Richard R. Lunt

Michigan State University, East Lansing, Michigan, United States

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Publications (39)276.26 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto-underutilized means of control over the absolute energy levels of QDs within electronic devices. Here we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.
    ACS Nano 05/2014; · 12.03 Impact Factor
  • Advanced Optical Materials. 05/2014;
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    ABSTRACT: We demonstrate efficient zinc sulfide cathode window layers in thin-film organic photovoltaics enabled by n-type doping zinc sulfide (ZnS) with aluminum sulfide (Al2S3) directly through co-deposition. By optimizing the Al2S3 concentration, the power conversion efficiency is improved from 0.6% ± 0.2% in undoped ZnS window layer devices to 1.8% ± 0.1%, identical to control devices. The mechanism for this performance enhancement is shown to stem from the enhanced conductivity and interface energetics of ZnS upon n-type doping. This work expands the catalog of efficient, inorganic, non-toxic, cathode side window layers that could be effective in a range of thin-film photovoltaic technologies.
    Journal of Applied Physics 01/2014; 115(19):194505-194505-5. · 2.21 Impact Factor
  • Yimu Zhao, Richard R. Lunt
    Advanced Energy Materials 09/2013; 3(9). · 10.04 Impact Factor
  • Yimu Zhao, Richard Lunt
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    ABSTRACT: Luminescent solar concentrators (LSCs) have recently regained attention as a route for integration into the building envelope. To date, however, these systems have been limited to absorption and emission (glow) in the visible part of spectrum. We have designed and fabricated novel transparent luminescent solar concentrators devices composed of synthesized metal halide nanocrystal phosphorescent luminophores that allow for efficient and selective harvesting of ultraviolet (UV) photons with a near perfect absorption cutoff at the edge of the visible spectrum (430nm) while efficiently down-converting emitted light with a massive stoke shift to the near-infrared (800nm). We have demonstrated transparent LSCs with power efficiency of 0.8% ± 0.5%, system external quantum efficiency exceeding 35%, and an average transmittance of 82% ± 1%. We show through experiments and modeling that this architecture has the potential to exhibit up to 1-2% power conversion over module areas 1 m^2. These concentrators present new opportunities for non-tinted and highly-adoptable solar- windows that can translate into improved building efficiency, enhanced UV-barrier layers, and lower cost solar harvesting systems.
    03/2013;
  • Sean Wagner, Richard Lunt, Pengpeng Zhang
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    ABSTRACT: Control of highly ordered organic molecular thin films is currently of intense interest for integration into modern electronics due to the tunable nature of organic molecules. Here, we study the initial growth of archetypal zinc phthalocyanine (ZnPc) and copper phthalocyanine (CuPc) on the deactivated Si(111) surface. Using scanning probe microscopy (SPM), we demonstrate access to a new quasi-epitaxial anisotropic step-flow growth for both ZnPc and CuPc with a single dominant long-range ordered relationship between the organic crystalline film and the substrate, uniquely distinct from inorganic epitaxial step-flow growth. This growth mode is largely attributed to the molecular diffusion and preferential nucleation at step edges enabled by the deactivated Si surface. We demonstrate the transition of growth modes by varying substrate temperature during deposition, altering the balance between diffusion and step- and island- nucleation rates. Access to the anisotropic step-flow growth offers new potential for the integration of highly-ordered organic thin films in silicon-based electronics.
    03/2013;
  • Sean R Wagner, Richard R Lunt, Pengpeng Zhang
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    ABSTRACT: We report the first demonstration of anisotropic step-flow growth of organic molecules on a semiconducting substrate using metal phthalocyanine thermally deposited on the deactivated Si(111)-B sqrt[3]×sqrt[3] R30° surface. With scanning probe microscopy and geometric modeling, we prove the quasiepitaxial nature of this step-flow growth that exhibits no true commensurism, despite a single dominant long-range ordered relationship between the organic crystalline film and the substrate, uniquely distinct from inorganic epitaxial growth. This growth mode can likely be generalized for a range of organic molecules on deactivated Si surfaces and access to it offers new potential for the integration of ordered organic thin films in silicon-based electronics.
