Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y38: Focus Session: Scalable Technologies for Photovoltaics II |
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Sponsoring Units: GERA FIAP Chair: Janelle Leger, Western Washington University Room: 347 |
Friday, March 22, 2013 8:00AM - 8:36AM |
Y38.00001: High Efficiency Photovoltaics -- The Key to Grid Parity Invited Speaker: David L. Young For three decades the photovoltaic (PV) industry has enjoyed roughly a 22\% price reduction for each doubling of cumulative production volume. Recently, the PV market has exceeded this trend with module prices dropping to all-time lows. This trend has come mainly from economies-of-scale, incremental efficiency increases, and over supply. However, this PV learning curve is likely to flatten (or even rise) as unsustainable profit margins weed competition and devices near minimal material usage and practical efficiencies. The current market climate, and the strong weighting factor of balance-of-system costs, favor higher efficiency devices. Technologies that cannot reach a minimum module efficiency of about 18\% will likely not be competitive. This paper will discuss several evolutionary and revolutionary scalable wafer and thin-film photovoltaic technologies that are likely to remain competitive, and will identify several areas within these technologies in need of scientific breakthroughs. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y38.00002: Local excitation and local collection of photocurrent in thin-film polycrystalline photovoltaic devices Nikolai Zhitenev, Heayoung Yoon, Marina Leite, Youngmin Lee, Sarah Ko, Yue Zhao, Anthony Gianfrancesco, Paul Haney, Alec Talin The power conversion efficiency of commercial solar modules based on thin-film chalcogenide materials is well below the theoretical limits. To understand the underlying physical mechanisms limiting the efficiency, we investigate local photovoltaic properties isolating the difference between the grain bulk (0.5-2 mkm in size) and the grain boundary in CdTe absorber. Local current-voltage measurements are performed using nano-contacts in conjunction with local electron-hole pairs generation comparing multiple injection techniques. First, the carriers are excited using variable energy electron beam enabling measurements with a spatial resolution down to 20 nm. Second, we have developed a novel approach for high-resolution and high-throughput photocurrent imaging downconverting electron beam into a near-field optical source using a thin film (\textless 50 nm) of phosphors. The electron beam is fully absorbed in the phosphors layer, and the cathodoluminescence is used as a local photon source. Third, we generate carriers using a near-filed optical microscope varying the excitation wavelength. The results show that, in a well-optimized material, a large fraction of grain boundaries displays higher photocurrent as compared to grain bulk effectively serving as a three-dimensional distributed photocurrent collector. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y38.00003: Development of InP Based Quantum Well Tunnel Junctions Michael Yakes, Matthew Lumb, Maria Gonzalez, Christopher Bailey, Igor Vurgaftman, Robert Walters In this presentation we demonstrate lattice-matched InAlGaAs quantum well tunnel junctions for an InP-based multi-junction cell. By including two 0.74 eV bandgap InGaAs quantum wells in InP-lattice matched InAlGaAs tunnel junctions with a 1.18eV bandgap, a peak tunnel current density of 113 A/cm$^{\mathrm{2}}$ was observed, 45 times greater than a baseline bulk InAlGaAs tunnel junction. The differential resistance of the quantum well device is 7.52 x 10$^{\mathrm{4}} \quad \Omega $cm$^{\mathrm{2}}$, a 15-fold improvement over the baseline device. The upper bound of the transmission loss to the bottom cell is estimated to be approximately 1.7{\%}. Strain balanced quantum wells will be discussed which have the same benefits of the latticed matched tunnel junctions, but can be made accessible to both InP and GaAs based multi-junction architectures. We will also show the results of a study where a bulk, double heterostructure design is used to mitigate the effects of dopant diffusion and maximize the peak tunnel current, achieving a 15 times improvement in peak tunnel current over the baseline device. We propose that quantum well tunnel junctions with bulk heterostructure diffusion barriers could play a key role in improving performance both at one sun and high sun concentrations. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y38.00004: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y38.00005: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y38.00006: Energy Alignment at Organic/Oxide and Organic/Metal Interfaces: The Effects of Molecular Overlayer Thickness on the HOMO/LUMO Gap and Interfacial Dipole Charles Ruggieri, Sylvie Rangan, Senia Coh, Robert Bartynski Dye-sensitized solar cells offer the potential for low-cost production with comparable efficiencies to traditional Si-based solar cells. Energy alignment of the dye orbitals with respect to the band edges of the oxide semiconductor substrate is a key parameter in device performance. Using direct and inverse photoemission, XPS, and STM we have investigated the electronic structure and bonding geometry of zinc tetraphenylporphyrin (ZnTPP) molecules adsorbed on a set of four oxide semiconductor and metallic substrates [TiO$_{2}$(110), ZnO(11-20), Ag(100) and Au(111)] at monolayer and multilayer coverages. The vacuum levels of the organic/oxide and organic/metal systems were also measured and the interface dipoles determined. The energy level shifts and the width of adsorbate spectral features are qualitatively different for molecules adsorbed on the oxide versus the metal substrates. The HOMO-LUMO energy separation decreases with decreasing molecular overlayer thickness, which is thought to be due to substrate screening properties, but these shifts occur in different ways for the two classes of substrates. Possible origins of this distinct behavior will be discussed. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y38.00007: Simulation and Testing of Type-II Strained-Layer Superlattices for Long Wavelength Thermophotovoltaics Abigail Licht, Dante DeMeo, Thomas Vandervelde In this presentation we detail our research on long wavelength thermophotovoltaic (TPV) cells, with cutoff wavelength in the 7-9 micron range, which hold the potential for a wide array of applications due to their ability to work with lower temperature sources. We will discuss simulation results on the optimization of structures utilizing type II strained-layer superlattice (SLS) cells and unipolar barriers. The performance of these simulated cells is compared with fabricated cells which were characterizing using calibrated blackbody sources. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y38.00008: Optimization of gain and energy conversion efficiency using front-facing photovoltaic cell luminescent solar concentrator design Melissa Osborn, Carley Corrado, Shin Woei Leow, Emory Chan, Ben Balaban, Sue Carter Luminescent solar concentrator (LSC) windows with front-facing photovoltaic (PV) cells were built and their gain and power efficiency were investigated. Conventional LSCs employ a photovoltaic (PV) cell that is placed on the edge of the LSC, facing inward. This paper describes a new design with the PV cells on the front-face allowing them to receive both direct solar irradiation and wave-guided photons emitted from a dye embedded in an acrylic sheet, which is optically coupled to the PV cells. Parameters investigated include the thickness of the waveguide, edge treatment of the window, cell width, and cell placement. The data allowed us to make projections that aided in designing windows for maximized overall efficiency. A gain in power of 2.2x over the PV cells alone was obtained with PV cell coverage of 5{\%}, and a power conversion efficiency as high as 6.8{\%} was obtained with a PV cell coverage of 31{\%}. Balancing the trade-offs between gain and efficiency, the design with the lowest cost per watt attained a power efficiency of 3.8{\%} and a gain of 1.6x. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y38.00009: Theoretical and Practical Limits for Transparent Photovoltaics Richard Lunt Transparent photovoltaics (TPVs) offer a new paradigm for solar energy harvesting, integration, and deployment. These devices have recently been shown to be enabled by exploiting the excitonic nature of molecular and organic semiconductors.\footnote{R. R. Lunt, and V. Bulovi\'{c}. Appl. Phys. Lett. 98, 113305, 2011.} Here, we present the theoretical and practical efficiency limits of these novel electronic architectures as a function of bandgap, transparency and aesthetic quality for both single and multi-junction cells. For example, power-production from ultraviolet and near-infrared photons alone leads to a theoretical single-junction efficiency of 21{\%} in completely transparent structures, compared to 33{\%} for opaque-junctions. This approach for transparent photovoltaics will be contrasted with other semi-transparent approaches, where TPVs with selective absorption offer the highest possible potential for combined transparency and efficiency. The impact of transparent PVs will be discussed for a range of applications from electronic displays to window integration. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y38.00010: Luminescent Solar Concentrators in the Algal Industry Katie Hellier, Carley Corrado, Sue Carter, Angela Detweiler, Leslie Bebout Today's industry for renewable energy sources and highly efficient energy management systems is rapidly increasing. Development of increased efficiency Luminescent Solar Concentrators (LSCs) has brought about new applications for commercial interests, including greenhouses for agricultural crops. This project is taking first steps to explore the potential of LSCs to enhance production and reduce costs for algae and cyanobacteria used in biofuels and nutraceuticals. This pilot phase uses LSC filtered light for algal growth trials in greenhouses and laboratory experiments, creating specific wavelength combinations to determine effects of discrete solar light regimes on algal growth and the reduction of heating and water loss in the system. Enhancing the optimal spectra for specific algae will not only increase production, but has the potential to lessen contamination of large scale production due to competition from other algae and bacteria. Providing LSC filtered light will reduce evaporation and heating in regions with limited water supply, while the increased energy output from photovoltaic cells will reduce costs of heating and mixing cultures, thus creating a more efficient and cost effective production system. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y38.00011: Monte Carlo Simulations of Luminescent Solar Concentrators with Front-Facing Photovoltaic Cells for Building Integrated Photovoltaics Shin Woei Leow, Carley Corrado, Melissa Osborn, Sue Carter Luminescent solar concentrators (LSCs) have the ability to receive light from a wide range of angles and concentrate the captured light on to small photo active areas. This enables LSCs to be integrated more extensively into buildings as windows and wall claddings on top of roof installations. LSCs with front facing PV cells collect both direct and concentrated light ensuring a gain factor greater than one. It also allows for flexibility in determining the placement and percentage coverage of PV cells when designing panels to balance reabsorption losses, power output and the level of concentration desired. A Monte-Carlo ray tracing program was developed to study the transport of photons and loss mechanisms in LSC panels and aid in design optimization. The program imports measured absorption/emission spectra and transmission coefficients as simulation parameters. Interactions of photons with the LSC panel are determined by comparing calculated probabilities with random number generators. Simulation results reveal optimal panel dimensions and PV cell layouts to achieve maximum power output. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y38.00012: Porous Silicon as Antireflecting Layer Gulsen Kosoglu, Mehmet Yumak, Selim Okmen, Ozhan Ozatay, Yani Skarlatos, Carlos Garcia The main aim in photovoltaic industry is to produce efficient and energy competitive solar cell modules at low cost. Efficient AntiReflection Coatings (ARC) improve light collection and thereby increase the current output of solar cells. Broadband ARCs are desirable for efficient application over the entire solar spectrum and porous silicon layers as antireflective coating layers provide successful light collection. In the study the most critical physical parameters of porous silicon are examined, homogeneous and uniform porous layers are produced. The photoluminescence spectrum and optical parameters of porous layers have been investigated, and we are now in the process of improving the efficiency of the device by modulating the structure of the porous silicon layers and studying its photovoltaic characteristics. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y38.00013: Transparent Luminescent Solar Concentrators for Large-area Solar Windows Yimu Zhao, Richard Lunt Luminescent solar concentrators (LSCs) have recently regained attention as a route for integration into the building envelope. To date, however, these systems have been limited to absorption and emission (glow) in the visible part of spectrum. We have designed and fabricated novel transparent luminescent solar concentrators devices composed of synthesized metal halide nanocrystal phosphorescent luminophores that allow for efficient and selective harvesting of ultraviolet (UV) photons with a near perfect absorption cutoff at the edge of the visible spectrum (430nm) while efficiently down-converting emitted light with a massive stoke shift to the near-infrared (800nm). We have demonstrated transparent LSCs with power efficiency of 0.8{\%} $\pm$ 0.5{\%}, system external quantum efficiency exceeding 35{\%}, and an average transmittance of 82{\%} $\pm$ 1{\%}. We show through experiments and modeling that this architecture has the potential to exhibit up to 1-2{\%} power conversion over module areas \textgreater\ 1 m$^{2}$. These concentrators present new opportunities for non-tinted and highly-adoptable solar- windows that can translate into improved building efficiency, enhanced UV-barrier layers, and lower cost solar harvesting systems. [Preview Abstract] |
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