Valeria G. Stoleru (On leave from The University of Delaware)
Andrew G. Norman National Renewable Energy Laboratory
N. J. Ekins-Daukes Imperial College London
N1: Multiple Exciton Generation and Electron Transport
Monday PM, December 01, 2008
Republic B (Sheraton)
9:30 AM - **N1.1
Multiple Exciton Generation in Colloidal Semiconductor Nanocrystals for Enhanced Solar Energy Conversion.
Randy Ellingson 1 , Matt Law 1 , Joseph Luther 1 , Justin Johnson 1 , Qing Song 1 , Barbara Hughes 1 , Wyatt Metzger 3 , Aaron Midgett 1 2 , Octavi Semonin 1 2 , Sean Sweetnam 1 , Matt Beard 1 , Arthur Nozik 1 2 Show Abstract
1 Center for Chemical and Biosciences, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Chemistry, University of Colorado, Boulder, Colorado, United States
Generation of multiple excitons per absorbed photon for colloidal nanocrystals (NCs) of PbS, PbSe, PbTe, CdSe, InAs, and Si provides an avenue for increasing the conversion efficiency of solar cells by increasing the photocurrent resulting from sunlight’s higher-energy photons. Our research addresses the measurement of multiple exciton generation (MEG) in NC solutions and films, and the development of NC-based solar cells which efficiently harvest multiple electrons per absorbed photon. MEG measurement techniques and results will be described, and recent advances in the performance of PbSe NC-based devices fabricated using a room temperature layer-by-layer technique will be discussed.
10:00 AM - N1.2
Charge Carrier Multiplication and Nature of Excited States in PbSe Quantum Dots.
Laurens Siebbeles 1 Show Abstract
1 , Delft University of Technology, Delft Netherlands
The exploitation of carrier multiplication (CM) in QDs receives a lot of interest, since this offers prospects for the development of solar cells with a maximum power conversion efficiency as high as 44%. In the CM process the excess energy of charge carriers produced by photons with energy greater than the band gap is utilized to produce additional charge carriers and is not lost by thermalization of the photo-excited electron. Efficient CM has been reported for several semiconductor QDs: PbSe, PbS, PbTe, CdSe, InAs and Si. Recently, some of these reports were challenged by studies claiming that CM does not occur in CdSe, CdTe, and InAs QDs, thus raising legitimate doubts concerning the occurrence of CM in the remaining materials. In the work to be presented conclusive evidence is given for occurrence of CM in PbSe QDs using femtosecond transient photobleaching. It is shown that a correct determination of CM efficiency requires spectral integration over the photobleach feature. The CM efficiency we obtain is significantly lower than has been reported previously.The nature of the electronic transition giving rise to the second peak in the optical absorption spectrum of PbSe QDs has been a subject of intense debate. Theoretical and experimental claims have been provided for the assignment of this feature as the 1Pe1Ph as well as the 1Sh,e1Pe,h transitions. We studied the nature of this absorption feature by femtosecond pump-probe spectroscopy and present evidence that the optical transition involves neither 1Se nor 1Sh states. This suggests that it is the 1Ph1Pe transition that gives rises to the second peak in the absorption spectrum of PbSe QDs.References M.T. Trinh, A.J. Houtepen, Juleon M. Schins, T. Hanrath, J. Piris, W. Knulst, A.P.L.M. Goossens and Laurens D.A. Siebbeles, Nano Lett., 8, 1713 (2008). M.T. Trinh, A.J. Houtepen, J.M. Schins, J. Piris and L.D.A. Siebbeles, Nano Lett., ASAP Article; DOI: 10.1021/nl8010963.
10:45 AM - N1.4
Sensing Charge Through Arrays of PbSe Nanocrystals with a Narrow MOSFET.
Tamar Mentzel 1 , Kenneth MacLean 1 , Scott Geyer 2 , Moungi Bawendi 2 , Marc Kastner 1 Show Abstract
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, MIT, Cambridge, Massachusetts, United States
Understanding electron transport through localized states is important for realizing novel approaches to solar cells. The efficiency of existing solar cells is limited in part by transport through localized states and proposed novel solar cell materials, such as arrays of semiconductor nanocrystals (NCs), are based on transport through localized states. Measurement of current through localized states gives information only about the states through which tunneling is fastest as that is what limits the current. To understand the full dynamics of transport through localized states, we measure the charge in addition to the current through the localized states of an array of PbSe NCs. We fabricate a narrow Si MOSFET charge sensor (~100 nm x ~60 nm ) in close proximity (~1 micron away) to a three dimensional array of PbSe nanocrystals. We simultaneously measure the noise current of the MOSFET caused by nearby charge fluctuations, and the current through the NC array, as a function of temperature and electric field.
N2: Multiple Exciton Generation and Nanocrystal Thin Film Devices
Monday PM, December 01, 2008
Republic B (Sheraton)
11:30 AM - **N2.1
Understanding Quantum Dots For Solar Cells.
Alex Zunger 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
12:00 PM - N2.2
Schottky-Quantum Dot Photovoltaics for Efficient Infrared Power Conversion.
Keith Johnston 1 , Andras Pattantyus-Abraham 1 , Jason Clifford 1 , Stefan Myrskog 1 , Dean MacNeil 1 2 , Larissa Levina 1 , Edward Sargent 1 Show Abstract
1 Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada, 2 Chemistry, Universite de Montreal, Montreal, Quebec, Canada
Third generation photovoltaics require efficiencies beyond the Schockley-Queisser single-junction limit at low costs. This can be achieved by fabrication of multiple junctions to efficiently convert visible and infrared wavelengths separately. We report here a solution-processed quantum dot device that achieves very promising efficiency in the infrared regime, which complements existing progress in the visible regime.Planar Schottky photovoltaic devices were prepared from solution-processed PbS nanocrystal quantum dot films with aluminum and indium tin oxide contacts. These devices exhibited up to 4.2% infrared power conversion efficiency at 975 nm, which is a three-fold improvement over previous results. Solar power conversion efficiency reached 1.8%. The simple, optimized architecture allows for direct implementation in multijunction photovoltaic device configurations. The mechanism for the photovoltaic effect in these devices will be discussed, and a simple model of device operation based on drift and diffusion will be presented.
12:15 PM - N2.3
Using a Nanometer Scale MOSFET as a Charge Sensor.
Kenneth MacLean 1 , Tamar Mentzel 1 , Scott Geyer 2 , Moungi Bawendi 2 , Marc Kastner 1 Show Abstract
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 Chemistry, MIT, Cambridge, Massachusetts, United States
Single electron charge sensing has been demonstrated as a powerful measurement technique, and has been primarily used to probe the quantum mechanical behavior of electrons at low temperatures. However, single electron charge sensing is not limited to the low temperature regime, so this technique may be used to investigate electronic behavior in a variety of technologically important materials, such as arrays of PbSe nanocrystals, which have been proposed as a novel photovoltaic material. We fabricate narrow Si MOSFET sensors designed to measure charge fluctuations in a material that is deposited near the channel of the device. We discuss our sensor design, sensitivity, and background charge fluctuations, and use it to study charge hopping in an array of PbSe nanocrystals.
12:30 PM - N2.4
Fourfold Efficiency Improvement in PbS Quantum Dot Photovoltaic Devices via Trap State Passivation by Ethanethiol.
Aaron Barkhouse 1 , Andras Pattantyus-Abraham 1 , Edward Sargent 1 Show Abstract
1 Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada
Ethanedithiol treatments on lead chalcogenide quantum dot films have been previously used to produce efficient photovoltaic devices through reduction in inter-dot spacing and increased crosslinking of dots. We report here that ethanethiol (EtSH) treatment, in addition to improving the hole mobility in PbS QD PV devices, also passivates PbS trap states in non-optimally exchanged nanocrystals. This is inferred from an improved PLQE in EtSH-treated dots and an increased built-in voltage (Vbi) in planar Schottky PV devices, consistent with a reduction in interface trap state density. The best EtSH-treated device had a monochromatic external quantum efficiency and power conversion efficiency of 22% and 2.56%, respectively, under 76 mW/cm2 illumination at 975nm. We show that the effects of EtSH on mobility, carrier lifetime, Vbi and depletion width are insufficient to account for the more than 4-fold increase in external quantum efficiency. We therefore believe that the increased efficiency comes from passivating traps states that favour non-radiative exciton recombination.
N3: Carbon Nanotubes: Carrier Multiplication and Photovoltaics
Monday PM, December 01, 2008
Republic B (Sheraton)
2:30 PM - **N3.1
Optical Properties of Single-Walled Carbon Nanotubes.
Tony Heinz 1 Show Abstract
1 Depts. of Physics and Electrical Engineering, Columbia University, New York, New York, United States
3:00 PM - N3.2
Study of Carrier Multiplication in Nanocrystal Quantum Dots and One-dimensional Carbon Nanotubes.
