cell which reduces the shadow losses in the device. A 1-sun AM1.5g cumulative efficiency of (26.2 ± 0.6)% has been achieved with this novel GaInP/Si 4-terminal tandem solar cell.
    Full-text · Conference Paper · Jun 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of ( $43.8 pm 2.2$)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of ($42.9 pm 2.1$ )% at 869 suns, which is the highest concentration measured. The $bf V!_bf oc$ increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.
    No preview · Article · Jan 2015 · IEEE Journal of Photovoltaics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high luminescence coupling to underlying subcells, both of which contribute to improved performance. Subcell voltages within two and four junction devices are measured by electroluminescence and the internal radiative efficiency is quantified as a function of recombination current using optical modeling. The performance of these concentrator multijunction devices is compared with the Shockley–Queisser detailed-balance radiative limit, as well as an internal radiative limit, which considers the effects of the actual optical environment in which a perfect junction may exist.
    No preview · Article · Jan 2015 · IEEE Journal of Photovoltaics
  • M. Ochoa · M. A. Steiner · I. García · J. F. Geisz · D. J. Friedman · C. Algora

    No preview · Article · Jan 2015 · Progress in Photovoltaics Research and Applications
  • [Show abstract] [Hide abstract]
    ABSTRACT: The variability of the solar spectra in the field may reduce the annual energy yield of multijunction solar cells. It would, therefore, be desirable to implement a cell design procedure based on the maximization of the annual energy yield. In this study, we present a measurement technique to generate maps of the real performance of the solar cell for a range of light spectrum contents using a solar simulator with a computer-controllable spectral content. These performance maps are demonstrated to be a powerful tool for analyzing the characteristics of any given set of annual spectra representative of a site and their influence on the energy yield of any solar cell. The effect of luminescence coupling on buffering against variations of the spectrum and improving the annual energy yield is demonstrated using this method.
    No preview · Article · Jan 2015 · IEEE Journal of Photovoltaics
  • Source
    Nabil Mohammad · Peng Wang · Daniel J Friedman · Rajesh Menon
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the enhancement of photovoltaic output power by separating the incident spectrum into 3 bands, and concentrating these bands onto 3 different photovoltaic cells. The spectrum-splitting and concentration is achieved via a thin, planar micro-optical element that demonstrates high optical efficiency over the entire spectrum of interest. The optic (which we call a polychromat) was designed using a modified version of the direct-binary-search algorithm. The polychromat was fabricated using grayscale lithography. Rigorous optical characterization demonstrates excellent agreement with simulation results. Electrical characterization of the solar cells made from GaInP, GaAs and Si indicate increase in the peak output power density of 43.63%, 30.84% and 30.86%, respectively when compared to normal operation without the polychromat. This represents an overall increase of 35.52% in output power density. The potential for cost-effective large-area manufacturing and for high system efficiencies makes our approach a strong candidate for low cost solar power.
    Preview · Article · Oct 2014 · Optics Express
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Quantitative electroluminescence (EL) and luminescent coupling (LC) analysis, along with more conventional characterization techniques, are combined to completely characterize the subcell JV curves within a fourjunction (4J) inverted metamorphic solar cell (IMM). The 4J performance under arbitrary spectral conditions can be predicted from these subcell JV curves. The internal radiative efficiency (IRE) of each junction has been determined as a function of current density from the external radiative efficiency using optical modeling, but this required the accurate determination of the individual junction current densities during the EL measurement as affected by LC. These measurement and analysis techniques can be applied to any multijunction solar cell. The 4J IMM solar cell used to illustrate these techniques showed excellent junction quality as exhibited by high IRE and a one-sun AM1.5D efficiency of 36.3%. This device operates up to 1000 suns without limitations due to any of the three tunnel junctions.
    Preview · Conference Paper · Sep 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Progressing beyond 3-junction inverted-metamorphic multijunction solar cells grown on GaAs substrates, to 4-junction devices, requires the development of high quality metamorphic 0.7 eV GaInAs solar cells. Once accomplished, the integration of this subcell into a full, monolithic, series connected, 4J-IMM structure demands the development of a metamorphic tunnel junction lattice matched to the 1eV GaInAs subcell. Moreover, the 0.7 eV junction adds about 2 hours of growth time to the structure, implying a heavier annealing of the subcells and tunnel junctions grown first. The final 4J structure is above 20 μm thick, with about half of this thickness used by the metamorphic buffers required to change the lattice constant throughout the structure. Thinning of these buffers would help reduce the total thickness of the 4J structure to decrease its growth cost and the annealing time. These three topics: development of a metamorphic tunnel junction for the 4th junction, analysis of the annealing, and thinning of the structure, are tackled in this work. The results presented show the successful implementation of an antimonide-based tunnel junction for the 4th junction and of pathways to mitigate the impact of annealing and reduce the thickness of the metamorphic buffers.
