Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L35: Thermoelectrics Low Dimensional MaterialsFocus
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Sponsoring Units: DMP Chair: Jeff Urban, Lawrence Berkeley National Laboratory Room: 338 |
Wednesday, March 16, 2016 11:15AM - 11:51AM |
L35.00001: \textbf{Reversible electron-hole separation in a hot carrier solar cell} Invited Speaker: Heiner Linke Hot-carrier solar cells are envisioned to utilize energy filtering to extract power from photogenerated electron-hole pairs before they thermalize with the lattice, and thus potentially offer higher power conversion efficiency compared to conventional, single absorber solar cells. The efficiency of hot-carrier solar cells can be expected to strongly depend on the details of the energy filtering process, a relationship which to date has not been satisfactorily explored. Here, we establish the conditions under which electron-hole separation in hot-carrier solar cells can occur reversibly, that is, at maximum energy conversion efficiency. We find that, under specific conditions, the energy conversion efficiency of a hot-carrier solar cell can exceed the Carnot limit set by the intra-device temperature gradient alone, due to the additional contribution of the quasi-Fermi level splitting in the absorber. To achieve this, we consider a highly selective energy filter such as a quantum dot embedded into a one-dimensional conductor. We also establish that the open-circuit voltage of a hot-carrier solar cell is not limited by the band gap of the absorber, due to the additional thermoelectric contribution to the voltage. Additionally, we find that a hot-carrier solar cell can be operated in reverse as a thermally driven solid-state light emitter. In addition this theoretical analysis, I will also report on first experimental results in a nanowire-based energy filter device. Ref: S Limpert, S Bremner, and H Linke, New J. Phys 17, 095004 (2015) [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L35.00002: Thermionic energy conversion in carbon nanotube networks Chen Li, Kevin Pipe We investigate whether efficient carrier ballistic transport in CNT networks can overcome the parasitic effects of high CNT thermal conductance to yield thermionic (TI) devices with high energy conversion efficiency and/or high cooling power density. We simulate semiconducting single-walled carbon nanotube (SWCNT) structures in which inter-tube junctions provide the necessary filtering of high-energy electrons. Using energy-dependent transmission functions, we compare the performances of various junction types in selective filtering, and then perform Monte Carlo (MC) simulations to study the subsequent relaxation of hot electrons within the SWCNTs. Finally, we examine the parasitic effects of high thermal conductance, accounting for reductions in phonon mean free path due to scattering at inter-tube junctions. The results of the junction transmission, MC, and phonon transport simulations suggest optimal CNT types, junction types, and inter-junction spacings that maximize energy conversion metrics such as efficiency and cooling power density. While certain aspects of electron transport and phonon transport in CNT networks remain unresolved, our simulations suggest that CNT-based networks show promise for TI energy conversion. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L35.00003: Enhanced Thermoelectric Properties in Tailored Semiconducting SWCNT Networks A.D. Avery, B.H. Zhou, J. Lee, E. Lee, E.M. Miller, R. Ihly, D. Wesenberg, K.S. Mistry, S.L. Guillot, B.L. Zink, Y. Kim, J.L. Blackburn, A.J. Ferguson Single-walled carbon nanotubes (SWCNTs) are a versatile electronic material being explored as cost-effective, high-performance alternative in a variety of renewable energy applications. In this talk, we present a series of experiments designed to probe the thermal and electrical transport through networks of semiconducting SWCNT dispersed in matrices of polyfluorene polymers. We measured electrical transport as a function of hole density to explore the coupling between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks. These networks exhibit large thermopowers $> 1000$ $\mu $V/K at very low hole densities. Thermopower values remain high at high doping levels, resulting in thermoelectric power factors greater than 340 $\mu $W/m K. Finally, we present measurements that demonstrate our doping process significantly reduces the thermal conductivity relative to undoped networks suggesting s-SWCNTs are a viable material for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents. [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L35.00004: Effects of Defects and Strain on Thermoelectric Properties of Single-walled Carbon Nanotubes Masato Ohnishi, Takuma Shiga, Junichiro Shiomi Carbon nanotubes (CNTs) have attracted much attention as a thermoelectric material. Although CNTs have large lattice thermal conductivity, CNT-based composites are promising candidates for thermoelectric material because the phonon transport is suppressed by scattering at contacts between CNTs. Therefore, previous studies have mainly focused on thermoelectric properties at contacts between CNTs. However, understanding the effects of defects and strain on the thermoelectric properties of CNTs themselves are important because they exist inevitably in real systems. In this study, we study the effects of defects, vacancy and Stone-Wales defect, and uniaxial compressive strain on single-walled CNTs (SWNTs) employing nonequilibrium molecular dynamics simulation and Green's function method. We find that the defects and buckling deformation significantly decrease electron conductance, and the effect is much stronger than that on thermal conductivity and Seebeck coefficient, resulting in severe reduction of the figure of merit. In addition, the estimation of thermoelectric performance including a inter-SWNT contact indicates that the effect of defects and strain can deteriorate the figure of merit of the SWNT networks. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L35.00005: Photoinduced thermoelectric transport in solution-processed semiconductors Nelson Coates, Fan Yang, Ayaskanta Sahu, Jeffrey Urban The ability to fabricate semiconductor materials directly from solution offers a number of advantages over traditional semiconductor processing routes. In addition to the generally lower costs of manufacturing and ability to scale device to large areas, solution-based fabrication techniques also easily allow for extensive physical and chemical tuning of the processed materials. Here, we examine ways to tune the photoinduced thermoelectric transport in solution-processed semiconductors, and in particular explore ways to leverage some of the inherent characteristics of solution processed semiconductors (such as electronic inhomogeneity and large trap densities) to improve the photoinduced thermoelectric response in these materials. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L35.00006: Large local temperature gradient induced by surface plasmon heating of periodic metal structure Ryoko Shimada, Hitomi Sakai Mixtures of several gas or solution having different concentration can be separated by the gradient of temperature. This is the so-called Soret effect. This phenomenon is quite important for chemical reaction and material condensation/separation. For activating large Soret effect, it would be useful to focus on the surface plasmon heating (SPH) of metal nanostructures that interact with light. In this work, a local temperate gradient was created with the aid of SPH achieved for periodic silver structures in a mesoscopic length scale fabricated by a nanosphere lithography method. Excitation of this periodic structure (by blue laser, for example) could create a localized periodic temperature gradient, as large as \textasciitilde 1,000 K/$\mu $m, as suggested from preliminary heat-transfer calculation. Experimental and theoretical results will be presented on site [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L35.00007: ABSTRACT WITHDRAWN |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L35.00008: Theoretical studies on performance evaluation of solar thermoelectronic energy converter with graphene emitter Olukunle Olawole, Dilip De In this paper we consider detailed energy dynamics of solar thermoelectronic energy converter using graphene as the emitter. The emitter is heated by solar energy concentrated by a parabolic mirror concentrator. We study the performance evaluation of the energy conversion using temperature dependent work function of graphene and model the space charge problem by introducing a factor in the emitter and collector current densities. We present computations on power output and efficiency as function of solar insolation, height of emitter from the base of the mirror, reflection coefficient of the mirror, temperature and work function of collector. Effect of molecular doping on the performance of the graphene solar tech is also discussed. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L35.00009: Graphene-based vdW heterostructure Induced High-efficiency Thermoelectric Devices. Shijun Liang, Lay KEE ANG Thermoelectric material (TE) can convert the heat into electricity to provide green energy source and its performance is characterized by a figure of merit (ZT) parameter. Traditional TE materials only give ZT equal to around 1 at room temperature. But, it is believed that materials with ZT \textgreater 3 will find wide applications at this low temperature range. Prior studies have implied that the interrelation between electric conductivity and lattice thermal conductivity renders the goal of engineering ZT of bulk materials to reach ZT \textgreater 3. In this work, we propose a high-efficiency van del Waals (vdW) heterostructure-based thermionic device with graphene electrodes, which is able to harvest wasted heat (around 400K) based on the newly established thermionic emission law of graphene electrodes instead of Seebeck effect, to boost the efficiency of power generation over 10{\%} around room temperature. The efficiency can be above 20{\%} if the Schottky barrier height and cross-plane lattice thermal conductivity of transition metal dichacogenides (TMD) materials can be fine-engineered. As a refrigerator at 260 K, the efficiency is 50{\%} to 80{\%} of Carnot efficiency. Finally, we identify two TMD materials as the ideal candidates of graphene/TMD/graphene devices based on the state-of-art technology. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L35.00010: Printable Graphene-based Thermoelectric Device with High Temperature Capability Tian Li, Yanan Chen, Dennis Drew, Liangbing Hu Thermoelectric devices are of particular interest due to their capability to convert heat into electrical power. We demonstrate the use of a Graphene-based thermoelectric device that can generate output voltages of hundreds of millivolts with an illuminating Graphene strip as the blackbody source.$\backslash $Our proposed device is superior for thermoelectric conversion mainly due to its high temperature capability that yields a maximum Carnot efficiency limit of 90{\%} (referenced to room temperature) and a high Seebeck coefficient. Our device is also macroscopic with good mechanical strength and stabilized performance, making it attractive for large scale and reliable thermoelectric devices. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L35.00011: Thermal conductivity and rectification study of restructured Graphene Anuj Arora Electronics' miniaturization, has led to search for better thermal management techniques and discovery of important transport phenomenon. Thermal rectification, directionally preferential heat transport analogous to electrical diode, is one such technique, garnering tremendous interest. Its possibility has been explored through structural asymmetry, introducing a differential phonon density of states in hot and cold regions. As of now, mass and shape asymmetries have been studied, both experimentally and theoretically. However, strict requirements of material length being shorter than phonon mean free path and phonon coherence preservation at surface, makes connecting two materials with different temperature-dependent thermal conductivities, a more natural approach. To avoid resultant thermal boundary resistance and integration complexities, we achieve the affect in single material, by restructuring a region of Graphene by introducing defects. The asymmetry impedes ballistic phonon transport, modulating temperature dependence of thermal conductivity in the two regions. We perform deviational Monte Carlo simulations based on Energy-based formulation to microscopically investigate phonon transport, possibility and optimal conditions for thermal rectification. The proposed method uses phonon properties obtained from first principle, treat phonon-boundary scattering explicitly with properties drawn from Bose-Einstein Distribution. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L35.00012: Experimental Studies of Graphene Antidot Lattices for Thermoelectric Applications Qing Hao, Dongchao Xu, Hongbo Zhao, Xu Du Pristine graphene has low thermoelectric performance due to its ultra-high thermal conductivity and zero band gap that leads to a low Seebeck coefficient ($S)$. Both issues can be addressed by patterning periodic nano- or sub-1-nm pores (antidots) across graphene, called graphene antidot lattices (GALs). In GALs, a geometry-dependent band gap can be opened up to dramatically increase S, with significantly reduced thermal conductivity ($k)$ due to phonon scattering by antidots. Above will lead to a high thermoelectric figure of merit (\textasciitilde 1.0 at 300 K by computations [1]) in GALs to be used for device cooling. Despite numerous calculations, experimental studies of GALs are restricted to electrical conductivity ($\sigma )$ measurements for GALs with \textasciitilde 10 nm patterns. The critical k measurements are still lacking. In this work, all three thermoelectric properties ($S$, $k$, and $\sigma )$ are measured on suspended GALs with sub-10 nm pores. In comparison, electrical properties are also characterized for GALs on a substrate. The results presented here provide important guidance on how to tailor transport properties of general two-dimensional materials with ALs. References: [1] Yan et al., Physics Letters A 376, 2425-2429 (2012). [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L35.00013: Ab initio design of low work function complex oxides for thermionic energy conversion Stephanie Mack, Guo Li, Jeffrey Neaton Understanding and controlling work functions, or band edge energies, is of interest for a variety of applications in optoelectronics and energy conversion. In particular, while recent advances in device design have improved the feasibility of thermionic generators, new low work function materials are needed to enable their widespread use. Perovskite-based oxides (ABO$_{3}$) are a diverse class of materials that, depending on the transition metal atoms on the A and B sites, can give rise to myriad emergent and collective phenomena. Here, we use density functional theory calculations to examine how the work function of one such oxide, SrRuO$_{3}$ (SRO), can be tuned by monolayers of SrTiO$_{3}$ (STO) and other polar or near-polar oxides. We find that SRO work functions can be tuned by over 1 eV with one layer of STO, although the calculated reduction in work function is an order of magnitude less than would be expected from the bulk polarization. We understand the variation in work function via a detailed analysis of Born effective charges at the surface, which are as small as 10\% of their bulk values, and charge rearrangement at the STO surface and SRO/STO interface. [Preview Abstract] |
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