Symposium Organizers
Bhushan Sopori National Renewable Energy Laboratory
Jeff Yang United Solar Ovonic LLC
Thomas Surek Motech Americas
Bernhard Dimmler Wurth Solar GmbH & Co. KG
P1: Si-based Materials, Solar Cells, Manufacturing
Session Chairs
Tuesday PM, December 02, 2008
Independence E (Sheraton)
9:30 AM - **P1.1
Challenges in Crystalline Silicon Photovoltaics Research.
David Carlson 1
1 , BP Solar, Frederick, Maryland, United States
Show AbstractPhotovoltaic (PV) electricity should become competitive with commercial grid power on a widespread basis in the next decade or so. However, in order to attain this goal of grid parity, the cost of manufacturing solar cells, modules and the balance of systems must all decline. Crystalline silicon PV technology currently dominates the PV business with about 89% of all solar cells shipped in 2007 based on either multicrystalline, monocrystalline or ribbon silicon. However, thin film PV technologies have recently started gaining market share as the cost of crystalline silicon PV modules have actually increased in the last few years largely due to the shortage of solar-grade silicon feedstock. This shortage should disappear in the next year or two as a large number of companies are constructing facilities to produce solar-grade silicon in significant quantities using mainly Siemens reactor technology, but some companies are using fluidized bed reactors and others are further refining metallurgical grade silicon. While lower feedstock prices should greatly assist in reducing the cost of silicon solar cells, a longer-term challenge is to find a more cost effective way to process the silicon feedstock into low-cost, high quality Si substrates than the current processes of pulling or casting silicon ingots and then sawing the ingots into wafers. Another challenge for crystalline silicon PV is to further reduce the processing costs of silicon solar cells. While most of the industry is using screen-print technology, companies such as SunPower and Sanyo have introduced higher performance silicon cells, but it is not clear that these approaches will result in overall lower PV system costs. However, a large amount of government and private money has been flowing into PV in the last several years so that the pace of PV technology development is accelerating, and many new approaches are being pursued to improve the performance and to reduce the cost of silicon based photovoltaics.
10:00 AM - **P1.2
Advances in Thin-film Silicon Photovoltaics.
Subhendu Guha 1
1 , United Solar Ovonic LLC, Auburn Hills, Michigan, United States
Show Abstract10:30 AM - **P1.3
Future Developments in High Efficiency Silicon Solar Cells.
Richard Swanson 1
1 , Sun Power Corporation, San Jose, California, United States
Show AbstractSilicon solar cells are undergoing rapid development resulting in increasing performance and reduced cost. This paper discusses the major and efficiency limitations and cost elements of silicon solar cells, and shows how these are being improved. A roadmap to achieving grid electricity cost parity by 2012 through halving the installed cost of photovoltaic systems will be outlined.
11:30 AM - **P1.4
Structural, Microelectrical and Microluminescent Characterization of Crystallized Si Films.
Mowafak Al-Jassim 1 , Manuel Romero 1 , Fude Liu 1 , Yanfa Yan 1
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
Show AbstractWith the continuing shortage of Si feedstock, the interest in thin film Si is rapidly increasing. However, carrier recombination at grain boundaries and intragrain defects is receiving less attention despite its detrimental effects. In this work, the structural, electrical and luminescnt properties of grain boundaries and intra-grain defects were studied in polycrystalline silicon layers obtained through aluminum-induced crystallization of amorphous Si followed by epitaxial thickening. Electron backscattered diffraction was used to study the crystallinity and texture in the films, while detailed TEM studies focused on the type, density and three-dimensional distribution of structural defects. Electron beam induced current measurements showed a strong minority carrier recombination activity at these defects. Further, two deep-level radiative transitions (0.73 eV and 0.94 eV) were found by cathodoluminescence, showing quite a uniform distribution. We believe that these transitions are not only associated with dislocations as is widely accepted, but also to point defects or defect complexes. These results and other findings will be presented and discussed.
