Symposium Organizers
Bhushan L. Sopori National Renewable Energy Laboratory
Bernhard Dimmler Würth Solar GmbH & Co. KG
Jeffrey Yang United Solar Ovonic LLC
Thomas Surek Surek PV Consulting
Q1: Crystalline Silicon Technologies
Session Chairs
Monday PM, November 30, 2009
Room 306 (Hynes)
9:30 AM - **Q1.1
Hydrogen Passivation for Crystalline Silicon Solar Cells.
Michael Stavola 1
1 Department of Physics, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractThe Si substrates that are often used for the fabrication of solar cells to reduce cost give rise to defect issues that must be addressed. Hydrogen is commonly introduced into silicon solar cells to reduce the deleterious effects of defects and to increase cell efficiency [1]. A process that is used by industry to introduce hydrogen is by the post-deposition annealing of a hydrogen-rich SiNx layer that is used as an antireflection coating [2]. A number of questions about this hydrogen introduction process and hydrogen’s subsequent interactions with defects have proved difficult to address because of the low concentration of hydrogen that is introduced into the Si bulk.Fundamental studies of hydrogen-containing defects in silicon provide a foundation for addressing issues of interest to the Si solar-cell community. Strategies have been developed by which hydrogen in silicon can be detected by IR spectroscopy with high sensitivity [3,4]. The introduction of hydrogen into Si by the post-deposition annealing of a SiNx coating has been investigated to reveal hydrogen’s concentration, diffusivity, and reactions with defects. The effect of processing variations on the concentration of hydrogen that is introduced into the Si bulk has also been studied. The contributions of F. Jiang, S. Kleekajai, V. Yelundur, A. Rohatgi, L. Carnel, J. Kalejs, and G. Hahn to our studies are gratefully acknowledged. This work has been supported by the Silicon Solar Research Center SiSoC Members through NCSU Subaward No. 2008-0519-02 and NSF Grant No. DMR 0802278.[1] J. I. Hanoka, C. H. Seager, D. J. Sharp, and J. K. G. Panitz, Appl. Phys. Lett. 42,618 (1983).[2] F. Duerinckx and J. Szlufcik, Sol. Energy Mater. Sol. Cells 72, 231 (2002).[3] F. Jiang et al., Appl. Phys. Lett. 83, 931 (2003).[4] S. Kleekajai et al., J. Appl. Phys. 100, 093517 (2006).
10:00 AM - Q1.2
A New, Ultrafast Technique for Mapping Dislocation Density in Large-area, Single-crystal and Multicrystalline Si Wafers.
Bhushan Sopori 1 , Przemyslaw Rupnowski 1 , Mathew Albert 2 , Chandra Khattak 2 , Mike Seacrist 3
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , GT Solar, Merrimack, New Hampshire, United States, 3 , MEMC, St. Peters, Missouri, United States
Show AbstractAverage dislocation density and spatial distribution of dislocations are routinely used as a measure of crystal quality of single- and multicrystalline Si (mc-Si) wafers. A variety of techniques have been developed to generate dislocation maps, including X-ray imaging, Cu decoration, and chemical delineation. The most common method is to defect etch the wafer with a suitable chemical etchant and then count the etch pits using an optical microscope. Commercial camera systems, with image analysis software, are available as microscope attachments that can count etch pits within the field of view and combine that information to produce maps of dislocation distribution over a wafer. An improved technique uses light scattered by etch pits to statistically count dislocations. The wafer is illuminated by a laser beam and the total scattered light, which is proportional to the number of etch pits in the illuminated region, is measured. An instrument based on this technique takes 30–60 minutes to map a 6-in x 6-in wafer.This paper describes a new technique that uses scattering from a defect-etched wafer to map dislocation distribution of the entire wafer in a single image. The measurement is very fast and compatible with large-area wafers. In this technique, the single- or multicrystalline wafer is polished to produce a damage-free polished surface. The wafer is then defect etched using Sopori etch (HF:CH3COOH:HNO3 in a 36:15:1 ratio) for 30 s to produce etch pits at dislocation sites. The shape of the etch pit depends on the direction of dislocation at the surface and does not depend on the orientation of the wafer or grain (in mc-Si). The wafer is then placed in a reflectometer where a set of lights, symmetrically placed around the wafer, illuminate it at an oblique incidence. The light scattered normal to the wafer is collected by a camera and imaged. The image corresponds to the local reflectance of the defect-etched wafer. Because local scattering is proportional to the density of etch pits, the camera image is proportional to the local variation in the dislocation density of the wafer. The system is calibrated by using a reference sample to convert the reflectance map into a dislocation map. This technique allows a fast (< 1 s) mapping of dislocations. An interesting feature of this etch is that the scattering cross-section of all dislocations (which can have circular, elliptical, or comet shapes) is the same. Thus, all dislocations are counted. An instrument based on this technique is now commercially available. We will show results that demonstrate: (i) repeatability of defect etching of large mc-Si wafers, (ii) variation of dislocation patterns over selected parts of a mc-Si ingot, (iii) a correlation between defect maps and photocurrent maps of commercial Si solar cells, and (iv) a correlation between defect distribution and the solar cell performance.This abstract is subject to government rights.
10:15 AM - Q1.3
Low-cost, High Efficiency Solar Cells on Scrapped CMOS Silicon.
Daniel Inns 1 , Joel de Souza 1 , K. Saenger 1 , H. Hovel 1 , D. Sadana 1
1 T. J. Watson Research Centre, IBM, Yorktown Heights, New York, United States
Show AbstractThe cost of scrapped Si from the CMOS industry is extremely low which makes it an attractive material for solar industry. However, the minority carrier lifetime of this material is very low and variable, typically ~ 1 µs compared to the lifetime of the original prime-Si wafer which is > 500 µs. Solar cells made on scrapped wafers therefore result in efficiencies which are inferior to that from a prime CMOS grade Si. We have developed a novel and effective low-cost metal gettering anneal process which allows the minority lifetime of the scrapped wafer to recover to close to its original value, a 100-500 fold increase. Since the efficiency of a solar cell is directly impacted by the minority carrier lifetime, cell efficiency of the improved scrapped Si is nearly equivalent to that from a prime-Si wafer. In order to erase the processing history of the wafer, surface etching is performed to remove ~ 20 µm of surface Si. Following this is a unique impurity gettering step that is performed at > 1300°C with chlorine-containing gas to enable efficient gettering of metals out of the substrate. An efficiency of ~15% has been demonstrated on both prime and improved scrapped wafers using rudimental device design to study the validity of our unique metal gettering process. This efficiency is being improved to much higher values by refinements in device design, anti-reflection coating(s) and surface passivation schemes.
10:30 AM - **Q1.4
Crystalline Silicon Technology for Solar Applications.
Aditya Deshpande 1 , Mike Seacrist 1 , Steve Kimbel 1 , Gang Shi 1 , Jihong Chen 1
1 , MEMC Electronic Materials, St Peters, Missouri, United States
Show AbstractThe use of crystalline silicon in solar applications exceeds the silicon consumed in semiconductor applications. Further, the growth rate of silicon use in solar applications has been higher than the growth rate of use in semiconductor applications over the past several years. Many similarities and synergies exist between manufacturing silicon for semiconductor and solar applications. These include producing polysilicon raw material, growing silicon crystals, and converting crystals into silicon wafers by wire slicing. For these reasons there is a strong motivation for silicon suppliers to participate in the crystalline silicon solar market. Crystalline silicon solar cells are the workhorse of the photovoltaic industry and have a significant portion of the market share of the world production of solar cells. The key driver and challenge for crystalline silicon in solar is cost which is influenced by both the silicon material cost and silicon performance. For silicon to maintain and improve on solar cell market share, further reductions in production cost as well as improvements in solar cell efficiency are necessary. The approach of a vertically integrated silicon supplier to the challenge of improving solar cell efficiency performance while also improving silicon manufacturing productivity and reducing cost will be described. A technical roadmap for crystalline silicon material will be presented and discussed. Key components of the cost are silicon feedstock, crystallization, and slicing. The approaches for commercial production of all these steps will be contrasted with other available methods. The use of directional solidification (DS) methods to grow multi-crystalline silicon (mc-Si) is a large fraction of the crystalline silicon market. The efficiency of mc-Si solar cells is usually lower than for single crystal silicon because of a high degree of material defects that include dislocations, random grain orientations, grain boundaries, impurity precipitates, and inclusions. Typical defects and impurities in mc-Si wafers and their influence on the device performance are reviewed. Detailed characterization of these defects is not straightforward. Methods developed for characterization of these defects will be presented.
11:30 AM - **Q1.5
Developments in Crystalline Silicon-based Photovoltaic Product Architecture and Manufacturing.
Juris Kalejs 1
1 , American Solar Technologies, Chelmsford, Massachusetts, United States
Show AbstractSolar electric (Photovoltaic) crystalline silicon (c-Si) product diversity has changed very little over three decades of development, including the last decade of unprecedented expansion of the industry. The dominant module product comprising over 90% of cumulative installations, which exceed 15 GW worldwide, still employs an ubiquitous configuration, a platform based on a planar laminate. This paper will review trends in module architecture and manufacturing methods for this currently dominant PV c-Si commodity module platform. The commodity flat-plate module contains typically 60-72 solar cells cut from multicrystalline blocks as 156 mm square areas, or 156 mm dimension pseudo-squares cut from single crystal boules. New module design and manufacturing approaches different from those of the commodity PV product are now in development and piloting. Developments which use innovations in manufacturing processes, i.e., stringing of cells and packaging in a laminate, will be discussed.
12:00 PM - **Q1.6
Contactless Measurement of Carrier Lifetime on As-Grown or Shaped Ingots, Sections, and Blocks.
Ronald Sinton 1 , Tanaya Mankad 1 , M. Forsyth 1 , James Swirhun 1
1 , Sinton Instruments, Inc., Boulder, Colorado, United States
Show AbstractThis work will describe recent developments in measurement techniques for assessing the bulk lifetime of ingots, sections, and blocks without surface preparation. This permits detailed characterization of the materials as they exist in the production environment. Prior to sawing into wafers, it is possible to characterize the quality of the feedstock and growth parameters of bulk silicon. This allows quick feedback in order to fully optimize the growth. This can also be used to qualify the quality and suitability of the crystalline silicon for particular solar cell processes. Measurement at this stage, compared to after wafering, is extremely useful and cost effective. The measurements are more sensitive to true bulk parameters before wafering, the entire ingot can be assessed quickly, and the cost of wafering can be adverted or modified if all or a portion of the piece “fails”. Therefore measurements in the ingots or blocks prior to sawing into wafers present an unusual combination of industrial and scientific advantages compared to measurements of wafers. After sawing, the unpassivated surfaces of the wafers can compromise electronic material measurements until at least the phosphorus-diffusion step in the process which acts to passivate the surface recombination.The parameters that are determined by Quasi-Steady-State Photoconductance, QSSPC, or transient photoconductance measurements on as-grown or shaped material are the bulk lifetime, the resistivity, and the “trapping” which can be a measure of crystalline quality. Patterns in any of these parameters, from top to bottom of the grown piece or across the diameter of a sectioned CZ ingot give valuable information concerning feedstock, external contamination during growth (from the crucible or growth furnace), and the thermal growth conditions.Three special cases will be described in some detail.1) Boron-doped CZ. The special characteristics of this material are the strongly injection-level-dependent lifetime and the B, O, and Fe spatial dependences that gives can give rise to strong lifetime variations in both the growth direction and radially. Typical ranges of bulk lifetime are 10-500 microseconds.2)B-doped multicrystalline silicon. This material, like B-CZ, has strong spatial dependence of the lifetime, ohm-cm, and trapping as a function of both the growth direction and the position of the block relative to the crucible.3)The highest efficiency cells in the industry use n-type CZ or FZ silicon. The desired sensitivity for these processes requires the accurate discrimination of differences between silicon in the 1-10 ms range. In this range, a significantly different measurement and analysis technique will be presented in detail.
12:30 PM - Q1.7
Commercial Production of Silicon Solar Cell Feedstock by Upgrade of Metallurgical Grade Silicon.
John Mott 1 , Julio Bragagnolo 1 , Michael Hayes 1
1 , Ohio Solar Energy, LLC, Alliance, Ohio, United States
Show AbstractIntroduction. The relationship between impurity content in Solar Grade Silicon (SGS) and solar cell quality is the subject of intensive research. The PV industry has developed around the use of silicon made by the Siemens process for the semiconductor industry, with impurity levels typically in the parts per billion by weight (ppbw) range. There is a growing consensus that SGS with impurities in the parts per million range (ppmw) can be obtained cost effectively from Metallurgical Grade Silicon (MGS) and used to yield solar cells with comparable performance (see for example ‘Beneficial Effects of Dopant Compensation on Carrier Lifetime in Upgraded Metallurgical Silicon’ by S. Dubois et al. in the 23rd European Photovoltaic Solar Energy Conference, Valencia, September, 2008). This provides insight on the success encountered by Timminco, an early SGS market entrant, in commercializing silicon material with [P] levels of the order of 2 ppmw. Current Work. Analysing data from 16 UDS runs on samples taken from the melt, before and after UDS, and a solid sample taken from the silicon frozen on the cold silicon collection surface, we note that the average values of [P] in the molten silicon samples increase from 11.9 ppmw before UDS to 15.9 ppmw after UDS. The average value of [P] in the solid silicon sample is 4.9 ppmw. This demonstrates an effective refining ratio of 0.41, even at a 50% solid fraction. This is important as UDS, by its nature, implies a loss of silicon, while little or no silicon is lost in B reduction. Performing a secondary UDS on silicon obtained from these primary UDS runs yields [P] around 2 ppmw.In addition to P and B reduction, in this paper we also discuss the hardware designed to implement this process in commercial production in volumes exceeding 4,000 MT per year. MB Scientific, the original process developer, and NC Consulting, an engineering company, have developed a plant design that can produce SGS at an estimated cost that will allow for profitable large scale production, and have joined in a new company, Ohio Solar Energy, to commercialize the large-scale production technology. Future Work. While the UDS equipment design is completed, we have so far succeeded in decreasing the B concentration to 30% of the initial value by using glass slagging, wherein molten glass devoid of boron is vigorously mixed with the silicon metal and made to ‘getter’ the boron in the silicon and is then removed from the silicon metal. Repeated with new glass each time, the number of steps is dependent on the starting concentration of the boron in the silicon. The difficulty with reducing B is related to the P levels in the glass constituents due to back-contamination with successive washes. A new furnace, with more powerful agitation and designed to prevent recontamination of the UDS-processed silicon, and purer glass will enable B removal to ≤1ppmw target levels.
12:45 PM - Q1.8
Efficient Single-crystal Black Silicon Solar Cells with Anti-reflection by a Nanocatalyzed One-step Etch.