Akihiro Ueda 1 , Takeshi Tayagaki 1 , Kazunari Matsuda 1 , Yoshihiko Kanemitsu 1 Show Abstract
1 Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan
The strongly confined carriers in low-dimensional semiconductor nanostructures (nanocrystal quantum dots and carbon nanotubes) show unique optical properties compared with the case of bulk semiconductors. Strong Coulomb interactions between carriers cause various interesting phenomena such as carrier multiplication and quantized Auger recombination. The carrier multiplication, production of two or more electron-hole pairs with one high-energy photon, is one of the most attracting phenomena because of its physical and applicative interests . Carrier multiplication phenomena have been observed in various semiconductor nanocrystals. However, the detailed mechanism of the carrier multiplication is unclear and several reports have pointed out the absence of carrier multiplication . In this work, we studied temporal changes of the carrier density in CdSe nanocrystals, nanorods and single-walled carbon nanotubes by means of time resolved photoluminescence (PL) and femtosecond transient-absorption (TA) spectroscopy.The samples used in this work were CdSe/ZnS core/shell nanocrystals, CdSe nanorods, and single-walled carbon nanotubes. The PL dynamics of CdSe nanocrystals and nanorods were studied by a streak camera. The excitation laser energies were 3.1 and 6.2 eV. The PL dynamics under strong excitation showed faster decay compared with that under weak excitation. The origin of this fast decay is quantized Auger recombination and the lifetime of Auger recombination depended on the size of nanocrystals and nanorods. The PL dynamics under weak excitation at 6.2 eV was almost the same as that under weak excitation at 3.1 eV, which means the absence of carrier multiplication or very low efficiency of carrier multiplication under our excitation conditions. On the contrary, we clearly find carrier multiplication in carbon nanotubes under high-energy excitation . The Auger recombination rate in carbon nanotubes is much faster than that in CdSe nanocrystals. From PL and TA studies on CdSe nanocrystals, nanorods, and carbon nanotubes, we discuss the efficiency and mechanism of carrier multiplication in zero- and one-dimensional semiconductor nanostructures.References R. D. Schaller and V. I. Klimov, Phys. Rev. Lett. 92, 186601 (2004). G. Nair and M. G. Bawendi, Phys. Rev. B 76, 081304(R) (2007).  A. Ueda, K. Matsuda, T. Tayagaki, and Y. Kanemitsu, Appl. Phys. Lett. 92, 233105 (2008).
3:15 PM - N3.3
Double-Walled Nanotube-Silicon Heterojunction Solar Cells.
Yi Jia 1 2 , Anyuan Cao 2 , Jinquan Wei 1 , Dehai Wu 1 Show Abstract
1 Department of Mechanical Engineering, Tsinghua University, Beijing China, 2 Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, United States
Carbon nanotubes (CNTs) show high charge mobility and excellent thermal stability, and their photovoltaic applications have been studied extensively, for example, CNTs were used as nanoscale fillers for polymeric solar cells to improve charge separation/transport, or as transparent electrode to improve charge collection. However, conjugated polymers used in constructing solar cells have very low charge mobility, a major factor that limits hole transport through the polymeric matrix to external electrodes and causes high charge recombination, and reported cell efficiencies remain at relatively low levels (<2%). The polymer-based hybrid solar cells also suffer from degradation in air despite that nanotubes are stable at much higher temperature. Several other recent reports involved high-conductivity nanotube films as transparent electrodes to replace indium tin oxide (ITO), however, the nanotubes were not involved into the photogeneration process and the cell efficiency is still limited by the polymeric materials forming the active layers. Here, we developed a novel method to configure double-walled carbon nanotube (DWNT)-silicon heterojunction solar cells. By simply coating thin nanotube films onto silicon wafers we have achieved power-conversion efficiencies of 5-7 % under one solar illumination at 100 milliwatts per square centimeter and negligible degradation of current density after hundreds of hour-exposure in air. The manufacturing process is simple and scalable, involving solution transfer of a porous, single-layer film of double-walled nanotubes onto silicon surface to form heterojunctions with silicon, and does not require separation of metallic and semiconducting nanotubes. We synthesized DWNTs by an improved chemical vapor deposition method. The as-grown DWNT films were treated in H2O2 and HCl solution to remove the impurities and spread to a single-layer film by solution processing. A piece of n-type silicon (n-Si) wafer with a window of pre-deposited insulating layer was immersed into water to pick up the DWNT film to form the heterojunction cell. The nanotubes serve multiple functions in our heterojunction cells, for example, as a heterojunction component for charge separation, as a highly-conductive percolated network for charge transport, and as a transparent electrode for light illumination and charge collection. Our method utilized the distinct properties of both crystalline silicon (e.g. diffusion length) and nanotubes (e.g. high mobility) to construct efficient solar cells and potentially could lead to lower-cost approaches based on nanomaterial-semiconductor heterojunction structures.
3:30 PM - N3.4
Electrical and Optical Transport of GaAs/Carbon Nanotube Heterojunctions.
Chen-Wei Liang 1 , Siegmar Roth 1 Show Abstract
1 , Max-Planck-Institute, Stuttgart Germany
From the semiconductor industry, people have learned that semiconductor heterostructures are very applicable in various technologies, like solar cells or high-frequency communication. Inspired by the history, our work is dedicated to the understanding of the interface between semiconductor and CNTs in order to expand the range of the application of nanotube electronics. We have made heterojunctions consisting of nanotubes and an industrialized semiconductor--GaAs. The transport properties of different types of heterojunctions, p-doped and n-doped GaAs, have also been studied and compared. We found that the p-doped GaAs forms an ohmic contact with the nanotubes while the n-doped GaAs/nanotube heterojunction are rectifying. Given the rectifying effect of the n-doped GaAs/SWNT junctions, transport measurements under illumination were also carried out. The photovoltaic effect therefrom was observed with the light sources of either green laser or regular desk lamp.
N4: CdSe and CdTe nanocrystals: Synthesis, Properties, and Devices
Monday PM, December 01, 2008
Republic B (Sheraton)
4:30 PM - **N4.1
The Use of Multi-Size Arrays of Colloidal Quantum Dots to Study Energy and Electron Transport in QD Junctions.
Emily Weiss 1 , Ryan Chiechi 2 , Scott Geyer 3 , Venda Porter 3 , Moungi Bawendi 3 , George Whitesides 2 Show Abstract
1 Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 3 Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
This paper describes the electrical characteristics of junctions containing 3D arrays of colloidal CdSe quantum dots (QDs) of either a single size or multiple sizes. The electrodes were indium tin oxide (ITO) covered with a thin layer of poly(3,4-ethylenedioxyl- thiophene):poly(styrene sulfonate) (PEDOT:PSS), and eutectic Ga/In. The turn-on voltage of the junctions depended on the size of the QDs next to the PEDOT:PSS. We describe this dependence using a Marcus model to estimate the barrier for charge transfer induced by the energy gap at the QD/PEDOT:PSS interface. Size-selective photoexcitation of the arrays of multiple sizes of QDs helped to determine the location of the interface at which photoinduced charge separation occurred, whether the energy absorbed by the QDs was redistributed before charge separation, and the dependence of the photovoltage on the locations of various sizes of QDs within the junction.
5:00 PM - N4.2
Temperature Dependent Characteristics of Thin Film Nanocrystal Solar Cells.
Yvonne Rodriguez 1 , Jeremy Olson 1 , Ingrid Anderson 1 , Glenn Gray 1 , Sue Carter 1 Show Abstract
1 , University of California, Santa Cruz, Santa Cruz, California, United States
The current-voltage characteristics of thin film solar cells made from layers of CdTe and CdSe nanocrystals were measured as a function of temperature in the dark and under illumination. Data collection was done between 150 K and 380 K. Device structures were fabricated by spin casting the CdTe and CdSe layers onto ITO substrates. After a CdCl2 treatment, the devices were sintered and aluminum contacts were evaporated on. Strong temperature dependence was observed in both the open circuit voltage and the short circuit current density of these sintered devices. The short circuit current density increased as the temperature increased. This suggests transport dominated by thermally activated hopping in these sintered films. The open circuit voltage increased as the temperature decreased. This behavior has been seen in bulk inorganic solar cells. An analysis of the trap density, mobility and junction dynamics are made based on these measurements. The effects of sintering were further examined by measuring the photovoltaic properties of non-sintered cells. Though the open circuit voltage did increase with decreasing temperature as with the sintered films, little temperature dependence of the short circuit current density was observed below room temperature. Above room temperature, the short circuit current density abruptly begins a steady decline. In addition, the effect of plasma treatment and electrode material on the temperature dependent characteristics of these solar cells was studied. Devices were compared with CdSe-P3HT blends and other nanoparticle based thin film devices.
5:15 PM - N4.3
Synthesis of New Organic/inorganic Heterostructures from CdSe Quantum Dots and Tetracyanoquinodimethane Derivatives.
