Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W38: Focus Session: Novel Photophysics and Transport in NanoPV III |
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Sponsoring Units: GERA DPOLY DCOMP Chair: Zhigang Wu, Colorado School of Mines Room: 347 |
Thursday, March 21, 2013 2:30PM - 3:06PM |
W38.00001: Novel Low-Loss Plasmonic Waveguides to Create HE PV from Ultra-Thin Organic and Low-Purity Earth Abundant Inorganic Layers Invited Speaker: Janelle Leger |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W38.00002: Embedded metal nanopatterns for near-field scattering-enhanced optical absorption Michael J. Burns, Fan Ye, Aaron H. Rose, Michael J. Naughton Simulations of metal nanopatterns embedded in a thin photovoltaic (PV) absorber show significantly enhanced absorbance within the semiconductor, with a more than 300\% increase for $\lambda$ = 800 nm. Integrating with AM1.5 solar irradiation, this yields a 70\% increase in simulated short circuit current density and thus power conversion efficiency (single junction $\eta$ = 13\%) in a 60 nm amorphous silicon film. Embedding such metal patterns inside an absorber maximally utilizes enhanced electric fields that result from intense, spatially organized, near-field scattering in the vicinity of the pattern. Appropriately configured (i.e., with a thin insulating coating), this optical metamedium architecture may be useful for increasing PV efficiency in thin film solar cells, including offering prospects for realistic ultrathin hot electron cells. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W38.00003: Improved electrical response of photovoltaic devices by photonic structuring Jeremy Munday We describe the use of dispersion engineered photonic materials to develop a new photovoltaic technology that can achieve much higher efficiencies than traditional devices through the modification of spontaneous emission. The limiting efficiency of photovoltaic energy conversion was determined by Shockley and Queisser using the theory of detailed balance, which described the balance between absorption and emission of photons. However, when the solar cell is formed from a photonic crystal or a similar material is placed on top of a solar cell, both the absorption and emission of photons is modified, a fact not considered in the original formalism. Here we show that photonic crystal structuring can improve the cell efficiency by either effectively modifying the semiconductor bandgap energy or reducing the spontaneous emission within the device, leading to higher carrier concentrations and hence higher open circuit voltages. [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W38.00004: Optical absorption of nanoporous silicon: quasiparticle band gaps and absorption spectra Guangsha Shi, Emmanouil Kioupakis Silicon is an earth-abundant material of great importance in semiconductors electronics, but its photovoltaic applications are limited by the low absorption coefficient in the visible due to its indirect band gap. One strategy to improve the absorbance is to perforate silicon with nanoscale pores, which introduce carrier scattering that enables optical transitions across the indirect gap. We used density functional and many-body perturbation theory in the GW approximation to investigate the electronic and optical properties of porous silicon for various pore sizes, spacings, and orientations. Our calculations include up to 400 atoms in the unit cell. We will discuss the connection of the band-gap value and absorption coefficient to the underlying nanopore geometry. The absorption coefficient in the visible range is found to be optimal for appropriately chosen nanopore size, spacing, and orientation. Our work allows us to predict porous-silicon structures that may have optimal performance in photovoltaic applications. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W38.00005: Absorption enhancement in amorphous silicon thin films via plasmonic resonances in nickel silicide nanoparticles Jordan Hachtel, Xiao Shen, Sokrates Pantelides, Ritesh Sachan, Carlos Gonzalez, Ondrej Dyck, Shaofang Fu, Ramki Kalnayaraman, Phillip Rack, Gerd Duscher Silicon is a near ideal material for photovoltaics due to its low cost, abundance, and well documented optical properties. The sole detriment of Si in photovoltaics is poor absorption in the infrared. Nanoparticle surface plasmon resonances are predicted to increase absorption by scattering to angles greater than the critical angle for total internal reflection (16$^{\circ}$ for a