Symposium Organizers
Christophe Ballif University of Neuchatel
Randy Ellingson National Renewable Energy Laboratory
Marko Topic University of Ljubljana
Miroslav (Miro) Zeman Delft University of Technology
Symposium Support
Kaneka Corp
Nuon Helianthos
OC Oerlikon Balzers Ltd
KK1: Modelling and Characterization
Session Chairs
Tuesday PM, March 25, 2008
Room 2018 (Moscone West)
9:30 AM - **KK1.1
Optical Properties of Ag/ZnO Back-reflectors for Thin Film Si Photovoltaics.
Robert Collins 1 , Deepak Sainju 1 , Lila Dahal 1 , Jian Li 1 , Jason Stoke 1 , N. Podraza 1 2 , Xunming Deng 1
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States, 2 Materials Research Laboratory, Penn State University, State College, Pennsylvania, United States
Show Abstract10:00 AM - KK1.2
Accurate Determination of Dielectric Functions of Thin Films and Interfaces in Photovoltaic Devices: Critical Issues in Optical Modeling.
Jian Li 1 , Robert Collins 1 , Jason Stoke 1 , Lila Dahal 1 , Deepak Sainju 1 , Jie Chen 1 , Anuja Parikh 1 , N. Podraza 1 2
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States, 2 Material Research Laboratory, Penn State University , State College Pensilvania, Pennsylvania, United States
Show AbstractThe optical properties of the thin film group IV and II-VI materials used in commercial photovoltaic devices depend sensitively on the preparation conditions due to variations in phase, composition, and microstructure such as void and crystalline fractions, hydrogen and alloy contents, grain size and orientation, etc. Furthermore, measurement of the optical properties can be strongly influenced by non-uniformities due to surface and interface micro-roughness and interface chemical mixing, as well as by variations in the microstructural characteristics with depth into the film. One goal of advanced optical characterization is to establish Kramers-Kronig consistent dielectric function parameterizations in which variations in fundamental parameters account for the variations in microstructure or composition due to processing conditions. Two examples from CdTe and Si:H based solar cell technologies will be presented. In one example, the ability to model the dielectric functions of CdS and CdTe of variable grain size and void density using a parabolic band critical point model with broadening parameters that vary in a predictable way with grain size will be demonstrated. In the second example, the ability to parameterize the dielectric function of a-Si1-xGex:H as a continuous function of composition, using a model that incorporates all previously observed features from below the band edge to well above, enables analysis of intentional compositional variations and unintentional fluctuations in solar cell fabrication. Another goal of advanced optical characterization is to develop optical models and optical property databases that can account for non-uniformities in the cell structure. Two additional examples from CdTe and a-Si:H technologies will be given. In one example, a parameterized dielectric function has been established that is appropriate for the Ag/ZnO interface region of the back-reflector of the a-Si:H based n-i-p solar cell and includes observed plasmon-polariton resonances. In the second example, optical property models have been established for the TEC-15 glass/CdS and CdS/CdTe interface regions of the CdTe solar cell structure. Most often, such features are not included in the optical modeling of solar cells due to their complexity; however, the importance of doing so for accurate determination of optical quantum efficiencies and thus assessment of light management will be emphasized.
10:15 AM - **KK1.3
Light Trapping by Means of Diffraction Gratings for Silicon Solar Cells.
Rudolf Morf 1
1 Condensed Matter Theory, Paul Scherrer Institute, Villigen Switzerland
Show Abstract10:45 AM - KK1.4
Quantification of Light Trapping Using a Reciprocity Between Electroluminescent Emission and Photovoltaic Action in a Solar Cell.
Thomas Kirchartz 1 , Anke Helbig 2 , Uwe Rau 1
1 IEF5-Photovoltaik, Forschungszentrum Juelich, Juelich Germany, 2 Institut für Physikalische Elektronik, Universität Stuttgart, Stuttgart, Baden-Württemberg, Germany
Show AbstractCombining Kirchhoff’s law - the fundamental reciprocity between absorption and emission of a body - with the Donolato reciprocity in electrical transport, leads to a connection between the two optoelectronic situations in a pn-junction diode: Electroluminescent emission and photovoltaic action. This newly found connection or reciprocity relation bases on the detailed balance between injection and radiative recombination of carriers on the one hand and generation followed by collection of photogenerated carriers on the other hand. The key result is that electrical and optical terminals of the device are connected by the same two parameters, namely the solar cell quantum efficiency and the internal voltage at the junction. The quantum efficiency describes both the photocurrent generation in the photovoltaic situation and the emission spectrum of the device if operated as a light emitting diode at a given voltage. The importance of this reciprocity relation lies in the need for better interpretation and simulation of electroluminescent measurements. Given the short measurement times and the simple setup, the application of spatially resolved luminescence imaging for in- and off-line measurements of solar cells and modules becomes increasingly important. Following the reciprocity relation, the information contained in these electroluminescence (EL) measurements corresponds to that in a quantum efficiency measurement, with the only difference of the spectral regime, which is limited to the region around the bandgap for luminescence. Thus we expect EL imaging to give information about both electronic and optical effects. That means we are able to detect for instance differences in the local recombination properties as well as information about light trapping properties of the device. To better understand the effects of light trapping on the spatially resolved EL, we first investigate the EL in a spectrally resolved way. Comparing spectra of solar cells with and without textured surfaces reveals the influence of light trapping on EL spectra. We show how to quantify the pathlength enhancement, i.e. the factor the light travels through the device for low absorption coefficients.