Anna Maria Laera 1 , Vincenzo Resta 1 , Emanuela Piscopiello 1 , Leander Tapfer 1 , Francesco Naso 2 , Francesco Babudri 2 , Gianluca Maria Farinola 2 , Antonio Cardone 3 Show Abstract
1 Department of Advanced Physics Technology and New Materials (FIM), ENEA, Brindisi Italy, 2 Dipartimento di Chimica, Università degli Studi di Bari, Bari Italy, 3 ICCOM, CNR, Bari Italy
Quantum dot (QD) – based hybrid materials currently attract great research and technological interest due to their potential applications in next-generation photovoltaics. The extraction of photoexcited electrons from QDs with the subsequent charge transfer to a suitable matrix represents the key point to efficiently generate a photocurrent. Several ligands for the extraction and transfer of holes are known and applied, while few compounds for the electron extraction/transfer are known and investigated. Due to their peculiar electronic properties, the tetracyanoquinodimethane (TCNQ) derivatives are potential candidates as electron acceptors. These compounds form anion radicals at very positive potentials and form a charge-transfer complex with many donors. Here, we report on the synthesis of tetracyanoquinodimethane and anthraquinone derivatives functionalized with a phosphonic acid group to coordinate the surface of CdSe QDs. The tunable absorption of CdSe QDs covers most of the solar spectrum and, therefore, CdSe is a very suitable material for photovoltaic applications. Furthermore, the possibility to generate multiple excited electrons for a single photon, through a carrier multiplication process, would improve the maximum attainable efficiency of solar photon conversion considerably. We investigated the extraction of photoexcited electrons from CdSe QDs by means of TCNQ derivatives. CdSe QDs were prepared by colloidal synthesis by using trioctylphosphineoxide (TOPO) as solvent. The TOPO capping layer was replaced by a process of ligand exchange in order to obtain the desired new nanocrystal-surfactant complexes. The QDs, typically in range of d=2nm-5nm, were characterized by X-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (TEM). The successful ligand exchange was monitored by FTIR spectroscopy, that reveals the complete disappearance of the CH-aliphatic stretching of TOPO. The inorganic-organic heterostructure was further analyzed by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Here, the complete quenching of fluorescence indicates that an electron transfer from the nanocrystals to the ligand may occur. The successful preparation of CdSe QDs functionalized by TCNQ-derivates and the hypothesized electron transfer indicates that this inorganic-organic structure may be a promising candidate to be used as building block in QDs-based hybrid photovoltaic cells.
5:30 PM - N4.4
Functionalized Semiconductor Nanocrystal Quantum Dots for Patterned, Multilayered Photovoltaic Devices.
Sung Jin Kim 1 2 , Won Jin Kim 2 , Alexander Cartwright 1 2 , Paras Prasad 1 2 3 Show Abstract
1 Electrical Engineering, University at Buffalo, State University of New York, Amherst, New York, United States, 2 Institute for Lasers, Photonics and Biophotonics, University at Buffalo, State University of New York, Amherst , New York, United States, 3 Chemistry, University at Buffalo, State University of New York, Buffalo, New York, United States
We report an approach for nanostructured photovoltaic devices using functionalized semiconductor nanocrystal quantum dots (NQDs) by incorporation of the functional ligand t-butoxycarbonyl (t-BOC) which has an acid-labile moiety. This change in the surface chemistry results in the ability to photo-pattern the NQDs where desired for a number of optoelectronic device geometries with sub-micron patterning capability and multi-layered nanostructure photovoltaic devices. We demonstrate an improved CdTe NQD photoconductive device using this patterning and a ligand modification process. In addition, we demonstrate a bi-layer solar cell using CdTe and CdSe NQDs. The photoconductor was fabricated using a metal semiconductor metal structure. The photoconductor with t-Boc functionalized NQDs showed at least 3 times better conductivity and photoresponse at an applied electric field of 300kV/cm when the ligand was shortened using UV excitation. The t-BOC functionalized NQDs enable all solution processing of multilayered and patterned structures without high temperature sintering or additional chemical treatment. By using this method, a bi-layered solar cell was fabricated. The preliminary results using an unoptimized structure show a 0.05% solar conversion efficiency and an open circuit voltage of 0.72V under AM1.5G 1 sun illumination.
Valeria G. Stoleru (On leave from The University of Delaware)
Andrew G. Norman National Renewable Energy Laboratory
N. J. Ekins-Daukes Imperial College London
N5: Intermediate Band Solar Cells
Tuesday AM, December 02, 2008
Republic B (Sheraton)
9:30 AM - **N5.1
Open Questions in the Implementation of the Intermediate Band (IB) Solar Cell.
Antonio Luque 1 , Antonio Marti 1 Show Abstract
1 Instituto de Energia Solar, Universidad Politecnica de Madrid, Madrid Spain
The concept behind intermediate band (IB) solar cells lies in absorbing sub-band-gap photons by means of a band located within the band gap while electrons are extracted at a voltage limited by the band gap. While the concept is very attractive, there are several issues that have prevented so far from obtaining a cell with promising efficiency.Quantum dots (QD) can form an IB material from the QD confined states. However, a too small sub-band-gap current is obtained because of the low density of QDs (limited to about 1E17 per cm3) as consequence of their large size. Furthermore, not too many QD layers can be grown without spoiling by stress the solar cell performance. Wavelength selective light confinement might be a solution.In practical QD IB cells, the voltage is reduced by the quantum well associated to the wetting layer that is formed at the time that the QDs are grown and also by the QD levels that appear closely spaced near the valence (VB) and conduction (CB) bands. Alternatively to the QD approach, bulk IB materials are being developed. In these materials, the density of IB states may be much higher, of 1E21 per cm3 and higher. However deep levels in the semiconductor band gap are known to be the origin of non-radiative recombination. It could be argued that the introduction of levels within a semiconductor band gap, at such a high density, might reduce excessively its non-radiative lifetime. However, there are theoretical arguments and experimental data that support the conclusion that, when the Mott transition is reached, the non-radiative recombination disappears.IB solar cells have been formed in high mismatched alloys on the basis of the band anti-crossing mechanism. In this way, Zn-Mg-Te-O alloys have been prepared by O ion implantation followed by pulsed laser melting. Also Ga-As-P-N alloys by N ion implantation. In both cases an IB has been found by spectral photo-reflectance measurements. An IB material has also been prepared by solvo-thermal synthesis in V-In-S compounds and optical absorption has been found in agreement with preceding ab initio calculations. The preparation method is not compatible with cell manufacturing but it is expected that other preparation procedures will be realized able to be integrated in thin film cells fabrication.Deep levels are IB precursors and the knowledge of their position as deep levels can be transferred to a large variety of materials if they are known in another material. Based on this, Ti and Fe are found to be good candidates to form an IB material in copper gallium disulphide. These materials are compatible with the manufacturing process of CIS thin film cells. With the same grounds, other impurities can be predicted to form IB materials in components closer to those used in multi-junction solar cells.
10:00 AM - N5.2
An Electronic Structure Study of Intermediate Band by Non-Magnetic Ion Defects in WO3.
Muhammad Huda 1 , Yanfa Yan 1 , Mowafak Al-Jassim 1 Show Abstract
1 , National Renewable Energy Lab, Golden, Colorado, United States
WO3 has a perovskite structure with one of the metal ion position vacant. The larger vacant space inside the crystal structure allows significant local relaxation in case of external doping. With an experimental band gap of around 2.6 eV this presents an interesting opportunity to produce intermediate band (IB) inside the forbidden gap to enhance photo-response. However, realizing IBs in metal oxides pose several problems. We have performed density functional theory (DFT) calculations to study these problems. We have found that non-magnetic ion defects did create IB over the valence band, however, these IBs depend highly on the defects concentration level and local magnetic effects. Moreover, localization of IB’s is one of the major concerns as this would define the performance of the semiconductor. We have found that spontaneous symmetry breaking associated with magnetic degrees of freedom could change the localization of these IBs. On the other hand very disperse doping would simply create a much localized unoccupied band in the gap, which would in turn act as a recombination center, hence degrading the conduction properties of the semiconductor. Over all, controlling these properties to obtain IB semiconductor are challenging issues. We would present our results to identify some of these challenges.
10:15 AM - N5.3
Band Structure Calculation and Material Identification for Quantum Dot Intermediate Band Solar Cells.
Som Dahal 1 , Stephen Bremner 2 , Christiana Honsberg 2 Show Abstract
1 Physics and Astronomy, University of Delaware, Newark, Delaware, United States, 2 Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States
10:30 AM - **N5.4
Applications of Highly Mismatched Alloys to Solar Power Conversion.
Wladyslaw Walukiewicz 1 Show Abstract
1 Materials Sciences Division, Lawerence Berkeley National Laboratory, Berkeley, California, United States
N6: Up/Down Conversion and Intermediate Band Solar Cells
Tuesday PM, December 02, 2008
Republic B (Sheraton)
11:30 AM - **N6.1
Up-Conversion in Multi-component Organic Systems: Enhancement of the Local Spectral Power Density of the Terrestrial Solar Irradiation?
