Daniel B. Turner-Evans

California Institute of Technology, Pasadena, California, United States

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Publications (27)234.37 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Wire arrays have demonstrated promising photovoltaic performance as single junction solar cells and are well suited to defect mitigation in heteroepitaxy. These attributes can combine in tandem wire array solar cells, potentially leading to high efficiencies. Here, we demonstrate initial growths of GaAs on Si0.9Ge0.1 structures and investigate III-V on Si1-xGex device design with an analytical model and optoelectronic simulations. We consider Si0.1Ge0.9 wires coated with a GaAs0.9P0.1 shell in three different geometries: conformal, hemispherical, and spherical. The analytical model indicates that efficiencies approaching 34% are achievable with high quality materials. Full field electromagnetic simulations serve to elucidate the optical loss mechanisms and demonstrate light guiding into the wire core. Simulated current-voltage curves under solar illumination reveal the impact of a varying GaAs0.9P0.1 minority carrier lifetime. Finally, defective regions at the hetero-interface are shown to have a negligible effect on device performance if highly doped so as to serve as a back surface field. Overall, the growths and the model demonstrate the feasibility of the proposed geometries and can be used to guide tandem wire array solar cell designs.
    Journal of Applied Physics 07/2013; 114(1). · 2.21 Impact Factor
  • Daniel B Turner-Evans, Hal Emmer, Christopher T Chen, Harry A Atwater
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    ABSTRACT: A transparent, flexible contact is developed using Ni nanoparticles and Ag nanowires and demonstrated on free-standing, polymer embedded, Si microwire solar cells. Contact yields of over 99% and a series resistance of 14 Ω cm(2) are demonstrated.
    Advanced Materials 06/2013; · 14.83 Impact Factor
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    ABSTRACT: Silicon microwire arrays have recently demonstrated their potential for low-cost, high-efficiency photovoltaics and photoelectrochemical fuel generation. A remaining challenge to making this technology commercially viable is scaling up of microwire-array growth. We discuss here a technique for vapor–liquid–solid growth of microwire arrays on the scale of six-inch wafers using a cold-wall radio-frequency heated chemical vapor deposition furnace, enabling fairly uniform growth over large areas with rapid cycle time and improved run-to-run reproducibility. We have also developed a technique to embed these large-area wire arrays in polymer and to peel them intact from the growth substrate, which could enable lightweight, flexible solar cells with efficiencies as high as multicrystalline Si solar cells. We characterize these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structure and fidelity, and we test their energy-conversion properties using a methyl viologen (MV$^{2+/+}$) liquid junction contact in a photoelectrochemical cell. Initial photoelectrochemical conversion efficiencies suggest that the material quality of these microwire arrays is similar to smaller (∼1 cm$^2$) wire arrays that we have grown in the past, indicating that this technique is a viable way to scale up microwire-array devices.
    IEEE Journal of Photovoltaics 01/2012; 2(3):294-297. · 3.00 Impact Factor
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    ABSTRACT: Many proposed next-generation photovoltaic devices have complicated nano- and micro-structured architectures that are designed to simultaneously optimize carrier collection and light absorption. Characterization of the electrical properties of these highly structured materials can be challenging due to the difficulty of creating electrical contacts, as well as the need to decouple the properties of the contact from that of the semiconductor. Regenerative photoelectrochemistry is a powerful technique to characterize the electrical properties of such systems, providing a conformal liquid contact that can be ohmic or rectifying, depending on the system used. We demonstrate the use of the methyl viologen regenerative electrochemical system to characterize different stages of the fabrication of radial junction Si microwire (SiMW) solar cells. Photoelectrochemical characterization, combined with other more traditional measurements allows evaluation of how the different processing steps affect the device performance, without having to construct a fully integrated device. We describe the operating principle of this technique, and demonstrate that it can be applied to semiconductor materials with complex architectures.
    Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
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    ABSTRACT: Crystalline Si wires, grown by the vapor–liquid–solid (VLS) process, have emerged as promising candidate materials for lowcost, thin-film photovoltaics. Here, we demonstrate VLS-grown Si microwires that have suitable electrical properties for high-performance photovoltaic applications, including long minority-carrier diffusion lengths (L_n » 30 µm) and low surface recombination velocities (S « 70 cm·s^(-1)). Single-wire radial p–n junction solar cells were fabricated with amorphous silicon and silicon nitride surface coatings, achieving up to 9.0% apparent photovoltaic efficiency, and exhibiting up to ~600 mV open-circuit voltage with over 80% fill factor. Projective single-wire measurements and optoelectronic simulations suggest that large-area Si wire-array solar cells have the potential to exceed 17% energy-conversion efficiency, offering a promising route toward cost-effective crystalline Si photovoltaics.
