Erik C Garnett

Stanford University, Palo Alto, CA, United States

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Publications (16)276.19 Total impact

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    ABSTRACT: The phase reversal that occurs when light is reflected from a metallic mirror produces a standing wave with reduced intensity near the reflective surface. This effect is highly undesirable in optoelectronic devices that use metal films as both electrical contacts and optical mirrors, because it dictates a minimum spacing between the metal and the underlying active semiconductor layers, therefore posing a fundamental limit to the overall thickness of the device. Here, we show that this challenge can be circumvented by using a metamaterial mirror whose reflection phase is tunable from that of a perfect electric mirror (φ = π) to that of a perfect magnetic mirror (φ = 0). This tunability in reflection phase can also be exploited to optimize the standing wave profile in planar devices to maximize light-matter interaction. Specifically, we show that light absorption and photocurrent generation in a sub-100 nm active semiconductor layer of a model solar cell can be enhanced by ∼20% over a broad spectral band.
    Nature Nanotechnology 06/2014; · 31.17 Impact Factor
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    ABSTRACT: Silicon has been driving the great success of semiconductor industry and emerging forms of silicon has generated new opportunities in electronics, biotechnology and energy applications. Here we demonstrate large-area free-standing ultrathin single-crystalline Si at the wafer scale as new Si materials with processibility. We fabricated them by KOH etching of the Si wafer and show their uniform thickness from 10 to sub-2 micrometers. These ultrathin Si exhibits excellent mechanical flexibility and bendability more than those with 20-30 micrometers in thickness in previous study. Unexpectedly, these ultrathin Si materials can be cut with scissors like a piece of paper and they are robust during various regular fabrication processings including tweezer handling, spin coating, patterning, doping, wet and dry etching, annealing, and metal deposition. We demonstrate the fabrication of planar and double-sided nanocone solar cells and highlight that the processibility on both sides of surface together with the interesting property of these free-standing ultrathin Si materials opens up exciting opportunities to generate novel functional devices different from the existing approaches.
    Nano Letters 07/2013; · 13.03 Impact Factor
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    ABSTRACT: Non-periodic arrangements of nanoscale light scatterers allow for the realization of extremely effective broadband light-trapping layers for solar cells. However, their optimization is challenging given the massive number of degrees of freedom. Brute-force, full-field electromagnetic simulations are computationally too time intensive to identify high-performance solutions in a vast design space. Here we illustrate how a semi-analytical model can be used to quickly identify promising non-periodic spatial arrangements of nanoscale scatterers. This model only requires basic knowledge of the scattering behaviour of a chosen nanostructure and the waveguiding properties of the semiconductor layer in a cell. Due to its simplicity, it provides new intuition into the ideal amount of disorder in high-performance light-trapping layers. Using simulations and experiments, we demonstrate that arrays of nanometallic stripes featuring a limited amount of disorder, for example, following a quasi-periodic or Fibonacci sequence, can substantially enhance solar absorption over perfectly periodic and random arrays.
    Nature Communications 07/2013; 4:2095. · 10.74 Impact Factor
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    ABSTRACT: Copper nanofiber networks, which possess the advantages of low cost, moderate flexibility, small sheet resistance, and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the premier transparent electrode. However, the chemical activity of copper nanofibers causes a substantial increase in the sheet resistance after thermal oxidation or chemical corrosion of the nanofibers. In this work, we utilize atomic layer deposition to coat a passivation layer of aluminum-doped zinc oxide (AZO) and aluminum oxide onto electrospun copper nanofibers and remarkably enhance their durability. Our AZO-copper nanofibers show resistance increase of remarkably only 10% after thermal oxidation at 160 °C in dry air and 80 °C in humid air with 80% relative humidity, whereas bare copper nanofibers quickly become insulating. In addition, the coating and baking of the acidic PEDOT:PSS layer on our fibers increases the sheet resistance of bare copper nanofibers by 6 orders of magnitude, while the AZO-Cu nanofibers show an 18% increase.