Stanislav Baluschev 1 , Tzenka Miteva 2 Show Abstract
1 , Max Planck Institute for Polymer Research, Mainz Germany, 2 , Sony Deutschland GmbH, Materials Science Laboratory, Stuttgart Germany
Luminescence concentrators in which photon conversion processes such as down-shifting (DS), down-conversion (DC) and up-conversion (UC) are used, can convert via re-emission the solar spectrum to match the absorption properties of different photovoltaic (PV) devices. All these conversion process are expected to lead to increase in the local spectral power density of the Sun-irradiation which means, that more photons with suitable photon energy can be delivered to the PV device. The “classical” concepts focus on developing inorganic or organic PV devices better matching the polychromatic solar spectrum but often when increasing the absorption in e.g. the red part of the spectrum the absorption in blue and green decreases. Another trade-offs within these concepts could be the deterioration in the charge generation and transport in the PV devices. The main advantage of the photon conversion technology is that the concentrators can be considered and optimized as independent optical devices, having no influence on the electrical properties of the operating PV device or its architecture. Consequently, the same photon conversion devices can be combined with variety of PV devices. Until now, experimental demonstration of photon conversion processes excited with Sun-light (non-coherent excitation) and its application for increase of the performance of Si or organic solar cells is made only with luminescence concentrators based on DS.During the last 3 years we have been actively working on the utilisation of the process of UC for Solar spectrum concentration. We investigate and apply for spectrum concentration an original photon UC process based on energetically conjoined triplet-triplet annihilation (TTA) supported bimolecular (sensitizer-emitter) process. In contrast to the all previously described UC-techniques (such as simultaneous or sequential absorption of two or more photons, second and higher harmonic generation, UC-systems based on rare-earth doped phosphors) the fundamental advantage of the TTA based photon UC is its inherent independence on the coherence of the excitation light. Another principal advantage of this UC process is the very low required intensity (as low as 20 mWcm-2) and extremely low spectral power density (as low as 125µWnm-1) of the excitation source needed - so it can in reality be the Sun. We present the summarised results of our investigations and experiments and state the general requirements for the materials’ parameters of the couples sensitizer/emitter in order to obtain efficient energetically conjoined TTA−UC. We also present our working UC-devices for spectrum concentration via UC with up-converted fluorescence quantum yield of 3.2% realized at ultra-low excitation intensities (~20 mWcm-2). We have applied these devices to different solar cells and to our knowledge we are the first to demonstrate dye-sensitized solar cell with performance improved by Sun-light excited photon UC concentrators.
12:00 PM - N6.2
Enhanced Upconversion by Plasmonic Field Concentration.
Ewold Verhagen 1 , L. (Kobus) Kuipers 1 , Albert Polman 1 Show Abstract
1 Center for Nanophotonics, FOM-Institute AMOLF, Amsterdam Netherlands
Plasmonic nanostructures, integrated with a solar cell, can lead to entirely new solar cell designs with lower materials costs or improved conversion efficiency. Here, we demonstrate how two novel plasmonic nanostructures, deposited onto a solar cell, can be used to upconvert infrared solar radiation to the visible, and inspire novel solar cell design that requires p-n junctions with only nanoscale dimension.Coaxial apertures, with overall diameters of ~400 nm, were made in a thin Ag film deposited onto SiO2 and Al2O3 substrates doped with optically active erbium ions. The apertures have localized plasmon resonances in the range 500-1800 nm, tunable by geometry, with typical bandwidths Δλ/λ=10. Finite-difference time domain simulations show field intensity enhancements in the nanoscale gaps of a factor 100. For arrays resonant at 1500 nm, optical measurements using a 1500 nm excitation source show that Er infrared-to-visible upconversion, integrated over the full array area, is enhanced more than 100 fold. While these experiments were done on samples made using electron-beam lithography, we will also demonstrate how soft-lithography techniques can be used to fabricate similar metal nanostructure over large areas characteristic for solar cells.In an alternative geometry, we couple infrared light into a hole array fabricated into a Ag film, that is coupled to a plasmonic nanotaper. Dispersion calculations, as well as near-field measurements of the plasmonic field symmetry, show large plasmon concentration toards the nanotaper, and a concomitant increased erbium upconversion. Calculations show that the field concentration in these nanotapers is – in principle – only limited by the sharpness of the taper tip. Calculations of geometries optimized for upconversion, taking into account Ohmic losses in the metal, will be presented.The metal hole array and nanotaper geometries can be readily integrated in existing solar cell metal contact designs, as will be discussed. Alternatively, they inspire entirely novel designs in which a nanoscale p-n junction is integrated with the plasmonic hotspot. Due to the small size, low-quality semiconductor material, with low minority carrier lifetime can be used, reducing cost. Several of these novel designs will be presented, and preliminary experimental data will be shown. We will elaborate on the pros and cons of implementing plasmonic nanostructures in solar cells in general, balancing losses and fabrication feasibility with the benefits of solar upconversion and reduced materials costs.
12:15 PM - N6.3
Near-Infrared Quantum Cutting in SrF2: Pr, Yb.
Andries Meijerink 1 , Bryan van der Ende 1 , Linda Aarts 1 Show Abstract
1 , Debye Institute, Utrecht Netherlands
Spectral conversion of the solar spectrum is a promising avenue to boost the energy efficiency of photovoltaic cells. Energy losses due to thermalization of hot electrons and holes can be minimized through near-infrared (NIR) quantum cutting, whereby one higher energy photon is converted into two NIR photons. Subsequent absorption of both NIR photons by the solar cell results in current doubling for the high energy part of the solar spectrum. Lanthanide ions are the prime candidates to realize efficient quantum cutting . In this work, we present evidence for a close to 200 % quantum cutting efficiency of visible photons into NIR photons in SrF2:Pr3+, Yb3+ through resonant two step energy transfer. Conclusive information on the transfer efficiency is obtained from comparison of the diffuse reflection spectra and the excitation spectra. The intensity of 3H4→3PJ, 1I6 lines relative to the 3H4→1D2 lines is twice as large in the excitation spectrum of the Yb3+ emission than in the diffuse reflection (absorption) spectrum. This shows that absorption of a single photon in the 3PJ, 1I6 spectral region results in the emission of two NIR photons by Yb3+. Energy transfer occurs through two sequential resonant energy transfer steps between Pr3+ and Yb3+ with 1G4 as an intermediate level: Pr3+(3P0→1G4)-Yb3+(2F7/2→2F5/2) followed by Pr3+(1G4→3H4)-Yb3+(2F7/2→2F5/2). The energy transfer is effective at relatively low Yb3+ concentrations (5%), where concentration quenching of the Yb3+ emission is limited. Comparison of emission spectra, corrected for the instrumental response, for SrF2:Pr3+ (0.1%) and SrF2:Pr3+ (0.1%), Yb3+ (5%) reveal an actual conversion efficiency of 140%. Optimization of the synthesis conditions are expected to further improve this efficiency by reducing losses due to energy transfer to quenching sites. A next step towards the conversion of the short wavelength part of the solar spectrum will involve the inclusion of a sensitizer for the 3P0 level of Pr3+ which is able to absorb efficiently over a broad wavelength range (300-500 nm) and subsequent energy transfer to the 3P0 level of Pr3+. It has been calculated that (ideal) spectral downconversion can increase the Shockley-Queisser limit for p-n semiconductor solar cells up to 40% for an (optimum) bandgap of 1.05 eV, very close to the bandgap of c-Si . The present results demonstrate the potential of spectral downconversion by lanthanide ions for reducing energy losses in photovoltaic devices and increasing the efficiency of c-Si solar cells to make them more cost-effective.  Wegh, R. T., Donker, H., Oskam, K. D. & Meijerink, A. Visible quantum cutting in LiGdF4:Eu3+ through downconversion. Science 283, 663-666 (1999). Trupke, T., Green, M. A. & Würfel, P. Improving solar cell efficiencies by down-conversion of high-energy photons. J. Appl. Phys. 92, 1668-1674 (2002).
12:30 PM - **N6.4
A Molecular Approach to the Intermediate Band Solar Cell.
Timothy Schmidt 1 , N. Ekins-Daukes 2 Show Abstract
1 School of Chemistry, University of Sydney, Sydney, New South Wales, Australia, 2 Blackett Laboratory, Imperial College, London United Kingdom
N7: Plasmonics in Photovoltaics
Tuesday PM, December 02, 2008
Republic B (Sheraton)
2:30 PM - **N7.1
Harry Atwater 1 Show Abstract
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
3:00 PM - N7.2
Gold Nanoparticles for Enhancing Dye Absorption.