    Energy & Environmental Science 03/2011; · 11.65 Impact Factor
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    ABSTRACT: Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n(+) emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H(2) evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies <0.2%, largely limited by the low photovoltage generated at the p-Si/H(2)O junction.
    Journal of the American Chemical Society 02/2011; 133(5):1216-9. · 10.68 Impact Factor
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    ABSTRACT: Silicon microwire arrays have recently demonstrated their potential for low cost, high efficiency photovoltaics. These high aspect ratio, radial junction wire arrays allow for the absorption of nearly all the incident sunlight while enabling efficient carrier extraction in the radial direction. One of the remaining challenges to make this technology commercially viable is scaling up of the microwire array growth. We discuss here a technique we have developed for vapor liquid solid growth of microwire arrays over full six-inch wafers using a cold-wall RF-heated chemical vapor deposition furnace. This geometry allows for fairly uniform growth over large areas, rapid cycle time, and improved run-to-run reproducibility. We have studied these large-area microwire arrays using scanning electron microscopy and confocal microscopy to assess their structural fidelity and uniformity. We have also developed a technique to embed these large-area arrays in polymer and peel them off the substrate, which could enable lightweight, flexible solar cells with efficiencies as high as crystalline Si solar cells. We have tested the energy conversion properties of these microwire array samples grown using a liquid junction contact and a photoelectrochemical cell. Initial efficiencies measured in this way suggest that the material quality of these microwire arrays is similar to earlier small-area wire arrays that we have grown, meaning that this technique is a viable way to scale up microwire array devices.
    01/2011;
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    ABSTRACT: Microwire solar cells have demonstrated promising optical and photovoltaic performance in arrays of single junction Si wires. Seeking higher efficiencies, we have numerically investigated III-V on Si1-xGex architectures as candidates for tandem microwire photovoltaics via optical and electronic transport modeling. Optical modeling indicates that light trapping is an important design criterion. Absorption is more than doubled by the presence of Al2O3 scattering particles around the wires, leading to high overall light collection despite low wire packing fraction. Texturing of the microwire outer surface, which was found to occur experimentally for GaP/Si microwires, is also shown to enhance absorption by over 50% relative to wires with smooth surfaces, allowing for the use of thinner layers. Finally, full optoelectronic simulations of GaAs on Ge structures revealed that current matching is attainable in these structures and that wire device efficiencies can approach those of planar cells.
    Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE; 01/2011
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    ABSTRACT: We report conformal, epitaxial growth of GaP layers on arrays of Si microwires. Silicon wires grown using chlorosilane chemical vapor deposition were coated with GaP grown by metal-organic chemical vapor deposition. The crystalline quality of conformal, epitaxial GaP/Si wire arrays was assessed by transmission electron microscopy and x-ray diffraction. Hall measurements and photoluminescence show p- and n-type doping with high electron mobility and bright optical emission. GaP pn homojunction diodes on planar reference samples show photovoltaic response with an open circuit voltage of 660 mV.
    Applied Physics Letters 12/2010; · 3.79 Impact Factor
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    ABSTRACT: Si microwire-array solar cells with Air Mass 1.5 Global conversion efficiencies of up to 7.9% have been fabricated using an active volume of Si equivalent to a 4 μm thick Si wafer. These solar cells exhibited open-circuit voltages of 500 mV, short-circuit current densities (J_(sc)) of up to 24 mA cm^(-2), and fill factors >65% and employed Al_2O_3 dielectric particles that scattered light incident in the space between the wires, a Ag back reflector that prevented the escape of incident illumination from the back surface of the solar cell, and an a-SiN_x:H passivation/anti-reflection layer. Wire-array solar cells without some or all of these design features were also fabricated to demonstrate the importance of the light-trapping elements in achieving a high J_(sc). Scanning photocurrent microscopy images of the microwire-array solar cells revealed that the higher J_(sc) of the most advanced cell design resulted from an increased absorption of light incident in the space between the wires. Spectral response measurements further revealed that solar cells with light-trapping elements exhibited improved red and infrared response, as compared to solar cells without light-trapping elements.
    Energy & Environmental Science 08/2010; · 11.65 Impact Factor