12:00 PM - P1.5
Atomic Force Microscopy-based Microelectrical Characterizations of Si-based Solar Cell Materials and Devices.
Chunsheng Jiang 1 , Helio Moutinho 1 , Baojie Yan 2 , Charlse Teplin 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , United Solar Ovonic LLC, Troy, Michigan, United States
Show AbstractBased on the atomic force microscopy (AFM) technique that measures two-dimensional corrugations on a sample surface, electrical signals between the probe (the AFM tip) and a sample surface can be detected and further used for measuring the microelectrical properties of a sample with fine resolutions ranging from nanometers to submicrometers. Examples of such electrical signals are electrostatic force, capacitance, and electrical current, which are used for measuring surface potential, carrier concentration, and local conduction paths, in the applications of scanning Kelvin probe force microscopy (SKPFM), scanning capacitance microscopy (SCM), and conductive-AFM, respectively. In recent years, we have established these AFM-based microelectrical characterizations techniques, and applied them to a wide range of solar cell materials and devices. In this presentation, we will review our recent progress on the microelectrical characterizations using these AFM-based techniques. We will present three examples of the characterizations of Si-based materials and devices, each using one of the technique above. The first example is a measurement of the electrical potential profile in an a-Si:H solar cell by SKPFM. The electric field shows highly non-uniform distributions in the nominal n-i-p structure, possibly due to high defect concentrations at the n/i and i/p interfaces. The SKPFM measurement further shows a significant improvement of the electric field uniformity by depositing buffer layers at the interfaces. The second example is a measurement of carrier depletion behavior on grain boundaries (GBs) of polycrystalline Si thin films by using SCM. The carrier depletion around the GBs can be a measure of the charged deep levels or impurities, and thus for the carrier recombination behavior that is critical for the photovoltaic performance. Then, the carrier depletion behavior is correlated with the GB misorientations, and a comprehensive understanding of the GB structural and electrical properties will be presented. The third example is a measurement of local conductivity on a nanocrystalline Si:H film (nc-Si:H) by using C-AFM. The local conduction path shows that the nc-Si:H grains aggregate to clusters, and electrical current flows mainly through the aggregations. Further comparison of the local conductivity after a light-soaking and a thermal annealing experimentally demonstrated the two-diode model for explaining the light-soaking-induced Voc increase in the nc-Si:H and a-Si:H mixed-phase devices.
12:15 PM - P1.6
SIMS Study of UMG-Si and Effect of Impurities on mc-Si Solar Cell Performance.
Larry Wang 1 , Richard Hockett 1
1 , Evans Analytical Group, Sunnyvale, California, United States
Show AbstractImpurities in PV-Si can adversely affect wafer yields and solar cell performance. Secondary Ion Mass Spectrometry (SIMS) is a very effective technique to quantitatively measure the elemental impurity concentrations in PV-grade silicon of many forms including powders, granules, flakes, chunks or recycled wafers. In this paper, we will present SIMS study of impurity uniformity of Upgraded Metallurgical Silicon (UMG), effect of impurities on solar cell performance.Case 1: Uniformity Study of B, Al, P, C, O, Ca and Fe in Upgraded Metallurgical Silicon (UMG-Si) by SIMSThe worldwide shortage of polysilicon for mono-crystalline and multi-crystalline Si PV has resulted in R&D and now commercialization of upgraded metallurgical silicon (UMG-Si) which has higher levels of impurities than traditional Siemens-based polysilicon but which can be used successfully in some PV solar cell designs. In this study, samples were taken from different locations from a large UMG-Si brick. Impurity levels for B, Al, P, C, O, Ca and Fe were determined using SIMS. Based on these analyses, we can conclude:-Impurity distributions in UMG-Silicon samples are generally not uniform.