Si/air interface), trapping the light in the film. Experiments confirm that nickel silicide nanoparticles embedded in amorphous silicon increases absorption significantly in the infrared. However, it remains to be seen if electron-hole pair generation is increased in the solar cell, or whether the light is absorbed by the nanoparticles themselves. The nature of the absorption is explored by a study of the surface plasmon resonances through electron energy loss spectrometry and scanning transmission electron microscopy experiments, as well as first principles density functional theory calculations. Initial experimental results do not show strong plasmon resonances on the nanoparticle surfaces. Calculations of the optical properties of the nickel silicide particles in amorphous silicon are performed to understand why this resonance is suppressed. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W38.00006: Progress Developing Hybrid Silicon Quantum Dot/Amorphous Silicon Thin Films for Photovoltaics Application Tianyuan Guan, Jeremy Fields, Grant Klafehn, Chito Kendrick, Robert Lochner, Zahra Nourbakhsh, Mark Lusk, Paul Stradins, Craig Taylor, Reuben Collins Quantum confined (QC) nanostructures exhibit novel, size tunable, quantum mechanical phenomena and their use in solar cell architectures may yield significant efficiency gains. We demonstrate QC hybrid silicon nanocrystal(nc-Si:H) -- hydrogenated amorphous silicon (a-Si:H) structures, which can potentially serve as photo-stable, thin film silicon, solar cell materials and provide higher open-circuit voltage compared to conventional materials. We deposit a/nc-Si:H films sequentially, where nc-Si:H and a-Si:H are grown layer-by-layer using separate plasma reactors in a common deposition chamber. X-ray diffraction, Raman spectroscopy, and electron microscopy results confirm the nanoparticles are the appropriate size to achieve QC (3-7nm). Photoluminescence spectroscopy reveals the QC. Co-planar electrical probe experiments investigate carrier transport in a/nc-Si:H, which could be limited by defects accompanying plasma interruption in the sequential deposition process. Defect spectroscopies, such as electron paramagnetic resonance and photothermal deflection spectroscopy are used to study this relationship. These studies reveal material quality limitations to be addressed for realizing film silicon materials that harvest QC to enhance PV device performance. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W38.00007: Computational spectroscopy of nanocomposites Marco Govoni, Tuan Anh Pham, Giulia Galli Most of the first principles calculations of the opto-electronic properties of nanoparticles appeared in the literature were conducted using structural models of isolated particles. However experiments are carried out on nanocomposites, e.g. nanoparticles in solution or embedded in solid matrices. Recent ab initio studies [1,2] pointed at the importance of taking into account interactions between nanoparticles and the environment surrounding them, in order to provide sensible predictions of their electronic properties, as well as interpretation of experiments. Here we report calculations of the relative position of energy levels of Si nanoparticles embedded in amorphous matrices, as obtained using many body perturbation theory, at the GW level. Our calculations were carried out using a newly developed method to obtain quasi particle energies, based on the spectral decomposition of the dielectric matrix [3].\\ $[1]$ T.S.Li, F.Gygi and G.Galli \textit{Phys. Rev. Lett.} 107, 206805 (2011)\\ $[2]$ M.Govoni, I.Marri and S.Ossicini \textit{Nature Photonics} 6, 672 (2012)\\ $[3]$ H-V.Nguyen, T.A.Pham, D.Rocca and G.Galli \textit{Phys. Rev. B} 85, 081101(R) (2012) [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W38.00008: Complementary transport channels in Si-ZnS nanocomposites: first principles simulations Stfan Wippermann, Marton Voros, Adam Gali, Gergely Zimanyi, Giulia Galli In solar energy conversion devices, nanoparticles (NPs) are often embedded in solid matrices, either crystalline or amorphous. At present a detailed understanding of the influence exerted by the embedding matrix on the absorption of sunlight by the nanoparticle, and the role of the nanoparticle-matrix interface remain elusive. We investigated Si NPs embedded in ZnS, a system that was used as a charge transport layer in recent experiments. A realistic model of the NP-matrix interface was created from ab-initio molecular dynamics simulations. We found that