Hao-Chih Yuan 1 , Vernon Yost 1 , Matthew Page 1 , Howard Branz 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractWithout using silicon nitride or another dielectric anti-reflection (AR) layer, we have fabricated confirmed 16.8%-efficient prototype solar cells on 2.7 ohm-cm, 300 um p-type single-crystal Si (100) substrates. Aside from the use of an inexpensive single-step nanocatalyzed liquid etch [Branz, Appl. Phys. Lett. 94, 231121 (2009)] to produce a nanoporous black silicon surface layer, processing of these cells is nearly identical to the present practice in PV silicon manufacturing, including a POCl3-diffused emitter and aluminum back-surface field. Open-circuit voltage (612 mV) and fill factor (80%) of the single-crystal black silicon cells is comparable to a planar control. The weighted average reflectance from 350 to 1000 nm of the single-crystal black silicon cells is below 2% compared with 34.3% of the planar control with no AR. As a result, the short-circuit current of the black silicon solar cells is 38% higher than the planar control. Nonetheless, the 34.7 mA/cm2 short-circuit current density of the black silicon solar cells is about 3 mA/cm2 below that predicted by the reflectance reduction alone. Our modeling shows that the current deficit is due to high recombination in the nanoporous layer, which impacts the short-wavelength spectral response. We also study the optical properties of the nanoporous black silicon surface layer and our measurements reveal some scattering in the nanoporous surface layer, but little internal reflection (light trapping). The studies on the optoelectronic and optical properties enable us to describe not only possible improvements to our solar cells, but general design considerations for high-efficiency solar cells based on density-graded black-silicon surfaces. Finally, we estimate the potential cost advantage of eliminating the vacuum-coated silicon-nitride anti-reflection equipment from the PV manufacturing line.
Q2: CdTe and GaAs Based Technologies
Session Chairs
Monday PM, November 30, 2009
Room 306 (Hynes)
2:30 PM - **Q2.1
Present Status of Research and Industrial Development of CdTe/CdS Solar Cells.
Ramesh Dhere 1 , David Albin 1 , Xiaonan Li 1 , Timothy Gessert 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractThin-film solar cells have attracted considerable attention due to their potential for low-cost production. CdTe has been one of the main contenders in this arena because its bandgap of 1.5 eV is ideally matched to the solar spectrum and the binary compound allows the freedom to choose a variety of fabrication techniques. This presentation will highlight key developments during the last forty years that have been stepping stones for progress in device performance. Industrial activity began in the early 1980s when Matsushita introduced screen-printed modules, and there were several players in the field. The field has expanded tremendously in the last five years with First Solar leading the way. First Solar is already the largest producers of photovoltaics in the United States, and with a planned expansion to 1 GW by the end of 2009, they will be contending for the top spot worldwide. We will present an overview of different industries involved in the field and their approaches. In addition, we will present the ongoing research at our laboratory (NREL) and others and will analyze the status of the research. The efficiency of the champion CdTe cell is 16.5%, and module efficiency based on present knowledge is expected to reach around 12.5%, which is well below its potential. We will present the analysis of the device performance and the main parameters affecting the performance. Further improvement in module performance is unlikely without better understanding these parameters. Other area of emphasis is the long-term reliability and accelerated life testing that is necessary to understand the effect of processing changes on product reliability. NREL is developing the Process Development and Integration Laboratory (PDIL) to facilitate interaction among industry and research groups. The presentation will provide an overview of the CdTe tool being developed and provide some details about the capabilities of the tool, in particular, and PDIL facility, in general. This abstract is subject to government rights.
3:00 PM - Q2.2
Study of the Electrical Properties of Cross Sections of CdTe/CdS Solar Cells Measured with Scanning Kelvin Probe Microscopy.
Helio Moutinho 1 , Ramesh Dhere 1 , Chun-Sheng Jiang 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe apply scanning Kelvin probe microscopy (SKPM) to analyze cross sections of working CdTe/CdS solar cells under different bias conditions. This technique is performed inside a scanning probe microscope (SPM), and it provides the distribution of the electrical potential inside the device with high spatial resolution. The SKPM and topographic images are compared to associate variations of the potential with topographic features. For instance, it is possible to compare the position of the p-n junction with the metallurgical junction between CdTe and CdS. In SKPM, we apply AC and DC bias between the tip of the SPM and the sample. The signal measures the difference between the work functions of the tip and sample surface, and it is proportional to the surface potential of the sample. By biasing the cell during measurement (reverse and forward polarizations), we avoided artifacts such as Fermi-level pinning, and we were able to investigate the distribution of the electrical potential inside a live device polarized under different conditions. By taking the derivative of the potential, we determined the distribution of the electric field, and by locating the maximum of the electric field, we located the position of the p-n junction.This work complements our other work presented in the 2009 Spring MRS Meeting that investigated different ways to prepare cross sections of samples, compared two different SKPM measurement procedures (using the first and second resonance cantilever peaks), and determined the position of the junction for a standard CdTe/CdS solar cell. In this work, we investigate the change in the electrical potential using different bias, forward and reverse, showing the change in the width of the depletion region. By calculating the derivative of the potential, we observed the distribution of the electric field inside the device and noticed the following: the field has a strong value concentrated on a thin layer of the device at the junction, and a much smaller value moving away from the junction, showing that there is a change on the electrical properties of the device at the interface. To investigate whether interdiffusion of Te and S is responsible for this effect, we analyzed solar cells without CdCl2 heat treatment, as well as cells produced without the CdS layer. In both cases, the distribution of the electrical potential and electric field was different than for a standard device. In this work, we will also present results of the distribution of the electrical potential on the back contact of the solar cells. In our case, we use graphite paste deposited on the CdTe film followed by the deposition of Ag film. We will show that, in general, the potential drop between the CdTe/graphite interface is much smaller than in the junction region. This abstract is subject to government rights.
3:15 PM - Q2.3
Finite Element Model to Understand the Effect of O2 on Closed Space Sublimation of CdTe.
Nirav Vora 1 , Ramesh Dhere 1
1 National Center for Photovoltaics, National Renewable Energy laboratory, Golden, Colorado, United States
Show AbstractIncorporating O2 in the closed space sublimation (CSS) of CdTe thin film has resulted in improved cell efficiencies. Many studies have been undertaken to understand this effect on cell efficiency. In this work we study the effect of oxygen on lateral uniformity of the deposited CdTe film. A finite element model has been developed to represent the mass and heat transfers involved in the CSS process. The model takes into consideration the effect of O2 by modeling its reaction with Cd vapors in the space between the source and the substrates. This reaction can decrease the amount of Cd available for condensation near the substrate if the diffusion of Cd from the source to the substrate is not fast enough. One of the factors affecting this reaction rate is the concentration of O2. So a gradient of O2 from the edges to the center of the substrate can result in a laterally non-uniform film. This gradient can be formed if the rate of diffusion of O2 is lower than that of its reaction with Cd. A steady state model will be solved at various temperatures, pressures, and separation distances to determine the optimum conditions for depositing a CdTe film with uniform thickness. Experiments will be carried out at these conditions and results compared with the simulation results. The comparison will help in determining the reaction rate constants, as there is a lot of variation in the values reported in the literature. A transient model will also be developed to better represent the experiments. Finally the model will be modified to represent the vapor transport deposition of CdTe. This abstract is subject to government rights.
3:30 PM - Q2.4
CdTe Thin Film Growth Using High Rate Sputtering for Photovoltaic Applications.
John Walls 1 , Paresh Nasikkar 1 , Hari Upadhyaya 1
1 Electronic and Electrical Engineering, Loughborough University, Loughborough United Kingdom
Show AbstractMagnetron sputtering has a number of important advantages for the deposition of thin films for use in photovoltaic devices. Sputtering provides control over thin film thickness with sub-nanometre precision using time only. This allows the thickness of the CdTe absorber layer to be optimized thereby minimizing materials usage and process manufacturing time. Using the closed field configuration, the thin films are super-smooth (< 1 nm rms roughness). This is especially important in the TCO base layer since roughness of the TCO can break through the CdS layer and cause “shunting” across cells. This paper describes a flexible reactive sputtering process in which adjacent unbalanced magnetrons are constructed of opposite magnetic polarity. The resulting closed magnetic field maintains a high density reactive plasma. In contrast to previous reactive sputtering strategies, the process does not require an auxiliary ion or plasma source and the associated use of high voltage ion acceleration. As a result, the deposition energy is optimized and insufficient to cause damage in the growing thin film. The substrate temperature is typically maintained below 100°C without the need for direct cooling. The thin films exhibit bulk optical properties, they are also dense and super-smooth. The thin films also have typically low compressive stress. The magnetron targets are simple metals or semiconductors for high rate deposition and are converted to compound thin films when required by using the appropriate reactive gas. This paper provides data derived from a medium throughput batch system with a 0.4 m diameter drum substrate carrier and four 0.6m linear magnetrons. However, the process geometry is scalable and adaptable to in-line deposition. The optical and electrical performance of each layer in the CdTe thin film photovoltaic stack will be presented together with preliminary device performance.
3:45 PM - Q2.5
Fabrication and Modeling of Three-Dimensionally Structured CdTe Thin Film Photovoltaic Devices with Self-Aligned Back-Contacts.
Jonathan Guyer 1 , Daniel Josell 1 , Carlos Beauchamp 1 , Suyong Jung 2 , Behrang Hamadani 2 , Lee Richter 3 , John Bonevich 1 , Nikolai Zhitenev 2 , Tom Moffat 1
1 Metallurgy Division, NIST, Gaithersburg, Maryland, United States, 2 Center for Nanoscale Science and Technology, NIST, Gaithersburg, Maryland, United States, 3 Surface and Microanalysis Science Division, NIST, Gaithersburg, Maryland, United States
Show AbstractOur goal is to provide industry with test structures and models ofnext-generation photovoltaics, with an initial focus on CdTe andCuInxGa1-xSe2 (CIS or CIGS) materials. These tools will enableinterpretation of measured external properties affected by geometry, grainstructure, and nanoscale phase separation, which will support improvedprocessing and design of Second Generation (thin film) and Third Generation(nanostructured) photovoltaic devices.
CdTe and CIGS are some of the most stable and efficient photovoltaicmaterials. A wide variety of deposition methods enable novel devicestructures at previously unobtainable dimensions, but optimal structuresand dimensions are unknown. Sensitivity of the microstructure (and,ultimately, the device efficiency) to deposition methods and processingconditions varies, and potential new and cheaper fabrication methods havenot been verified.
We are adapting our experience with the electrochemical "superfill" ofmetal in sub-micrometer-scale trenches and vias [1] to the fabrication ofnovel photovoltaic structures. We have fabricated electrodeposited CdTedevices that use interdigitated back-contact electrodes both forindependent electrode-position of n- and p-type material during fabricationand for collecting the photo-generated current in the fabricated devices[2]. The devices enable quantitative evaluation of bulk and interfaceproperties of 3-d devices through controlled variation of length scales,independent of the absorber thickness, including electrode pitch andcross-section.
To guide and interpret the experimental measurements, we have implemented aphotovoltaic device model that gives us complete control over geometry andmicrostructure. We are using the model, in conjunction with experimentalmeasurements, to extract discrete materials properties from complex structures.We implemented our device model in theFiPy partial differential equation solver package [3]. This freely available package allows us to easilydistribute our photovoltaic codes to researchers in industry and elsewhereas they are validated.
[1] D. Josell, D. Wheeler, W. H. Huber & T. P. Moffat Phys. Rev. Lett. 87 (2001) 016102
[2] D. Josell, C. Beauchamp, S. Jung, B.H. Hamadani, A. Motayed, L. Richter, M. Williams, J.E. Bonevich, A. Shapiro, N. Zhitenev & T.P. Moffat J. Electrochem. Soc., in press
[3] J. E. Guyer, D. Wheeler & J. A. Warren Comput. Sci. Eng. 11 (2009) 6 http://www.ctcms.nist.gov/fipy
4:30 PM - Q2.6
Thin-film ITO/InP Solar Cells on Flexible Plastic Substrates.
Kuen-Ting Shiu 1 2 , Jeramy Zimmerman 2 , Hongyu Wang 2 , Stephen Forrest 2
1 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Materials Science and Engineering, Electrical Engineering and Computer Science and Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPhotovoltaic cells made with amorphous silicon, II-VI compounds such as CdTe [1], and copper-indium-gallium-selenide (CIGS) [2] thin-films have been adopted for use in high specific power, light-weight solar cell panel applications. In this work, we demonstrate single-crystal thin-film InP Schottky-type solar cells mounted on flexible plastic substrates by cold-welding a metallic film on the solar cell to one deposited on the plastic sheet. The lightly p-doped InP cell is grown epitaxially on an InP substrate via gas source molecular beam epitaxy. The InP substrate is removed via selective chemical wet-etching after the epitaxial layers are bonded to a metal layer pre-deposited onto the surface of an 25 m thick Kapton sheet, and indium tin oxide (ITO) top contacts are then deposited. The power conversion efficiency under 1 sun is up to 10.2±1.0% and its specific power is 2.0±0.2 kW/kg. Stress tests indicate that the thin film solar cells can tolerate both tensile and compressive stress by bending over a 1 cm radius without damage. Full materials and device characterization will be presented. This work provides an alternative method for adopting III-V semiconductor solar cells for portable and space applications where very high specific power efficiency is required. 1. Romeo, et.al, Solar Energy Materials & Solar Cells, 90, 3407 (2006).2. Otte, et.al, Thin Solid Films, 511-512, 613 (2006)
4:45 PM - Q2.7
Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells.
Tingyi Gu 1 , Kai Yang 1 , Mohamed El-Emawy 1 , Andreas Stintz 1 , Luke Lester 1
1 Center for High Technology Materials, University of New Mexico, Albuquerque, New Mexico, United States
Show AbstractRecent interest in using InAs quantum dots (QDs) in the absorbing region of solar cells has focused primarily on the predicted increase in quantum efficiency due to the intermediate band effect or simply larger short circuit current density. However, the three-dimensional carrier confinement inherent to QDs endows them with unique carrier transport capabilities that have not been previously explored in the context of solar cells. In this work, it is observed that InAs/InGaAs dots-in-a-well (DWELL) structures efficiently suppress lateral carrier diffusion. Therefore, not only do the DWELL structures enhance photocurrent by extending the absorption edge, but they should also inhibit the spreading of current to the perimeter of a device where edge recombination can dominate. In this paper, we examine this premise by comparing the dark current behavior of DWELL cells and GaAs control cells of varying area. The results are promising for applications such as concentration and flexible surfaces where shrinking the size of the device while maintaining high charge collection efficiency are of paramount importance.The InAs/InGaAs DWELL solar cell grown by MBE is a standard pin diode structure with six layers of InAs QDs embedded in InGaAs quantum wells placed within a 200-nm intrinsic GaAs region. The GaAs control wafer consists of the same pin configuration but without the DWELL structure. The typical DWELL solar cell exhibits higher short current density while maintaining nearly the same open-circuit voltage for different scales, and the advantage of higher short current density is more obvious in the smaller cells. In contrast, the smaller size cells, which have a higher perimeter to area ratio, make edge recombination current dominant in the GaAs control cells, and thus their open circuit voltage and efficiency severely degrade. The open-circuit voltage and efficiency under AM1.5G of the GaAs control cell decrease from 0.914V and 8.85% to 0.834V and 7.41%, respectively, as the size shrinks from 5*5mmsq to 2*2mmsq, compared to the increase from 0.665V and 7.04% to 0.675V and 8.17%, respectively, in the DWELL solar cells.The lower open-circuit voltage in the smaller GaAs control cells is caused by strong Shockley-Read-Hall (SRH) recombination on the perimeter, which leads to a shoulder in the semi-logarithmic dark IV curve. However, despite the fact that the DWELL and GaAs control cells were processed simultaneously, the shoulders on the dark IV curve disappear in all the DWELL cells over the whole processed wafer. As has been discussed in previous research on transport in QDs, it is believed that the DWELL cells inhibit lateral diffusion current and thus edge recombination by collection first in the InGaAs quantum well and then trapping in the embedded InAs dots. This conclusion is further supported by the almost constant current densities of the different area DWELL devices as a function of voltage.
