Richard R Lunt

Michigan State University, East Lansing, Michigan, United States

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Publications (31)213.72 Total impact

  • 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. · 7.94 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
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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: 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
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    Richard R. Lunt, Vladimir Bulovic
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    ABSTRACT: We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.
    Applied Physics Letters 03/2011; 98(11):113305-113305-3. · 3.79 Impact Factor
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    ABSTRACT: Crystalline order and orientation influence both the electronic and optical properties of thin organic crystalline films. Here, we demonstrate the quasiepitaxially ordered growth via organic vapor phase deposition of two organic materials, 1,4,5,8-naphthalene-tetracarboxylic-dianhydride and dibenzotetrathiafulvalene-tetracyanoquinodimethane on single-crystal substrates. To understand the quasiepitaxial orientations between the organic-inorganic and organic-organic lattices we compare geometrical lattice registry to full-structure van der Waals potential calculations, and find that only the complete description of the atomistic potential correctly matches experimentally observed orientations. We also demonstrate single-crystalline film growth of alternating multiple quasiepitaxial layers of these two organic semiconductors, and discuss this phenomenon in the context of incommensurate quasiepitaxy and surface energy matching, which is distinct from the lattice-matching criterion for inorganic heteroepitaxy.
    Physical review. B, Condensed matter 01/2011; 83(6).
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    ABSTRACT: Spin-cast 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQ) thin films only 62 A thick are converted from amorphous to polycrystalline via postannealing at elevated temperatures. The surface roughness of the SQ films increases by a factor of 2, while selected area electron diffraction spectra indicate an increase in the extent of postannealed film crystallinity. Dichloromethane solvent annealing is also demonstrated to increase the exciton diffusion length of SQ by a factor of 3 over thermally annealed SQ films as a result of further enhancement in crystalline order. We find that the roughened surface features have a length scale on the order of the exciton diffusion length. Hence, coating the donor SQ with the acceptor, C(60), results in a nearly optimum controlled bulk heterojunction solar cell structure. Optimized SQ/C(60) photovoltaic cells have a power conversion efficiency of eta(p) = 4.6 +/- 0.1% (correcting for solar mismatch) at 1 sun (AM1.5G) simulated solar intensity, and a corresponding peak external quantum efficiency of EQE = 43 +/- 1% even for the very thin SQ layers employed.
    Nano Letters 09/2010; 10(9):3555-9. · 13.03 Impact Factor
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    ABSTRACT: We demonstrate that organic photovoltaic cell performance is influenced by changes in the crystalline orientation of composite layer structures. A 1.5 nm thick self-organized, polycrystalline template layer of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) orients subsequently deposited layers of a diindenoperylene exciton blocking layer, and the donor, copper phthalocyanine (CuPc). Control over the crystalline orientation of the CuPc leads to changes in its frontier energy levels, absorption coefficient, and surface morphology, resulting in an increase of power conversion efficiency at 1 sun from 1.42 ± 0.04% to 2.19 ± 0.05% for a planar heterojunction and from 1.89 ± 0.05% to 2.49 ± 0.03% for a planar-mixed heterojunction.
    Optics Express 09/2010; 18 Suppl 3:A444-50. · 3.55 Impact Factor
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    Advanced Materials 07/2010; 22(25):2780-3. · 14.83 Impact Factor
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    Advanced Materials 03/2010; 22(11):1233-6. · 14.83 Impact Factor
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    ABSTRACT: We demonstrate that material utilization efficiencies of 50% and deposition nonuniformities ≤2.5% are achievable over substrate diameters of 200 mm using a simplified, organic vapor phase deposition (OVPD) system. The OVPD system is used to demonstrate doped electrophosphorescent organic light emitting diodes whose performance is comparable to those grown by vacuum thermal evaporation. Through continuum modeling, we demonstrate that analogous systems whose chamber dimensions are comparable to the substrate width are scalable to substrate sizes of at least 1500×1800 mm <sup>2</sup> with deposition nonuniformities between 1.5% and 2.5%. These results indicate that OVPD is useful in the large area deposition of displays, lighting, and other organic electronic devices.
    Applied Physics Letters 01/2010; · 3.79 Impact Factor

Publication Stats

165 Citations
213.72 Total Impact Points

Institutions

  • 2011–2012
    • Michigan State University
      • Department of Chemical Engineering and Materials Science
      East Lansing, Michigan, United States
    • 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