Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F34: Focus Session: Novel Photophysics and Transport Mechanisms for Nanostructured Photovoltaics |
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Sponsoring Units: GERA Room: 210A |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F34.00001: Electronic Spectroscopy at the Solid-Liquid Interface: A Window to Electrochemistry and Solvation Phenomena Invited Speaker: Miquel Salmeron Basic phenomena in electrochemistry, and environmental science occur at solid-liquid interfaces. To obtain information on the atomic structure, composition, electronic levels of surface and adsorbed molecules optical (IR, SFG), structural (STM, AFM, GIXS), and x-ray spectroscopy techniques (XAS-XES) can be used. In fluorescence yield mode, XAS provides information on the electronic structure of sample materials within the escape/penetration depth of x-rays, which for soft x-rays is about 100-1000 nm. In my laboratory we use a well-known variant known as electron yield mode (EY-XAS), by measuring the electrode current which is the balance between emission and capture of secondary electrons. We applied EY-XAS to study electrode interfaces in aqueous electrolytes. In the case of graphene we studied reactions leading to the oxidation. In the case of gold we determined the orientation of water molecules near the interface, due to interaction with gold and as a function of applied bias. In the case of Platinum we studied oxidation-reduction reactions in acid electrolytes. Because XAS spectra are dependent on x-ray absorption rules and transition probabilities in the presence of core holes, interpretation of the spectra is not trivial and requires theoretical calculations for a meaningful interpretation. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F34.00002: Straddling to staggered band gap transition and optics of double-walled carbon nanotubes Alex Kutana, Vasilii I. Artyukhov, Boris I. Yakobson Single-walled carbon nanotubes have outstanding optical properties that show pronounced dependence on their helicity. Techniques for helicity separation or selective growth enabled extensive studies of their optics. However, in double-walled nanotubes (DWNT) the inner wall helicity is hard to control. Only recently effective techniques for DWNT sorting and helicity assignment were developed,\footnote{A. Loiseau \emph{et al.}, NT14, Los Angeles, CA (2014)} and experiments on helicity-controlled DWNT reveal remarkable new optical effects.\footnote{K. Liu \emph{et al.}, \emph{Nat. Phys.} 10, 737 (2014)} Here we study the effect of breaking of $\pi$ band symmetry by wall curvature, creating a potential difference between the inside and outside of a nanotube. In DWNT, this intrinsic flexoelectric voltage, $V\propto1/R$, shifts the bands of the inner wall, and above a certain threshold the voltage exceeds the band offset ($\propto1/R^{2}$), marking the transition from conventional straddling to staggered band structure, where frontier electronic bands are localized on different walls. This has dramatic optical implications which we study using density functional theory and many-body $GW$+BSE calculations.\footnote{V. I. Artyukhov, A. Kutana, B. I. Yakobson, in preparation} [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F34.00003: Free Charge Carriers in Lead Iodide Perovskites Revealed by Transient Absorption Spectroscopy Minh Tuan Trinh, Xiaoyang Zhu Lead iodide perovskites have emerged as the most-promising materials for low-cost thin film photovoltaics and other optoelectronic materials. Much experimental effort has been devoted to understanding fundamental properties and device performances, however the nature of photo-excitation in these materials remains debated. Using transient absorption spectroscopy, we study the dynamics of electrons and holes in photo-excited lead iodide perovskites. We show that, upon excitation with photon energy above the bandgap, hot electrons and holes are created. The e-h pair possess a transient dipole moment, which induces a transient Stark effect seen in subsequent optical transitions by the probe pulse. At high excitation densities, we find that carrier decay is well described by a third-order kinetic process, as expected from Auger recombination of free carriers. We also provide direct evidence for charge carrier traps on the surfaces and excitonic traps below the optical gaps in these materials. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F34.00004: Enhanced Multiple Exciton Generation in Amorphous Silicon Nanoparticles Andrei Kryjevski, Deyan Mihaylov, Dmitri Kilin Multiple exciton generation (MEG) in nm-sized hydrogen-passivated silicon nanowires (NWs), and quasi two-dimensional nanofilms depends strongly on the degree of the core structural disorder as shown by the many-body perturbation theory (MBPT) calculations based on the DFT simulations. Here, we use the HSE exchange correlation functional. In MBPT, we work to the 2nd order in the electron-photon coupling and in the approximate screened Coulomb interaction. We also include the effect of excitons for which we solve Bethe-Salpeter Equation. We calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, in 3D arrays of ${\rm Si}_{29}{\rm H}_{36}$ quantum dots, NWs, and quasi 2D silicon nanofilms, all with both crystalline and amorphous core structures. Efficient MEG with QE of 1.3 up to 1.8 at the photon energy of about $3E_g$, where $E_g$ is the gap, is predicted in these nanoparticles except for the crystalline NW and film where ${\rm QE}\simeq 1.