5:00 PM - Q2.8
Virtual Single Crystalline GaAs Epitaxial Thin Films on Flexible Polycrystalline Metallic Substrates.
Alex Freundlich 1 , Venkat Selvamanickam 1
1 , University of Houston, Houston, Texas, United States
Show AbstractDevelopment of high quality III-V epitaxial layers on inexpensive flexible substrates is a desirable feature to many civilian and military applications. In particular it may be a game-changing enabler toward significantly reducing the cost and increasing the efficiency of thin film solar cells, as it offers the possibility of combining the unsurpassed performance of GaAs based multi-junction technologies (1 sun efficiency >36%) with a conventional roll to roll processing standard of thin film industry as afforded by polycrystalline metallic foil technology.Here we report our recent results on the development of virtual single crystalline GaAs thin film on thin (50 microns) flexible polycrystalline metallic substrates. The flexible poly-crystalline Ni-based substrates were coated with a few hundred nm oxide-ceramic epitaxial buffer, adapted from a previously developed structure for high Tc superconductor wire technology, followed by a very thin (<50nm) Ge epilayer. After introduction in the molecular beam epitaxy chamber Ge native oxide was thermally removed and a subsequent high temperature annealing was implemented resulting in the formation of clear c(2x2) (mixed (2x1)(1x2) ) reconstruction, typical of (001) Ge surface. The GaAs growth was initiated at relatively low temperature (~400C) and a thin nucleation layer of GaAs was deposited , followed by an annealing step under As2, subsequently 1 micron-thick GaAs was deposited in standard growth conditions (growth rate ~ 1ML/sec T~550C). The entire growth sequence was monitored by reflection high energy electron diffraction (RHEED). The self -annihilation of anti-phase boundaries (mixed 2x4, 4x2 RHEED diagram), was observed for thicknesses exceeding 100 nm where a 2x4 RHEED diagram typical of a single domain (001) GaAs was recorded. Epilayers exhibited a specular morphology. High resolution X-ray diffraction analysis confirmed the single crystalline (001) nature of GaAs. Temperature dependent photoluminescence (PL) analysis revealed a strong PL in as-grown samples (Fig. 2). The low temperature photoluminescence was found to be dominated with DA –eA like bands in the 1.4-1.5 eV range and a relatively broad deeper luminescence band at 1.3- 1.35 eV. At low temperature the GaAs excitonic emission was detected at 1.526 eV (FWHM~20 meV) and was found to be slightly red shifted compared to the typical A0X exciton (1.512) in homoepitaxial GaAs. The magnitude of this red-shift (~14 meV) suggested the absence of any significant thermoelastic/lattice mismatch strain in the epilayers. In summary the development of high quality single crystalline (001) GaAs on flexible metal substrates is demonstrated. Samples exhibit high optical and structural quality as stressed by the RHEED, X-ray and luminescence properties of the as-grown epilayers. Development of GaAs based thin film single junction solar cells is underway and preliminary device results will be presented at the meeting.
5:15 PM - Q2.9
Neodymium Luminescent Solar Concentrator.
Phil Reusswig 1 , Carmel Rotschild 1 , Marc Baldo 1
1 EECS, MIT, Cambridge, Massachusetts, United States
Show AbstractLuminescent solar concentrators (LSCs) are promising in photovoltaic applications because they do not need to track the sun to obtain high optical concentration factors. However, loss mechanisms that are associated with optical self-absorption decrease the efficiency of LSCs at increasing values of the geometric gain, G, which is defined as the ratio of the facial area to the edge area. In this work, we demonstrate LSCs based on a lanthanide infrared emitter, neodymium (Nd3+). Neodymium is nearly the optimal infrared LSC material: inexpensive, abundant, efficient, and spectrally well matched to high-performance silicon solar cells. Neodymium is a natural four level system, making it reasonably transparent to its own emission enabling high optical concentrations and geometric gain. Neodymium’s one disadvantage is its absorption. It has relatively poor overlap with the visible spectrum, meaning that it will require sensitization. LG-760 and APG-1 phosphate glass was purchased from Schott with Nd3+ doping concentrations of 1.0%, 2.0% and 3.0% with thicknesses of 1mm and 5mm. For the neodymium glass without sensitizer, peak optical quantum efficiencies (OQE) of 50% have been measured with an estimated power efficiency of 2.5%. The neodymium glass was sensitized by spin casting thin films doped with BASF Lumogen Violet 570, Gelb 083, and Orange 240 in a matrix of poly(methyl methacrylate) (PMMA). Together the dyes exhibit broad absorption below λ = 550 nm. Foerster energy transfer was used to couple the dyes non-radiatively such that all radiative emission was matched to the neodymium absorption lines at approximately λ = 580 nm. Initial results indicate peak OQE of 45% with an increase in estimated power efficiency of 3.5%. We will also discuss the sensitization of neodymium glass using nanocrystals in a polymer matrix.
Q3: Poster Session I
Session Chairs
Tuesday AM, December 01, 2009
Exhibit Hall D (Hynes)
9:00 PM - Q3.1
Electron Reflector Strategy on Micron-Thickness CdTe Solar Cells.
Kuo-Jui Hsiao 1 , James Sites 1
1 Physics, Colorado State University, Fort Collins, Colorado, United States
Show AbstractIncorporation of an electron reflector is a proposed strategy to improve Voc for CdTe thin-film solar cells. An electron reflector is a conduction-band barrier at the back surface, which can reduce the recombination resulting from the electron flow to the back surface. It should be particularly valuable at sub-micron thicknesses. For optimal improvement with an electron reflector, reasonable carrier lifetime (1 ns or above) and full depletion are required. Numerical simulation is used to investigate the electron reflector strategy for thin CdTe cells. Theoretically, a 200 mV increase in voltage and 3% in efficiency should be possible for a micron-thickness CdTe cell with a 0.2-eV electron reflector barrier, assuming 2x1014-cm-3 hole density and 1-ns lifetime, which are currently achieved. For the electron reflector to be beneficial, the CdTe needs to be fully depleted at its typical operating voltage. With the electron reflector, good CdTe cell performance at thicknesses as low as 0.3 μm should be possible.
9:00 PM - Q3.10
Mechano-chemical Synthesis, Deposition and Structural Characterization of CIGS.
Vidhya Bhojan 1 , Velumani Subramaniam 1 , Jesus A.Arenas-Alatorre 2 , Rene Asomoza 1
1 Electrical Engineering, CINVESTAV, Mexico,D.F. Mexico, 2 Institute of Physics, Universidad Nacional Autónoma de México, Mexico ,D.F. Mexico
Show AbstractCuInGaSe2 (CIGS) is a prominent thin-film photovoltaic material. However, commonly used physical vapour deposition and sputtering techniques to fabricate CIGS thin-film photovoltaic (PV) devices are complex and expensive. Therefore non-vacuum deposition techniques such as paste coating, spray pyrolysis and electro deposition are gaining more attention in recent years. Our intention is to choose a low cost non-vacuum technique like mechano-chemical synthesis of CIGS powder, followed by screen printing. Mechano chemical synthesis is a process that induces physical and/or chemical change in the compounds by mechanical energy, such as pulverization, friction or compression. This method has some advantages for the mass production of CIGS solar cells, high productivity and short processing cycle time. In the present work CIGS powders suitable for screen-printing ink has been prepared by ball milling. High purity elemental copper granules (>99.9% pure), selenium and indium powders (>99.9% pure) and fine chips of gallium (>99.9% pure) were used as the starting materials. Ball milling was carried out for an optimized composition of CuIn0.75Ga0.25Se2 using a SPEX-8000 mixer/mill at 1200 rpm for 1.5 hours. X-ray diffraction analysis of the milled powder shows the presence of (112), (220)/ (204), (312)/ (116), (400) and (332) peaks corresponding to CIGS chalcopyrite structure with a preferential orientation along (112) peak. The average grain size calculated by Scherrer’s formula is about 13.8 nm. Crystallographic structure of the prepared CIGS powder was analyzed by Rietveld analysis using X-ray powder diffraction data. Geometry optimization of the structure was performed and the basic structural properties have been evaluated by density functional approximation and compared with experimental results. Density of states has been simulated for the same structure inorder to have a better understanding of the distribution of atomic orbitals. FESEM analysis shows the agglomeration of nano particles. Particle size varied from 11 to 30nm. Final composition of the milled powder studied by Energy dispersive X-ray analysis gives 24.39 at% Cu, 21.42 at% In, 7.22 at% Ga and 46.97 at% Se. HRTEM analysis reveals the presence of nano crystalline particles. The interplanar distance (d-spacing) corresponding to (112), (220)/ (204), (512)/ (417) and (620)/ (604) diffraction peaks has been estimated and compared with standard values and the corresponding diffraction pattern has been simulated with simulaTEM software. CIGS ink is prepared by proper mixing of the powder with a suitable organic binder (ethyl cellulose). Screen printing is carried out on glass substrates, followed by annealing at 5 different temperatures of 300,350,400,450 and 500 degrees, in order to form a porous CIGS film. XRD and SEM analysis were carried out to study the structure and morphology of screen printed CIGS .Cross section of the screen printed CIGS thin film has been analyzed by HRTEM.
9:00 PM - Q3.11
Photo-patternable Polythiophenes Having Methacrylate Pendant Group for Organic Photovoltaics.
Yuna Kim 1 , Jeonghun Kim 1 , Sehwan Kim 1 , Eunkyoung Kim 1
1 Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractNew series of poly(3-hexylthiophene) copolymer functionalized with methacrylate group were synthesized for the application to organic photovoltaics. The regioregularity of the polythiophenes was controlled by the ratio of thiophene methacrylate to 3-hexylthiophene. As synthesized copolymer showed good solution processible and photo-patternable properties. Photoconversion of the side chain cross-linking of the polymer was examined by isothermal photo-DSC and FT-IR study under UV irradiation. By the photo cross-linking reaction, the ordering of poly(3-hexylthiophene) copolymer itself and the phase separation in fullerene blended films were significantly changed as observed by UV-Vis absorbance, PL, AFM and XRD. The photo-induced charge transport of polymer films were also changed by the photo cross-linking reaction. The organic photovoltaics were fabricated to show different photo conversion efficiency from the polythiophenes depending on the degree of the side chain cross-linking.
9:00 PM - Q3.13
Thermally and Chemically Stable Transparent Conducting Multi-layered Al-doped ZnO and Its Applications for Dye Sensitized Solar Cells.
Jun Hong Noh 1 , Hyun Soo Han 1 , Sangwook Lee 1 , Hyun Suk Jung 2 , Kung Sun Hong 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Advanced Materials Engineering, Kookim University, Seoul Korea (the Republic of)
Show Abstract Thermally and chemically stable NTO/AZO multilayered TCO fabricated by pulsed laser deposition for application in DSSC. Nb doped TiOx (NTO) layer was deposited at room temperature with amorphous and the layer crystallized to anatase and Ti3+ ions in the TiOx layer oxidized Ti4+ ions during air annealing at 450 oC. Owing to the crystallization and oxidation, NTO layer prevented from penetrating oxygen into AZO layer and consequently the AZO layer was in quasi-reducing atmosphere. As if the AZO layer is annealed under reducing atmosphere, oxygen vacancy was produced and substitutional Al atoms were activated in the AZO, thereby the conductivity of NTO/AZO was enhanced. In addition, the NTO layer prevented from forming the aggregate Zn2+-dye molecule on surface of AZO so that high transmittance of the NTO/AZO was maintained. The DSSC using the stable NTO/AZO TCO showed twice efficiency as high as the AZO-DSSC. However, the NTO/AZO-DSSC showed lower fill factor compared to the conventional FTO-DSSC. The cause for low fill factor might be electronic barrier which was formed by the non-degenerated thin AZO:Oi layer.
9:00 PM - Q3.14
Fabrication, Characterization and Optical Studies of Cu(In1-xGax)3Se5 Bulk Compounds.
Dayane Habib 1 , Georges El Haj Moussa 1 , Roy Al Asmar 1 , Michael Ibrahim 1 , Mario El Tahchi 1 , Claude Llinares 2
1 LPA, Lebanese University, Jdeidet Lebanon, 2 Centre Electronique et Micro-optoélectronique de Montpellier (CEM2), University of Montpellier 2, Montpellier France
Show AbstractIn this paper we present the structural and optical properties of Cu(In1-xGax)3Se5 ternary and quaternary compounds crystals fabricated by horizontal Bridgman technique. The Cu(In1-xGax)3Se5 materials were characterized by Energy Dispersive Spectrometry (EDS), hot point probe method, X-ray diffraction, Photoluminescence (PL), and Optical response (Photoconductivity). The Cu(In1-xGax)3Se5 have an Ordered Vacancy Chalcopyrite-type structure with lattice constants varying as a function of the x composition. A good stœchiometry given by the EDS characterization method is well observed in our samples and its magnitude deviation Δy is slight; so, our samples present a nearly perfect stœchiometry (Δy = 0) [1]. X-Ray diffraction patterns show the presence of many preferential orientations according to the planes (112), (220) and (312) of all the samples [2]. Also, it shows a linear shifting of peaks towards the higher magnitudes of 2θ when the x composition increases. These compounds can be of stanite structure [3] or an Ordered Vacancy Chalcopyrite structure (OVC) [4] or Ordered Defect Chalcopyrite Structure (ODC).We observe a large shift of the main PL and optical response emission peak versus x composition. The band gap energy of Cu(In1-xGax)3Se5 compounds is found to vary from 1.23 eV to 1.85 eV as a function of x.[1] Migual A. Contreras, Holm Wiesner, Rick Mtson, John Tuttle, Kanna Ramanathan, Rommel Noufi, Mat. Res. Soc. Symp. Proc. Vol. 426 (1996) 243-254.[2] Ariswan, G. El Haj Moussa, M. Abdelali, F. Guastavino, C. Llinares, Solid State Communications 124 (2002) 391-396. [3] M. Suzuki, T. Uenoyama, T. Wada, T. Hanada, Y. Nakamura, Jpn. J. Appl. Phys. 36 L1139 (1997).[4] Kristjan Laes, Sergei Bereznev, A. Tverjanovich, E.N. Borisov, Tiit Varema, Olga Volobujeva , Andres Öpik. Thin Solid Films 517 (2009) 2286–2290.
