D. J. Friedman

National Renewable Energy Laboratory, گلدن، کلرادو, Colorado, United States

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Publications (210)351.69 Total impact

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    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.
    Energy Procedia 08/2015; 77:464-469. DOI:10.1016/j.egypro.2015.07.066
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    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.
    Solar Energy Materials and Solar Cells 07/2015; 143:48-51. DOI:10.1016/j.solmat.2015.06.036 · 5.34 Impact Factor
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    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.
    42nd IEEE Photovoltaic Specialists Conference, New Orleans, Louisiana; 06/2015
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    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.
    IEEE Journal of Photovoltaics 01/2015; 5(1-1):432-437. DOI:10.1109/JPHOTOV.2014.2364132 · 3.17 Impact Factor
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    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.
    IEEE Journal of Photovoltaics 01/2015; 5(1-1):418-424. DOI:10.1109/JPHOTOV.2014.2361014 · 3.17 Impact Factor
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    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.
    IEEE Journal of Photovoltaics 01/2015; 5(1-1):438-445. DOI:10.1109/JPHOTOV.2014.2364128 · 3.17 Impact Factor
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    Nabil Mohammad · Peng Wang · Daniel J Friedman · Rajesh Menon
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    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.
    Optics Express 10/2014; 22 Suppl 6(21):A1519-25. DOI:10.1364/OE.22.0A1519 · 3.49 Impact Factor
  • Emmett E Perl · William E McMahon · John E Bowers · Daniel J Friedman
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    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.
    Optics Express 08/2014; 22(S5):A1243-A1256. DOI:10.1364/OE.22.0A1243 · 3.49 Impact Factor
  • I. García · J. F. Geisz · R. M. France · J. Kang · S.-H. Wei · M. Ochoa · D. J. Friedman
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    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.
    Journal of Applied Physics 08/2014; 116(7):074508. DOI:10.1063/1.4892773 · 2.18 Impact Factor
  • Daniel J. Friedman · John F. Geisz · Myles A. Steiner
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    ABSTRACT: We analyze the implications of luminescent coupling on multijunction cell design and performance, using a recently developed formalism that uses the measured luminescent coupling parameters as inputs to an analytical model of the full current–voltage (J–V) characteristic of the cell. This calculation of the full J–V curve allows the determination of the cell open-circuit voltage, short-circuit current, fill factor, and efficiency in the presence of luminescent coupling. We show that luminescent coupling affects critical aspects of the cell design that include the optimal junction thicknesses and bandgaps, and affects the dependence of the cell performance on the spectral content of the light illuminating it.
    IEEE Journal of Photovoltaics 05/2014; 4(3):986-990. DOI:10.1109/JPHOTOV.2014.2308722 · 3.17 Impact Factor
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    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.
    IEEE Journal of Photovoltaics 05/2014; 4(3):962-967. DOI:10.1109/JPHOTOV.2014.2304359 · 3.17 Impact Factor
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    ABSTRACT: We report on a photolithographic and electro-deposition process that results in an optimized front grid structure for high efficiency multi-junction III–V concentrator solar cells operating under flux concentrations up to 1000 suns. Two different thick photoresists were investigated to achieve a 6 µm wide grid line with an aspect ratio of 1:1. A positive photoresist, SPR220 manufactured by Rohm and Haas was compared with a negative photoresist, nXT15 manufactured by AZ. A gold sulfite electrolyte was employed to prevent underplating as well as for environmental and safety considerations. An initial layer of nickel was discovered to be necessary to prevent delamination of the fingers during the removal of the contact layer. When deposited on a purpose grown, heavily doped GaAs contact layer, this Ni/Au contact exhibits an acceptable specific contact resistance in the low 10−4 to mid 10−5 Ohm cm2 range along with excellent adhesion without sintering. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 03/2014; 23(5). DOI:10.1002/pip.2490 · 7.58 Impact Factor
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    ABSTRACT: A novel bonding approach with an interface consisting of a metal and dielectric is developed, and a “pillar-array” metal topology is proposed for minimal optical and electrical loss at the interface. This enables a fully lattice-matched two-terminal, four-junction device that consists of an inverted top two-junction (2J) cell with 1.85 eV GaInP/1.42 eV GaAs, and an upright lower 2J cell with ~1 eV GaInAsP/0.74 eV GaInAs aimed for concentrator applications. The fabrication process and simulation of the metal topology are discussed along with the results of GaAs/GaInAs 2J and (GaInP + GaAs)/GaInAs three-junction bonded cells. Bonding-related issues are also addressed along with optical coupling across the bonding interface. Copyright © 2014 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 01/2014; 23(5). DOI:10.1002/pip.2468 · 7.58 Impact Factor
  • Peng Wang · Jose Dominguez-Callabero · Daniel J. Friedman · Rajesh Menon
