D. J. Friedman

National Renewable Energy Laboratory, Golden, Colorado, United States

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Publications (200)344.67 Total impact

  • 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 · 3.53 Impact Factor
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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 · 3.53 Impact Factor
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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 · 2.19 Impact Factor
  • D.J. Friedman, J.F. Geisz, M.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 · 3.00 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 · 3.00 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 · 9.70 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 · 9.70 Impact Factor
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    ABSTRACT: We describe a novel approach to improve efficiency of photovoltaics via a microstructured broadband diffractive optic that spectrally splits and concentrates incident sunlight onto multiple laterally separated solar cells of different bandgaps.
    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 · 2.19 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 · 3.00 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
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    ABSTRACT: The three-junction 1.8/1.4/1.0-eV inverted metamorphic multijunction solar cell can be extended to four junctions by adding another lattice-mismatched GaInAs junction with a bandgap of 0.7 eV. However, this requires a significant amount of mismatch to GaAs substrates, i.e., 3.8%, which is difficult to obtain while maintaining high-quality material. In this paper, we perform an in-depth investigation of a GaInP compositionally graded buffer varying in composition between Ga 0.5In 0.5P and InP in order to identify limitations to dislocation glide and sources of excess dislocation formation. In situ wafer curvature, cathodoluminescence, and X-ray diffraction (XRD) are used to analyze dislocation glide; transmission electron microscopy, atomic force microscope, and XRD are used to analyze material structural properties. Composition nonuniformities and roughness are observed, and a region in the compositionally graded buffer where a significant number of excess dislocations are formed is identified. The formation of these dislocations is related to atomic ordering, which has a large influence on the dislocation behavior. Adding thickness to the region in the buffer where dislocations are formed reduces the threading dislocation density an order of magnitude. Metamorphic 0.74 eV solar cells grown on this template have internal quantum efficiency > 90% and Voc > 0.3 V with Jsc set to 13 mA/cm2, which is the expected current in a multijunction device. These results are compared with lattice-matched GaInAs/InP solar cells to evaluate the loss associated with the lattice-mismatch.
    IEEE Journal of Photovoltaics 09/2013; DOI:10.1109/JPHOTOV.2013.2281724 · 3.00 Impact Factor
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    ABSTRACT: We demonstrate 1.81 eV GaInP solar cells approaching the Shockley-Queisser limit with 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80–90% internal radiative efficiency at one-sun AM1.5 global conditions. Optically enhanced voltage through photon recycling that improves light extraction was achieved using a back metal reflector. This optical enhancement was realized at one-sun currents when the non-radiative Sah-Noyce-Shockley junction recombination current was reduced by placing the junction at the back of the cell in a higher band gap AlGaInP layer. Electroluminescence and dark current-voltage measurements show the separate effects of optical management and non-radiative dark current reduction.
    Applied Physics Letters 07/2013; 103(4). DOI:10.1063/1.4816837 · 3.52 Impact Factor
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    ABSTRACT: Efficiencies exceeding 40% have already been achieved with GaAs-based multijunction (MJ) solar cells. In this talk, we will discuss the unique advantages and challenges of fabricating hybrid InGaN-GaAs MJ cells for ultrahigh efficiency device designs.
    CLEO: Applications and Technology; 06/2013
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    ABSTRACT: Ultra-broadband Antireflection Coatings (ARCs) are essential to realizing the potential efficiency gains of four-junction photovoltaic devices that absorb to longer wavelengths than state-of-the-art three-junction cells. In this work, 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 the reflected AM1.5D power by 10-45 W/m2 compared to conventional thin-film ARCs. A hybrid ARC is designed and fabricated on a sample consisting of approximately 1μm of indium gallium phosphide (InGaP) on gallium arsenide (GaAs). For the hybrid coating, we measure a reflection loss of just 23.9 W/m2 corresponding to less than a 3% power reflection.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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    ABSTRACT: The paper discusses how the Terrestrial simulator approach can be applied to the AM0 spectrum; Reduce the need for balloon flight standards. The paper shows how the cell design can be improved through better understanding of internal physics (e.g. luminescent coupling). A description of spectrally adjustable multisource simulators at NREL and measurement techniques and algorithms for an arbitrary number of junctions. Examples are given for 3, 5, and 6 junction solar cells. Finally the talk describes the characterization of luminescent coupling of subcell currents
    Space Power Workshop, Manhattan Beach, CA; 04/2013
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    ABSTRACT: Metal-interconnected multijunction solar cells offer one pathway toward efficiencies in excess of 50%. However, if a three- or four-terminal configuration is used, optical losses from the interfacial grid can be considerable. Here, we examine an alternative that provides an optimal interconnection for two-terminal bonded devices. This “pillar-array” topology is optimized by minimizing the sum of all power losses, including shadow losses and numerically computed electrical losses. Numerical modeling is used to illustrate the benefit of a pillar-array interfacial metallization for some two-terminal configurations.
    IEEE Journal of Photovoltaics 04/2013; 3(2):868-872. DOI:10.1109/JPHOTOV.2012.2234208 · 3.00 Impact Factor
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    ABSTRACT: A unique aspect of the inverted metamorphic multijunction (IMM) solar cell is the bandgap tunability of each junction, creating extremely flexible device designs. The optimal structure has subcell photocurrents that are matched for a given spectrum. However, the subcell photocurrents depend on the cell operating temperature, and therefore, the bandgaps need to be optimized for a certain range of operating conditions. In addition, imperfect material quality results in a loss of voltage and current that depends on the cell bandgap and thickness. In this case, an iterative process of multijunction design and subcell characterization is necessary to determine the optimal design. We compare two different three-junction devices to demonstrate the effect of bandgap selection and lattice-mismatched material quality on device performance at different temperatures. The triple-junction (3J)-IMM design with two lattice-mismatched junctions of perfect material quality (2MMJ) is theoretically optimal at room temperature but experimentally performs similarly to a simpler design with one mismatched junction (1MMJ) at higher temperature because of material quality tradeoffs and the temperature dependence of the designs. Significant progress in the growth, processing, and measurement has led to a 1MMJ design with (42.6 ± 2.1)% peak efficiency at 327 suns and (40.9 ± 2.0)% efficiency at 1093 suns under the direct spectrum.
    IEEE Journal of Photovoltaics 04/2013; 3(2):893-898. DOI:10.1109/JPHOTOV.2013.2239358 · 3.00 Impact Factor
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    ABSTRACT: High-quality, direct-bandgap solar cells emit significant luminescence at their band edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III-V multijunction solar cell. We describe a technique that uses a set of light-emitting diodes (LEDs) and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.
    IEEE Journal of Photovoltaics 04/2013; 3(2):879-887. DOI:10.1109/JPHOTOV.2012.2228298 · 3.00 Impact Factor
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    ABSTRACT: The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by the model are in good agreement with the data.
    Journal of Applied Physics 03/2013; 113(12, 28 March 2013). DOI:10.1063/1.4798267 · 2.19 Impact Factor

Publication Stats

4k Citations
344.67 Total Impact Points


  • 1993–2014
    • National Renewable Energy Laboratory
      • National Center for Photovoltaics
      Golden, Colorado, United States
  • 2005
    • University of Delaware
      • Institute of Energy Conversion (IEC)
      Ньюарк, Delaware, United States
  • 1985–1987
    • Stanford University
      • Department of Applied Physics
      Palo Alto, California, United States