Symposium Organizers
Christophe Ballif University of Neuchatel
Randy Ellingson National Renewable Energy Laboratory
Marko Topic University of Ljubljana
Miroslav (Miro) Zeman Delft University of Technology
KK8: Advanced Light Management
Session Chairs
Wednesday PM, March 26, 2008
Room 2018 (Moscone West)
3:00 PM - **KK8.1
Periodic Structures for Improved Light Management in Thin-film Silicon Solar Cells.
Janez Krc 1 , Stefan Luxembourg 2 , Thomas Soderstrom 3 , Andrej Campa 1 , Miro Zeman 2 , Christophe Ballif 3 , Marko Topic 1
1 , University of Ljubljana, Faculty of Electrical Engineering, Ljubljana Slovenia, 2 , Delft University of Technology – DIMES, P.O. Box 5053, 2600 GB Delft Netherlands, 3 , Institute of Microtechnology, University of Neuchatel, CH-2000 Neuchatel Switzerland
Show Abstract3:30 PM - KK8.2
Harvesting Photons in Thin Film Solar Cells with Photonic Crystals.
Rana Biswas 1 , Zhou Dayu 2
1 Ames Laboratory & Microelectronics Res Ctr, Physics/Astronomy & ECpE, Iowa State University, Ames, Iowa, United States, 2 Microelectronics Research Ctr, Electrical & Computer Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractAbsorption of photons with energies just above the band edge is very difficult in solar cells, because of the large photon absorption lengths. To enhance photon harvesting we have designed solar cell configurations using loss-less photonic crystals, that can have considerable advantages over traditional textured metallic back reflectors. Using a-Si:H as a prototype, we utilize a distributed Bragg reflector with a omni-directional band gap in the red and near IR wavelengths, behind the absorber layer. We have simulated in detail two competing designs where the two-dimensional photonic crystal 1) resides between the DBR and the absorber at the back of the solar cell, or 2) where the two dimensional photonic crystal is in between the absorber and anti-reflecting layer at the front of the solar cell. The lattice period, structure, and thickness of the photonic crystal layer, together with the other solar cell parameters were optimized using a rigorous scattering matrix simulation [1] that solves Maxwell’s equations in this 3-D multi-layer solar cell, to achieve best performance. The photonic crystal diffracts light at oblique angles within the absorber layer, thereby enhancing photon path lengths, particularly at diffraction resonances. These solar cell architectures provide an average absorption enhancement of >60% or enhancement ratio upto a factor of 20 for near IR photons. The principles behind this light-harvesting will be described. Conductive DBRS are also considered for easier current collection. The short circuit current is estimated using AMPS for both configurations, and is favored in the first configuration. We have also tested our designs on c-Si solar cells and confirm similar absorption enhancement, and compare c-Si with a-Si:H solar architectures. Complexities of using 3-D photonic crystals in solar cells will be discussed.We acknowledge support from the Catron Solar Foundation.[1] R. Biswas et al, Phys. Rev. B 74, 045107 (2006). R. Biswas, D. Zhou, MRS. Symp. Proc. 989, A03.02 (2007).
3:45 PM - KK8.3
Enhanced Light-trapping in Solar Cells by Directional Selective Optical Filters.
Carolin Ulbrich 1 , Thomas Kirchartz 2 , Uwe Rau 2
1 Institut für Physikalische Elektronik, Universität Stuttgart, Stuttgart Germany, 2 IEF5-Photovoltaik, Forschungszentrum Jülich, Jülich Germany
Show Abstract4:00 PM - KK8.4
Monte-Carlo Simulations of Fluorescent Photovoltaic Collectors.