Markus Hallermann 1 , Tapan Sau 1 , Enrico Da Como 1 , Andrey Rogach 1 , Jochen Feldmann 1 , Omar Stern 2 Show Abstract
1 Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-Universität, Munich Germany, 2 , GE Global Research Europe, Garching b. Munich Germany
Thin film solar cells are one of the most promising technologies for the development of third generation photovoltaic devices. While the thin film technology takes advantage from the easy implementation of such devices in many different applications, the low optical density limits the amount of solar energy harvested. Nanostructures able to enhance the absorption cross-section of the sensitizer are highly desirable in order to achieve higher efficiencies. Plasmonic nanostructures based on coupled metal nanoparticles can generate local fields of high intensity with spectral tunability across the visible spectral range, acting as nanoantennas . A major challenge in the exploitation of this concept in the existing thin film solar cell technology is the spatial control of enhancement, avoiding loss mechanism such as charge carrier trapping or recombination. It is, therefore, important to design well localized enhancement effects involving the sensitizer.In this communication we present the combination of metal nanoparticles with the system dye/titanium dioxide (TiO2). The synthesis of the nanoparticles has been performed in solution ensuring a facile implementation in the low cost solution processing of dye/TiO2. We demonstrate the self-assembly of nanoparticles on TiO2 thin films coated with a monolayer of dye. By comparing the absorption spectra of the hybrid sample with the reference samples of dye/TiO2 and nanoparticle/TiO2, we show an enhancement of the absorption cross-section in the spectral regions corresponding to the nanoparticle plasmon resonance.  M. Ringler, A. Schwemer, M. Wunderlich, A. Nichtl, K. Kürzinger, T. A. Klar, J. Feldmann, Phys. Rev. Lett., 100, 203002 (2008)
3:15 PM - N7.3
Thin-Film Silicon-Solar Cells and Metallic Tin Nanoparticles.
Mads Kjeldsen 1 , J. Hansen 1 , J. Chevallier 1 , A. Larsen 1 Show Abstract
1 Department of physics and astronomy, Aarhus University, Aarhus Denmark
3:30 PM - N7.4
Coupling Au Nanoparticles Surface Plasmon Resonance with A-Si:H/c-Si Heterojunction Solar Cells for Enhanced Absorption and Efficiency.
Giovanni Bruno 1 , Maria Giangregorio 1 , Alberto Sacchetti 1 , Pio Capezzuto 1 , Maria Losurdo 1 Show Abstract
1 PlasmaChemistry, IMIP-CNR, Bari Italy
A variety of approaches for increasing optical absorption in silicon solar cells is being investigated, with preference to light-absorption and trapping mechanisms that do not increase recombination losses. In this frame, exploitation of the localized absorption of metal nanoparticles (NPs) via surface plasmon resonance (SPR) has been proposed since the wavelength-selective photon absorption can be tailored by the NPs’ size, shape and local dielectric environment.Among the various solar cell technologies, over 85% of the current production is based on the well-established silicon wafer technology. In this frame, a-Si:H/c-Si heterojunction solar cells have been the subject of extensive research especially since Sanyo has demonstrated efficiencies >21% for heterojunction solar cells produced by plasma enhanced chemical vapor deposition (PECVD) of a-Si:H on crystalline silicon (c-Si). So far, there are no reports in literature about exploitation of SPR metal nanoparticles coupled directly with hydrogenated amorphous silicon (a-Si:H). Furthermore, although many studies and approaches reported in literature are mainly based on the use of Au NPs colloidal solutions requiring transferring and stabilization of the metal nanoparticles on the semiconductor/device platform of interest and additional steps of solvent evaporation, we are seeking and investigating dry processes for direct formation of metal NPs compatible with silicon-based photovoltaic technologies.In this contribution, we report on the integration of the a-Si:H PECVD with the Au sputtering process for depositing Au NPs directly on a-Si:H to exploit the metal nanoparticles surface plasmon resonance in enhancing light absorption. This approach allows Au nanoparticles to be added at the late stage of an integrated PV cell processing at a temperature of approximately 200°C by direct sputtering of Au NPs onto PECVD a-Si:H. The impact of Au nanoparticles density and size and of sputtering temperature on the heterojunction performance will be presented and discussed. The correlation among the electrical, morphological and optical properties of the Au-NPs/(n)a-Si:H/(p)c-Si heterojunctions is discussed, exploiting spectroscopic ellipsometry (UVISEL-Jobin Yvon), which is demonstrated to be useful for determining the a-Si:H film optical properties as well as the surface plasmon resonance of Au nanoparticles, whose diameter is determined by atomic force microscopy (AFM). The I-V characteristic of devices show that engineering Au nanoparticles’ diameter and density makes possible improvements in conversion efficiency. In particular, we demonstrate an improvement of 20% of the short-circuit current, Jsc, an increase of 3% of the fill factor, FF, an increase of 25% of the power output, Pmax, and, consequently, of the energy efficiency conversion for (n-type)a-Si:H/(p-type)c-Si heterojunctions with 20 nm Au nanoparticles (NPs), which are characterized by SPR at 572 nm.
3:45 PM - N7.5
Light Trapping with Particle Plasmons.
Kylie Catchpole 1 2 , Fiona Beck 2 , Ruud Schropp 3 , Albert Polman 1 Show Abstract
1 Center for Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands, 2 Centre for Sustainable Energy Systems, Australian National University, Canberra, ACT, Netherlands, 3 Debye Institute, University of Utrecht, Utrecht Netherlands
The excitation of surface plasmon resonances on metal nanoparticles is a promising way of increasing the efficiency of thin film solar cells. We develop a number of design principles for increasing the efficiency of solar cells (or light-emitting diodes) using metal nanoparticles. We show that potential path length enhancements of up to a factor of 28 can be achieved in the near-infrared for cylindrical and hemispherical Ag particles on Si, because the incident light leads to a dipole-type excitation very close to the interface with the substrate. (In fact, the path length enhancement for a horizontal electric point dipole is over twice the Lambertian value). In contrast, spherical particles have a low potential path length enhancement of 5-10, due to the low fraction of their volume close to the surface. Because of the exponential decay of high in-plane wavevector components of the scattered field, this results in poor coupling to the substrate. We also show that the scattering cross-section of a particle depends not only on the particle size and shape, but also its position relative to the substrate. For example, increasing the thickness of a Si3N4 spacer layer from 10nm to 30nm increases the scattering cross-section of a 100nm Ag particle on Si by a factor of 2. Finally, we show that Ag particles give much higher path length enhancements than Au particles. Optimizing plasmon mediated light trapping in thin film solar cells thus requires balancing scattering cross-section, fraction of light scattered into the substrate and the angular distribution of the scattered light.We apply the design principles to 100μm thick bifacial c-Si silicon solar cells and 350nm thick p-i-n a-Si:H solar cells deposited on glass substrates with Ag nanoparticles formed by evaporation followed by annealing, and show substantially increased near-infrared photocurrent. We also demonstrate that fine control over nanoparticle size and shape can be achieved with a process that is scalable to large areas using nanoimprinting, and we report results for silicon solar cells with Ag nanoparticles fabricated using nanoimprinting.
N8: Photon Management and Si Quantum Dot Solar Cells
Tuesday PM, December 02, 2008
Republic B (Sheraton)
4:30 PM - N8.1
3D Photonic Spectrally Selective and Diffractive Intermediate Filter for Micromorph Tandem Cell.
Andreas Bielawny 1 , Johannes Upping 1 , Paul Miclea 1 , Ralf Wehrspohn 1 , Seung-Mo Lee 2 , Mato Knez 2 , Marius Peters 3 , Andreas Lambertz 4 , Reinhard Carius 4 Show Abstract
1 Physics, Martin-Luther-University of Halle-Wittenberg, Halle Germany, 2 , Max-Planck-Institute for Microstructure Physics, Halle Germany, 3 FMF (Freiburg Centre for Material Research), University of Freiburg, Freiburg Germany, 4 Institute of Energy Research, Forschungszentrum Jülich, Jülich Germany
4:45 PM - N8.2
Very Effective Light Trapping in Grating Thin-Film Solar Cells.
Mukul Agrawal 1 , Peter Peumans 1 Show Abstract
1 Electrical Engineering, Stanford University, Stanford, California, United States
5:00 PM - N8.3
An Array of Nanocones with Sharp Tips as Efficient Antireflection Layers for Solar Cells.
Jia Zhu 1 , Zongfu Yu 1 , George Burkhard 1 , Ching-Mei Hsu 1 , Stephen Connor 1 , Yi Cui 1 Show Abstract
1 , Stanford, Stanford, California, United States
Graded refractive-index layer has been actively pursued for broadband antireflection properties. However, current technologies are either limited to specific materials or requiring complicated process. Here an array of nanocones with sharp tips (~5nm) fabricated by a novel process is reported. The nanocones array can suppress the reflection of light at a wide rage of spectrum and angles of incidence. More importantly, the process is very simple and can be applied to different kinds of materials. Simulation results are also provided to explain the results. This technique could have many important applications in electro-optical devices, especially in Solar Cells and Light Emitting Diodes.
5:15 PM - N8.4
Bandgap Engineering of Silicon Quantum Dot Nanostructures for High Efficient Silicon Solar Cell: The Tandem Approach.