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    ABSTRACT: Si wire arrays have recently demonstrated their potential as photovoltaic devices. Using these arrays as a base, we consider a next generation, multijunction wire array architecture consisting of Si wire arrays with a conformal GaN<sub>x</sub>P<sub>1-x-y</sub>As<sub>y</sub> coating. Optical absorption and device physics simulations provide insight into the design of such devices. In particular, the simulations show that much of the solar spectrum can be absorbed as the angle of illumination is varied and that an appropriate choice of coating thickness and composition will lead to current matching conditions and hence provide a realistic path to high efficiencies. We have previously demonstrated high fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques, and we have now successfully grown conformal GaP coatings on these wires as a precursor to considering quaternary compound growth. Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray diffraction, Hall measurements, and optical absorption of polymer-embedded wire arrays using an integrating sphere were performed. The GaP epilayers have high structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them promising for future multijunction wire array solar cells.
    Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE; 07/2010
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    ABSTRACT: Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array’s volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit18 for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.
    Nature Material 02/2010; 9(3):239-244. · 35.75 Impact Factor
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    ABSTRACT: Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics. To achieve competitive conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array's volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.
    Nature Material 02/2010; · 35.75 Impact Factor
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    ABSTRACT: Silicon wire arrays, though attractive materials for use in photovoltaics and as photocathodes for hydrogen generation, have to date exhibited poor performance. Using a copper-catalyzed, vapor-liquid-solid-growth process, SiCl4 and BCl3 were used to grow ordered arrays of crystalline p-type silicon (p-Si) microwires on p+-Si(111) substrates. When these wire arrays were used as photocathodes in contact with an aqueous methyl viologen(2+/+) electrolyte, energy-conversion efficiencies of up to 3% were observed for monochromatic 808-nanometer light at fluxes comparable to solar illumination, despite an external quantum yield at short circuit of only 0.2. Internal quantum yields were at least 0.7, demonstrating that the measured photocurrents were limited by light absorption in the wire arrays, which filled only 4% of the incident optical plane in our test devices. The inherent performance of these wires thus conceptually allows the development of efficient photovoltaic and photoelectrochemical energy-conversion devices based on a radial junction platform.
    Science 01/2010; 327(5962):185-7. · 31.20 Impact Factor
  • ChemInform 01/2010; 41(14).
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    ABSTRACT: The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes.
    Applied Physics Letters 10/2009; · 3.79 Impact Factor
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    ABSTRACT: Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1-6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells.
    Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE; 07/2009
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    ABSTRACT: Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.
    Nano Letters 03/2008; 8(2):710-4. · 13.03 Impact Factor
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    ABSTRACT: Recent advances demonstrated metallic nanowires as structures allowing selective coupling of photons to fluctuations in the surface density of electrons, and the propagation of these plasmon modes along the wire. We report here the observation of such propagating plasmons in heterogeneous metal/semiconductor/metal nanowires. Specifically, we excite one end of a Au/CdSe/Au nanowire with focused laser light and demonstrate the coupling of photons into the plasmon modes of the wire. These modes propagate along the wire, being emitted as elastically scattered photons, exclusively at the metal/semiconductor interfaces and the distal end. Through control of the excitation wavelength and wire composition, we gain insights about the nature of the plasmon propagation through CdSe, allowing direct comparison with standard metal studies. This contributes to the growing interest in plasmonics within nanoscale devices by extending it to semiconductor materials, and goes towards the integration of optics with nanotechnology.
    03/2008;
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    ABSTRACT: Solar cells based on arrays of CVD-grown Si nano- or micro-wires are being considered as a potentially low-cost route to implementing a vertical multijunction cell design via radial p-n junctions. This geometry has been predicted to enable efficiencies competitive with planar multicrystalline Si designs, while reducing the materials and processing costs of solar cell fabrication [1]. To further assess the potential efficiency of cells based on this design, we present here experimental measurements of minority carrier diffusion lengths and surface recombination rates within nanowires via fabrication and characterization of single-wire solar cell devices. Furthermore, we consider a potential Si wire array-based solar cell design, and present device physics modeling of single-wire photovoltaic efficiency. Based on experimentally observed diffusion lengths within our wires, we model a radial junction wire solar cell capable of 17% photovoltaic energy conversion efficiency.
    Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE; 01/2008

Publication Stats

1k Citations
234.37 Total Impact Points

Institutions

  • 2008–2013
    • California Institute of Technology
      • Division of Chemistry and Chemical Engineering
      Pasadena, California, United States
  • 2006–2007
    • Yale University
      • Department of Biomedical Engineering
      New Haven, CT, United States