Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L52: Solar Energy Including Concentrated Solar, and Solar Thermal |
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Sponsoring Units: GERA FIAP Room: Hilton Baltimore Holiday Ballroom 3 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L52.00001: Free-Carrier Absorption in Silicon from First Principles Guangsha Shi, Emmanouil Kioupakis The absorption of light by free carriers in semiconductors such as silicon results in intraband electron or hole excitations, and competes with optical transitions across the band gap. Free-carrier absorption therefore reduces the efficiency of optoelectronic devices such as solar cells because it competes with the generation of electron-hole pairs. In this work, we use first-principles calculations based on density functional theory to investigate direct and phonon-assisted free-carrier absorption in silicon. We determine the free-carrier absorption coefficient as a function of carrier concentration and temperature and compare to experiment. We also identify the dominant phonon modes that contributing to phonon-assisted free-carrier absorption processes, and analyze the results to evaluate the impact of this loss mechanism on the efficiency of silicon solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L52.00002: Design principles of shift current photovoltaics Ashley Cook, Benjamin Fregoso, Fernando de Juan, Joel Moore While the basic principles and limitations of conventional solar cells are well understood, relatively little attention has gone toward evaluating and maximizing the potential efficiency of photovoltaic devices based on shift currents. In this work, a sum rule approach is introduced and used to outline design principles for optimizing shift currents for photon energies near the band gap, which depend on wavefunctions via Berry connections as well as standard band structure. Using these we identify two new classes of shift current photovoltaics, ferroelectric polymer films and orthorhombic monochalcogenides, both of which exhibit peak photoresponsivities larger than predictions for previously-known photovoltaics of this type. Using physically-motivated tight-binding models, the full frequency dependent response of these materials is obtained. Exploring the phase space of these models, we find photoresponsivities that can exceed 100 mA/W. These results show that considering the microscopic origin of shift current via effective models allows one to improve the possible efficiency of devices using this mechanism and better grasp their potential to compete with conventional solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L52.00003: Complete voltage recovery due to suppression of capture to quantum dots Andrei Sergeyev, Kimberly Sablon, Alex Varghese, Michael Yakimov, Vadim Tokranov, Serge Oktyabrsky, Vladimir Mitin Decrease of the open circuit voltage in quantum dot (QD) solar cells with respect to the reference cell is an essential drawback of QD devices. Despite numerous efforts, the complete voltage recovery in QD cells has been demonstrated only at low temperatures. We propose and investigate a new approach that combines nanoscale engineering of band structure and potential profile. In this work we (i) fabricated and investigated GaAs solar cells with various nano-engineered InAs QD media, (ii) identified the key photocarrier processes responsible for the voltage reduction, (iii) optimized QD devices and demonstrated the complete voltage recovery with respect to the reference cell together with some improvements in the short circuit current. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L52.00004: Enhancing photovoltaic efficiency through radiative cooling of solar cells below ambient temperature. Taqiyyah Safi, Jeremy Munday Sunlight heats up solar cells and the resulting elevated solar cell temperature adversely effects the photovoltaic efficiency and the reliability of the cell. Currently, a variety of active and passive cooling strategies are used to lower the operating temperature of the solar cell. Passive radiative cooling requires no energy input, and is ideal for solar cells; however, previously demonstrated devices still operate above the ambient, leading to a lower efficiency as compared to the ideal Shockley-Queisser limit, which is defined for a cell in contact with an ideal heat sink at ambient temperature (300 K). In this talk, we will describe the use of radiative cooling techniques to lower the cell temperature below the ambient temperature. We show that by combining specifically designed radiative cooling structures with solar cells, efficiencies higher than the limiting efficiency achievable at 300 K can be obtained for solar cells in both terrestrial and extraterrestrial environments. We show that these structures yield an efficiency 0.87{\%} higher than a typical PV module at operating temperatures in a terrestrial application. We also demonstrate an efficiency advantage of 0.4-2.6{\%} for cells in an extraterrestrial environment in near-earth orbit. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L52.00005: Tin alloyed acanthite Cu$_{\mathrm{2}}$S using cluster expansion method and their stability analysis using density functional theory Sajib Barman, Muhammad Huda Cu$_{\mathrm{2}}$S is a widely known semiconductor which has the potential to be used as an efficient solar absorber material. However, complex phase structures and phase instabilities due to spontaneous Cu vacancy formation are big issues which need to be addressed. Based on a recent theoretical study which has predicted acathite like Cu$_{\mathrm{2}}$S to be more favorable than other known crystal structures, we have used cluster expansion method to look for the most favorable tin alloyed acanthite Cu$_{\mathrm{2}}$S. We have used density functional theory systematically to assess the stabilities of those tin alloyed acanthite Cu$_{\mathrm{2}}$S structures. In addition, effect of Cu vacancies in Sn alloying has also been investigated. [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L52.00006: The Upper Bound on Solar Power Conversion Efficiency Through Photonic Engineering yunlu xu, Jeremy Munday The power conversion efficiency is a key parameter by which different photovoltaic devices are compared. The maximum value can be calculated under steady-state conditions where the photon flux absorbed by the device equals the outgoing flux of particles (also known as the principle of detailed balance). The photonic engineering of a solar cell offers a new alternative for boosting efficiency. We show that, for an ideally photonic engineered solar cell, its efficiency is subject to an upper bound dictated by a generalized form of detailed balance equation where nano-concentration is taken into account. Results under realistic operating conditions and recent experimental studies will also be discussed. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L52.00007: Nonlinear Response in Silicon Solar Cells Behrang Hamadani, John Roller, Andrew Shore, Howard Yoon A light emitting diode (LED)-based system utilizing a combinatorial flux addition method was used to measure the nonlinear relationship in silicon solar cells between the output current of the cell and the incident irradiance level. The light flux was controlled by the supplied current to two sets of LEDs, of either monochromatic light or a combination of various wavelengths. The (non)linearity of a variety of cells were measured over many orders of magnitude of light intensity and various trends were observed, including a transition from nonlinear to linear behavior for some cells as a function of intensity or a complete nonlinear response throughout the probed range. Furthermore, nonlinearity was found to be spectral dependent. An explanation for the observed behavior based on fundamental physics will be provided. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L52.00008: Contactless Spectral-dependent Charge Carrier Lifetime Measurements in Silicon Photovoltaic Materials John Roller, Behrang Hamadani, Mario Dagenais Charge carrier lifetime measurements in bulk or unfinished photovoltaic (PV) materials allow for a more accurate estimate of power conversion efficiency in completed solar cells. In this work, carrier lifetimes in PV-grade silicon wafers are obtained by way of quasi-steady state photoconductance measurements. These measurements use a contactless RF system coupled with varying narrow spectrum input LEDs, ranging in wavelength from 460 nm to 1030 nm. Spectral dependent lifetime measurements allow for determination of bulk and surface properties of the material, including the intrinsic bulk lifetime and the surface recombination velocity. The effective lifetimes are fit to an analytical physics-based model to determine the desired parameters. Passivated and non-passivated samples are both studied and are shown to have good agreement with the theoretical model. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L52.00009: Effect of Long-Range Polar Electron-Phonon Interaction on the Hot Carrier Dynamics of GaAs Chin Shen Ong, Marco Bernadi, Steven G. Louie Hot carrier dynamics plays an important role in the functionality of electronic and photovoltaic devices. Recent interest in harvesting the energy of hot electrons before it is lost through thermalization has led to renewed interest in the microscopic details of hot electron energy loss mechanisms. Gallium arsenide (GaAs) is of particular interest because amongst its many advantages, it is a direct-gap semiconductor, has high electron mobility and is a high-performing candidate for electronic and photovoltaic applications. GaAs is a polar material, and long-range polar (Fr\"{o}lich) electron-phonon interaction has non-trivial effects on the carrier dynamics in the material. In this work, we investigate the effect of this interaction on the hot carrier dynamics of GaAs. This work is supported by NSF grant No. DMR15-1508412 and the DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at Lawrence Berkeley National Laboratory's NERSC facility. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L52.00010: Nano-scale Characteristics of Copper poor ordered defect compound at grain boundary of CuInGaSe2 Yaping Ma This work investigates the copper poor ordered defect compound (ODC) layer at grain boundaries (GB) for Cu$_{\mathrm{X}}$(Ga$_{\mathrm{0.3}}$In$_{\mathrm{0.7}})$Se$_{\mathrm{2}}$ with different Cu composition ratio (x $=$ 0.9 and 0.68). Same chemical composition while widened ODC layer at GBs with lower Cu ratio were first reported determined by the energy dispersive spectroscopy in scanning transmission microscopy mode. Band structure of the ODC layer was directly measured by scanning tunneling spectroscopy showing a downward offset for conduction band and valance band of 200 eV and 350 eV, respectively. This result was further confirmed by photocurrent accumulation and higher schottky barrier at GBs measured by the conducting probe atomic force microscopy (CP-AFM). Local photovoltaic performance measurements of individual grain boundaries with different ODC width were investigated, using CP-AFM and the disappearance of the differences of open circuit voltage and shunt resistance between grain interior and grain boundary at low illumination provides a direct evidence for the reduced recombination at widened ODC grain boundary which greatly supports the hole barrier theory for the high efficiency of the Copper Indium Ga Selenide solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L52.00011: Employing Time-Resolved Terahertz Spectroscopy to Analyze Carrier Dynamics in Cu$_{\mathrm{2}}$ZnSn(S,Se)$_{\mathrm{4}}$ Absorber Layers Jason Baxter, Glenn Guglietta, Siming Li, Kaushik Roy Choudhury, Jonathan Caspar, Douglas Bishop, Michael Lloyd, Brian McCandless We report the application of time-resolved terahertz spectroscopy (TRTS) to measure photoexcited carrier lifetimes and mobility, and to determine recombination mechanisms in Cu$_{\mathrm{2}}$ZnSn(S,Se)$_{\mathrm{4}}$ (CZTSSe) thin films and single crystals. Ultrafast time resolution permits tracking the evolution of carrier density to determine recombination rates and mechanisms. The carrier generation profile was manipulated by varying the photoexcitation wavelength and fluence to distinguish between surface, Shockley-Read-Hall (SRH), radiative, and Auger recombination mechanisms and determine rate constants. Surface and SRH recombination are the dominant mechanisms for the air/CZTSSe/SiO$_{\mathrm{2}}$/Si film stack. Diffusion to, and then recombination at, the air-CZTSSe interface occurred on the order of 100 picoseconds, while SRH recombination lifetimes were 1 - 2 nanoseconds. Analogous measurements on single crystals reveal the effects of eliminating grain boundaries, reducing point defects and secondary phases, and applying surface treatments to reduce surface recombination velocity. TRTS measurements can provide information that is complementary to conventional time-resolved photoluminescence measurements and can direct the design of efficient thin film photovoltaics. Ref: Guglietta et al., APL, 2014. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L52.00012: Surfactant Antimony enhanced Indium incorporation on InGaN --C plane Yiou Zhang, Junyi Zhu InGaN is an ideal alloy system for optoelectronic devices due its tunable band gap. Yet high-quality InGaN with high In concentration is still a challenging issue that limits its use in green-light LEDs and other devices. In this presentation, we report the surfactant effect of Sb on the In incorporation on InGaN (000-1) surface via first-principles approaches. We constructed surface phase diagram to determine surface structures under different growth conditions. By analyzing surface stress under different structures, we found that Sb adatom can induce tensile sites in the cation layer, enhancing the In incorporation. These findings may provide fundamental understandings and guidelines for the growth of InGaN with high In concentration. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L52.00013: Probing the Surface Defect States of Gallium Nitride Nanowires Lauren Simonsen, Yuchen Yang, Nicholas Borys, Anil Ghimire, James Schuck, Shaul Aloni, Jordan Gerton In this work, we investigate gallium nitride nanowires (NWs) as a potential system for solar-driven water splitting. Although bulk GaN has a UV bandgap, the synthesized NWs exhibit strong absorption and fluorescence emission across the visible spectrum. Density functional theory calculations suggest that this visible fluorescence originates from mid-gap surface-defect states along the triangular facets of the NWs. The orientation of the NWs can be controlled during MOCVD growth, leading to different exposed crystallographic surface terminations with different electronic structures. High resolution microscopy techniques using AFM and confocal hyper-spectral imaging show spectral inhomogeneity across the widths of the NWs, providing evidence that various crystallographic terminations produce different surface states. These NWs also exhibit wave guiding properties, leading to Fabry-Perot fringes and high intensity spectra and the ends of the wires. Photoluminescence excitation spectroscopy reveals a non-linear dependence of the emission spectral features on excitation wavelength, indicating a complex distribution of mid-gap defect states. Time-resolved spectroscopy reveals non-exponential decay dynamics through a complicated manifold of mid-gap states. [Preview Abstract] |
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