Georges Bremond 1 , Bechir Rezgui 1 Show Abstract
1 INL, INSA Lyon, Villeurbanne France
A tandem approach is proposed using Silicon nanostructures to increase the efficiency of so-called third generation photovoltaic solar cells.Si quantum dot nanostructures (or silicon nanocrystals)are synthesized by depositing silicon-rich nitride (SRN) layers using plasma-enhanced chemical vapour deposition (PECVD). We have shown the intrinsic formation of silicon nanocrystals (nc-Si) in non-stoechiometric amorphous hydrogenated silicon nitride (a-SiNx:H) layers using pure silane (SiH4) and ammonia (NH3) as reactants. The NH3 would provide more hydrogen in the silicon nitride film leading to an improvement of the crystallinity of Si quantum dots (QD) by favouring the disorder-to-order transition. Furthermore, hydrogen dissociated from the NH3 would passivate the surface of a Si QD more effectively.Transmission Electron Microscopy (TEM) was employed to explore the microstructure of the as-deposited Si-in-SiNx composite films. The chemical bonds of these films were examined by using Fourier Transform Infrared (FTIR) spectroscopy in the wavenumber range from 400 to 4000 cm-1 with a resolution of 4 cm-1.The photoluminescence (PL) property of silicon nanocrystals in silicon-rich nitride (SRN) layers are also investigated. The peak position of PL could be controlled by adjusting the flow rates of ammonia and silane . Two types of luminescent mechanisms, such as radiative defects in the film and the quantum confinement effect (QCE) in silicon nanocrystals, have been proposed to explain the origin of light emission from these structures. These two mechanisms are inherently coexisting in our samples and the photoluminescence spectrum depends on the contribution of each other.The optical absorption properties of the deposited films are obtained and analyzed from light transmittance measurements. Spectroscopique ellipsometry have been performed in order to analyse the refractive index and the extension coefficient. All these measurements were carried out at room temperature. These techniques have given good correlation in the extraction of the absorption coefficient induced by the Si nanocrystal in the visible /UV energy range. Measurements of photocurrent have shown a great increase of the induced currrent in the visible/UV energy range for an optimum of deposition conditions. These results will be discussed in order to reach a better knowledge of the physical properties of this third generation photovoltaic all silicon included material for the tandem solar cell application approach.
5:30 PM - N8.5
Metamaterial Based on the Nanostructured Si for Multistage PV Conversion.
Zbigniew Kuznicki 1 , Patrick Meyrueis 1 Show Abstract
1 , Photonic Systems Laboratory, Illkirch France
Completing one-step PV conversion by additional new low-energy mechanisms is one of the most important challenges of modern photovoltaics. Silicon is a basic PV material which is not efficient enough to convert light into electricity in its bulk or thin film form because of its indirect bandgap. Progress in conversion efficiency requires breakthroughs. One way has been indicated by low-dimensional or nanostructured Si materials as, for example, nanoscale Si-layered systems combined with an active interface with its crystalline defects.We have demonstrated low-energy carrier multiplication experimentally under attenuated solar excitation in nanostructured Si. The investigation has been limited to comparative measurements of the short-circuit currents: reference and modified. Their ratio shows steps which allow estimating a characteristic energy E = 0.274 eV, previously determined by us from spectral response and modeling. The effect is particularly visible under weak incident beams.Another investigation of the modified surface activity, presented in, shows that the state of the semiconductor front surface/interface dominates first the optoelectronic properties of the MIND (multi-interface novel device) and, as a consequence, its PV performance. This behavior raises a series of paradoxes in comparison with well known Si photovoltaics. Most of these paradoxes can be explained on the basis of electronic and optical features as could be confirmed in simulations. The nonlinear behavior of the surface region versus incident light intensity shown by us previously results from the high free-carrier density. The incident flux acts through its intensity and spectral composition. The necessary modification consists of the introduction of a nanoscale Si-layered system into the surface zone of a conventional material (Si wafer). The nanoscale and multiinterface approaches modify the existing semiconductor, such as single-crystal Si, e.g. by making use of geometrical free-carrier confinement due to the CCL. This allows introducing useful energy levels and a ''new'' band structure, leading to a multilevel multistage PV conversion. A good “tuning” of device energy levels to the whole solar spectrum seems to be possible. In its simplest form it could be obtained by a superposition of only two groups of mechanisms having different energy thresholds: larger primary (conventional conversion stage) and smaller secondary (new conversion stages). The secondary mechanism combined with confinement peculiarities can be at the origin of multiple subpopulations distinguished from each other by their average energy. Certain non obvious features of the continuous nanoscale Si-layered system resulting from the buried amorphization can be noted.
5:45 PM - N8.6
Nanostructured Silicon-based Layers for Photovoltaic Applications.
Fabrice Gourbilleau 1 , Bechir Rezgui 2 , Abel Sibai 2 , Georges Bremond 2 Show Abstract
1 CIMAP, ENSICAEN, Caen Cedex 04 France, 2 INL, Insa Lyon/Université Lyon, Villeurbanne France
N9: Poster Session I
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - N9.1
RF-Sputtered Ge-ITO Nanocomposite Thin Films for Photovoltaic Applications.
Grace Shih 1 , Tracie Bukowski 1 , Barrett Potter 1 , Joseph Simmons 1 Show Abstract
1 Materials Science and Engineering, University of Arizona, Tucson, Arizona, United States
The use of inorganic quantum dot (QD) ensembles as an absorptive component in nanostructured photovoltaic junction designs is of current interest as an alternative to organic sensitizers. QD composition and quantum-size effects both can provide tuning of the QD optical absorption within the solar spectrum with the promise of improved energy conversion efficiency. For sensitization of thin film heterojunctions for photovoltaic energy conversion, the introduction of QD’s into junction structures represents an important challenge, requiring control of the mesoscale QD phase distribution along the film depth (e.g. within the depletion region of the heterojunction) and the QD-matrix interfacial electronic character (for efficient carrier extraction and transport). The current work introduces the use of a sequential, dual-source, RF-sputtering deposition technique to produce Ge-indium-tin-oxide (ITO) nanocomposite thin films with controlled QD phase distribution within the ITO matrix. Single and multilayer structures of Ge embedded in an ITO matrix have been formed and the technique has also been used to produce a QD layer in the vicinity of an ITO-Si heterojunction. Modification of local Ge volume fraction has been demonstrated through the control of substrate exposure times to each target. Transmission electron microscopy and optical absorption spectroscopy has confirmed the formation of Ge quantum dots upon post-deposition thermal annealing and photoconductivity has been observed in the Ge-ITO nanocomposite. Photovoltaic characterization of these structures will also be presented.
9:00 PM - N9.10
A New Method for Forming Surfactant-free PbSe Quantum Dot Films and Quantum Dot-polymer Composites for Excitonic Solar Cells.
Gayan Dedigamuwa 1 , Xiaomei Jiang 1 , Jian Zhang 1 , Pritish Mukherjee 1 , Sarath Witanachchi 1 Show Abstract
1 Department of Physics, University of South Florida, Tampa, Florida, United States
Multiple exciton generation in semiconductor quantum dots (QD) promises a new generation of solar devices that include flexible inorganic-organic hybrid structures. In these devices incorporation of PbSe or PbS quantum dots in a polymer matrix without a surfactant barrier at the QD-polymer interface is important for the dissociation of excitons and subsequent collection of carriers. We have developed a laser-assisted spray process to deposit surfactant-free crystalline PbSe nanoparticles on substrates. In the first step of the process, nanoparticles of PbSe were synthesized by the reaction between tributyl-phosphine selenium and lead oxide in the presence of Oleic acid and 1-octadecene. After washing the particles several times to remove the excess surfactant, they were dispersed in hexane. In the second step, toluene solution containing 8-10 nm size PbSe particles was atomized by a nebulizer to form an aerosol. SF6 was used as the carrier gas to inject the aerosol into the growth chamber through a nozzle. Heating of the SF6 gas by a CO2 laser beam at the nozzle caused the solvent and the surfactants to evaporate while PbSe nanoparticles were deposited on a substrate. Nanoparticles within the film grown by this method were uniformly distributed and in intimate contact with each other. TEM studies confirmed both the single crystal nature of each QD and the absence of surfactants. A second nebulizer was used to inject aerosols of P3HT polymer dissolved in toluene onto the substrate. SEM and TEM analysis of the co-deposited films showed a uniform distribution of PbSe particles within the polymer matrix. Conductivity of PbSe QD films and QD-polymer composite films have been measured in several electrode configurations. Comparison of these results with films formed with surfactant- coated QD will be presented.
9:00 PM - N9.11
Solar Cell Efficiency Enhancement above the Shockley-Queisser Limit using Low Dimensional Absorbers.