    ACS Nano 05/2012; 6(6):5150-6. · 12.03 Impact Factor
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    ABSTRACT: Recently, hybrid Si/organic solar cells have been studied for low-cost Si photovoltaic devices because the Schottky junction between the Si and organic material can be formed by solution processes at a low temperature. In this study, we demonstrate a hybrid solar cell composed of Si nanocones and conductive polymer. The optimal nanocone structure with an aspect ratio (height/diameter of a nanocone) less than two allowed for conformal polymer surface coverage via spin-coating while also providing both excellent antireflection and light trapping properties. The uniform heterojunction over the nanocones with enhanced light absorption resulted in a power conversion efficiency above 11%. Based on our simulation study, the optimal nanocone structures for a 10 μm thick Si solar cell can achieve a short-circuit current density, up to 39.1 mA/cm(2), which is very close to the theoretical limit. With very thin material and inexpensive processing, hybrid Si nanocone/polymer solar cells are promising as an economically viable alternative energy solution.
    Nano Letters 04/2012; 12(6):2971-6. · 13.03 Impact Factor
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    ABSTRACT: Nanoscience provides many strategies to construct high-performance materials and devices, including solar cells, thermoelectrics, sensors, transistors, and transparent electrodes. Bottom-up fabrication facilitates large-scale chemical synthesis without the need for patterning and etching processes that waste material and create surface defects. However, assembly and contacting procedures still require further development. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into large interconnected networks. The small gaps that form naturally at nanowire junctions enable effective light concentration and heating at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a physical connection between the wires is made. The localized nature of the heating prevents damage to low-thermal-budget substrates such as plastics and polymer solar cells. This work opens new avenues to control light, heat and mass transport at the nanoscale.
    Nature Material 01/2012; 11(3):241-9. · 35.75 Impact Factor
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    ABSTRACT: In recent photovoltaic research, nanomaterials have offered two new approaches for trapping light within solar cells to increase their absorption: nanostructuring the absorbing semiconductor and using metallic nanostructures to couple light into the absorbing layer. This work combines these two approaches by decorating a single-nanowire silicon solar cell with an octahedral silver nanocrystal. Wavelength-dependent photocurrent measurements and finite-difference time domain simulations show that increases in photocurrent arise at wavelengths corresponding to the nanocrystal's surface plasmon resonances, while decreases occur at wavelengths corresponding to optical resonances of the nanowire. Scanning photocurrent mapping with submicrometer spatial resolution experimentally confirms that changes in the device's photocurrent come from the silver nanocrystal. These results demonstrate that understanding the interactions between nanoscale absorbers and plasmonic nanostructures is essential to optimizing the efficiency of nanostructured solar cells.
    Nano Letters 11/2011; 11(12):5189-95. · 13.03 Impact Factor
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    ABSTRACT: The nanowire geometry provides potential advantages over planar wafer-based or thin-film solar cells in every step of the photoconversion process. These advantages include reduced reflection, extreme light trapping, im-proved band gap tuning, facile strain relaxation, and increased defect toler-ance. These benefits are not expected to increase the maximum efficiency above standard limits; instead, they reduce the quantity and quality of mate-rial necessary to approach those limits, allowing for substantial cost reduc-tions. Additionally, nanowires provide opportunities to fabricate complex single-crystalline semiconductor devices directly on low-cost substrates and electrodes such as aluminum foil, stainless steel, and conductive glass, ad-dressing another major cost in current photovoltaic technology. This review describes nanowire solar cell synthesis and fabrication, important charac-terization techniques unique to nanowire systems, and advantages of the nanowire geometry.
    Annual Review of Materials Research 01/2011; 4141:269-95. · 13.07 Impact Factor