9:00 PM - Q3.15
Reactive Sputtering of Magnesium Hydride Thin Films for Photovoltaic Applications.
Charlotte Platzer-Bjorkman 1 , Smagul Karazhanov 1 , Jan-Petter Maehlen 1 , Erik Marstein 1 , Arve Holt 1
1 , Institute for Energy Technology, Kjeller Norway
Show AbstractMetal hydrides have been intensively studied for hydrogen storage [1], battery [2] and smart window [3] applications. For these purposes, fast and repeatable switching of material properties and hydrogen content are crucial. Many metal hydrides are also semiconducting or insulating and have recently been suggested for application in photovoltaic devices either as transparent conducting layers, antireflective coatings or even absorber layers [4]. The advantage of using metal hydrides for these applications are the large abundance of the constituent elements, high hydrogen content possibly improving bulk and surface passivation of silicon-based devices and suitable band gap range of several of the materials. In contrast to the applications based on hydrogenation/dehydrogenation, high stability is required for photovoltaic applications. In this work, we investigate the possibilities for in-situ deposition of MgH2 using reactive sputtering from a metallic target in Ar/H plasma. For low H2/Ar ratio, crystalline MgH2 is formed together with metallic Mg while for increasing ratio films are amorphous, partly transparent and insulating. Previous studies of in-situ deposition of MgHx films by activated reactive evaporation showed difficulties in obtaining single phase MgH2 [5]. In the present study, sputtering process parameters such as RF power, gas ratio, pressure and substrate temperature are varied. Film properties are monitored using x-ray diffraction, resistivity measurements and optical characterization. Depositions on both glass and silicon substrates are reported as well as studies of stability under annealing and light exposure.References1P. Chen, Z. Xiong, and J. Luo, Nature 420, 302 (2002).2L. Schlapbach and A. Zuttel, Nature 414, 535 (2001).3J. Huiberts, R. Griessen, and J. Rector, Nature 380 (1996).4S. Karazhanov, A. Ulyashin, P. Vajeeston, and P. Ravindran. Phil. Mag. 88(16), 2461 (2008).5R. Westerwaal, C. Broedersz, R. Gremaud, et al., Thin Solid Films 516, 4351 (2008).
9:00 PM - Q3.16
Wide Energy Bandgap Materials (LiF and Al2O3) as Hole Blocking Layers in Organic Solar Cells.
Chen Cheng Hong 1 , Wu Jui Chi 1 , Chen Chun Wei 1 , Chen Jen Sue 1
1 Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Show AbstractMaterials of different energy bandgaps are investigated as hole blocking layers in organic solar cells with P3HT:PCBM heterojunction active layer and Al cathode. In this study, by inserting LiF (Eg: 14.6 eV) or Al2O3 (Eg: 10.8 eV) between P3HT:PCBM and Al as the hole blocking layer, we demonstrate the power conversion efficiency (PCE) of the solar cell devices reaches 3.05% or 2.57%, respectively, which is much higher than that of the solar cell without the interlayer (only 1.32%). The enhancement of efficiency is associated with the increase of short circuit current and fill factor. The mechanism for the high efficiency is based on the tunneling effect. Owing to the wide energy bandgap of the hole blocking layer, the band offset between HOMO of PCMB and valance band edge of hole blocking layer is significantly large and the barrier height for tunneling of holes will be subsequently elevated. Consequently, the electron-hole recombination at the Al cathode will be appreciably reduced. Therefore, the use of wide bandgap materials as the hole blocking layer in solar cell devices will achieve higher efficiency.
9:00 PM - Q3.17
Ti-doped Gallium Phosphide Layers with Concentrations Above the Mott Limit.
David Pastor 1 , Javier Olea 1 , Maria Toledano-Luque 1 , Ignacio Martil 1 , German Gonzalez-Diaz 1 , Jordi Ibanez 2 , Ramon Cusco 2 , Luis Artus 2
1 Dpto. de Física Aplicada III, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid Spain, 2 Institut Jaume Almera, Consejo Superior de Investigaciones Científicas (CSIC), Barcelona Spain
Show AbstractIntermediate band solar cell (IBSC) materials exhibit highly attractive properties that could allow exceeding the solar conversion efficiency limit for single junction solar cells. An intermediate band (IB) material is formed by the introduction of a new band inside the bandgap of a semiconductor. In these compounds, photons with energy below the bandgap can be absorbed, pumping electrons from the valence band to the conduction band with the IB as an intermediary step. To form the IB material, deep center impurities with a concentration above the Mott limit (∼5x1019 cm-3) have to be introduced. Theoretical calculations have predicted Ti-doped Gallium Phosphide as one of the most efficient IB materials. For the introduction of such high Ti concentrations, well-above the solid solubility limit of Ti in GaP, here we explore the use of ion beam implantation to produce a GaP-based IB material. For the post-implantation annealing treatment of these heavily implanted samples, non equilibrium thermal processes such as pulsed laser melting (PLM) have to be used instead of thermal equilibrium treatments to avoid impurity out-diffusion. We perform a structural characterization by means of Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), Raman Spectroscopy and Glancing Incidence X-Ray Diffraction (GIXRD) of GaP layers with Ti concentrations above the Mott limit. The samples were produced by implantation of Ti into GaP layers with a dose of 8x1014, 8x1015 and 1.6x1016 cm-2 and subsequent PLM annealing at energy densities of 0.2, 0.7 and 1.2 J/cm2. The electrical properties of the samples were also studied by Hall effect measurements in the Van der Pauw configuration. ToF-SIMS Ti depth profiles after the PLM indicate that a Ti impurity concentration above the Mott limit is achieved. The Raman spectra show forbidden TO modes in all the annealed samples, which indicates that implanted GaP is polycrystalline after the PLM. Both the LO and TO peaks redshift by about 5 cm-1 after PLM at 0.2 J/cm2, which can be attributed to strain induced by the presence of Ti. The observed shifts disappear after PLM at higher energy densities, which may be related to a change of Ti location in the GaP lattice. GIRXD corroborates these results and suggests that TiP of GaTi alloys could be formed after PLM at the highest energy densities. The Hall effect measurements show an increase of the sheet resistivity and a suppression of the mobility.We discuss the possibility to achieve an IB material by Ti implantation and subsequent PLM annealing in GaP. We show that a high concentration of Ti impurities above the Mott limit is still present in the samples after PLM. However, the PLM process alters substantially the crystal quality of GaP, yielding a highly polycrystalline lattice and a deleterious effect on the electrical properties.
9:00 PM - Q3.18
Optical Characterization of Cu2ZnSnSe4 Grown by Thermal Co-evaporation.
Do young Park 1 , Hyeonsik Cheong 1 , Sunghun Jung 2 , Jaeho Yun 2 , Sejin An 2 , Jihye Gwak 2 , Kyeong-hoon Yoon 2
1 Physics, Sogang university, Seoul Korea (the Republic of), 2 Solar Cells Research Group, Korea Institute of Energy Research, Daejun Korea (the Republic of)
Show AbstractCuInGaSe2 (CIGS) is widely studied due to its promise as low-cost material for high-efficiency thin film solar cells. However, since indium and gallium are becoming scarce and expensive, high-efficiency and low cost solar cell absorber materials to replace CIGS such as quaternary Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) have been suggested. In previous years, the direct bandgap energy of CZTSe was suggested as being around 1.5eV, estimated from optical absorption data [1]. However, recent theoretical analysis and photoluminescence measurements suggest it to be around 1.0eV [2, 3]. In this study, we investigated quaternary CZTSe grown by thermal co-evaporation various optical spectroscopic techniques, including Raman scattering, photoluminescence and contactless electroreflectance (CER). Since CER is a modulation technique, the critical points of a semiconductor is pronounced in CER spectra, which helps identify the bandgap energy. Raman spectra of CZTSe show three main peaks at 173 cm-1, 197 cm-1, and 231 cm-1 [3]. The photoluminescence spectra show a peak at around 1.0 eV, which is consistent with previous measurements. There seems to be a correlation between the presence of the 173 cm-1 Raman peak and the intensity of the photoluminescence. Photoluminescence and Raman results are compared with CER measurements. [1] T. Tanaka, T. Nagatomo, D. Kawasaki, M. Nishio, Q. Guo, A. Wakahara, A. Yoshida, and H. Ogawa, J. Phys. Chem. Solids 66, 1978 (2005).[2] S. Chen, X. G. Gong, A. Walsh, and S.-H. Wei, Appl. Phys. Lett. 94, 041903 (2009).[3] M. Grossberg, J. Krustok, K. Timmo and M. Altosaar, Thin Solid Films 517 2489 (2009).
9:00 PM - Q3.19
Influence of Deposition Parameters on Surface Texturing of ZnO:Al Films Prepared by In-line RF Magnetron Sputtering.
Jun-Sik Cho 1 , Young-Jin Kim 1 , Jeong Chul Lee 1 , Sang-Hyun Park 1 , Jinsoo Song 1 , Kyung Hoon Yoon 1
1 Photovoltaic Research Center, Korea Institute of Energy Research, Deajeon Korea (the Republic of)
Show Abstract A systematic study of the effect of sputtering deposition parameters on material properties of Al-doped ZnO (ZnO:Al) films prepared by in-line rf magnetron sputtering and on surface morphologies of the films after wet etching was carried out. After deposition, the as-deposited films were surface-textured to improve optical properties such as haze and angle resolved distribution of scattered light on the film surfaces for application of silicon thin film solar cells.High quality ZnO:Al films with electrical resistivity of 5 × 10-4 ohm cm and optical transmittance of 85% in the visible range are obtained at high substrate temperature and low working pressure. The surface morphologies and optical properties of textured ZnO:Al films are changed significantly depending on the deposition conditions, whereas the electrical properties such as resistivity, carrier density and mobility are seldom affected.The photovoltaic characteristics of silicon thin film solar cells prepared on the textured ZnO:Al films will be presented in comparison of commercial Asahi U glass.
9:00 PM - Q3.2
Incorporation of Sodium into Low-Temperature Deposition of CIGS Flexible Solar Cells.
Hendrik Zachmann 1 , Stefan Puttnins 1 , Thomas Lange 1 , Felix Daume 1 , Andreas Rahm 1 , Karsten Otte 1 , Tobias Eisenbarth 2 , Raquel Caballero 2 , Christian Kaufmann 2 , Hans-Werner Schock 2
1 , Solarion AG, Leipzig Germany, 2 , Helmholtz-Zentrum für Materialien und Energie, Berlin Germany
Show AbstractIt is a well known fact that the quality of Cu(In,Ga)Se2 (CIGS) absorber layers can be enhanced by the addition of sodium. However, the way how sodium influences the electrical and structural parameters of CIGS absorber layers is still under discussion. Different methods for sodium incorporation are known for the use of sodium-free substrates like stainless steel or polyimide foil. In this work we compare sodium free CIGS-layers with CIGS-layers, where sodium was incorporated extrinsically at different growth stages or by a sodium-containing precursor. The CIGS samples were prepared via a roll-to-roll process with ion beam assistance (Solarion) or by a multi-stage low temperature co-evaporation process (HZB) respectively. The elemental depth profiles of the samples, especially for sodium, were studied via secondary ion mass spectroscopy (SIMS). In addition, we investigated a set of solar cell samples by means of quantum efficiency measurements and capacitance spectroscopy. It was observed that the depth profile for sodium is not directly correlated with the gallium gradient when an extrinsic sodium incorporation process is used. For sodium containing precursor layers the sodium gradient follows the gallium depth profile. The electrical parameters of the solar cells are correlated with the amount of sodium supplied either via a precursor, deposited prior to the absorber layer, or by extrinsic incorporation in the deposition process together with the metallic elements. With increasing amounts of sodium an increase of Voc is observed for both techniques. In contrast, Jsc decreases for extrinsic sodium incorporation while it remains unaffected for a precursor layer. Changes in Jsc are correlated with the quantum efficiency in the long wavelength region as well as in the short wavelength region. Higher amounts of sodium lead to a higher acceptor concentration which is usually correlated with a higher built-in voltage. This result is confirmed by capacitance spectroscopy investigations. In summary the beneficial effects of the addition of sodium to the growing CIGS-layer are shown. The efficiency of the CIGS-solar cells on polyimide foil was improved from 10 % (sodium-free) to up to 13.5 % for an optimal sodium content.
9:00 PM - Q3.20
Low Band Gap Poly(Thieno[3,4-b]Furan): A Potential Light-harvesting Material for Organic Photovoltaic Cells.
Tanmoy Dey 1 2 , Jayesh Bokria 1 2 , Michael Invernale 1 2 , Daminda Navarathne 1 2 , Zeki Buyukmumcu 3 , Gregory Sotzing 1 2
1 Department of Chemistry, University of Connecticut, Storrs, Connecticut, United States, 2 Polymer program, University of Connecticut, Storrs, Connecticut, United States, 3 Department of Chemistry, Erciyes University, Kayseri Turkey
Show AbstractThe field of optically transparent conducting polymers is an active research area due to their low density, flexibility and relatively low processing cost. Apart from their high number of conducting applications, these materials are used as hole-injection layers in photovoltaics. Our group has reported the polymerization of thieno[3,4-b]thiophene (T34bT) to yield a low bad gap conjugated polymer with Eg = 0.85 eV. The success of PEDOT-PSS as a commercially available low band gap conjugated polymer motivates us to study the properties of new conjugated polymers with fused ring structures, such as T34bF. Electrochemically prepared Poly(thieno[3,4-b]furan (PT34bF) was anion dominant upon redox cycling; its band gap was measured to be 1.04 eV by spectroelectrochemistry. This indicates the possibility of applications for the polymer as a transparent conductor or an ion-storage layer for electrochromic devices. Density Functional Theory (DFT) calculations using B3PW91 hybrid functional have been done for each of the possible connections between the three open α-positions. Besides optimizing the geometry, properties in relation to band structure such as band (energy) gaps, band widths, and effective masses were calculated for each connection. Based on the calculations, it can be concluded that the 4-6 connectivity is the most probable and dominant structure. The calculated energy gap of 1.01 eV for polymerization via the 4-6 connection corresponds well with the experimentally observed value. PT34bF has a good spectral match to the solar flux and exhibits appropriate band energies for exciton travel. With these two elements in mind, PT34bF could serve well as a potential light harvesting material in the active layer of an organic photovoltaic device.
9:00 PM - Q3.21
Photovoltaic Electrochemical Optical Rectennas of Carbon Nanotubes with Self-assembled Monolayers.