-The range of non-uniformity is element specific-Boron in this study is the most uniformly distributed element in UMG-Silicon-Some impurities in some sections show trends (not random distribution) – most likely process dependent-Sensitive analytical measurements can help manufacturers to improve their processes-Analytical results can be used to determine, for instance, what material to exclude (e.g., edge)-Analytical results can be used to select where representative samples should be taken.-Sampling volume for techniques such as SIMS, GDMS or ICPMS is not relevantCase 2: Purity of PV-Si material on the performance of MC-Si solar cellTo reduce the cost of c-Si solar cell, high purity Si materials are often mixed (at 5% to 20% mixture ratio) with lower purity materials. In this study, high purity Si (>8N) was mixed with low purity Si (5N) at 80:20 ratio to make multi-crystalline solar cell wafer. SIMS analysis for C, O, B, P, Al, Cr, Fe, Mn, Ni and Cu were performed on initial raw materials, and on mc-Si solar cell manufactures using high purity Si and mixed Si. Solar cell made from mixed Si material has higher B (5x), O (2x) and P (80x, > E16 at/cm3)) comparing to cell made from high purity Si. Higher impurity levels leaded lower conversion efficiency, higher leakage current and lower resistivityCase 3: Effect of O on solar cell conversion efficiencySIMS impurity analysis (bulk and profile) were performed on two mc-Si solar cells with large efficiency difference (15.4% vs. 6%). The “bad” cell shown much higher oxygen levels. In addition, oxygen precipitates were detected on the “bad” cell.
12:30 PM - P1.7
Mixed-Phase Solidification of Si films for Photovoltaics.
Monica Deep 1 , P. van der Wilt 1 , U. Chung 1 , A. Chitu 1 , A. Limanov 1 , James Im 1
1 Program in Material Science and Engineering, Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States
Show AbstractThin polycrystalline Si film-based PV technology continues to be recognized and appreciated as a potentially viable route for realizing low-cost/high-efficiency Si solar cells. One technical approach for realizing such a PV device consists of a two-step procedure that involves (1) generating a low-defect-density polycrystalline Si film on top of a low-cost/large-area substrate, and (2) creating the absorber layer (with a commensurately high level of microstructural integrity and, therefore, device performance) on this seed layer via a subsequent epitaxial thickening process.For this “seed layer” method, it is possible to identify the ability to effectively and efficiently produce a high-quality polycrystalline Si seed layer on top of a variety of substrates as an all-significant factor; in this paper, we propose a radiative-heat-source-induced melt-mediated crystallization approach, referred to as mixed-phase solidification (MPS), as a potentially viable and competitive means for accomplishing such a task. The MPS process employs a radiative heat source to transitionally (and typically iteratively) induce dynamically stabilized coexistence of liquid and solid domains within the irradiated region. When implemented optimally, the MPS method can convert an initially amorphous film into a large-columnar-grained and intra-grain-defect-free polycrystalline Si film with a very strong (100) surface texture; these microstructural attributes correspond to the very seed-layer material characteristics that are presently being viewed as being beneficial, and which cannot otherwise be obtained using other crystallization techniques.In this presentation we will (1) provide experimental results obtained from MPS (using 532 nm frequency doubled Nd:YVO4 cw-laser scanning or xenon arc-lamp exposure) of 50- to 150-nm-thick a-Si films on various substrates (glass, glass-ceramic, quartz, oxidized-Si wafers, etc.) and that are characterized using various analytical tools (TEM, AFM, EBSD, and SEM), (2) discuss the experimental results in terms of the two fundamental characteristics that enable, define, and dictate the process (i.e., the thermodynamically dominated near-equilibrium nature of the transitions as well as the reflectance-change-dominated stabilization of the coexisting superheated solid and supercooled liquid regions), and (3) suggest possible technical approaches to practically implementing the process in a cost-effective manner.
12:45 PM - P1.8
Measurement of Boron and Phosphorus Concentrations in Solar Grade Silicon Feedstock Source by High Resolution Glow-discharge Mass Spectrometry.