this nanocomposite exhibits complementary transport channels, where electron transport occurs by hopping between NPs and hole transport through the ZnS-matrix. In analogy to Si NPs embedded in SiO2 [1] we found a strong gap reduction and corresponding red-shifted optical absorption, caused by chemical shifts at the NP-matrix interface. \\[4pt] [1] T. Li, F. Gygi, G. Galli, Phys. Rev. Lett. 107, 206805 (2011) [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W38.00009: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W38.00010: Quantum Monte Carlo Characterization of Excited States and Energy-Level Alignment of Oligomer/Quantum-Dot Interfaces Jonathan Dubois, Donghwa Lee, Yosuke Kanai Charge separation of excitons in materials is one of the most important physical processes to utilize the solar energy in diverse devices including solar cells and photo-catalysts. Heterogeneous interfaces with the so-called type-II character are often employed to infer the interfacial charge transfer in this context. As a simple criterion for designing such an interface, the energy alignment of the quasi-particle states together with the exciton binding energy of electron-donating materials is often discussed in the literature. However, an accurate description of the effect of exciton binding at the interface has not been investigated extensively. Although density functional theory (DFT) is a powerful method to investigate various electronic properties of materials, incomplete description of many-body interactions can lead to an incorrect interpretation of the energy level alignment. While Many-Body Perturbation Theory and Quantum Monte Carlo are promising in this context, much more work is necessary to assess how well these methods perform in practice. In this talk, we will discuss our preliminary results using diffusion Quantum Monte Carlo to calculate the excited states and energy-level alignment at an Oligomer/Quantum-Dot interface -- a system that is often discussed in context of solar energy conversion. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W38.00011: Binding mechanism of CdSe quantum dots to carbon nanotubes/graphene Jie Jiang, Sohrab Ismail-Beigi Decorating carbon nanotube or graphene with CdSe quantum dots (QDs) is one approach to creating next generation high efficiency photovoltaics. We have used first principles methods to calculate the binding mechanisms of oleic acid (OA) to CdSe QDs as well as how -COOH functional groups can link the QD to graphene. In both cases, the strongest binding involves the terminating double-bonded oxygen atom in the -COOH group covalently bonding to a surface Cd atom while the hydrogen (from the OH part of the -COOH) aligns to make a weak hydrogen-like bond to a neighboring surface Se. We find a strong defect enhanced binding of the QD to graphene via -COOH: when the -COOH links the QD to a defect site on the graphene, the binding energy of the complex is ~ 0.5 eV larger than when a -COOH links the QD to a pristine graphene region. These results are consistent with available edge X-ray absorption fine structure (EXAFS) data and also rationalize the growth procedure by which ultrasonication of the OA functionalized QDs leads to the replacement of some QD-OA bonds by QD-COOH-graphene bonds, which strongly link the QDs to the graphene surface. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W38.00012: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W38.00013: ZnO Transistor Interfaces Sensitized with Photo Donor Molecules Josef Spalenka, Lushuai Zhang, Padma Gopalan, Paul Evans A better understanding of the physics at interfaces between semiconducting oxides and monolayers of covalently bonded organic molecules is relevant to important applications such as inexpensive chemical sensors and improved dye-sensitized solar cells. We use field-effect transistor (FET) structures in which electrical measurements are made before and after functionalizing the surface of ZnO nanocrystalline films, which form the channel of the FET, with organic dye molecules based on rhenium-bipyridine complexes that act as electron donors during illumination with monochromatic light. Measurements of the charge transfer as a function of light intensity and dye coverage give the ratio between the rates of charge transfer and recombination between the dyes and the ZnO, an important parameter to maximize to further improve the efficiency of solar cells based on donor functionalized oxides. [Preview Abstract] |
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