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    ABSTRACT: Recently, there have been several studies demonstrating that highly ordered nanoscale texturing can dramatically increase performance of applications such as light absorption in thin-film solar cells. However, those methods used to make the nanostructures are not compatible with large-scale fabrication. Here we demonstrate that a technique currently used in roll-to-roll processing to deposit uniform thin films from solution, a wire-wound rod coating method, can be adapted to deposit close-packed monolayers or multilayers of silica nanoparticles on a variety of rigid and flexible substrates. Amorphous silicon thin films deposited on these nanoparticle monolayers exhibit 42% higher absorption over the integrated AM 1.5 spectrum than the planar controls. This simple assembly technique can be used to improve solar cells, fuel cells, light emitting diodes and other devices where ordered nanoscale texturing is critical for optimal performance.
    Nano Letters 08/2010; 10(8):2989-94. · 13.03 Impact Factor
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    Erik Garnett, Peidong Yang
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    ABSTRACT: Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n(2) approximately 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between improved absorption and increased surface recombination; for nanowire arrays fabricated from 8 mum thick silicon films, the enhanced absorption can dominate over surface recombination, even without any surface passivation. These nanowire devices give efficiencies above 5%, with short-circuit photocurrents higher than planar control samples.
    Nano Letters 03/2010; 10(3):1082-7. · 13.03 Impact Factor
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    ABSTRACT: We demonstrate the basic operation of an organic/inorganic hybrid single nanowire solar cell. End-functionalized oligo- and polythiophenes were grafted onto ZnO nanowires to produce p-n heterojunction nanowires. The hybrid nanostructures were characterized via absorption and electron microscopy to determine the optoelectronic properties and to probe the morphology at the organic/inorganic interface. Individual nanowire solar cell devices exhibited well-resolved characteristics with efficiencies as high as 0.036%, J(sc) = 0.32 mA/cm(2), V(oc) = 0.4 V, and a FF = 0.28 under AM 1.5 illumination with 100 mW/cm(2) light intensity. These individual test structures will enable detailed analysis to be carried out in areas that have been difficult to study in bulk heterojunction devices.
    Nano Letters 12/2009; 10(1):334-40. · 13.03 Impact Factor
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    ABSTRACT: Silicon nanowires are expected to have applications in transistors, sensors, resonators, solar cells and thermoelectric systems. Understanding the surface properties and dopant distribution will be critical for the fabrication of high-performance devices based on nanowires. At present, determination of the dopant concentration depends on a combination of experimental measurements of the mobility and threshold voltage in a nanowire field-effect transistor, a calculated value for the capacitance, and two assumptions--that the dopant distribution is uniform and that the surface (interface) charge density is known. These assumptions can be tested in planar devices with the capacitance-voltage technique. This technique has also been used to determine the mobility of nanowires, but it has not been used to measure surface properties and dopant distributions, despite their influence on the electronic properties of nanowires. Here, we measure the surface (interface) state density and the radial dopant profile of individual silicon nanowire field-effect transistors with the capacitance-voltage technique.
    Nature Nanotechnology 06/2009; 4(5):311-4. · 31.17 Impact Factor
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    Erik C Garnett, Peidong Yang
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    ABSTRACT: We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n-p core-shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.
    Journal of the American Chemical Society 08/2008; 130(29):9224-5. · 10.68 Impact Factor
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    ABSTRACT: Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.
    Nature 01/2008; 451(7175):163-7. · 38.60 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2008; 39(14).
  • Erik C Garnett, Wenjie Liang, Peidong Yang