Karol Putyera 1 , Richard Hockett 2 , Larry Wang 2
1 , EAG-NY Shiva Technologies, Syracuse, New York, United States, 2 EAG-CA, EAG California, Sunnyvale, California, United States
Show AbstractThe calibration factors for the determination of boron and phosphorus concentration in solar grade silicon samples by high resolution glow-discharge mass spectrometry (GDMS) are examined, with the aim of comparing the behavior of single-crystalline, float-zone and Czochralski type samples. It is shown that the generalized calibration factors, derived from calibrations using MG grade Si SRM, cannot be generally applied to all solar grade Si types and forms. However, using single crystalline Si wafers with known boron and phosphorus contents traceable to NIST reference materials can be used for more accurate calibrations and for developments of analytical procedures for variety of sample forms. Additionally, the GDMS results will be compared to carrier mobility results between different solar grade Si samples, in order to examine if a conversion algorithm to deduce the carrier concentrations from the resistivity measurement is appropriate for applying for these types of materials.
P2: Optical Effect, Light Trapping, Crystallization
Session Chairs
Tuesday PM, December 02, 2008
Independence E (Sheraton)
2:30 PM - P2.1
Realization of Significant Efficiency Enhancement in Thin Film Silicon Solar Cells with Textured Photonic Crystal Backside Reflector.
Lirong Zeng 1 , Peter Bermel 2 , Yasha Yi 1 , Bernard Alamariu 3 , Kurt Broderick 3 , Jifeng Liu 1 , Ching-yin Hong 1 , Xiaoman Duan 1 , John Joannopoulos 2 , Lionel Kimerling 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThin film silicon solar cells are already emerging as a leading next generation photovoltaic technology, due to their potential of low cost. However, currently they suffer from low efficiencies due to the weak absorption of red and near-infrared photons. Effective light trapping schemes are crucial to overcome the weak absorption of these long wavelength photons and increase cell efficiencies. Here we report the first experimental realization of huge efficiency enhancement in thin film Si solar cells based on a novel light trapping scheme, the textured photonic crystal backside reflector. Differing from traditional light trapping utilizing geometrical optics, this light trapping technique, based on wave optics theory, allows us to effectively target the longer wavelengths requiring trapping. Combining a one-dimensional photonic crystal and reflection grating, textured photonic crystal can significantly enhance absorption of red and near-infrared photons due to the nearly 100% reflectivity of the 1D photonic crystal in a wide omnidirectional bandgap and the large angle diffraction by the grating, rendering greater absorption of the long wavelength photons admitted while simultaneously reducing the thin film interference effect at the top surface of the solar cell, improving light coupling into the cell. Silicon-on-insulator (SOI) wafers were used for solar cell fabrication to avoid complication of materials quality issues and make the optical effect obvious. Textured photonic crystal backside reflector was successfully integrated onto monocrystalline thin film Si solar cells made from SOI wafers through an active layer transfer technique. Short circuit current density was improved by 19%, and power conversion efficiency by 10% in 5 μm thick Si solar cells. With process optimization and better surface passivation, the efficiency enhancement can be as high as 28% as predicted by simulation. The textured photonic crystal backside reflector we developed can be applied directly to monocrystalline and polycrystalline Si solar cells, and its principle is broadly applicable to other materials systems.
2:45 PM - P2.2
Characterization of SixNy AR Coatings on Textured Si Photovoltaic Cells.
Mario Saenger 1 , Martin Schaedel 3 , James Hilfiker 2 , Jianing Sun 2 , Mathias Schubert 1 , John Woollam 1 2
1 Electrical Engineering, and Nebraska Center for Materials and Nanoscience, University of Nebraska - Lincoln, Lincoln, Nebraska, United States, 3 , Q-Cells A.G. , Thalheim, Saxony, Germany, 2 , J. A. Woollam Co., Inc., Lincoln, Nebraska, United States
Show Abstract3:00 PM - P2.3
An Analysis of Light Trapping Configurations for Thin Film Solar Cells based on Shaped Substrates.