Ian Ballard 1 , Keith Barnham 1 , Nicholas Ekins-Daukes 1 Show Abstract
1 EXSS Physics, Imperial College London, London United Kingdom
The detail-balance efficiency limit for photovoltaic solar energy conversion assumes isotropic, radiative emission as the sole means for carrier recombination. This is a good approximation for bulk semiconductors, but the valance bands in low dimensional structures can give rise to strongly directional emission, polarised in either the TE or TM direction. Typically, TE emission (perpendicular to the plane of the quantum well) couples to heavy-hole valance states, with some weaker coupling to light-hole states. TM emission couples exclusively to light-hole states. The degree of TE and TM emission can therefore be controlled by adjusting the energetic alignment of heavy and light hole bands via strain. In this way it is possible to engineer a material with intrinsic directional emission, either in TE or TM directions. In terms of detail balance efficiency limits, the highest efficiency occurs when the etendue of the incident radiation matches that of the escaping radiation. We show that this intrinsic directional emission gives rise to a new, marginally higher limiting efficiency for a single junction solar cell when low dimensional semiconductors are employed. Further we show how directional emission is useful in real, constrained devices that employ distributed Bragg reflectors to recycle photons.
9:00 PM - N9.13
Effect of Back-Surface Reflectors on the Performance of Strain-Balanced Quantum Well Solar Cells.
Jessica Adams 1 , Keith Barnham 1 , Ravin Ginige 1 , James Connolly 1 , Geoff Hill 2 , John Roberts 2 , Tom Tibbits 3 Show Abstract
1 Department of Physics, Imperial College London, London United Kingdom, 2 , EPSRC National Centre for III-V Technologies, Sheffield United Kingdom, 3 , QuantaSol Ltd., Richmond upon Thames United Kingdom
The strain-balanced quantum well solar cell (SB-QWSC) is a GaAs p-i-n cell with multiple quantum well layers inserted into the i-region, confining carriers in the wells to discrete energy levels and extending the absorption edge to beyond that of the bulk. This results in a net efficiency increase over a conventional GaAs cell, as the loss in open-circuit voltage is over-compensated by the gain in photocurrent from the well region .Recombination processes in a GaAs/InGaAs SB-QWSC have been found to be radiatively dominated at the high bias at which solar concentrator cells operate , and so cell efficiency can be improved by the incorporation of a reflector at the back of the n-region. For example, use of a distributed Bragg reflector (DBR) substrate can lead to an effect known as photon recycling , where a proportion of the radiative recombination is reflected back into the well region and reabsorbed, thus lowering the net radiative dark current. Cell efficiency is further increased as incident radiation not absorbed on the first pass is reflected back into the bulk material, increasing the short-circuit current.We present the characterisation of SB-QWSC devices grown by metal-organic vapour-phase epitaxy (MOVPE). Devices were compared from growth on doped and un-doped substrates. Different back-reflectors were investigated, including mirrored, polished, and saw-tooth etch. Furthermore, different device mountings were investigated, including gold epoxy, black paint, and an air gap between device and substrate. Dark current and quantum efficiency measurements were made on the anti-reflection coated test devices.Quantum efficiency measurements were fitted using in-house simulation software SOL, and parameters were taken from the fits to model device dark currents. A close match to experimental data was obtained. The software was then used to predict device efficiencies from the theoretical short-circuit current and open-circuit voltage.We will present details of experimental and modelling results found on these and on other devices.1.K.W.J. Barnham et al., “Quantum Well Solar Cells and Quantum Dot Concentrators”, Chapter 16, Nanostructured Materials for Solar Energy Conversion, T.Soga (Ed.), Elsevier B.V., (2006), p.517.2.J.P. Connolly, I.M. Ballard, K.W.J. Barnham, D.B. Bushnell, T.N.D. Tibbits, J.S. Roberts, “Efficiency Limits of Quantum Well Solar Cells”, Proc. 19th European Photovoltaic Solar Energy Conference, Paris, France, (2004), pp. 355-359.3.D. C. Johnson et al., Appl. Phys. Letters, 90, 213505, (2007).
9:00 PM - N9.15
Influence of Buffer Layer Patterning on Droplet Epitaxy and Photovoltaic Properties of InAs/GaAs Quantum Dots.
Leon Webster 1 , Jia-Hung Wu 2 , Christopher Proctor 4 , Levi Thompson 3 , Rachel Goldman 2 Show Abstract
1 Applied Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Physics, University of Michigan, Ann Arbor, Michigan, United States, 3 Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Low-dimensional systems exhibiting quantum confinement have been predicted to lead to high efficiency intermediate-band solar cells (IBSCs). For example, highly ordered arrays of quantum dots (QDs) are expected to enable the formation of mini-bands of intermediate energy between the valence and conduction bands of the host matrix. In principle, this would allow two-photon carrier excitation processes, followed by resonant carrier transport between the QDs, thereby lowering cell series resistance and improving efficiency . Although a number of groups have attempted to experimentally demonstrate high efficiency IBSCs based upon arrays of QDs, the performance to date has been limited, likely by the relatively large QD sizes and low QD densities resulting from the strain-driven Stranski-Krastanow (SK) QD growth process. An alternative approach to achievement of ultra-high QD densities is droplet epitaxy, which involves synthesis of In nanodroplets, followed by conversion to InAs QDs via exposure to an arsenic flux . In this case, the QD size and density is determined primarily by the surface droplet size and density, rather than by the QD/matrix strain. Thus, surface patterning, prior to droplet epitaxy, provides a promising means for significant increases (decreases) in QD densities (sizes), while maintaining a specific spatial arrangement of QDs. For this purpose, we are exploring the influence of GaAs buffer layer patterning  on droplet epitaxy of InAs QDs. Our QDs consist of 2, 4, and 6 ML equivalent of In deposited on GaAs and subsequently exposed to As2 at 150°C, followed by annealing at 400°C to form InAs. Atomic force microscopy reveals an increase (decrease) in QD diameter (areal density) with increasing In exposure. Prior to QD formation, GaAs buffer layers were grown under several different conditions, involving a variety of growth rates, substrate temperatures, V/III ratios, and/or annealing sequences. For buffers grown at 580°C and As2/Ga = 13, 1–2 ML height step-terraces elongated along [-110] are apparent. Interestingly, QDs preferentially nucleate at the step-edges, similar to the case of SK QDs reported earlier . We will discuss the effects of various buffer layer patterns on the QD sizes, densities, and spatial arrangements. Similar studies using focused-ion-beam and laser pre-patterned surfaces will be presented. Finally, the effects of buffer layer patterning on the photovoltaic properties of p-i-n structures incorporating droplet epitaxy QDs as the active region will be discussed.This work was supported in part by DOE and AFOSR. A. Luque and A. Martí, Phys. Rev. Lett. 78, 5014 (1997) S. Ohkouchi, Y. Nakamura, H. Nakamura, and K. Asakawa, Thin Solid Films, 464, 233 (2004).  W. Ye, S. Hanson, M. Reason, X. Weng, and R. S. Goldman, Journal of Vacuum Science & Technology B, 23, 1736 (2005).
9:00 PM - N9.2
Application of the Design of Experiments Method to the Study of the Intermediate-Band InAs/GaAs Quantum-dot Solar Cell.
Alysha Grenko 1 , John Walker 2 , Ibrahim Kimukin 2 , Brian Hoskins 1 , Elias Towe 1 2 Show Abstract
1 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Conventional single-junction photovoltaic devices can only convert a narrow band of the solar spectrum into electric energy. Appropriately engineered semiconductor quantum-dots—inserted into the active regions of solar cells—could potentially capture a broader spectrum, thus improving device performance. Specifically, the quantum dots can be embedded in the i-region of a simple homojunction p-i-n device architecture. By controlling the size, shape, and composition of the quantum dots, their discrete energy levels can be made to form an “intermediate band” located within the energy gap of the homojunction semiconductor material. This intermediate band should lead to the absorption of low energy photons that would normally be lost to the photoelectric conversion process in a conventional single-junction device. This novel device architecture is expected to have a higher conversion efficiency than a single-junction cell because of the additional photocurrent that would be collected from the low energy photons. This paper studies the impact of various parameters on the characteristics of the intermediate-band solar cell device. The devices studied in this work are fabricated from material synthesized by the method of molecular beam epitaxy. The basic device architecture consists of 10 periods of InAs/GaAs quantum-dot layers inserted between the p-GaAs and n-GaAs layers of an otherwise conventional p-n junction structure. Several parameters have a major impact on the performance characteristics of the intermediate-band solar cell. These parameters include (i) the thickness of the InAs wetting layer from which the dots are formed; (ii) the substrate temperature at which a fraction of the spacer layer thickness between dots is grown; (iii) the thickness of the inter-dot spacer layer; and (iv) the magnitude of an electron sheet carrier concentration inserted between the inter-dot spacer layers during a growth interruption. To uncover the impact of each parameter on a performance characteristic, a structured approach is indispensable. In this work, we use a systematic statistical method, known as the design of experiments (DOE) method. The essence of the method is to uncover the impact of a factor (parameter) affecting the process and the output (characteristic) of the device. By varying the parameters listed above, and analyzing the resulting device characteristics, we have uncovered the factors that impact the spectral location and width of the intermediate band. Furthermore, we have identified the parameters that can be used to improve the photoresponse, including how to reduce non-radiative recombination losses in the quantum-dot region. This paper discusses the details of the application of the DOE method to the design of an intermediate-band solar cell structure, and highlights preliminary results obtained through use of the method.