    Preview · Conference Paper · Sep 2014
  • Emmett E Perl · William E McMahon · John E Bowers · Daniel J Friedman
    [Show abstract] [Hide abstract]
    ABSTRACT: The successful development of multijunction photovoltaic devices with four or more subcells has placed additional importance on the design of high-quality broadband antireflection coatings. Antireflective nanostructures have shown promise for reducing reflection loss compared to the best thin-film interference coatings. However, material constraints make nanostructures difficult to integrate without introducing additional absorption or electrical losses. In this work, we compare the performance of various nanostructure configurations with that of an optimized multilayer antireflection coating. Transmission into a four-junction solar cell is computed for each antireflective design, and the corresponding cell efficiency is calculated. We find that the best performance is achieved with a hybrid configuration that combines nanostructures with a multilayer thin-film optical coating. This approach increases transmitted power into the top subcell by 1.3% over an optimal thin-film coating, corresponding to an increase of approximately 0.8% in the modeled cell efficiency.
    No preview · Article · Aug 2014 · Optics Express
  • I. García · J. F. Geisz · R. M. France · J. Kang · S.-H. Wei · M. Ochoa · D. J. Friedman
    [Show abstract] [Hide abstract]
    ABSTRACT: Lattice-matched and pseudomorphic tunnel junctions have been developed in the past for application in a variety of semiconductor devices, including heterojunction bipolar transistors, vertical cavity surface-emitting lasers, and multijunction solar cells. However, metamorphic tunnel junctions have received little attention. In 4-junction Ga0.51In0.49P/GaAs/Ga0.76In0.24As/Ga0.47In0.53As inverted-metamorphic solar cells (4J-IMM), a metamorphic tunnel junction is required to series connect the 3rd and 4th junctions. We present a tunnel junction based on a metamorphic Ga0.76In0.24As/GaAs0.75Sb0.25 structure for this purpose. This tunnel junction is grown on a metamorphic Ga0.76In0.24As template on a GaAs substrate. The band offsets in the resulting type-II heterojunction are calculated using the first-principles density functional method to estimate the tunneling barrier height and assess the performance of this tunnel junction against other material systems and compositions. The effect of the metamorphic growth on the performance of the tunnel junctions is analyzed using a set of metamorphic templates with varied surface roughness and threading dislocation density. Although the metamorphic template does influence the tunnel junction performance, all tunnel junctions measured have a peak current density over 200 A/cm(2). The tunnel junction on the best template has a peak current density over 1500 A/cm(2) and a voltage drop at 15 A/cm(2) (corresponding to operation at 1000 suns) lower than 10mV, which results in a nearly lossless series connection of the 4th junction in the 4J-IMM structure. (C) 2014 AIP Publishing LLC.
    No preview · Article · Aug 2014 · Journal of Applied Physics
  • Emmett E. Perl · William E. McMahon · John E. Bowers · Daniel J. Friedman
    [Show abstract] [Hide abstract]
    ABSTRACT: Multijunction photovoltaic devices with four or more junctions require low reflection over a wavelength range that is nearly 50% wider than what is required for a triple-junction design. Antireflective nanostructures can drastically reduce reflection across this range; however careful design is necessary for integration with multijunction devices. In this work, we address the design trade-offs imposed by material availability by modeling absorption and reflection loss for various configurations. We find that the best performance is obtained using a hybrid design that combines antireflective nanostructures with a thin-film optical coating. Our models show that this configuration can increase transmitted power into the solar cell by 2.1% compared to the best standalone nanostructure configuration and 1.3% compared to an optimal thin-film antireflection coating. We also detail a fabrication process for integrating this hybrid design onto an active photovoltaic device.
    No preview · Conference Paper · Jun 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ultrabroadband and wide-angle antireflection coatings (ARCs) are essential to realizing efficiency gains for state-of-the-art multijunction photovoltaic devices. In this study, we examine a novel design that integrates a nanostructured antireflection layer with a multilayer ARC. Using optical models, we find that this hybrid approach can reduce reflected AM1.5D power by 10–50 W/m$^{2}$ over a wide angular range compared to conventional thin-film ARCs. A detailed balance model correlates this to an improvement in absolute cell efficiency of 1–2%. Three different ARC designs are fabricated on indium gallium phosphide, and reflectance is measured to show the benefit of this hybrid approach.
    Full-text · Article · May 2014 · IEEE Journal of Photovoltaics

Publication Stats

6k Citations
383.26 Total Impact Points

Institutions

  • 1993-2014
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      Golden, Colorado, United States
    • Lund University
      • MAX-Lab
      Lund, Skåne, Sweden
  • 2005
    • University of Delaware
      • Institute of Energy Conversion (IEC)
      Ньюарк, Delaware, United States
  • 2004
    • Virginia Commonwealth University
      • Electrical Engineering
      Ричмонд, Virginia, United States
  • 1985-1989
    • Stanford University
      • Department of Applied Physics
      Palo Alto, California, United States