Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B36: Photovoltaics: Novel Approaches and System Issues |
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Sponsoring Units: GERA Chair: Fatima toor, National Renewable Energy Laboratory Room: C142 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B36.00001: Surpassing the classical light-trapping limit in thin film solar cells Jeremy Munday, Dennis Callahan, Harry Atwater We describe a methodology for designing thin film solar cells that have light-trapping intensity and absorption enhancements that exceed the classical, ergodic light-trapping limit. From thermodynamic arguments, Yablonovitch and Cody determined the maximum absorption enhancement in the ray optics limit for a bulk material to be $4n^2$, where n is the index of refraction of the absorbing layer. Stuart and Hall expanded this approach to study a simple waveguide structure; however, for the waveguide structures they considered, the maximum absorption enhancement was $<4n^2$. Using a combination of analytical and numerical methods, we describe why these structures do not surpass the ergodic limit and show how to design structures that can. We present here a physical interpretation in terms of the waveguide dispersion relations and optical density of states. We further describe the necessary criteria for surpassing the classical limit and provide examples of waveguide structures with absorption enhancements in excess of $4n^2$. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B36.00002: Resonance shifting: A simple, all-optical method for circumventing the reabsorption problem in luminescent concentrators Noel Giebink, Gary Wiederrecht, Michael Wasielewski Luminescent concentrators (LSCs) were developed over three decades ago as a simple route to obtain high concentration ratio for photovoltaic cells without tracking the sun. In principle, high concentration ratios $>$100 are possible for commonly used chromophores. In practice, however, there is typically an overlap between the chromophore absorption and emission spectra that, although small, ultimately leads to unacceptable reabsorption losses, limiting the concentration ratio to $\sim $10 and hence the utility of LSCs to date. We introduce a simple, all-optical means of avoiding reabsorption loss by ``resonance shifting'' from a bilayer cavity that consists of an absorber/emitter waveguide lying upon a low refractive index layer supported by a transparent substrate. Emission is evanescently coupled into the substrate at sharply defined angles and hence, by varying the cavity thickness over the device area, the original absorption resonance can be avoided at each bounce, allowing for extremely low propagation loss to the substrate edges and hence an increase in the optical concentration ratio. We validate this concept for absorber/emitter layers composed of both a typical luminescent polymer and inorganic semiconductor nanocrystals, demonstrating near-lossless propagation in each case. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B36.00003: Computational Design of All-Carbon Photovoltaics Marco Bernardi, Jeffrey C. Grossman We employ ab-initio computational approaches to study interfaces between different carbon nanomaterials (graphene nanoribbons, carbon nanotubes, graphene fragments) with different structures and surface chemistries. The presence of suitable type-II band alignment at these interfaces and significant light-absorption in the visible and infrared make all-carbon heterojunctions appealing as the active material in next-generation flexible photovoltaic devices, particularly given their greatly enhanced stability compared with polymer-based cells. Results for a wide range of carbon nanomaterials interfaces will be presented, and we will discuss possible applications of such a technology, extending the analysis to the thin-film device scale. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B36.00004: Continuum and KMC simulations of realistic bulk heterostructure solar cell photovoltaic devices Kanokkorn Pimcharoen, Daniel Olds, Phillip Duxbury Design of novel solar cell architectures is significantly assisted by reliable continuum device models, and computational methods capable of solving these models in one, two and three dimensions. We are developing computational methods for these models and are validating them using Kinetic Monte Carlo simulations in the same morphologies. We present simulations using idealized morphologies to test approximations in the continuum models, and we present results for bulk heterostructure morphologies deduced by refining digital nanostructures to experimental neutron relectometry and small angle scattering data. In particular we discuss the ability of one dimensional device models to capture the physics of photovoltaic response of realistic bulk heterostructures. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B36.00005: Computational Materials Design for High Efficiency Photovoltaic Solar Cells and Transparent Conducting Sulfides Hiroshi Katayama-Yoshida, Yoshimasa Tani, Kazunori Sato Based on the first-principles electronic structure calculations we propose computational materials design for high efficiency and low price (In free) solar cell materials based on CuInSe2. Firstly, to avoid the use of In, we try to substitute In by Zn and Sn, or by Ga. The electronic structure calculations are performed by using the KKR-CPA method. To calculate band gap energy correctly, we use self interaction corrections proposed by Filippetti et al. It is found that the direct band gap does not collapse and there appears no deep impurity state in the gap, thus it should be possible to avoid In without any deterioration of photovoltaic effect. From the calculations of mixing energy, we predict that the present system favors the spinodal decomposition and we can expect the formation of nano-wire by two dimensional spinodal nano-decomposition. When the nano-wires are formed, we can expect Type 2 band alignment between host material and the nano-wires. Due to this band alignment, efficient electron hole separation is expected leading to highly efficient photovoltaic effect. As an extension of the present design, we also propose a new class of n-type and p-type transparent conducting sulfides with the negative activation energy for the application of high-efficiency photovoltaic solar-cells. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B36.00006: Efficient Black Silicon Solar Cells with Multi-Scale Surface Texture Fatima Toor, William Nemeth, Matthew Page, Qi Wang, Howard Branz, Hao-Chih Yuan A nanostructured, density-graded surface layer can replace conventional quarter-wavelength coatings as the anti-reflection layer in photovoltaics. If the layer is comprised of structures smaller than the wavelength of the incident light and the density is graded across more than about half the wavelength of the light, reflection is strongly suppressed (H. M. Branz et al., APL \textbf{94} 2009). We developed an inexpensive liquid etch technique for silicon to produce ``black Si'' based upon this physics. However, the problem of high carrier recombination within this nanostructured layer must be overcome to improve beyond the present best solar cell with its confirmed 16.8{\%} black silicon sunlight-to-electricity conversion efficiency (H-C. Yuan et al., APL \textbf{95} 2009). In this work, we combine the black Si layer with conventional KOH-etched pyramidal surface texture (Y. Xiu et al., Langmuir \textbf{24 }2008) at micron-scale. Pyramids contribute anti-reflection based on geometric optics. Combining the pyramids with nanostructures only 100 nm deep provides reflectivity below 2{\%} across a wavelength range from 350 -- 1000 nm. To-date, we have obtained a solar cell efficiency of 17{\%} with a V$_{oc}$ of 613 mV, J$_{sc}$ of 35 mA/cm$^{2}$ and fill-factor of 78{\%}. These cells have improved blue response compared to the best planar black Si cells. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B36.00007: Scalability of Nanocoax PV Architecture Michael J. Naughton, Zhifeng Ren, Kris Kempa The radial junction nanocoax-based nanowire solar architecture offers the prospect of high conversion efficiency with thin film PV, due to enhanced light trapping and ultrathin absorbers. We critique the potential applicability of this structure for various PV media. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B36.00008: Physical effects of ultrathin photovoltaic junctions T. Kirkpatrick, K. Kempa, M.J. Naughton Hot carrier photovoltaic cells have potential to increase conversion efficiency beyond the Shockley-Queisser limit. In addition to implementing selective energy filters into the device in order to extract the hot carriers at elevated energies beyond the band edges, a possible requirement, of particular importance for non-crystalline material, is that the device also be constructed ultrathin in order to extract the hot carriers as usable energy on time scales of less than one picosecond, after which thermalisation sets in. Ultrathin amorphous silicon p-i-n junctions have been shown to extract hot carriers as usable energy at fixed short circuit current density for p- and n- region thicknesses of 5 nm, and i-layer thickness less than 50 nm [Appl. Phys. Lett. \textbf{95}, 233121 (2009)]. Physical effects on device performance in ultrathin cells, such as optical absorption, scattering, band structure, and transport are discussed. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B36.00009: Amorphous Silicon-Carbon Nanostructure Solar Cells Maria Schriver, Will Regan, Matthias Loster, Alex Zettl Taking advantage of the ability to fabricate large area graphene and carbon nanotube networks (buckypaper), we produce Schottky junction solar cells using undoped hydrogenated amorphous silicon thin films and nanostructured carbon films. These films are useful as solar cell materials due their combination of optical transparency and conductance. In our cells, they behave both as a transparent conductor and as an active charge separating layer. We demonstrate a reliable photovoltaic effect in these devices with a high open circuit voltage of 390mV in buckypaper devices. We investigate the unique interface properties which result in an unusual J-V curve shape and optimize fabrication processes for improved solar conversion efficiency. These devices hold promise as a scalable solar cell made from earth abundant materials and without toxic and expensive doping processes. [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B36.00010: Prediction of enhanced photovoltaic performance of amorphous silicon solar cells with filled nanopores Jeffrey Grossman, Joo-Hyoung Lee We propose a novel hybrid structure for improving the efficiency of thin-film amorphous silicon solar cells. Using {\it ab initio} calculations, we demonstrate that nanoporous, amorphous silicon (pa-Si), when filled with polythiophene (PT) inside the pores, forms a staggered gap (type II) heterojunction at the interfaces, where both the highest occupied and the lowest unoccupied molecular orbitals of PT are positioned in energy higher than those of pa-Si. Furthermore, we find that while the absorption coefficient ($\alpha$) of pa-Si is significantly reduced from that of bulk amorphous Si (a-Si), inclusion of PT recovers $\alpha$ to the values of a-Si and even higher at thicknesses of $\sim 1$$\mu$m. These results suggest that such a hybrid material, which from a manufacturing standpoint may be substantially easier to scale up than nanowire-based approaches, could greatly enhance the hole mobility in the active layer, which is one of the main reasons for poor efficiency in a-Si solar cells. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B36.00011: Optimizing materials for photon-enhanced thermionic emission Jared Schwede, Daniel Riley, Igor Bargatin, Samuel Rosenthal, Roger Howe, Nicholas Melosh, Zhi-Xun Shen We recently described a novel process for solar energy harvesting called photon-enhanced thermionic emission (PETE) based on a semiconductor cathode and a low-workfunction anode separated by a vacuum gap. Previous work explored the limiting theoretical efficiency of a PETE device, which was shown to exceed the Shockley-Queisser limit on single-junction photovoltaic cells, and described experiments that showed strong evidence the PETE effect. In this presentation, I will describe challenges for making the PETE process efficient, some of which were encountered in these proof-of-concept measurements. I will also describe experimental paths to overcoming these challenges and improving efficiency. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B36.00012: Optimization and Characterization of Nanostructured Surfaces for Photon-Enhanced Thermionic Emission and Photoemission cathodes Daniel Riley, Vijay Narasimhan, Joel Jean, Igor Bargatin, Jared Schwede, Zhi-Xun Shen, Roger Howe, Nick Melosh In the cathode of an energy converter based on photon-enhanced thermionic emission (PETE) photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum and therefore should be confined close to the surface. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. ~Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied. {\_}We have recently reported preliminary theoretical results from a suite of simulation tools to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. In this work we use the simulation suite to optimize nanostructures for applications including PETE-based solar energy converters, photodetectors and electron sources. The samples are then characterized, and the emission efficiency measured in an ultra-high vacuum test chamber under application-centric conditions. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B36.00013: Design and characterization of transparent thin film nanostructure device Uday Trivedi, Utpal Joshi Indium tin oxide (ITO) is one of the most widely used transparent conducting oxides (TCO) because of its electrical conductivity and optical transparency.We have grown ``all oxide'' transparent $p-n$ junction thin film nanostructure device by using chemical solution deposition and e-beam evaporation onto SiO$_{2}$ substrate. The oxide $p-n$ junction was characterized by GIXRD, AFM, UV-Vis. spectroscopy and I-V measurements. Combined GIXRD and AFM confirm phase pure, mono-disperse 30 nm NiO and ITO nanocrystallites. More than 70{\%} optical transparency is achieved across 160 nm thick $p-n$ junction. The forward bias current is greater than the reverse bias current by approximately a factor of 10$^{4}$ in the measured voltage s weeping range. A small leakage current as low as 12 nA was observed at a reverse bias of --5 V. Previously, Tonooka and co-authors [3] reported the average turn on voltage of their n-ZnO / p-Cu-Al-O diode $\sim $ 0.5 V, which is higher than our $p$-NiO/$n$-ITO diode. This is mainly because of the large variations in the carrier concentrations as well as larger lattice mismatch between the oxides forming the $p-n$ junction. The observed optical and electrical properties of oxide transparent diode are attributed to the heteroepitaxial nature and carrier diffusion at the junction interface. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B36.00014: Interface properties of chalcopyrite heterocontacts Christian Pettenkofer, Andreas Hofmann, Eike Janocha, Carsten Lehmann Interface properties of heterocontacts determine the device performance of thin film solar cells. We investigated well defined chacopyrite interfaces and heterocontacts of MBE grown samples by electron spectroscopy to obtain informations on the morphology and electronic properties of the contact phases. In particular CuInSe2 and CuInS2 (001) and (112) surfaces were grown by MBE and studied with respect to contact formation to ZnO, ZnS and ZnSe. Due to Cu back diffusion into the bulk even for stoichiometric samples Cu poor interfaces were observed giving rise to interdiffused Zn3In2X6 (X=S,Se) layers in the contact plane. Band alignments obtained fort the prepared heterocontacts will be compared to models given by M\"{o}nch and Wei et al. The influence of contact preparation on the properties of the interface will be discussed in detail. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B36.00015: Effects of Back Contact Materials on Substrate Configuration CdTe Solar Cells Nathan G.F. Reaver, Kristopher Wieland, Alvin D. Compaan Substrate configuration CdTe photovoltaics has the potential to provide both a reduction in the production costs and improved power to mass ratio. In this study the effect of copper placement in the cells, sequence of CdCl$_{2}$ treatment, and the effect of back contact material on cell performance was examined. Cells were deposited on a Mo coated conductive substrate, on stainless steel or on TCO coated glass, using RF magnetron sputtering. Three different back contacts were used, copper-gold as used in superstrate configuration cells, Sb$_{2}$Te$_{3}$, and ZnTe:N. Cells were measured using a solar simulator at one sun to obtain current density vs. voltage curves and cell efficiencies. The structure that gave the best performance was stainless steel/Mo/Sb$_{2}$Te$_{3}$/CdTe/CdS/ZnO/ZnO:Al, with the best cell having an efficiency of 5.34{\%}. [Preview Abstract] |
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