9:00 PM - N9.3
Fabrication of Optically Active Thin Films by Chemical Solution Deposition.
Edita Garskaite 1 , Mari-Ann Einarsrud 1 , Tor Grande 1 Show Abstract
1 IMT, NTNU, Trondheim Norway
Conversion of light in doped thin films is very attractive for increasing the efficiency of photovoltaic cells. Chemical solution deposition of thin films of such oxide materials is very attractive due to low costs and environmental issues. Here we present our resent data on preparation of optically active thin films prepared by aqueous sol-gel dip-coating route. Several different Er-doped optically transparent materials such as YAG, ZnO and In2O3 have been prepared. The influence of the precursor sol composition, deposition parameters, heat treatment of the films on the nanostructure and homogeneity of thin films on silicon substrates will be discussed. Finally, up-conversion process in thin oxide-based films has been demonstrated.
9:00 PM - N9.4
Growth of Copper Indium Disulfide Nano-rods using Anodized Aluminum Nano-mask.
Hong Lam 1 , Zhong Wei Zhang 1 2 , Chung Pui Chan 1 , Zhuo Chen 1 , Charles Surya 1 , Chang Fei Zhu 2 1 Show Abstract
1 Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, Hong Kong, China, 2 Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui, China
Nano-structured semiconductor material is considered advantageous in photovoltaic applications. In this work, Copper Indium Disulfide (CIS) nano-rods were electrodeposited by utilizing anodized aluminum oxide (AAO) nano-mask. The AAO nano-mask was obtained using a two-step anodization process. The anodization was carried out in a 0.05 to 0.3M oxalic acid solution at 1°C and a potential of 40 to 100V was applied. The alumina yielded in the first anodization step was dissolved by immersing the sample in a mixed acid solution with phosphoric acid and chromic oxide solution (6wt% H3PO4+1.8wt% H2CrO4) at 60°C. By controlling the time of the second step, we can control the ultimate thickness of the AAO film. Subsequently, the AAO foil was subjected to chemical etching (in 5wt.% H3PO4 for 30 to 50 minutes at 30°C) to enlarge the pores and remove the barrier layer at the bottom of AAO template. Such two-step process has shown to help improving the pore order in the AAO foil. A continuous layer of Cu and In film on the back side of the AAO layer was obtained by sequential deposition of elemental 5N Cu and In by e-beam evaporation. The total thickness of the stack was about 1μm. The stoichiometric ratio of Cu and In was maintained at 1:1. A 1μm thick layer of molybdenum was then deposited by e-beam on top of the Cu/In layers. To provide mechanical support, 100μm of copper was then electroplated onto the Mo layer. Depositions of Cu and In through the nanometer-scale windows in the AAO were performed by electrodeposition technique. We used a three electrode system with aquatic solutions containing copper and indium salts. The metal salts concentration was adjusted so that deposition rate of copper and indium ions is approximately 1:1. The electrodeposition process was conducted in constant current mode at 1mA and that deposition time ranged from 10 to 30 minutes. Columns of stoichiometric copper indium nano-rods were obtained and embedded inside AAO nano-holes. Finally, the AAO mask was eliminated using phosphoric acid and the nanowires were sulfurized in a quartz tube furnace with sulfur vapor at 450°C using argon as the carrier gas.X-ray diffraction characterization studies confirmed the presence of copper indium disulfide crystal structures in the material. Scanning electron microscope pictures of the columnar CIS nano-rods were taken. The structures were characterized for their optical and electrical properties.
9:00 PM - N9.5
Wet-Chemical Route to ZnO/CuInS2 Core-Shell Nanowire Array for Photovoltaic Applications.
Jih-Jen Wu 1 , Wan-Ting Jiang 1 Show Abstract
1 Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan
Well-aligned ZnO/CuInS2 core-shell nanowire (NW) arrays have been synthesized on fluorine-doped tin oxide substrates using a simple wet-chemical route, i.e., aligned ZnO NW array first formed by an aqueous chemical bath deposition (CBD) and then conformal CuInS2 shells deposited on the ZnO NWs using the successive ionic layer absorption and reaction (SILAR) method. The thickness of the CuInS2 shell is variable by the cycle number of SILAR. The absorption edge of the ZnO/CuInS2 core-shell NWs is at ~700 nm after annealing in sulfur atmosphere. The ZnO/CuInS2 core-shell NW arrays with a NW length of 3 μm have been employed to be the n-type semiconductor and the absorber for fabricating the eta (extremely thin absorber) solar cells. Na2S liquid electrolyte and CuSCN were utilized as the p-type material in the eta solar cells. The results show that a current density and an efficiency of 3.4 mA/cm2 and 0.46%, respectively, are achieved in the ZnO/CuInS2/Na2S electrolyte eta solar cells, which is comparable to those of the N3-sensitized ZnO NW solar cells. Synthesis and characterization of the ZnO/CuInS2 core-shell NW arrays as well as the photovoltaic measurements of the ZnO/CuInS2 eta solar cells will be reported in the presentation in detail.
9:00 PM - N9.6
Microstructure and Local Electronic Transport Properties of nc-Si:H Thin Films: Effect of Hydrogen Plasma Annealing.
Pavel Dutta 1 , Sanjoy Paul 1 , David Galipeau 1 , Venkat Bommisetty 1 Show Abstract
1 EE, South Dakota State University, Brookings, South Dakota, United States
9:00 PM - N9.7
Effect of Hydrogen Plasma Annealing on Optoelectronic Properties Nanocrystalline Silicon.
Sanjoy Paul 1 , P. Dutta 1 , Dorin Cengher 1 , David Galipeau 1 , Venkat Bommisetty 1 Show Abstract
1 EE, South Dakota State University, Brookings, South Dakota, United States
Nanocrystalline silicon (nc-Si) is an important photovoltaic material due to bandgap tunability, high optical absorption and low cost fabrication. Also, nc-Si can be deposited at low temperatures and on a variety of substrates making it potential candidate for tandem solar cell structures for efficient conversion of broadband solar spectrum. Improving optoelectronic properties requires detailed understanding of the interface structure between Si nanocrystals and surrounding amorphous tissue . Deposition temperature, hydrogen dilution and plasma conditions are known to play an important role in determining nc-Si/a-Si interface [2,3]. This report presents optoelectronic properties of low temperature deposited nc-Si:H using RF sputtering and the use of post-deposition atomic hydrogen plasma treatment to improve crystallinity and optoelectronic properties. Nanocrystalline Si thin films of 100 nm thick were deposited on glass substrates using reactive sputter deposition and characterized using Raman, UV-vis spectroscopy, temperature dependent I-V, transient current measurement, transmission electron microscopy and atomic force microscopy. Hydrogen dilution impacts the Fermi level (EF) position and density of states (DOS) in the films significantly resulting change in optical and transport properties. Optical bandgap of nc-Si varied between 1.6 to 2.14 eV. Bulk defect density of all nc-Si thin films was estimated in the order of 1016 cm-3 from transient IV measurements. Electrical conductivity was about 10-5 S/cm. Arrhenius plots showed three distinct activation regions: intrinsic, extrinsic and partial ionization. Thermal activation energy calculated from high temperature region of the Arrhenius plot ranged between 0.73 and 0.88 eV. Post-deposition hydrogen plasma treatment improved crystallinity and electrical conductivity of nc-Si. Mild H2 plasma treatment (less than 1 W/cm2 plasma exposure for 10 s) at room temperature improved both structural and optoelectronic properties of nc-Si. While the effect of hydrogen plasma exposure was most significant during initial few seconds of exposure, higher plasma power and longer exposure periods showed incremental improvement in optoelectronic properties. Results suggest the potential use of hydrogen plasma during initial stages of nc-Si deposition to improve its structural and optoelectronic properties. Seung Yeop Myong, Koeng Su Lim and Makoto Konagai, Appl. Phys. Lett., 88, 103120 (2006). Yu. L. et. al., J. Appl. Phys., 94, 443 (2003) S. Veprek, F. A. Sarott and Z. Iqbal, Phys. Rev. B36, 3344 (1987)
9:00 PM - N9.8
Nano Si Structures for Solar Cell Application.
Branko Pivac 1 , Pavo Dubcek 1 , Nikola Radic 1 , Hrvoje Zorc 1 , Sigrid Bernstorff 2 , Branislav Vlahovic 3 Show Abstract
1 , R. Boskovic Institute, Zagreb Croatia, 2 , Sincrotrone Trieste, Trieste Italy, 3 Physics Department, North Carolina Central University, Durham, North Carolina, United States