    Physical Review Letters 02/2013; 110(8):086107. · 7.73 Impact Factor
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    ABSTRACT: PbS colloidal quantum dot heterojunction solar cells have shown significant improvements in performance, mostly based on devices that use high-temperature annealed transition metal oxides to create rectifying junctions with quantum dot thin films. Here, we demonstrate a solar cell based on the heterojunction formed between PbS colloidal quantum dot layers and CdS thin films that are deposited via a solution process at 80 °C. The resultant device, employing a 1,2-ethanedithiol ligand exchange scheme, exhibits an average power conversion efficiency of 3.5%. Through a combination of thickness-dependent current density-voltage characteristics, optical modeling, and capacitance measurements, the combined diffusion length and depletion width in the PbS quantum dot layer is found to be approximately 170 nm.
    Nano Letters 02/2013; · 13.03 Impact Factor
  • Yimu Zhao, Richard R. Lunt
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    ABSTRACT: Transparent luminescent solar concentrators are a new standard for selective light harvesting allowing for non‐tinted, transparent, and highly scalable solar windows. These devices are enabled by exploiting phosphorescent hexanuclear nanocrystalline‐polymer blends that exhibit massive Stokes‐shifts, high phosphorescent quantum yield, high stability, and perfectly tuned absorption/emission around the visible spectrum.
    Laser Physics Review 01/2013; 3(9). · 10.04 Impact Factor
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    ABSTRACT: Integration of transparent photovoltaics into the building envelope creates unique opportunities to reduce the levelized electricity cost of solar power. However, this integration warrants consideration of the angular dependence of these devices as illumination around the building envelope is rarely at normal incidence. Here we correctly update transfer-matrix and equations to accurately model the quantum efficiency and optical properties under oblique illumination. We use this model to demonstrate the various angular performance characteristics possible for proof-of-concept optimizations of transparent planar-heterojunction solar cells and discuss considerations needed to fully account for optical, electrical, and positional configurations in this optimization.
    Applied Physics Letters 01/2013; 103(13):133304-133304-5. · 3.79 Impact Factor
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    ABSTRACT: Organic photovoltaics devices typically utilize illumination through a transparent substrate, such as glass or an optically clear plastic. Utilization of opaque substrates, including low cost foils, papers, and textiles, requires architectures that instead allow illumination through the top of the device. Here, we demonstrate top-illuminated organic photovoltaics, employing a dry vapor-printed poly(3,4-ethylenedioxythiophene) (PEDOT) polymer anode deposited by oxidative chemical vapor deposition (oCVD) on top of a small-molecule organic heterojunction based on vacuum-evaporated tetraphenyldibenzoperiflanthene (DBP) and C60 heterojunctions. Application of a molybdenum trioxide (MoO3) buffer layer prior to oCVD deposition increases the device photocurrent nearly 10 times by preventing oxidation of the underlying photoactive DBP electron donor layer during the oCVD PEDOT deposition, and resulting in power conversion efficiencies of up to 2.8% for the top-illuminated, ITO-free devices, approximately 75% that of the conventional cell architecture with indium-tin oxide (ITO) transparent anode (3.7%). Finally, we demonstrate the broad applicability of this architecture by fabricating devices on a variety of opaque surfaces, including common paper products with over 2.0% power conversion efficiency, the highest to date on such fiber-based substrates.