Seung-Bum Rim 1 , Peter Peumans 1
1 Electrical Engineering, Stanford University, Stanford, California, United States
Show AbstractLight trapping approaches are used in thin-film solar cells to overcome the trade-off between achieving efficient optical absorption and high internal quantum efficiencies. It was recently shown that structuring of the transparent substrate on a scale much larger than the film thickness can be used to build effective light traps based on multiple reflections of incident rays that work particularly well for low-index media [1, 2]. In this study, we provide a general understanding of this type of light traps. We determine the optimal shape and evaluate the angular response and parasitic absorption of the light traps. We show that, for low-index media, these light traps approach the theoretical limit to light trapping performance. The analysis is performed using raytracing calculations to model light trapping and concentration. Transfer matrix calculations were used to accurately model Fresnel reflections at each interface. The analysis was performed for archetypal organic solar cells and amorphous and microcrystalline silicon cells. We show that inverted pyramid and V-shaped light traps exhibit the best performance, showing a 2.5-fold improvement in power conversion efficiency (PCE) compared to an unmodified cell at normal incidence for a bilayer organic solar cell. Our study reveals that total internal reflection at air/substrate interfaces can be exploited to achieve optimal light trapping using this approach. The analysis also shows that the angular response of the light traps considered is better than that of the unmodified cells. Inverted pyramid and V-shaped traps exhibit a 2.8-fold higher short circuit current density (JSC) averaged over a day compared to an unmodified cell. Finally, parasitic absorption caused by incomplete reflection from metal electrodes limits the performance that can be achieved with these light traps. [1] S.-B.Rim, S.Zhao, S.R.Scully, M.D.McGehee and P. Peumans, Appl. Phys. Lett. 91, 24(2007)[2] K. Tvingstedt, V. Andersson, F. Zhang, and O. Inganas, Appl. Phys. Lett. 91, 12 (2007)
3:15 PM - P2.4
Large-Area Convective Assembly for Solution Processed Omni-Directional Antireflection Coatings.
Yuehui Wang 1 2 , Li Chen 1 , Hongjun Yang 1 , Qing Guo 3 , Weidong Zhou 1 , Meng Tao 1 3
1 Electrical Engineering, University of Texas at Arlington, Arlington, Texas, United States, 2 Department of Chemistry and Biology, University of Electronic Science and Technology of China, Zhongshan Institute, Zhongshan, Guangzhou, China, 3 , ZT Solar, Inc., Fort Worth, Texas, United States
Show AbstractOmni-directional antireflection coatings (Omni-ARC) and surface texturing are highly desirable for high performance solar cells. The organic-free coating ensures minimal absorption in the coating layer and the long term reliability of the structure. Large area solution processed in-organic omni-ARC can be formed with self-assembled monolayers of micro-spheres partially immersed into spin-on-glass thin films. We report here a convective assembly process for the formation of large-area self-assembled monolayers of microscale silica spheres on glass, quartz, and silicon substrates. The structure of the self-assembled monolayers and their spatial extent are significantly influenced by sphere concentration in the suspension, dispersed suspension volume, solvent, coating plate speed, and wedge angle. Uniformly coated monolayers of 2-μm silica spheres were achieved on large 6”x6” glass substrate, comparable with the current commercial 6” silicon solar cells. Coating on even larger substrates is feasible with the incorporation of continuous injection system. The coating of large-area uniform monolayers of silica microspheres is characterized with scanning electron microscopy, optical microscopy, laser diffraction, and optical transmission/reflection measurements. It is found that the omni-ARC improves the transmittance of quartz wafer from 89.2% to 92.7% around 400 nm and from 90.8% to 92.5% around 1,100 nm, demonstrating its broad-spectrum nature. Less angle-dependent reflection is also obtained for incident angles up to 300. The performance of solution processed omni-AR coatings on flexible solar cells will be discussed, with increased efficiency at large incident angles. The solution processed all in-organic omni-ARC structure offers an attractive solution to antireflection in crystalline silicon solar cells, as well as thin-film, quantum dot, and other flexible solar cells.