Juan Duque 1 , Matteo Pasquali 1 , Howard Schmidt 1
1 Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas, United States
Show AbstractCarbon nanotubes (CNT) and CNT-gold hybrid nanostructures are shown to produce rectified photocurrents without pre-bias when illuminated in the presence of aqueous electrolytes containing anionic surfactants. Graphite nanoplatelets, multi-wall carbon nanotube mats, single-wall carbon nanotube (SWNT) arrays and carbon black-loaded polymer all display this novel photovoltaic effect, and share a unique isolated photocurrent peak at ~300 nm. This feature apparently originates with their shared π-plasmon absorption. Rectified photocurrents are obtained only in the presence of anionic surfactants known to form self-assembled monolayers on graphitic surfaces. Carbon nanotube electrodes also produce minor photocurrents in the near infra-red. When coated with 5 nm of gold, single-walled carbon nanotube arrays generate similar photocurrents when treated with thiol-terminated anionic surfactant molecules. The Au-SWNT composite nanostructure displays a broad photocurrent spectrum spanning the entire visible region, along with a pronounced peak at ~400 nm. These features appear to arise from photoabsorption by gold nanorod plasmon hybridized with SWNT π-plasmons. A model explaining these results based on hot carriers traversing a rectifying tunnel barrier is presented and discussed. This mechanism is consonant with the concept of an optical rectifying antenna (rectenna), and could enable development of high efficiency photodetectors, photovoltaics and photochemical converters.
9:00 PM - Q3.22
Effects of Discotic-liquid Crystals as an Additive on the Performance of Polymer Solar Cells.
Seonju Jeong 1 , Cham Kim 1 , Jong Tae Kim 1 , Yun Seon Do 1 , Yoon Soo Han 1
1 , Daegu Gyeongpook Science & Technology, Daegu Korea (the Republic of)
Show AbstractAlthough an polymer solar cell is promising and new renewable resources for generation of electrical energy, its low power conversion efficiency (PCE) is considered as a major obstacle to its commercial utilization. The most promising method to improve power conversion efficiency of polymer solar cell is to control the morphology between the electron donor and the electron acceptor, which are conjugate polymer and fullerene derivatives, respectively, because exciton diffusion length is limited to 10~20 nm within organic polymer semiconductor. Besides, discotic liquid crystals (DLCs) are considered as promising organic semiconductors for application to molecular electronics, optoelectronics, photoconductor, photovoltaic solar cell and organic light emitting diode (OLED) devices. Organic devices with DLCs materials showed their high charge mobilities of up to 1 and 0.5 cm2/Vs in the crystalline solid phase and the columnar liquid crystalline phase, respectively. Especially, p-type discotic liquid crystals, triphenylene derivatives substituted with flexible alkoxy chains are widely studied because they show very high charge carrier mobility in various mesophase. With the concepts, by very simply incorporating a small portion of triphenylene derivatives such as DLC 1 and DLC 2, polymer photovoltaic cells with the configuration of ITO/PEDOT:PSS(30 nm)/P3HT:PCBM:DLC 1 or 2 (120 nm)/LiF (10 Å)/Al (100 nm) were fabricated. It was observed that electric - and optical properties were improved by the addition of a small amount of DLC 1 and DLC 2 into the film composed of P3HT and PCBM, caused by the change of morphology in active layer. Open circuit voltage (Voc) and fill factor (FF) of fabricated photovoltaic cells containing DLC 1 and DLC 2 were improved from 0.59 V and 57.19 (without DLC) to 0.65 V and 63.60 (with DLC 1) and 0.67 V and 64.94 (with DLC 2), respectively. This was considered to be due to the increased charge carrier mobility and the increased contact area between active layer and electrode by the introduction of columnar DLCs, as well as the wider absorption band. As the results, PCEs of the polymer cells were greatly improved from 3.0 % to 3.9 % (DLC 1) and 3.97 % (DLC 2), respectively.
9:00 PM - Q3.23
Effect of Annealing Solvent Solubility on the Performance of Poly(3-hexylthiophene)/Methanofullerene Solar Cells.
Jong Hwan Park 1 , Jong Soo Kim 1 , Ji Hwang Lee 1 , Wi Hyoung Lee 1 , Kilwon Cho 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractThe effect of the solubility of the annealing solvent on the performance of poly(3-hexylthiophene) (P3HT):C61-butyric acid methyl ester (PCBM) solar cells is studied. The short-circuit current (Jsc) and the fill factor (FF) increase remarkably, regardless of the type of annealing solvent, whereas a reduction of the open-circuit voltage (Voc) (of 0.1~0.2V) is observed after solvent annealing. Interestingly, both the value of Jsc and the power conversion efficiency (PCE) are higher for the poor-solvent-annealed devices than for the good-solvent-annealed ones. A good solvent vapor induces better self-organization of P3HT than a poor solvent vapor. However, the exciton loss increases due to excessive phase separation. A study of the space-charge-limited current reveals no significant differences between the carrier mobilities of good- and poor-solvent-annealed devices. Furthermore, the measured photocurrent suggests that the space charges no longer limit the values of Jsc and FF for all the solvent-annealed devices. These results indicate that the higher Jsc and PCE values obtained for the poor-solvent-annealed devices can be attributed to the optimized phase separation of the active layers, which induces balanced carrier mobility and minimum exciton loss.Acknowledgement. This work was supported by a grant (F0004021-2008-31) from the Information Display R&D Center under the 21st Century Frontier R&D Program and Creative Research Initiative-Acceleration Research. (R17-2008-029-01001-0)
9:00 PM - Q3.24
Tuning of Absorbance Spectra in CdS/CdSe Quantum Dot Co-sensitized Solar Cells.
Dongho Lee 1 2 , Jeremy Nevins 1 3 , David Watson 1 3 , Alexander Cartwright 1 2 , Paras Prasad 1 2 3
1 Institute for Lasers, Photonics and Biophotonics, University at Buffalo, State University of New York, Buffalo, New York, United States, 2 Department of Electrical Engineering, University at Buffalo, State University of New York, Buffalo, New York, United States, 3 Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York, United States
Show AbstractWe investigate the solar power conversion efficiency (PCE) in quantum dot sensitized solar cells (QDSSC) using in-situ precipitation of a) CdS quantum dots, b) CdSe quantum dots, and c) cascade CdS/CdSe nanostructures. The sensitizing CdS and CdSe QDs were deposited directly on the TiO2 surface by chemical bath deposition (CBD) using cadmium, sulfur and selenium sources. Deposition of the CdS nanostructures on the TiO2 resulted in a gradual increase, and red-shift, of the absorbance as the number of CdS CBD cycles increased. Similarly, deposition of CdSe nanostructures on TiO2 resulted in an increase of the absorption for wavelengths less than the bandgap of CdSe (~700nm). Detailed analysis of the deposition of a cascade structure of CdS deposition followed by CdSe deposition resulted in the ability to tune the spectral response of the absorptive material. In general, the cascade structure resulted in better solar power conversion efficiency than the CdS or CdSe single QD structure solar cell due to the spectral response engineering. The shorter wavelength absorption (<550nm) is mainly due to CdS with a contribution from CdSe, while the longer absorption (550~700nm) is due to CdSe. The optimum structure was obtained with a CdS:CdSe cycling strategy of 6:6 cycles. This resulted in a 1.55% PCE, 0.53V open circuit voltage (Voc) and short circuit current density (Jsc) of 8.1mA/cm2 under AM1.5G one sun illumination. In general, increasing the CdSe cycles on the CdS QD layer increases the Voc, Jsc, and fill factor. However, excess cycles of either CdS or CdSe beyond 6:6 decreases the device performance.
9:00 PM - Q3.25
Sputter Deposition of CuInSe2 and CuGaSe2 from Composite Targets on (100) Si.
Okechukwu Akpa 1 , Shoieb Shaik 2 , Trenton Thompson 2 , Tamara Isaacs-Smith 3 , Philip Anderson 4 , Supapan Seraphin 4 , Kalyan Das 2
1 Material Science and Engineering, Tuskegee University, Tuskegee, Alabama, United States, 2 Electrical Engineering, Tuskegee University, Tuskegee, Alabama, United States, 3 Physics, Auburn University, Auburn, Alabama, United States, 4 Material Science and Engineering , University of Arizona, Tucson, Arizona, United States
Show AbstractCopper indium-gallium diselenide (CIGS) is an important material in thin film photovoltaics. Finding better ways to deposit stoichiometric high quality CIGS is a necessary part of furthering the use of this photovoltaic material. The growth and characterization of CuInSe2 and CuGaSe2 p-type material systems on n-type (100) silicon was studied to examine the characterization of chalcopyrite/Si heterojunctions for future use as solar cells. In addition chalcopyrite/Si heterojunctions using a graded CIGS structure were studied and will be analyzed. Film deposition was obtained by RF magnetron sputtering using stoichiometric targets at various temperatures. Visual inspection indicated that sputter deposited CuInSe2 and CuGaSe2 was translucent and highly reflective when deposited in thin layers. Rutherford backscattering spectroscopy (RBS) indicated that CuInSe2 films with a composition of Cu0.85In1.5Se2.5 were deposited on Si. The RBS profile also showed that CuGaSe2 layers were Se-rich and Cu-poor with a ratio of Cu0.5Ga1.5.Se3.0. Transmission electron microscopy (TEM) micrographs of earlier samples showed a high degree of contamination at the Si/chalcopyrite interface. The high levels of contamination were confirmed by energy-dispersive X-ray spectroscopy (EDS) and RBS. Despite the contamination, the materials were polycrystalline in nature and the diffraction patterns confirmed that the crystals were of reasonable quality. Cleaner chalcopyrite/Si interfaces were obtained using Fl terminated Si substrates. Micrographs, from TEM of these samples, showed that the native oxide was completely eliminated and EDS showed that contamination levels were significantly lower. The Hall-mobility experiments and electrical characterization of the heterojunction will be reported. (Work reported here was partially supported by NSF through RISE, CREST and IGERT grants. The authors would like to acknowledge Dr. John Williams of the Auburn University Department of Physics for the use of RBS facilities and Mr. Charles Ellis of the AMSTC Microelectronics Laboratory at Auburn University for providing laboratory facilities)
9:00 PM - Q3.26
Effect of Cu/Ga Molar Ratio on Optical and Electrical Properties of Spin Coated-CuGaSe2 Thin Films for Tandem Solar Cell Applications.
Ik Jin Choi 1 , Bhaskar Chandra Mohanty 1 , Yeon Hwa Jo 1 , Deuk Ho Yeon 1 , Yong Soo Cho 1
1 Materials Science & Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractThe wide-band gap chalcopyrite compound, CuGaSe2 is a promising material to achieve the highest levels of tandem-junction device performance. The CuGaSe2-based photovoltaic devices (PV) were fabricated with transparent back contact for use as a top cell in a tandem junction PV cells. The CuGaSe2 thin films were prepared by spin coating of solutions having different molar ratio of Cu and Ga (0.8 to 1.2) on Al-doped ZnO deposited glass substrate followed by subsequent selenization at various temperatures ranging from 350 to 550 °C. The effects of the Cu/Ga molar ratio and selenization temperature on various properties of the films were investigated. Below the selenization temperature of 450 °C, in addition to the chalcopyrite phase, secondary phases of Cu and Ga selenides were observed in the X-ray diffraction patterns irrespective of the starting compositions. However, phase pure chalcopyrite CuGaSe2 was obtained for films having Cu to Ga molar ratio of only 0.9, 1.0, and 1.1 and selenized at 500°C and above. Grain size of the films decreased with increase in deviation of Cu/Ga ratio from stoichiometric composition, i.e., 1.0. Results of Hall measurements for phase-pure samples indicate that CuGaSe2 films have p-type conductivity. Mobility of these films was found to be higher than those of the samples containing secondary phases. In this work, the highest conversion efficiency of about 4.1% has been achieved under the standard air mass (AM) 1.5 spectrum.
9:00 PM - Q3.27
High Quality Se-Excessive Cu(In, Ga)Se2 Films Prepared by Screen Printing.
Deuk Ho Yeon 1 , Bhaskar Mohanty 1 , Yeon Hwa Jo 1 , Ik Jin Choi 1 , Yong Soo Cho 1
1 Materials Science & Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractCIGS-based photovoltaic devices were prepared by the simple and cost effective screen printing method. The CIGS powder synthesized by the mechanochemical process was mixed with excess Se with Se content varying from 0 to 30 wt %. The excess Se was added with the intention of compensating the Se loss during subsequent process steps. This mixture (i.e., stoichiometric CIGS powder and Se) was added to an organic vehicle to prepare the CIGS paste. The paste was screen-printed on the substrate using 500 mesh screen and the organic residue was removed by sintering the obtained films at 300, 400, 500, and 600 oC in N2. Crystal structure and surface microstructure of the films were studied using X-ray diffraction and scanning electron microscopy. The thickness of the CIGS film was about 5~10μm. It was observed that thickness of the films prepared with increased Se amount was lower than those prepared with lower Se content. This suggests that addition of higher Se helped in densification of the films. Our cell fabricated in Glass/Mo/CIGS/CdS/ZnO structure showed an efficiency of about 1.7 % under AM 1.5 condition.
9:00 PM - Q3.28
Effect of Deposition Temperature on the Structural, Optical and Electrical Properties of ZnO:Al Deposited by Pneumatic Spray Pyrolysis.
Jagadeesh Bellam 1 , Vidhya Bhojan 1 , Arturo Maldonado 1 , Velumani Subramaniam 1
1 Electrical Engineering, CINVESTAV, Mexico .D.F. Mexico
Show AbstractTransparent Conducting Al-doped ZnO(AZO) thin films were prepared on glass substrates by Pneumatic spray pyrolysis method. The effect of substrate temperature on structural, optical and electrical properties of ZnO:Al thin films were investigated. 0.2M of Zinc Acetate Ac(Zn), 0.2M of Aluminum Pentanedionate P(Al) and a mixture of methanol, acetic acid, DI water were used as starting material, dopant source and solvent respectively. The films were grown at different substrate temperatures of 450°C, 460°C, 475°C, 485°C and 500°C. Structural, optical and electrical properties of the deposited films were investgated by X-ray diffraction, SEM, UV-VIS transmittance spectroscopy, Profilometry and Hall measurements. In addition films were also characterized by photoluminanscence(PL) studies and Raman spectroscopy. Simulation studies were carried out by Density Functional Theory (DFT).XRD measurements showed that the films were crystallized in wurtzite phase with preferential (002) orientation. FWHM, grain size, lattice constants and strain in the films were calculated. The structure of ZnO:Al has been simulated with close-packed O layers in h stacking, Zn in tetrahedral voids ,where some of the Zn atoms are replaced by Al atoms. Lattice constants a=3.248 Å and c=5.205Å were employed. Geometry optimization of the simulated structure has been carried out using Local-Density Approximation (LDA). Simulated XRD pattern is found to be in good agreement with the experimental pattern. SEM images revealed the surface morphology of the films. Influence of deposition temperatures on the optical properties of the films has been studied. The average optical transmittances of all films were 85% in the visible range of wavelength 400 to700 nm. The calculated direct band gap value was found to be in the range from 3.320 eV to 3.350 eV.Resistivity of the films varied from 1.42×10-2 Ω-cm to 1.62×10-3 Ω-cm. Minimum resistivity is obtained for the film deposited at 485°C. PL studies showed a strong near band edge UV emission peak at 380nm and a weak deep level emission centered at about 500nm. In the Raman analysis, the active mode of Al doped ZnO films were observed at 125cm-1(low) and 497cm-1(high).