    Advanced Energy Materials 11/2012; 2(11). · 10.04 Impact Factor
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    ABSTRACT: We demonstrate series-integrated multijunction organic photovoltaics fabricated monolithically by vapor-deposition in a transposed subcell order with the near-infrared-absorbing subcell in front of the green-absorbing subcell. This transposed subcell order is enabled by the highly complementary absorption spectra of a near-infrared-absorbing visibly-transparent subcell and a visible-absorbing subcell and motivated by the non-spatially-uniform optical intensity in nanoscale photovoltaics. The subcell order and thicknesses are optimized via transfer-matrix formalism and short-circuit current simulations. An efficient charge recombination zone consisting of layers of BCP/Ag/MoOx leads to negligible voltage and series-resistance losses. Under 1-sun illumination the multijunction solar cells exhibit a power conversion efficiency of 5.5 ± 0.2% with an FF of 0.685 ± 0.002 and a V(OC) of 1.65 ± 0.02 V, corresponding to the sum of the V(OC) of the component subcells. These devices exhibit a broad spectral response (in the wavelength range of 350 nm to 850 nm) but are limited by subcell external quantum efficiencies between 20% and 30% over the photoactive spectrum.
    Physical Chemistry Chemical Physics 09/2012; 14(42):14548-53. · 3.83 Impact Factor
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    ABSTRACT: We demonstrate a near-infrared photodetector that consists of a thin film of the J-aggregating cyanine dye, U3, and transparent metal-oxide charge transport layers. The high absorption coefficient of the U3 film, combined with the use of a reflective anode and optical spacer layer, results in a zero-bias external quantum efficiency of 16.1 ± 0.1% (λ = 756 nm) for a device containing an 8.1 ± 0.3 nm-thick U3 film. The specific detectivity (D∗) and response speed (f3dB) of a fully optimized device are measured to be (4.3 ± 0.1) × 1011 cm Hz1/2 W−1 and 92 kHz, respectively.
    Applied Physics Letters 09/2012; 5(9). · 3.79 Impact Factor
  • Richard R. Lunt
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    ABSTRACT: Transparent photovoltaics (PVs) provide a potentially facile route to building-integrated PVs and seamless energy-harvesting within non-window surfaces such as electronic displays, autonomously powered electronic-glazings, and mobile-electronic accessories. Such devices have been enabled by manipulation of excitons in organic and molecular semiconductors that allow for selective ultraviolet and near-infrared solar conversion. Here, the theoretical efficiency limits of transparent photovoltaics are determined as a function of transparency. Power-production from ultraviolet and near-infrared photons alone leads to a theoretical single-junction efficiency of 21% in transparent structures, compared to 33% for opaque-junctions. Reducing thermal losses via transparent multi-junction stacking these limits increase to 37%.
    Applied Physics Letters 07/2012; 101(4). · 3.79 Impact Factor
  • Richard R. Lunt, Vladimir Bulovic
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    ABSTRACT: The practical efficiency limits for nanostructured photovoltaics including organic small molecule, dye-sensitized, polymer, and colloidal-quantum-dot architectures are assessed a posterori. Five decades since Shockley and Queisser derived the theoretical power conversion efficiency limit of single-junction photovoltaic cells, researchers have still not demonstrated such high performance for any photovoltaic device system. Hence, in evaluating the achievable performance of a comparatively new photovoltaic technologies, such as nanostructured PVs, it is prudent to estimate the upper limit of achievable efficiencies based on trends of the best technical demonstrations across the nanostructured platforms. This analysis is utilized to give a clear perspective on the potential market viability of these technologies in the near future and outline the challenges necessary to overcome this threshold. These technologies are compared and contrasted to provide an overview for the potential of each for reducing thermal losses with ``Third Generation'' concepts accessible to nanostructured PVs that can subsequently impact cost structures.