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    ABSTRACT: Silicon nanowires (Si NWs) will likely revolutionize a wide variety of applications ranging from field-effect transistors [1–4] (FETs) and other nanoelectronics to chemical and biological sensing, [5] and even solar cells. [6,7] These nanowire devices must be integrated with more traditional electronic or optical components to make a complete usable system, which will probably require standard silicon clean-room processing. Nearly all Si NWs are made using a gold (or gold-based) cata-lyst and the well-known vapor–liquid–solid (VLS) growth mechanism first discovered by Wagner and Ellis. [7,8] Because Au creates mid-gap trap states in silicon, it poisons device per-formance and typically is not allowed for use in electronics-fabrication labs and clean rooms. [9] Therefore a new, electron-ics-friendly catalyst is critical not only for nanowire electron-ics, but also for integrated devices incorporating Si NWs in any capacity. Several groups have successfully grown Si NWs with alternative catalyst thin films such as Ti, [10] Al, [11] Pt, [12] and PtSi [13] but extensive electrical characterization that is very important for many device applications has not been conducted. In this report, Pt was chosen as a catalyst because it has a high melting point, can be made into nanoparticles with a tight size distribution and shows orders-of-magnitude-lower leakage current when incorporated into silicon diodes compared to gold. [9,14,15] We have developed a chemical-va-por-deposition (CVD) synthesis based on our previous experi-ence with gold catalysts [16] to grow high-quality single-crystal-line size-controlled epitaxial Si NWs from various sized Pt nanoparticles. The nanowires were characterized by using scanning electron microscopy (SEM) and transmission elec-tron microscopy (TEM) to determine their size distribution, growth direction, and alignment, whereas their electrical properties were tested by making planar FETs. Unlike the Au–Si system, Pt does not form a simple eutectic with Si; rather, there are several stable platinum silicide com-pounds in the 800–1000 °C temperature range where Si-NW growth occurs. [17] There is a eutectic formed between PtSi and Si at 979 °C, so at temperatures above this point and at high Si concentrations, it is thermodynamically favorable to precipi-tate pure Si. [17,18] Si-NW growth below 979 °C can be ex-plained by two possible mechanisms. First, because the Pt nanoparticles begin melting (at least surface melting) around 600 °C, [19,20] which is about 1000 °C lower than the bulk melt-ing point, the bulk phase diagram may not accurately repre-sent the phase transitions occurring in the catalyst nanoparti-cle tip. In a very simplistic view, all the phase boundaries should shift down in temperature, with the Pt-rich phases being affected more strongly than the Si-rich phases. With a shift of over 1000 °C at the pure platinum side of the phase diagram, a 180 °C shift for the PtSi–Si eutectic down to 799 °C at 67 % Si seems likely. Several reports also show that Pt nanoparticles annealed in a hydrogen atmosphere at tempera-tures as low as 600 °C on silica substrates form Pt x Si y [21,22] Ad-ditionally, Wagner and Ellis found that even Pt thin films as thick as 100 nm on Si formed a liquid surface layer at temper-atures as low as 850 °C, further supporting a significant tem-perature decrease of the PtSi eutectic point from the bulk val-ue. [23] The second possible explanation is that the Pt nanoparticles do not completely melt and instead act as an active site for rapid SiCl 4 decomposition and diffusion, leading to a vapor– solid–solid (VSS) rather than VLS growth mechanism. The VSS mechanism has been proposed to explain the growth of several other semiconducting nanowires, particularly III–V compounds, that were originally thought to grow according to the VLS mechanism. [24,25] In a recent report, Pt thin films de-posited on Si were annealed at 800 °C in a hydrogen atmo-sphere to form PtSi islands which in turn were used to cata-lyze Si-NW growth at temperatures between 500 and 700 °C through a proposed VSS mechanism. [13] Considering the strong in situ TEM evidence from the literature mentioned above that the Pt nanoparticles begin melting well below their reaction temperatures, the island formation observed for Pt/Si films near 800 °C, and the evidence of strong eutectic-point depression seen for Pt thin films on Si, the Si NWs in this study most likely grow via the VLS mechanism. However, the VSS mechanism cannot be ruled out without in situ TEM evi-dence. Pt nanoparticle catalysts with average diameters of (9.3 ± 1.2) nm were used to synthesize Si NWs with average diameters of (11.3 ± 1.6) nm (Fig. 1). The standard deviations of the starting colloid and the resulting wire diameters were
    Advanced Materials 08/2007; 19:2946-2950. · 14.83 Impact Factor

Publication Stats

1k Citations
276.19 Total Impact Points

Institutions

  • 2010–2013
    • Stanford University
      • • Geballe Laboratory for Advanced Materials
      • • Department of Materials Science and Engineering
      • • Department of Electrical Engineering
      Palo Alto, CA, United States
  • 2008–2011
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, MO, United States