9:00 PM - Q3.29
Effect of Pitch and Height on the Performance of Textured Polymer Solar Cells.
Kanwar Nalwa 2 , Joong-Mok Park 2 , Wai Leung 2 , Kai-Ming Ho 2 , Sumit Chaudhary 2
2 , Iowa State Univ, Ames, Iowa, United States
Show AbstractPolymer based photovoltaics (PPVDs) are attractive for solar-electric conversion due to potentially solution-processible, low-cost, roll-to-roll manufacturing capability. The charge carrier mobility in conjugated polymers remains low, which demands thin (tens of nm) active photovoltaic layers for high efficiencies. However, efficient optical absorption is only ensured by thicker films (100 - 200 nm). Thus, arises a trade-off situation between the optical and electronic phenomena. Introducing micron- and submicron-scale textures in such PPVDs is one of the possible ways to achieve efficient light absorption in very thin active layers. We fabricated PPVDs on such textured (grating-type) surfaces realized by laser holographic lithography. PPVDs on 600 nm pitch and 300 nm height grating showed higher power conversion efficiency than those on 2 micron pitch and 1.5 micron height owing to more uniform spin coated active organic layer and lower series resistance. We also observed that the variation in pitch and height of such grating-type structures has a significant effect on spectral responsivity; indicating that wavelength tunable photodetection can readily be achieved with the same materials deposited on different geometries. For example, 600nm pitch and 300 nm height PPVDs demonstrated better relative spectral response at wavelengths less than 500nm, as compared to the flat PPVDs. Our observations also include the conflicts between the promising optics and challenges of solution processible deposition on textured surfaces.
9:00 PM - Q3.3
Titanium-Silicon Dioxide as a Transparent Conducting Oxide and an Anti-Reflection Contact for Photovoltaic Applications.
John Chivers 1 , Chandler Downs 1 , Thomas Vandervelde 1
1 ECE, Tufts University, Medford, Massachusetts, United States
Show AbstractWe report on the use of Earth-abundant silicon-dioxide and titanium-dioxide as a transparent conducting oxide (TCO) and antireflective (AR) coating. The varied band gap and index of refraction conditions inherent in the SiTiO2 system allow for an easily adaptable set of conditions, conferring the growth of these materials monolithically. TCOs are a critical component in modern photovoltaic devices, used as a front-side contact that won’t block incident light. At present, many TCOs require rare-Earth materials (e.g. Indium), which will likely be problematic for widespread, long-term distribution. The abundant, well characterized materials used here can be integrated into an existing line quickly and cheaply. Some TCOs also act as an AR coating, further increasing the light absorbed. The ideal AR coating would gradually change from the index of refraction of air to that of the underlying semiconductor. Most AR coatings used today make this transition in a small number of steps, which limits their efficacy. In this work, we use a PECVD process that slowly grades the index of refraction while maintaining conductivity and transparency. In the drive towards grid parity, the quality of the TCO and AR coatings could prove crucial. This work would contribute significantly to that effort.
9:00 PM - Q3.30
Optical and Micro-structural Characterization of Printed Cu2ZnSnS4 Thin Films.
Inyoung Kim 1 , Ki Woong Moon 2 , Kwang-Won Jeon 2 , Jongryoul Kim 2 , Jong-Su Yu 1 , Jeongdai Jo 1 , Dong-Soo Kim 1
1 Nano-Mechanical Systems Research Division, Korea Institute of Machinery & Materials, Daejeon Korea (the Republic of), 2 Department of Metallurgy and Materials, Hanyang University, Ansan Korea (the Republic of)
Show AbstractPrinting technologies, inkjet, roll-to-roll, gravure and so on, have been applied to the fabrication of electronics devices such as TFT, OLED, RFID, LCD color filters. Nowadays their application fields have been expanded to the energy field such as solar cell and battery. One of the main reasons, why many researchers have been interested in printing technology as a manufacturing method, is the reduction of manufacturing cost. In particular, the urgent subject of solar cells for triggering the market growth is also the reduction of cost per watt. For the fabrication of printed the solar cells, printing solutions were developed in several ways but they were usually toxic and scarce materials used [1-3]. However, it is inconsistent idea to use these materials for the cheap production of clean energy. In this study, ink for printed Chalcopyrite solar cell was synthesized using modified sol-gel method. For the free of toxic and scarce materials, CuZnSnS (CZTS), was selected. CZTS ink was printed on the Mo coated SLG glasses and sintered at the ranges of 300 ~ 550 degrees Celsius. Parametric studies were conducted in order to obtain printed Cu2ZnSnS4 phase changing precursor synthesis conditions, ink formulation, binder addition and thermal treatment. Optical and microstructural aspects were discussed for the application of photovoltaic devices of printed CZTS films. [1] D. B. Mitzi, M. Yuan, W. Liu, A. J. Kellock, S. J. Chey, L. Cignac, A. G. Schrott, Thin Solid Films 517 (2009) 2158-2162.[2] J. A. Hollingsworth, K. K. Banger, M. H. -C. Jin, J. D. Harris, J.E. Cowen, E. W. Bohannan, J. A. Switzer, W. E. Buhro, A. F. Hepp, Thin Solid Films 431-432 (2003) 63-67.[3] V. K. Kapur, A. Bansal, P. Le, O. I. Asensio, Thin Solid Films 431-432 (2003) 53-57.
9:00 PM - Q3.5
The Effects of Bathocuproine Incorporation on the Properties of Organic Photovoltaic Devices.
Ching-Chun Chang 1 , Chi-Feng Lin 2 , Jian-Ming Chiou 3 , Tzung-Han Ho 4 , Kuei-Hsien Chen 1 5 , Li-Chyong Chen 5
1 Institute of Atomic and Molecular Sciences, Academia Sinica , Taipei Taiwan, 2 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei Taiwan, 3 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei Taiwan, 4 Department of Physics, National Taiwan University, Taipei Taiwan, 5 Center for Condensed Matter Science, National Taiwan University, Taipei Taiwan
Show AbstractIn the present work, we have investigated the effects of cathode buffer layers on the poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) blend photovoltaic cells. The results indicate that the performance of the bulk-heterojunction photovoltaic devices is improved considerably by using ultra-thin layer (20 Å) of bathocuproine (BCP). We find that the thin BCP layer increases both the open circuit voltage (Voc) and the fill factor (FF) of the device, yielding an increased power conversion efficiency (PCE). Compared with the bufferless device, obvious enhancements of Voc from 0.35 to 0.62 V and FF from 41 to 65% are obtained, which make the overall device PCE improved by three times (from 1.3% to 4.1%). The significant enhancement may be attributed to the passivation for the contacts by preventing aluminum compounds and dipole moment formation at the interfaces between active materials and Al cathodes. Thus, the introduction of cathode buffer layer such as lithium fluoride (LiF) and tris(8-hydroxyquinolinato)aluminium (Alq3) are also studied for clarification purpose. The interfacial reactions between P3HT:PCBM blending layers and Al cathodes were investigated by using X-ray photoelectron spectroscopy (XPS) depth profiles. For the bufferless device, Al cathode reacts with the underneath active materials to form Al-C compounds, which decreases the built-in potential in the device and thus lower Voc. Nevertheless, for the device with BCP buffer, the outer-diffusion of carbon from P3HT:PCBM blending layer can be prohibited and charge injection will be improved under forward bias. Therefore, the device shows even better performance than the one with commonly used LiF thin film.
9:00 PM - Q3.6
Effects of Non-noble Metal Current-collecting Grids on Internal Resistance for Dye-sensitized Solar Cells.
Seon Hee Seo 1 , Hyun-Ju Kim 1 , Bo-Kun Koo 1 , Dong Yoon Lee 1
1 , Korea Electrotechnology Research Institute, Changwon, Kyungsangnam-do, Korea (the Republic of)
Show AbstractWe studied the effectiveness of various non-noble metals used as current-collecting grids on fluorine-doped SnO2 (FTO) for dye-sensitized solar cells (DSCs). Non-noble metal grids were deposited by dc magnetron sputtering and mechanical adhesion of non-noble metals was improved by a 300-nm-thick Cr interlayer between FTO and grid. The metals were evaluated on the basis of J-V characteristics and impedance spectra. Analysis using an equivalent circuit model showed that such metal grids affect the internal ohmic resistance, which is inversely related with fill factor. A 1-um-thick NiCr grid afford a fill factor of 0.639—as much as that with Ag—but with a slightly lower efficiency of 6.4 %, which supports the feasibility of using stable non-noble metal grids in DSCs, albeit with a 5 % reduction in photovoltaic efficiency.
9:00 PM - Q3.8
Photosensitization of TiO2 by Electron Beam Irradiated Silica and Titania Based Hybrid Polymer for PEC Cell Application.
Seung Hwa Yoo 1 , Sung Oh Cho 1
1 Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show Abstract In this study, mesoporous titania nanoparticle was sensitized by electron beam irradiated silica and titania based polymer. Titania nanoparticle was fully dispersed in ethanol by ultrasonication and was spin coated on the substrate. The prepared titania film was immersed in a dilute solution of the polymer overnight to sufficiently cover the porous film. After drying at ambient condition, the sample was subjected to electron beam irradiation. The beam fluence was controlled to observe the absorbance change of the samples by UV-VIS measurement. The band structure of the irradiated polymer was studied by several methods including UV-VIS spectroscopy, cyclic voltammetry and PES(photoelectron spectroscopy). Yet only the valance band of the irradiated polymer was estimated by PES. This sensitized titania film coated on ITO glass was installed in a PEC(photoelectochemical) cell including a platinum electrode as counter electrode and SCE(saturated calomel electrode) as reference electrode. NaOH aqueous solution (pH 11) was applied as electrolyte. Certain photocurrent was measured under 500W Xe lamp with a 420nm cut-off filter. Estimation of the band structure and chemical structure of the irradiated polymer are remaining questions in this study. Also improvement of the photocurrent generation is required in further study.
9:00 PM - Q3.9
Synthesis of Organic Semiconducting Materials by Electron Irradiation and Their Application to Organic Solar Cell.
Hyeok Moo Lee 1 , Sung Oh Cho 1
1 Nuclear and Quantum Engineering , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractElectron irradiation is one of the efficient methods to change the optical properties of organic materials. Chemical structures of polymeric materials are easily changed by electron irradiation. This suggests that physical and chemical properties of the materials, including optical property, can be modified by the irradiation. Based on the fact, we present a novel route to synthesize organic semiconducting materials by electron irradiation. Finally, we proposed a possibility that synthesized semiconducting materials can be used for organic solar cell as an electron donor. Electron irradiation was carried out in a vacuum chamber under a pressure of less than 2×10-5 torr using a thermionic electron gun and energy of an electron beam was 35 keV and the beam current density was 4 μA×cm-2. In order to synthesize proper semiconducting materials which have low bandgap, high hole mobility and high absorption coefficients in visible range various non-conducting polymer, semiconducting polymer and organic hole-transport materials are used as a precursor. The optical properties and electronic band structure of synthesized semiconducting materials were characterized by photoluminescence spectroscopy, UV/Vis absorption spectroscopy, and photoelectron spectroscopy in air. The mock-up organic solar cells are simply fabricated using ITO glass and Al electrode. Power conversion efficiency of solar cell was calculated under AM 1.5G solar illumination at 100mW×cm-2.
Symposium Organizers
Bhushan L. Sopori National Renewable Energy Laboratory
Bernhard Dimmler Würth Solar GmbH & Co. KG
Jeffrey Yang United Solar Ovonic LLC
Thomas Surek Surek PV Consulting
Q4: PV Devices and Module Issues
Session Chairs
Tuesday AM, December 01, 2009
Room 306 (Hynes)
9:30 AM - Q4.1
Investigation on the Optical and Electrical Properties of a Novel Synthesized Copolymer Designed for Efficient Polymer Solar Cells.
Fengling Zhang 1 , Weiwei Li 1 2 , Ruiping Qin 2 3 , Yi Zhou 1 , Fenghong Li 1 , Clemens Veit 4 , Hans-Frieder Schleiermacher 4 , Yi Thomann 5 , Mattias Andersson 1 , Viktor Andersson 1 , Olle Inganas 1 , Uli Wuerfel 4 5 , Zhishan Bo 2
1 , Linkoeping University, Linkoeping Sweden, 2 , Institute of Chemistry, Chinese Academy of Sciences, Beijing China, 3 , Beijing Normal University, Beijing China, 4 , Fraunhofer Institute for Solar Energy Systems ISE, Freiburg Germany, 5 , Freiburg Materials Research Center, Freiburg Germany
Show AbstractWe present a novel synthesized copolymer (HXS-1) with 2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiazole) as the backbone of the electron donor polymer for polymer photovoltaic application. HXS-1 is designed with two octyloxy side chains on the benzothiazole ring and an octyl chain on carbazole ring. The solubility of HXS-1 in 1,2-dichlorobenzene(DCB) is not good, but it can be dissolved by heating DCB. HXS-1 film shows broad absorption from UV to 700 nm. Bulk heterojunction solar cells were fabricated from HXS-1 mixed with PC71BM in DCB mixed with 2.5% 1,8-diiodooctane (DIO). The devices were optimized with varied stoichiometries, thicknesses and DIO concentration. A short-circuit current of 9.5 mA/cm2, open-circuit voltage of 0.79 V, fill factor of 0.72 and a power conversion efficiency of 5.4% under AM1.5 (100 mW/cm2, corrected for spectral mismatch) were achieved with the stoichiometry of 1:2.5 by weight, the thickness of 100±10 nm from DCB:DIO, The FF of 0.72 at Jsc =9.5 mA/cm2 indicates that HXS-1 has decent and balanced charge carrier mobility, as verified by field effect transistor measurements. The high photovoltage, extensive spectral coverage and superior electrical transport is attractive. Combining these merits in one polymer makes HXS-1 a very promising polymer for photovoltaic application. The optical and electrical properties of HXS-1 were intensively investigated with photoluminescence and electroluminescence in neat and blend films. The morphologies of active layers of solar cells were imaged with both AFM and electron tomography.
9:45 AM - Q4.2
Polymer Tandem Solar Cells with 3 Terminal Parallel Connected Structure.