Liv Proenneke 1 , Gerda Glaeser 1 , Uwe Rau 2
1 Institut für Physikalische Elektronik, Universität Stuttgart, Stuttgart Germany, 2 IEF5-Photovoltaik, Forschungszentrum Jülich, Jülich, Nordrhein-Westphalen, Germany
Show AbstractKK9: Organic and Sensitized Solar Cells
Session Chairs
Wednesday PM, March 26, 2008
Room 2018 (Moscone West)
4:30 PM - **KK9.1
New Strategies for Improved Light Management in Organic based Photovoltaics.
Jan Kroon 1
1 Solar Energy, ECN, Petten Netherlands
Show AbstractIn the past decade, there is a growing interest for new generations of PV technologies and conversion concepts aiming at either very low cost or very high efficiency or a combination of both. It has been recognized that organic based PV can be considered as such a high risk-high potential option.A multitude of device concepts have been investigated to date like liquid and solid state versions of the dye sensitized solar cells, molecular as well as polymer organic solar cells. The research focus is on understanding the working principles and improving efficiency and stability, which are all necessary to show the potential of organic based PV for commercial applications. Despite the enormous progress made in this field, it is clear that most of the Organic based PV options are still in their early infancy. Several research issues must still be addressed before Organic based PV will become a practical technology. These include a further understanding of operation (interfaces, charge transport, morphology control) and degradation of these cells while novel materials are needed for a better spectrum utilization (band gap engineering, new device architectures). This should ultimately lead to much higher power conversion efficiencies (>10 %) and lifetimes that are sufficiently long for practical use. It is clear that scientific and technological breakthroughs are needed to show that Organic based PV will finally realize its potential as a low cost, economically viable PV-technology with a significant impact. An important way to realize the required increase of the power conversion efficiency of Organic based PV concepts is to make more efficient use of the solar spectrum, thereby increasing the current density of the devices. A common strategy to increase the light harvesting properties of organic materials is the engineering of new materials with band gaps that show a better match with the solar spectrum. However, these materials will only effectively operate in solar cells if they fulfill additional requirements such as high charge carrier mobilities and absorption bands with high extinction coefficients over a broad spectral range. For almost all organic based PV devices, the optimal layer thickness is often too small to absorb all the photons within the absorption bands. Thicker layers will absorb more light but due to limited charge transport properties of many organic materials this does not afford larger currents in general. The application of advanced light management and light harvesting strategies to increase and optimize the photon absorption in the photoactive layers is therefore be of utmost importance in the design of highly efficient organic based thin film PV devices.In this contribution a summary will be given of various research approaches towards improved light management in Organic based solar cell i.e. Dye sensitized and polymer based solar cells.
5:00 PM - KK9.2
The Influence of Particle Sizes on the Optical Characteristics of Nanocrystalline TiO2 Films for Dye-Sensitized Solar Cells.
Peter Chao-Yu Chen 1 , Guido Rothenberger 1 , Michael Graetzel 1
1 Institute of Chemical Sciences and Engineering, EPFL, Lausanne, VD, Switzerland
Show Abstract5:15 PM - KK9.3
Application of Long-range Ordered Mesoporous TiO2 Films to Dye-sensitized Solar Cell.
Wan In Lee 1 , Yong Joo Kim 1 , Jia Hong Pan 1
1 Department of Chemistry, Inha University, Incheon Korea (the Republic of)
Show Abstract5:30 PM - KK9.4
Quantum Dot Sensitized Nanostructured Solar Cells.
Jun Wang 1 , Jun Xu 1 , Zhiqun Lin 1
1 Department of Materials Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractWe report a rational design and engineering of quantum dot sensitized nanostructured solar cells (QDSNSC) by utilizing quantum dots (QDs) (e.g., CdSe QDs) into vertically oriented TiO2 nanotubes fabricated by electrochemical anodization. A water soluble bifunctional group (i.e., dithiocarbamate) capped CdSe QDs is synthesized via a biphasic ligand exchange. The -COOH group at the dithiocarbamate functionalized CdSe QDs surface reacts with the -OH group at the TiO2 nanotubes surface, thereby facilitating interfacial interaction between the electron donor (i.e., CdSe) and electron accepter (i.e., TiO2). The effects of the size of CdSe QDs and the aspect ratio of nanotubes on the performance of QDSNSC are studied.
Symposium Organizers
Christophe Ballif University of Neuchatel
Randy Ellingson National Renewable Energy Laboratory
Marko Topic University of Ljubljana
Miroslav (Miro) Zeman Delft University of Technology
KK10: Nanostructures I
Session Chairs
Thursday AM, March 27, 2008
Room 2018 (Moscone West)
9:15 AM - KK10.1
Size-dependent Intrinsic Radiative Decay Rates of Silicon Nanocrystals at Large Confinement Energies.