$ MEG in the amorphous nanoparticles is enhanced by the electron localization due to structural disorder. The exciton effects significantly red-shift QE$({\rm E}_{photon})$ curves. Nanometer-sized amorphous silicon NWs and films are predicted to have effective MEG within the solar spectrum range. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F34.00005: Hot electron-generated plasmon resonance in ultrathin solar absorbers: Theory Jiantao Kong, Chaobin Yang, Juan M. Merlo, Michael J. Burns, Michael J. Naughton, Krzysztof Kempa It has been proposed in a simple model calculation that hot electrons excited in a semiconductor can emit plasmons in an adjacent metallic nanostructure at very high rate, exceeding that of phonon emission [1]. We demonstrate by FDTD simulations and quantum mechanical calculations that an ultrathin solar absorber with a composite metamaterial/plasmonic collector can ``hold'' and make use of the excess energy of hot electrons, and thus yield PV efficiency in excess of the Shockley-Queisser limit. The composite collector has a dual function: it is designed to efficiently trap light and it is a plasmonic resonator tuned to absorb the energy of hot electrons, thus ``protecting'' them from phonon losses. We propose a specific structure where observation of this phenomenon can occur.\\[4pt] [1] K. Kempa, Phys. Status Solidi RRL \textbf{7}, 465 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F34.00006: Hot electron-generated plasmon resonance in ultrathin solar absorbers: Experiment Chaobin Yang, Juan M. Merlo, Aaron H. Rose, Jiantao Kong, Michael J. Burns, Krzysztof Kempa, Michael J. Naughton We describe experimental progress on a hot electron PV structure based on hot electron plasmon protection (HELPP) [1] that provides a path to solar efficiency in excess of the Shockley-Queisser limit. It combines hot electron recovery in ultrathin junctions with superabsorption in metamaterial/plasmonic nanosystems and a HELPP/ plasmon resonance energy transfer (PRET) mechanism. Measurements of optical absorbance (via reflectance and transmittance) of Ag nanopatterns on p- and n-type crystalline and amorphous Si absorbers were performed at incident wavelengths from 350 to 2,500 nm. In samples prepared with Ag nanopatterns with dimensions tuned to provide a resonance near 1,600 nm, we indeed observed such a resonance. We discuss these and other experimental results associated with hot electron-facilitated plasmon resonances.\\[4pt] [1] K. Kempa, ``Plasmonic protection of the hot-electron energy,'' Phys. Stat. Sol. RRL, \textbf{7}, 465-468 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F34.00007: Colloidal Nanoparticles for Intermediate Band Solar Cells Marton Voros, Giulia Galli, Gergely Zimanyi The Intermediate Band (IB) solar cell concept is a promising idea to transcend the Shockley-Queisser limit.[1] Using the results of first principles calculations, we proposed that colloidal nanoparticles (CNPs) are a viable and efficient platform for the implementation of the IB solar cell concept. We focused on CdSe and we showed that intragap states present in the isolated dots with reconstructed surfaces combine to form an IB in arrays of NPs, which is well separated from the valence and conduction band edges. We also showed that in solution such IB may be electron doped using, e.g. decamethylcobaltocene, thus activating an IB-induced absorption process. Our results, together with the recent report of a nearly 9\% efficient CNP solar cell[2] indicate that colloidal nanoparticle intermediate band solar cells are a promising platform to overcome the Shockley-Queisser limit.\newline [1] Antonio Luque and Antonio Mart\'i, Phys. Rev. Lett. 78, 5014 (1997). [2] Chia-Hao M. Chuang et al., Nature Materials 13, 796 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F34.00008: The Molecular Photo-Cell: Quantum Transport and Energy Conversion at Strong Non-Equilibrium Shigeru Ajisaka, Bojan Zunkovic, Jonatan Dubi The molecular photo-cell is a single molecular donor-acceptor complex attached to electrodes and subject to external illumination. Besides the obvious relevance to molecular photo-voltaics, the molecular photo-cell is of interest being a paradigmatic example for a system that inherently operates in out-of-equilibrium conditions and typically far from the linear response regime. Moreover, this system includes electrons, phonons and photons, and environments which induce coherent and incoherent processes, making it a challenging system to address theoretically. Here, using an open quantum systems approach, we analyze the non-equilibrium transport properties and energy conversion performance of a molecular photo-cell, including for the first time both coherent and incoherent processes and treating electrons, photons, and phonons on an equal footing. We find that both the non-equilibrium conditions and decoherence play a crucial role in determining the performance of the photovoltaic conversion and the optimal energy configuration of the molecular system. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F34.00009: Experimental evidence of enhanced optical absorption in ultra-thin film CdTe PV absorbers via embedded metal nanopatterns Aaron H. Rose, Michelle L. Solomon, Michael J. Burns, Michael J. Naughton We show an increase of optical absorbance in ultra-thin film cadmium telluride solar absorbers by embedded metal nanopatterns (EMN) in the absorber material. Our experimental results are supported by FEM simulations, which also indicate minimal energy loss in the metal. As opposed to locating the metal nanostructures outside of the active region, e.g. at the back or top contact of a solar cell, embedding the structures within the active region gives the greatest absorbance enhancement [1-3]. Previous work has shown a similar increase in absorbance for an amorphous silicon solar medium [1-3], suggesting that EMN is a general light management scheme that can be applied to any thin film photoabsorber. Such light trapping may further yield a viable route to ultra-thin hot electron solar cells [4,5]. \\ $[1]$ F. Ye, M.J. Burns, M.J. Naughton, Proc. SPIE $\bf{8111}$, 811103 (2011). \\ $[2]$ $\it{Idem.}$, Phys. Status Solidi A $\bf{209}$, 1829 (2012). \\ $[3]$ $\it{Idem.}$, $\it{Ibid.}$ (2014). DOI:10.1002/pssa.201431544. \\ $[4]$ K. Kempa, M.J. Naughton, Z.F. Ren, A. Herczynski, T. Kirkpatrick, J. Rybczynski, and Y. Gao, Appl. Phys. Lett. $\bf{95}$, 233121 (2009). \\ $[5]$ K. Kempa, Phys. Status Solidi RRL $\bf{7}$, 465 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F34.00010: Broadband Solar Energy Harvesting in Single Nanowire Resonators Yiming Yang, Xingyue Peng, Steven Hyatt, Dong Yu Sub-wavelength semiconductor nanowires (NWs) can have optical absorption cross sections far beyond their physical sizes at resonance frequencies, offering a powerful method to simultaneously lower the material consumption and enhance photovoltaic performance. The degree of absorption enhancement is expected to substantially increase in materials with high refractive indices, but this has not yet been experimentally demonstrated. Here, we show that the absorption efficiency can be significantly improved in high-index NWs, by a direct observation of 350{\%} external quantum efficiency (EQE) in lead sulfide (PbS) NWs. Broadband absorption enhancement is also realized in tapered NWs, where light of different wavelength is absorbed at segments with different diameters analogous to a tandem solar cell. Our results quantitatively agree with the finite-difference-time-domain (FDTD) simulations. Overall, our single PbS NW Schottky solar cells taking advantage of optical resonance, near bandgap open circuit voltage, and long minority carrier diffusion length exhibit power conversion efficiency comparable to single Si NW coaxial p-n junction cells, while the fabrication complexity is greatly reduced. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F34.00011: Disentangling the roles of free-carrier density and mobility in the performance of CH$_{3}$NH$_{3}$PbI$_{3}$ perovskite films Elbert E.M. Chia, Chan La-o-vorakiat, M.T. Khuc, R. Haselsberger, Maria-Elisabeth Michel-Beyerle, T. Salim, Haibin Su, Y.M. Lam, Rudolph A. Marcus Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by two parameters carrier mobility and carrier density. They indicate how many and how fast the free carriers drift away from the light-harvesting medium before loss mechanisms, such as carrier recombination, occur. However, these parameters are usually entangled as a product. Using time-resolved terahertz spectroscopy, the number density, mobility and quantum yield of charge carriers in a perovskite film have been disentangled. The free carrier recombination mechanism and rates were determined, and hence the diffusion length. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F34.00012: Bond pathway analysis of NMR spectra for Li$_{1.2}$Mn$_{0.4}$Co$_{0.4}$O$_{2}$: pristine material Hakim Iddir, Baris Key, Fulya Dogan, John Russell, Brandon Long, Javier Bareno, Jason Croy, Roy Benedek NMR has been applied extensively to lithium ion battery cathode materials, of which layered-layered composites $x$Li$_{2}$MnO$_{3}\cdot(1-x)$Li$M$O$_{2}(M=$Mn,Co,Ni) are of particular interest, owing to their high energy density. In this work, NMR spectra are measured for the model layered-layered system $x$Li$_{2}$MnO$_{3}\cdot(1-x)$LiCoO$_{2}$ and Bond-Pathway-model analysis is applied to elucidate the atomic arrangement and domain structure of this material (in its pristine state, before electrochemical cycling). The simplest structural element of a domain consists of a stripe of composition LiMn2 parallel to an in-layer crystallographic axis in a metal layer of the composite. A simple model of the composite structure may be constructed by a superposition of such stripes in an LiCoO background. We show that such a model can account for most of the features of the observed NMR spectra. Support from the Vehicle Technologies Program U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F34.00013: Roll-to-Roll Production of Spray Coated N-doped Carbon Nanotube Electrodes for Supercapacitors Mehmet Karakaya, Jingyi Zhu, Achyut Raghavendra, Ramakrishna Podila, Samuel Parler, James Kaplan, Apparao Rao Although nanocarbons are being increasingly used in energy storage, there has been a lack of inexpensive, continuous and scalable synthesis methods. Here we present a scalable roll-to-roll spray coating process for synthesizing supercapacitors from randomly oriented multi-walled carbon nanotubes electrodes on Al foils, which yield high power and energy densities ($\sim$ 700 mW/cm$^{3}$ and 1 mWh/cm$^{3})$ and cycle stability (\textgreater 10000 cycles) on par with Li-ion thin film batteries. Our cost analysis shows that the R2R spray coating process can produce supercapacitors with 10 times the energy density of conventional activated carbon devices at $\sim$ 17{\%} lower cost. [Preview Abstract] |
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