Srinivas Sista 1 , Ziruo Hong 1 , Mi-Hyae Park 1 , Yang Yang 1
1 Materials Science & Engineering, University of California Los Angeles, Los Angeles, California, United States
Show AbstractHere we present a 3 terminal tandem cell where in the two sub cells are connected in parallel as opposed to in series that has been reported in literature so far. In this device architecture the two sub-cells are connected in parallel through a transparent conducting interlayer that acts as a common electrode to the two sub-cells. We fabricated a 3 terminal tandem cell with poly(3-hexyl thiophene) as the front cell and poly[(4,4′-bis(2-ethylhexyl) dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3 benzothiadiazole)-4,7-diyl] (PSBTBT) as the rear cell. The front P3HT cell has inverted structure while the rear cell fabricated from PSBTBT has regular structure. The interlayer connecting the two cells acts as an efficient hole collector from the two cells and delivers it to the external circuit. The front P3HT:PCBM cell showed power conversion efficiency (PCE) of 3.5% with short circuit current (Jsc) of -10.1mA/cm2, open circuit voltage of (Voc) of 0.58V and fill factor (FF) of 60.0%. While from the rear PSBTBT:PCBM we obtained PCE of 1.3%,, Jsc of -5.1mA/cm2, Voc of 0.6V and FF of 39.5%. The two sub cells were connected in parallel with the interlayer acting as the anode and the cathodes of the bottom and front cells shorted to form the cathode connection. The resulting tandem cell showed a Jsc of -15.1mA/cm2, Voc of 0. 60V, PCE of 4.80% and FF of 52.5%. The main advantage of this structure is that the efficiencies of the sub-cells can be measured independently in the tandem structure and as well as when connected in parallel. Another advantage of this device structure is that the current matching between the two sub-cells is not a necessary criterion for achieving an efficient parallel connected tandem cell. With the help of an efficiently conducting and transparent interlayer, we observe an added up short circuit current from the two sub-cells. This 3 terminal device configuration is particularly important for enhancing the absorption range by utilizing two polymer with complementary absorption range or the absorption itself when the same polymer is used for the two sub cells. Thus this device configuration poses great promise in further enhancing the polymer solar cell efficiencies.
10:00 AM - Q4.3
Analysis of Tunnel Junction Suitable for 4000 Suns in Tandem High Efficiency Solar Cell Structures.
Salah Bedair 1 , John Hauser 1 , Donggeun Jung 2
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Physics, Sungkyunkwan University, Kyonggi-do Korea (the Republic of)
Show AbstractThe road to achieve ultra high efficiency is through multi junction solar cells operating at high solar concentration, larger than 1000 suns. Critical to the success of this approach is the development of tunnel junction (TJ) that serve as electrically low loss interconnections yet optically transparent using high band gap semiconductor material systems. We have previously reported the fabrication of TJ made of n+-InGaP/ p+-AlGaAs with a band gap about 1.9 eV using Se and C doping respectively. This TJ structure has a peak current density of 80A/cm2 allowing it to be implemented in a three junction cell structure for use at solar concentration as high as 4000 suns (x4000). Almost all reported conversion efficiencies higher than 40% have used this tunnel junction. This unexpected very high peak current density, in these high band gap material systems represents good news for the multi junction solar community. The high current seems to be due the fact that the InGaP/AlGaAs interface has a staggered band gap lineup. We will present the effect of this band line up at the hetero interface and its effect on the width of the depletion region and the peak current density. We will also compare the current result from this hetero structure junction with an artificial homo junction made of n+-AlGaAs/ p+-AlGaAs doped to the same levels as that of the hetero junction. Results from the homo junction showed that peak current density is about one half of that obtained from the hetero junction at the same doping levels. A reasonable match between experimental result and the model was obtained when a value of 150 meV was used for ΔEc, the conduction band discontinuity at the interface. Both experiment and theory predicted that at a current density of 80A/cm2, there is only about a few tens of meV voltage drop across the TJ. This will have minimal effects on the over all efficiency of the tandem solar cell structure when used at high solar concentrations.We will also report on the effect of thermal annealing at 650°C and 750°C for 30 minutes to simulate the post growth conditions after the formation of the TJ, on its electrical properties. We will also report on the effect of inter diffusion of impurities at the junction interface on the width of the deletion region and peak current density. The peak current density was reduced by about 20% due to thermal annealing at 650°C.
10:15 AM - Q4.4
Domain Wall Driven Anomalous Photovoltaic Effect in a Complex Oxide.
Seung-Yeul Yang 1 , Jan Seidel 2 3 , Steve Byrnes 2 3 , Padraic Shafer 1 , Chanho Yang 3 , Marta Rossell 4 , Joel Ager 2 , Lane Martin 2 , Ramamoorthy Ramesh 1 2 3
1 MSE, U.C. Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Physics, U.C. Berkeley, Berkeley, California, United States, 4 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract While progress in photovoltaic systems proceeds mainly using standard semiconductor p-n junction or heterojunction-based devices, alternative approaches are beginning to receive a renewed interest. We show evidence for an anomalous photovoltaic effect in the complex oxide material BiFeO3 under visible light illumination leading to photovoltages larger than the band gap of the material (2.67 eV). BiFeO3 films with two different domain patterns, possessing 109° and 71° domain walls, were prepared on single crystalline (110) DyScO3 substrates by metal-organic chemical vapor deposition (MOCVD). In the direction of parallel to the net in-plane polarization, a large photo-induced VOC of 16 V was measured, with in-plane current density Jsc ~ 1.2×10-4 A/cm2 from 200μm electrode spacing. The effect arises from a new mechanism, namely structurally driven steps of the electrostatic potential that occur at ferroelectric domain walls. These potential steps—and, in turn, the photovoltaic effect—can be fully controlled through the domain structure in these films.
10:30 AM - Q4.5
Downconversion for Photovoltaics in Lanthanide Doped Low Phonon Energy Host Lattices.
Timon van Wijngaarden 1 , Karl Kraemer 2 , Hans-Ulrich Guedel 2 , Joren Eilers 1 , Linda Aarts 1 , Bryan van der Ende 1 , Andries Meijerink 1
1 , Debye Institute, Utrecht Netherlands, 2 , Department of Chemistry, Bern Switzerland
Show AbstractState-of-the-art commercial crystalline Si (c-Si) solar cells dominate the market and have energy efficiencies around 15%. The main energy losses (over 70%) are related to the spectral mismatch.[1] IR photons with energies lower than the bandgap are not absorbed while for photons with energies exceeding the bandgap, the excess energy is lost as heat during the fast thermalization. Two methods are capable of reducing spectral mismatch losses: upconversion and downconversion.[2] In case of upconversion, two infrared photons are ‘added up’ to give one higher energy photon that can be absorbed. The opposite process, downconversion, involves ‘cutting’ of one high energy photon into two lower energy photons. This process can reduce energy losses related to thermalization.In the present work we explore the possibilities for downconversion of a single visible or ultraviolet photon into two near infrared (NIR) photons using lanthanide ions doped into low frequency host lattices. The unique and rich energy level structures of lanthanide ions allow for efficient up- and downconversion. The Yb3+ ion has a single excited state some 10,000 cm-1 ground state. The absence of other energy levels allow Yb3+ to ‘pick up’ energy packages of 10,000 cm-1 from other lanthanide ions and emitting ~980 nm photons that can be absorbed by c-Si. Efficient downconversion using Yb3+ as acceptor requires donor ions with an energy level around 20,000 cm-1 and an intermediate level around 10,000 cm-1. Evaluation of the Dieke diagram reveals that potential downconversion couples are (Er3+, Yb3+), (Nd3+, Yb3+) and (Pr3+, Yb3+). Recently we demonstrated efficient downconversion for the (Pr3+, Yb3+).[3] Work on the (Er3+, Yb3+), (Nd3+, Yb3+) couples showed that fast multi-phonon relaxation from the starting level around 20 000 cm-1 prevented efficient downconversion in fluorides. Here we demonstrate the feasibility of efficient downconversion for both(Er3+, Yb3+) and (Nd3+, Yb3+) by doping them into bromides and chlorides. The low phonon energies in these host materials reduce multi-phonon relaxation rates and the level around 20 000 cm-1 is sufficiently long lived to achieve efficient two-step energy transfer. The dynamics and efficiency of the downconversion process are evaluated by steady state and time resolved luminescence measurements. Especially in the bromides (Cs3Y2Br9 and CsCdBr3) efficient downconversion is observed leading to strong infrared emission from Yb3+ around 980 nm upon excitation in the 4F7/2 level of Er3+ or the 4G9/2 level of Nd3+. The low phonon energies also aid in reducing quenching processes for the Yb-emission. This makes these systems promising for efficient downconversion for c-Si solar cells. [1] B. S. Richards, Sol. En. Mat. Sol. Cells 2006, 90, 2329-2337.[2] T. Trupke, M.A. Green, P. Würfel, J. Appl. Phys. 2002, 92, 1668; J. Appl. Phys. 2002, 92, 4117.[3] B. van der Ende, L. Aarts and A. Meijerink, Advanced Mater., in press.
10:45 AM - Q4.6
Photovoltaic Devices Based on Carbon Nanostructured Materials.
Zhongrui Li 1 , Viney Saini 1 , Yang Xu 1 , Enkeleda Dervishi 1 , Meena Mahmood 1 , Alexandru Biris 1
1 , University of Arkansas at Little Rock, Little Rock, Arkansas, United States
Show AbstractSuperior properties of nanostructural carbon materials (carbon nanotubes and graphene) represent attractive materials for photovoltaic devices. Single-wall carbon nanotubes (SWNTs) can be directly configured as energy conversion materials to fabricate thin-film solar cells, with nanotubes serving as both photogeneration sites and charge carriers collecting/transport layers. SWNTs can be modified into either p-type conductor through chemical doping (like thionyl chloride, or just exposure to air) or n-type conductor through polymer (like polyethylene imine) functionalization. The solar cells consist of either a semitransparent thin film of p-type nanotubes deposited on an n-type silicon wafer or a semitransparent thin film of n-type SWNT on p-type substrate to create high-density p-n heterojunctions between nanotubes and silicon substrate to favor charge separation and extract electrons and holes. The high aspect ratios and large surface area of nanotubes could be beneficial to exciton dissociation and charge carrier transport thus improving the power conversion efficiency. Initial tests have shown a power conversion efficiency of >4%, proving that p-n SWNT configuration is a potentially suitable configuration for making solar cells. Another prototype employ the use of conducting polymeric materials combined with graphene layers to achieve higher efficiency of such photovoltaic devices. A higher dissociation/transportation of the charge carriers is expected for such nanocomposite materials while used as p/n layers.
11:30 AM - **Q4.7
Using Accelerated Testing to Reduce PV Module Material Costs without Sacrificing Reliability and Lifetime.
John Wohlgemuth 1 , Zhiyong Xia 1 , Daniel Cunningham 1
1 , BP Solar International Inc., Frederick, Maryland, United States
Show AbstractFor photovoltaics (PV) to compete with other forms of electricity, the cost of PV module materials and processes must be reduced from today’s levels. However, this should be accomplished without negatively impacting module reliability and lifetime. This paper will discuss the types of accelerated testing and engineering analysis that can be done to determine whether proposed changes of materials or processes have reduced the ability of the modules to survive in the field. Outdoor testing is essential for all PV technologies in order to identify failure mechanisms. The long term durability and reliability of today’s crystalline silicon modules means that changes in module performance occur slowly outdoors. Module manufacturers can not wait for 20 to 25 years to see if a new lower cost material (e.g. encapsulant) continues to perform well in the field. Therefore, accelerated stress tests like thermal cycling, humidity freeze in combination with thermal cycling, and damp heat have been developed to assess the modules susceptibility to known failure mechanisms. The critical issue discussed in this paper is how to utilize these accelerated tests to evaluate the potential impact on longevity of using new materials and processes in module construction.
12:00 PM - Q4.8
Self-Cleaning Solar Panels and Solar Concentrators with Integrated Transparent Dust Screens.
Malay Mazumder 2 1 , Mark Horenstein 2 , Rajesh Sharma 3 , Hidetaka Ishihara 1 , Jeremy Stark 1
2 ECE Department, Boston University, Boston, Massachusetts, United States, 1 Applied Science, University of Arkansas - Little Rock, Little Rock, Arkansas, United States, 3 Renewable Energy, Arkansas State University, West Memphis, Arkansas, United States
Show AbstractIf only 1% percent of our land is dedicated to the use of solar panels for harvesting energy with only 10% efficiency, most of need for electricity can be met. The investment will be huge but it will be paid off within a reasonable time period. Long term operation of the photovoltaic (PV) and photothermal (PT) systems with a high efficiency requires that the panel surface and the surface of the optical concentrators should be clean for efficient conversion of solar energy by the solar energy systems. Deposition of dust on the light harvesting surfaces, particularly when these are installed in dusty areas, desert, or along the highways, could severely minimize solar-to-power output efficiency. Under a NASA project, we have developed transparent electrodynamic screens (EDS) for protecting solar panels against obscuration by dust deposition on the surface of the moon and Mars. We will describe the applications of this technology for terrestrial application worldwide. The electrodynamic screens (EDS) are made of transparent polymer sheets, such as PET (for its UV radiation resistance) or glass plates and a set of parallel conducting transparent electrodes made of Indium Tin Oxide (ITO) embedded under a thin film transparent coating. The basic principle of EDS operation is based upon the electrostatic and dielectrophoretic forces applied by the electric field to remove the dust particles deposited on the surface of the panel. Mathematical models and experimental studies show that when the screens are energized, dust removal efficiency higher than 95% can be achieved within a short time period (< 1min.). The method is effective for both charged and uncharged particles over a wide range particle size and particle dielectric constant. The results show that EDS technology is applicable for protecting solar panels and solar panels against dust hazards effectively in both space and terrestrial applications. The power requirements will be approximately 10 watts per square meter of the surface to be cleaned when dust removal is needed. Under normal conditions, dust cleaning of solar panels may not be required for more than a few minutes per day while the average power produced per square meter of the panels during the peak hours is approximately 500 watts. For solar concentrators, such as mirrors and Fresnel lenses, the transparent dust shields can be powered by using solar panels. A brief description of the principles self-cleaning system, experimental results, and their automated operation will be presented.
12:15 PM - Q4.9
Photocharge Generation on the Surface of Mesoscopic PCBM Crystals Grown by Dip-coating as Visualized by Kelvin Probe Microscopy.