Milan Sykora 1 , Lorenzo Mangolini 2 , Richard Schaller 1 , Uwe Kortshagen 2 , David Jurbergs 3 , Victor Klimov 1
1 , LANL, Los Alamos, New Mexico, United States, 2 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 3 , Innovalight, Inc., Santa Clara, California, United States
Show AbstractBulk silicon (Si) is the most commonly used material in the semiconductor industry. Because of highly developed processing capabilities, there is great interest in the realization of Si-based optoelectronic devices such as light emitting diodes and lasers. However, bulk Si is an indirect-gap semiconductor, and thus, the radiative recombination in this case can only occur via low-efficiency, phonon-assisted processes. Following the initial observation of efficient photoluminescence (PL) from porous Si, there have been numerous reports on greatly enhanced emission efficiencies in various types of Si nanostructures. For example, a recently developed plasma-synthesis and organic surface passivation technique produces Si NCs with a PL quantum yield >60%. However, the mechanism for high-efficiency PL from Si nanostructures is still under debate. For example, “surface-state” models ascribe it to recombination of carriers trapped at surface sites, while in “quantum-confinement” models, the PL is explained by recombination across the fundamental nanostructure band gap. In the latter case, the observed increase in the radiative rate is attributed to confinement-induced relaxation in momentum conservation, which opens an additional radiative decay channel via zero-phonon, pseudodirect transitions. To better understand the role of the quantum confinement and surface states in Si NCs, in the present work, we analyze the evolution of PL spectra of Si NCs with diameter <4 nm on sub-picosecond to sub-microsecond time scales using different spectroscopic techniques. The early time PL spectra (<1 ns), which show strong dependence on NC size, are attributed to emission involving NC quantized states. The PL spectra recorded for long delays (>10 ns) are almost independent of NC size and are likely due to surface-related recombination. Based on instantaneous PL intensities measured 2 ps after excitation, we determine intrinsic radiative decay rates for NCs of different sizes. These rates sharply increase for confinement energies greater than ~1 eV indicating a fast, exponential increase in the oscillator strength due to opening of zero-phonon, pseudodirect transitions. Similar size-dependent trends are observed for samples prepared by two different gas-phase techniques, which strongly suggests that our results are not sample-specific but rather reflect intrinsic properties of Si NCs.
9:30 AM - **KK10.2
Multiple Exciton Generation in Semiconductor Quantum Dots and Novel Molecules: Applications to 3rd Generation Solar Photon Conversion.
Arthur Nozik 1 , Randy Ellingson 1 , Matt Beard 1 , Joseph Luther 1 , Justin Johnson 1 , Matt Law 1 , Qing Song 1 , James Murphy 1
1 Center for Chemical Sciences and Biosciences, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWednesday, March 26New Presenter*KK10.2 @ 8:30 AMMultiple Exciton Generation in Semiconductor Quantum Dots and Novel Molecules: Applications to 3rd Generation Solar Photon Conversion. Matt C. Beard
10:00 AM - KK10.3
Study of Carrier Multiplication in Semiconductor Nanocrystals by Transient Photoluminescence Spectroscopy.
Gautham Nair 1 , Scott Geyer 1 , Moungi Bawendi 1
1 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe enhancement of carrier multiplication (CM) is an important aim that could increase solar cell performance and widen the range of materials suitable for future solar technologies. Pump-probe measurements have shown evidence ofstrongly enhanced CM in lead chalcogenide, InAs, and CdSe nanocrystals (NCs). However, the nature of the enhancement mechanism is not well understood. We have carried out an experimental assessment of CM yields in semiconductor NCsby carefully studying exciton and biexciton signatures in transient photoluminescence decays. In the case of CdSe NCs, though the technique is particularly sensitive due to the biexciton's strong radiative rate, we have found no evidence for CM up to photon energies as high as 3.1 Eg. The implications of our findings on the efficiency and material dependance of CM are discussed within a general physical framework.
10:15 AM - KK10.4
Time-resolved Photoluminescence Studies of Semiconductor Nanocrystals and Coupled-nanocrystal Films.
Qing Song 1 , Wyatt Metzger 4 , Joseph Luther 1 3 , Matt Law 1 , Matt Beard 1 , Arthur Nozik 1 2 , Randy Ellingson 1
1 Chemical and Biosciences Center, National Renewable Eenergy Laboratory, Golden, Colorado, United States, 4 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Department of Applied physics, Colorado School of Mines, Golden, Colorado, United States, 2 Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States
Show Abstract10:30 AM - **KK10.5
Up Conversion for Photovoltaic Cells.
Gavin Conibeer 1 , Thorsten Trupke 1 , Avi Shalav 1
1 ARC Photovoltaics Centre of Excellence, University of New South Wales, Sydney, New South Wales, Australia
Show Abstract