    02/2012;
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    ABSTRACT: We demonstrate single-walled carbon nanotube (SWCNT)/P3HT polymer bulk heterojunction solar cells with an AM1.5 efficiency of 0.72%, significantly higher than previously reported (0.05%). A key step in achieving high efficiency is the utilization of semiconducting SWCNTs coated with an ordered P3HT layer to enhance the charge separation and transport in the device active layer. Electrical characteristics of devices with SWCNT concentrations up to 40 wt % were measured and are shown to be strongly dependent on the SWCNT loading. A maximum open circuit voltage was measured for SWCNT concentration of 3 wt % with a value of 1.04 V, higher than expected based on the interface band alignment. Modeling of the open-circuit voltage suggests that despite the large carrier mobility in SWCNTs device power conversion efficiency is governed by carrier recombination. Optical characterization shows that only SWCNT with diameter of 1.3-1.4 nm can contribute to the photocurrent with internal quantum efficiency up to 26%. Our results advance the fundamental understanding and improve the design of efficient polymer/SWCNTs solar cells.
    Nano Letters 12/2011; 11(12):5316-21. · 13.03 Impact Factor
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    ABSTRACT: The significant research interest in the engineering of photovoltaic (PV) structures at the nanoscale is directed toward enabling reductions in PV module fabrication and installation costs as well as improving cell power conversion efficiency (PCE). With the emergence of a multitude of nanostructured photovoltaic (nano-PV) device architectures, the question has arisen of where both the practical and the fundamental limits of performance reside in these new systems. Here, the former is addressed a posteriori. The specific challenges associated with improving the electrical power conversion efficiency of various nano-PV technologies are discussed and several approaches to reduce their thermal losses beyond the single bandgap limit are reviewed. Critical considerations related to the module lifetime and cost that are unique to nano-PV architectures are also addressed. The analysis suggests that a practical single-junction laboratory power conversion efficiency limit of 17% and a two-cell tandem power conversion efficiency limit of 24% are possible for nano-PVs, which, when combined with operating lifetimes of 10 to 15 years, could position them as a transformational technology for solar energy markets.
    Advanced Materials 11/2011; 23(48):5712-27. · 14.83 Impact Factor
  • Advanced Materials 08/2011; 23(31):3499. · 14.83 Impact Factor
  • Advanced Materials 08/2011; 23(31):3499-3505. · 14.83 Impact Factor
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    ABSTRACT: The ability to engineer interfacial energy offsets in photovoltaic devices is one of the keys to their optimization. Here, we demonstrate that improvements in power conversion efficiency may be attained for ZnO/PbS heterojunction quantum dot photovoltaics through the incorporation of a MoO(3) interlayer between the PbS colloidal quantum dot film and the top-contact anode. Through a combination of current-voltage characterization, circuit modeling, Mott-Schottky analysis, and external quantum efficiency measurements performed with bottom- and top-illumination, these enhancements are shown to stem from the elimination of a reverse-bias Schottky diode present at the PbS/anode interface. The incorporation of the high-work-function MoO(3) layer pins the Fermi level of the top contact, effectively decoupling the device performance from the work function of the anode and resulting in a high open-circuit voltage (0.59 ± 0.01 V) for a range of different anode materials. Corresponding increases in short-circuit current and fill factor enable 1.5-fold, 2.3-fold, and 4.5-fold enhancements in photovoltaic device efficiency for gold, silver, and ITO anodes, respectively, and result in a power conversion efficiency of 3.5 ± 0.4% for a device employing a gold anode.
    Nano Letters 06/2011; 11(7):2955-61. · 13.03 Impact Factor

Publication Stats

330 Citations
276.26 Total Impact Points

Institutions

  • 2011–2014
    • Michigan State University
      • Department of Chemical Engineering and Materials Science
      East Lansing, Michigan, United States
  • 2011–2012
    • Massachusetts Institute of Technology
      • • Department of Materials Science and Engineering
      • • Department of Physics
      Cambridge, MA, United States
  • 2007–2011
    • University of Michigan
      • • Department of Materials Science and Engineering
      • • Department of Electrical Engineering and Computer Science (EECS)
      Ann Arbor, MI, United States
  • 2008–2010
    • Princeton University
      • • Department of Chemical and Biological Engineering
      • • Institute for Science and Technology of Materials
      Princeton, NJ, United States
  • 2009
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States