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    ABSTRACT: A semiconductor absorber with a single bandgap is unable to convert broadband sunlight into electricity efficiently. Photons with energy lower than the bandgap are not absorbed, whereas those with energy far higher than the bandgap lose energy via thermalization. In this Article, we demonstrate an approach to mitigate these losses via a thin, efficient broadband diffractive micro-structured optic that not only spectrally separates incident light but also concentrates it onto multiple laterally separated single-junction semiconductor absorbers. A fully integrated optoelectronic device model was applied in conjunction with a nonlinear optimization algorithm to design the optic. An experimental demonstration is presented for a dual-bandgap design using GaInP and GaAs solar cells, where a 20% increase in the total electric power is measured compared with the same cells without the diffractive optic. Finally, we demonstrate that this framework of broadband diffractive optics allows us to independently design for the number of spectral bands and geometric concentration, thereby enabling a new class of multi-bandgap photovoltaic devices with ultra-high energy conversion efficiencies. Copyright
    Optics for Solar Energy; 11/2013
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    ABSTRACT: Ordering-induced effects on dislocations in metallic alloys have been extensively studied due to their importance in technology applications. We demonstrate that dislocation behavior in ordered III-V semiconductor alloys can be drastically different. This is because ordering in bulk metallic alloys is generally stable, whereas the surface-stabilized group-III sublattice ordering of a III-V alloy is only metastable in the bulk. Here, we show that dislocation glide can release some of the energy stored by ordering of III-V alloys, enhancing the glide of any dislocation which cuts through the ordered layers to create an antiphase boundary in the ordering pattern. This leads to an experimentally observed glide-plane switch which is unique to ordered III-V alloys. Implications for other unique strain-relaxation processes in III-V ordered alloys are also discussed.
    Journal of Applied Physics 11/2013; 114:203506-203506. DOI:10.1063/1.4833244 · 2.18 Impact Factor
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    ABSTRACT: For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. We demonstrate a detailed model for solar cells that calculates the external luminescent efficiency and discuss the relationship between the external and internal luminescence. The model accounts for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell, and assumes isotropic internal emission. For single-junction cells, the calculation leads to V-oc, and for multijunction cells, the calculation leads to the V-oc of each junction as well as the luminescent coupling constant. In both cases, the effects of the optics are most prominent in cells with high internal radiative efficiency. Exploiting good material quality and high luminescent coupling, we demonstrate a two-junction nonconcentrator cell with a conversion efficiency of (31.1 +/- 0.9)% under the global spectrum.
    IEEE Journal of Photovoltaics 10/2013; 3(4):1437-1442. DOI:10.1109/JPHOTOV.2013.2278666 · 3.17 Impact Factor
  • D.J. Friedman · J.F. Geisz · M.A. Steiner
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    ABSTRACT: Luminescent coupling in multijunction solar cells is the phenomenon in which a junction in forward bias radiates photons that are absorbed in and converted to photocurrent by the junction beneath the radiating one. This effect can be significant in modern high-efficiency multijunction cells. We have previously developed a combined measurement and analytical approach to characterize the short-circuit current including the effects of nonlinear coupling in terms of measurable parameters η and φ that describe the coupling strength and linearity, respectively. Here, we develop an analytical model for the full current-voltage characteristic V (J) of a multijunction cell in the presence of luminescent coupling, in terms of the η and φ parameters. We compare the model with the measured V (J) parameters of GaInP/GaAs two-junction cells that exhibit differing degrees of luminescent coupling, and show that the model well describes the measurements. We then use the model to explore the consequences of luminescent coupling on the operating parameters of an idealized two-junction cell as a function of the top-junction thickness, focusing on the open-circuit voltage, fill factor, and efficiency. The results demonstrate that the strong luminescent coupling can significantly alter the dependence of cell efficiency on junction thickness, and that consequently the well-known optical-thinning design rules must be modified.
    IEEE Journal of Photovoltaics 10/2013; 3(4):1429-1436. DOI:10.1109/JPHOTOV.2013.2275189 · 3.17 Impact Factor
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    ABSTRACT: In this editorial, we report on the conclusions of a concentrator photovoltaics (CPV) industry group convened in July 2012 to develop pathways to large-scale CPV deployment, specifically targeting the installation of 100 GW of CPV in the United States by 2030. The group identified technical and financial barriers to this goal and developed a corresponding set of recommendations for overcoming these barriers. These recommendations focus on technical improvements at the system and cell levels and on activities needed to support the commercialization.
    IEEE Journal of Photovoltaics 10/2013; 3(4):1460-1463. DOI:10.1109/JPHOTOV.2013.2270341 · 3.17 Impact Factor
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    ABSTRACT: In high quality solar cells, the internal luminescence can be harnessed to enhance the overall performance. Internal confinement of the photons can lead to an increased open-circuit voltage and short-circuit current. Alternatively, in multijunction solar cells the photons can be coupled from a higher bandgap junction to a lower bandgap junction for enhanced performance. We model the solar cell as an optical cavity and compare calculated performance characteristics with measurements. We also describe how very high luminescent coupling alleviates the need for top-cell thinning to achieve current-matching.
    09/2013; DOI:10.1063/1.4822199

Publication Stats

5k Citations
351.69 Total Impact Points


  • 1993–2015
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      گلدن، کلرادو, Colorado, United States
  • 2005
    • University of Delaware
      • Institute of Energy Conversion (IEC)
      Ньюарк, Delaware, United States
  • 1985–1988
    • Stanford University
      • Department of Applied Physics
      Palo Alto, California, United States