Reza Dabirian 1 , Vincenzo Palermo 1 , Luca Ortolani 2 , Vittorio Morandi 2 , Xinliang Feng 3 , Klaus Muellen 3 , Paolo Samori 1 4
1 Istituto per la Sintesi Organica e la Fotoreattività, Consiglio Nazionale delle Ricerche, Bologna Italy, 2 Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delle Ricerche, Bologna Italy, 3 , Max-Planck Institute for Polymer Research, Mainz Germany, 4 Nanochemistry Laboratory , ISIS/CNRS Université de Strasbourg, Strasbourg France
Show AbstractIn organic electronics, the ability to self-assemble well-defined nanostructures with tunable nanoscale electronic functions, such as charge generation and transport, is fundamental for both basic research and device fabrication. Herein we present the preparation of mesoscopic crystals of a soluble fullerene, PCBM ([6,6]-phenyl C61-butyric acid methyl ester) using a novel dip-coating technique. PCBM is a widely used electron-acceptor in organic electronics.Mesoscopic PCBM crystals, with a diameter ranging from 1 to 100 μm and a thickness from 20 to 500 nm, have been obtained by dip-coating the substrates into a solution containing PCBM. These crystals have been prepared on a wide variety of surfaces such as silanized SiOx, Au, Cu, graphite, ITO and amorphous carbon. The size, shape and distribution of the crystals can be tuned by regulating the temperature and dip coating speed. Their multiscale characterization has been accomplished by atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM), transmission electron microscopy (TEM) and optical microscopy. TEM measurements revealed exclusively large single crystals with a confirmed hexagonal symmetry. AFM studies showed that the crystal surface is made of regular, molecularly flat terraces, up to over 1 μm in size, with a roughness Rrms=0.6±0.2 nm over an area of 1 μm2. The obtained crystals represent an ideal optically-active substrate upon which a second material can easily be deposited, ideally using an orthogonal solvent. By using this multiple layer deposition strategy, the PCBM crystals were partially coated with solution-processed nanofibers of a functionalized electron-donor (hexa-peri-hexabenzocoronene, or HBC), allowing quantitative understanding of the photovoltaic activity in bi-component electron-acceptor and donor systems. The different photovoltage build-up upon illumination on neat PCBM crystals respect to those covered by either a few HBC nanofibers or a tens of nm thick HBC film has been measured by KPFM. The measurements of these nanostructured interfaces in darkness revealed potential differences between the two components that increase by an average of 50 mV upon illumination, providing new insight into the physics of charge generation at the acceptor-donor interface.
12:30 PM - Q4.10
Raman and Rutherford Backscattering Characterization of Ti-implanted Si above the Mott Limit.
David Pastor 1 , Javier Olea 1 , Ignacio Martil 1 , German Gonzalez-Diaz 1 , Jordi Ibanez 2 , Ramon Cusco 2 , Luis Artus 2
1 Dpto. de Física Aplicada III, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid Spain, 2 Institut Jaume Almera, Consejo Superior de Investigaciones Científicas (CSIC), Barcelona Spain
Show AbstractOne of the most relevant ideas to increase the efficiency in solar cell technology is the intermediate band solar cell (IBSC), which could improve significantly the solar conversion efficiency limit for single junction solar cells. The active material of an IBSC exhibits a new band located between the conventional valence band (VB) and the conduction band (CB) of the semiconductor. This intermediate band (IB) divides the band gap into two low energy sub bandgaps. The electrons generated by photon absorption of energy below the band gap can be pumped from the VB to the CB in two stages using the IB as intermediate step. Given that Si solar cell technology is widely developed, Si-based IB materials are highly attractive. The formation of the IB material requires the introduction of deep centers in the semiconductor with a concentration above the Mott limit (∼5x1019 cm-3). Ti is one of the most promising impurities to form the IB in Si. However, the high concentration required clearly exceeds the solubility limit of Ti in Si (∼4x1014 cm-3). Ion implantation is an appropriate technique to achieve the high impurity concentration needed to form the IB material; however, the lattice damage generated by the ion implantation has to be removed. Non equilibrium thermal processes like pulsed laser melting (PLM) are necessary to recover the crystal lattice and avoid impurity out-diffusion. In the present work, we have used Raman spectroscopy and Rutherford Backscattering (RBS) to asses the degree of lattice recovery achieved after PLM annealings in Ti-implanted silicon with concentrations well above the Mott limit. The analysis of the RBS data provide us information about the location of Ti in the lattice of PLM annealed Si, where IB-related electrical behavior has already been observed in this material. We have analyzed samples implanted with Ti doses of 1015, 5x1015, 1016 and 5x1016 cm-2 and subsequently annealed at energy densities of 0.2 and 0.8 J/cm2. Raman measurements performed with 458 and 514 nm excitation wavelengths show a reduction in the lattice crystallinity with increasing implantation dose. For the sample implanted with the lowest dose, a high degree of lattice recovery is achieved after the PLM annealings at the highest energy density. The RBS data corroborate these results and show that after the PLM annealings most of the Ti impurities are in interstitial lattice sites.In this study, we evaluate the potential of the silicon layers implanted with very high Ti doses and subsequently PLM annealed at different energy densities in order to obtain an IB material. We show that a high quality Si crystal lattice can be achieved for the lowest implanted dose at the maximum energy density investigated. We find a massive interstitial non equilibrium position of Ti impurities after PLM annealing, susceptible of IB formation as predicted by theory.
Q5: Metalization, Microcracks and New Manufacturing Initiatives
Session Chairs
Tuesday PM, December 01, 2009
Room 306 (Hynes)
2:30 PM - **Q5.1
A Review of Advanced Metallization for Si Solar Cells.
Aziz Shaikh 1
1 , Ferro Corporation, Vista, California, United States
Show AbstractContact metallizations continue to play a key role in improvement of conversion efficiency of Crystalline Silicon based Photo Voltaic devices. The standard Si Solar Cell efficiency improvements are achieved by forming low resistance contact to a passivated higher ohmic emitter while maintaining good diode quality and minimizing shadowing associated with metallizations. Achieving good quality contact poses significant materials development challenges in terms of control of microstructure in the contact region. This paper discusses issues associated with making contacts to Si devices with varying surface chemistries.The second set of important factors affecting the electrical efficiency is the strength of Back Surface Field, the reflection from the back surface and the back surface recombination velocity. These properties are governed by alloying of Al with Si wafer during the firing process. This paper discusses the evolution of Aluminum conductors for next generation Si Solar Cells.The solar industry is moving towards a goal of achieving > 24 % Efficiency through low cost fabrication techniques.. To achieve these goals various device schemes are being pursued. These schemes require making contacts to wide range of surfaces such as n-doped, p doped surfaces with varying concentration and, with and without passivation coatings. This paper provides an overview of these schemes.
3:00 PM - Q5.2
Screen-Printed Al Back Contacts on Si Solar Cells: Issues and Some Solutions.
Vishal Mehta 1 , Bhushan Sopori 1 , Robert Reedy 1 , Bobby To 1 , Helio Moutinho 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractCommercial Si solar cells are fabricated with screen-printed contacts consisting of gridded, Ag-based front contacts and continuous, Al-based back contacts. Typically, the contacts are co-fired, which yields cells with efficiencies in the range of 14% to 17%. However, many issues remain that prevent achieving optimum performance of the contacts. This paper explores issues related to Al back-contact formation that make it difficult to achieve a high-efficiency cell. Formation of a back contact requires creation of a thick melt of Si-Al alloy that, upon solidification, can produce a stratified structure consisting of a uniform P+ layer as a back-surface field (BSF) and low-resistivity Al layer. Unfortunately, a liquid layer of Al on a Si surface is not stable, and creates bumps, dimples, spheres on the back side of the cell, and shunting. Other problem is that it can cause staining of the back contact. The objective of this work is to study the kinetics of Si-Al alloy formation/solidification, identify mechanisms that cause problems in contact formation, and explore the possibility of controlling them to improve the back-contact performance. Experiments were done on two types of samples: (i) single-crystal Si wafers with evaporated Al layers of different thicknesses, and (ii) screen-printed, multicrystalline-Si (mc-Si) cells. The samples were fired in a static optical furnace under different process conditions. The fired samples were examined to determine structure, composition, and thickness of various layers. Solar cells were characterized by current-voltage measurements and their cell parameters were determined. The following electro-optical characterization techniques were used on planar cross-sectioned cells: SEM, C-AFM, SKPM, TEM, EDX, and SIMS. Results include the following: (a) A rapid diffusion of Si into Al occurs during the temperature ramp-up. Our results show that diffusivity of Si in Al is very high (~10 -8 cm2/s at 500°C), and the solubility is about 1% at 545°C. (b) Si diffusion can be deployed to create a Si-Al eutectic melt as an interface to promote the adhesion and improve stability of the Al melt. (c) Obtaining a thick BSF (~10 microns) requires a controlled cooling of the cell. Our best results (i.e., open-circuit voltages in excess of 620 mV), on mc-Si solar cells, were produced with a cooling rate of 20°C/s from 800°C to 600°C. (d) Presence of glass in paste hinders Al flow toward the interface; thus, the entire amount of Al is not used in forming the Si-Al melt. (e) Series resistance of single-crystal Si wafers with evaporated Al increased by 3%–4% due to rapid Si diffusion. We will present details of our investigation and discuss methods to improve back-contact properties of screen-printed mc-Si solar cells. This abstract is subject to government rights.
3:15 PM - Q5.3
Use of Direct Write Metallization for Photovoltaics.
Maikel van Hest 1 , Alex Miedaner 1 , Calvin Curtis 1 , Robert Pasquarelli 2 1 , John Kreuder 3 1 , Peter Hersh 4 1 , David Ginley 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Colorado School of Mines, Golden, Colorado, United States, 3 , Rochester Institute of Technology, Rochester, New York, United States, 4 , Heliovolt, Austin, Texas, United States
Show AbstractCurrently photovoltaics are becoming an increasing part of the energy supply mix, however to have a really significant impact they must become cost competitive with more conventional and other alternative energy sources. Direct write methods could help toward this significant cost reduction. We are investigating solution precursors and ink based atmospheric deposition approaches to explore metallization for a variety of solar cell materials. We are studying is inkjet printing of contacts for Si, Cu(InxGa(1-x))Se2 (CIGS) and organic photovoltaics. Metallization materials and requirements for each of these types of photovoltaics are different. We have developed metal organic decomposition inks for metals such as: silver, nickel, copper and aluminum. All of these can be deposited in lines with 30-40 µm width and conductivities close to that of bulk metals. For silicon devices most commonly screen printed silver is used. The screen printing pastes have an additive to facilitate contact formation between the silver and the silicon. We have developed a metal organic ink equivalent of this paste additive, which has resulted in improved electrical properties. For CIGS devices pure silver contacts and bilayer contacts of nickel and silver have been studied. For organic photovoltaic devices pure silver contacts deposited and processed at moderate temperatures (<140°C) were explored. For silicon, CIGS and organic photovoltaic devices with printed contacts, efficiencies similar to those with contacts deposited by alternative means were obtained. Additionally to the metallization results also an overview will be given of the new direct write capabilities for large area deposition (Up to 157 mm x 157 mm) at NREL.
3:30 PM - Q5.4
Investigation of Fired and Non-fired Si-SiNx Interface Properties by Deep-level Transient Spectroscopy measurements.
Chun Gong 1 , Eddy Simoen 1 , Rui Yang 1 , Niels Posthuma 1 , Emmanuel Van Kerschaver 1 , Jef Poortmans 1 , Robert Mertens 1
1 , IMEC, Leuven Belgium
Show AbstractSilicon nitride (SiNx) films deposited by plasma-enhanced chemical vapor deposition (PECVD) are widely used in silicon solar cell fabrication as passivation layers . The use of hydrogenated silicon nitride is one of the most significant technological evolutions that has taken place in solar cells industry, due to its ability to act simultaneously as antireflective coating as well as a source of hydrogen for surface and bulk passivation. Moreover, the firing step after SiNx film deposition is assumed to improve the passivation quality by forming hydrogen terminated dangling bonds. However, until now, most of the published data on the Si-SiNx interface have been based on C-V measurements of MIS structures which mainly give interface states density (Dit) information. The aim is to investigate both fired and non-fired Si-SiNx interface properties by deep-level transient spectroscopy measurements which can provide insight on both Dit and capture cross sections.Monocrystalline FZ 0.7 Ω.cm p- and CZ 2 Ω.cm n-type silicon wafers with a <100> orientation were used. Prior to the plasma deposition, wafers received a standard RCA clean and an HF dip prior to SiNx deposition. The deposition of Si-SiNx films was performed by a direct-plasma PECVD reactor with two parallel plates. The thickness of the deposited films was around 80 nm. After deposition, samples were diced in half, half received an extra firing step. Then circular gate contacts consisting of 1 μm aluminum were deposited onto the SiNx films by e-beam evaporation through a shadow mask. InGa/In was used as the rear ohmic contact. Finally, DLTS measurements were performed and analyzed using a Boonton C-V bridge operating at 1MHz; C-V measurements were also performed at temperatures varying between 77 and 310 K. The temperature scan of non-fired n-Si exhibits a rather deep interface peak at ~Ec-0.7 eV which is close to the Et-Ev = 0.4-0.5 eV position of Schmidt et al., APL, 71, 252 (1997). The non-fired p-type sample show a clear peak at 200K for a saturating voltage pulse which corresponds to B defects published by Schmidt et al., APL, 71, 252 (1997). It should be noted that plasma deposition techniques may vary which cause Si-SiNx interface variation. The frequency scan of fired and non-fired, p- and n-type samples show that the DLTS signal is much lower for fired samples. This indicates that the extra firing step can improve the passivation quality by reducing Dit, due to the formation of hydrogen terminated bonds.In conclusion, using the DLTS technique, B defect states are identified at the Si-SiNx interface fabricated by direct PECVD. Fired samples show much lower DLTS signals, which suggests a lower Dit. Small pulse measurements of the activation energy and capture cross section will be carried out and presented in the final paper.
4:30 PM - **Q5.6
CENTESIL: A Pilot Plant for R&D in Polysilicon.
Antonio Luque 1 3 , Carlos del Canizo 1 3 , Gabriel Ovejero 2 3
1 Instituto de Energia Solar, Universidad Politecnica de Madrid, Madrid, Madrid, Spain, 3 , CENTESIL, Madrid, Madrid, Spain, 2 Departamento de Ingenieria Quimica, Universidad Complutense de Madrid, Madrid, Madrid, Spain
Show AbstractThe growth photovoltaics is explosive nowadays. The main part of it has been based in the silicon solar cells, up to 89,5% in 2008 according to Photon International, and it is to be expected that the introduction in the market of new technologies —thin film of high efficiency concentrators— will probably not deprive this technology of its condition of workhorse of the PV development, at least in the next ten years.This incredible expansion and the relative avidity for silicon of the solar cell technology has resulted in a dramatic change of the polysilicon industry structure. While in the past the polysilicon was manufactured almost exclusively for the semiconductor industry in 2008 47,700 tm of polysilicon were consumed by the solar industry vs.23,300 by the semiconductor industry. The consequence is that while in 2000 virtually only 7 companies supplied all the polysilicon consumed in 2008 there were 11 major suppliers and there are numerous new ventures entering this market.Based on this in 2006 CENTESIL was founded in Spain as a new private-public partnership venture, with vocation of opening to an international partnership, to deal with the polysilicon research. For it, a pilot plant is in advanced state of construction that has been preceded of some laboratory-size implementations. The pilot plant is designed for a production capacity of 100 tm year. The purpose is to allow the photovoltaic companies worldwide to count with an independent research centre to help them to establish their own polysilicon plant. This is a profitable option for companies producing more than 100 MW/year.The purpose of CENTESIL is to produce solar silicon but the very concept of solar silicon is uncertain. In this respect in CENTESIL we have analyzed the permitted concentration of a number of impurities in the feedstock to allow for 15% efficiency solar cells. It has also been determined that improving efficiency in a certain percentage allows to increase polysilicon cost ~6 times this percentage for equal module cost. Therefore CENTESIL is aligned with the production of highly purified polysilicon for high efficiency solar cells.To fulfill these objectives the development contains four areas of activity: (1) synth