3:30 PM - P2.5
Computer Simulation of Edge Effects in a Small-Area Mesa N-P Junction Diode.
Jesse Appel 1 2 , Bhushan Sopori 1 , Nuggehalli Ravindra 2
1 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Physics Department, New Jersey Institute of Technology, Newark, New Jersey, United States
Show AbstractWe have modeled influence of edges on the performance of small-area solar cells using a modified commercial, finite-element, modeling software package. Small-area mesa diode arrays, fabricated by chemical etching, are typically used for detailed characterization of photovoltaic substrates. These devices can be probed with an automatic prober to provide local solar cell parameters such as Voc, Jsc, and FF, and other materials related parameters such as resistivity, defect density, and minority carrier diffusion length. Our computer simulations include generation/recombination at the diode edges as well as the influence of light on the recombination characteristics of the edges. This model takes into account the charge accumulation at the sub-oxide layer and the loss of carriers caused by the carrier flow into the edge regions. The results of our calculations show that an increase in the positive surface charge along the vertical edge of the mesa diode causes the edges to be depleted and then inverted as the charge in the oxide is increased. This results in a higher forward biased dark current, particularly at low voltages. As the edge becomes inverted, carrier recombination around the edge of the device increases, which results in more dark current. The influence of edges on the illuminated characteristics of the diode depends strongly on the size of the diode and the bulk properties of the device. The results are also applicable to other device structures such as interdigitated contacts on high efficiency cells and low magnification concentrator cells. We will present a description of our model, dark and illuminated characteristics of devices with various charge concentrations, and the dynamics of carrier generation/recombination.
3:45 PM - P2.6
Arrays of Monocrystalline Silicon Solar Micro-cells for Modules with Ultra-thin, Mechanically Flexible, Semi-transparent and Micro-optic Concentrator Designs.
Jongseung Yoon 1 2 3 , Alfred Baca 3 5 , Sang-Il Park 1 2 3 , Paulius Elvikis 4 , Joseph Gedds 1 , Lanfang Li 2 5 , Rak Hwan Kim 1 2 3 , Jianliang Xiao 6 , Shuodao Wang 6 , Tae-Ho Kim 1 2 3 , Motala Michael 3 5 , Ralph Nuzzo 2 3 5 , Placid Ferreira 4 , Yonggang Huang 6 7 , Angus Rockett 2 , John Rogers 1 2 3
1 Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 5 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 4 Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 6 Department of Mechanical Engineering, Northwestern University, Evanston, Illinois, United States, 7 Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois, United States
Show Abstract4:30 PM - P2.7
Thermal Crystallization of a-Si:H from Laser Generated Nucleation Sites.
Matthew Dabney 1 , A. Mahan 1 , Philip Parilla 1 , David Ginley 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractPolycrystalline silicon has shown to be beneficial in microelectronics technology over the past few decades, however the ultimate aim of obtaining high-quality, large-grain c-Si films on low cost substrates has proven difficult. The recent understanding of what constitutes a nucleation site in silicon has led to this investigation of laser-induced nucleation in amorphous silicon films. Here we report studies of the use of laser irradiation to produce sparse “nucleation by design” centers in thin a-Si:H layers which, upon annealing, result in large-grained films. While direct crystallization of a-Si:H by laser annealing has been previous examined, the novelty in this approach is the creation of ordered a-Si regions (nucleation centers) with a laser fluence too small to directly crystallize the a-Si:H layer, thus avoiding multiple crystallite growth from each nucleation site. In this study, arrays of 2 μm wide laser spots of wavelengths between 248 nm and 1064 nm were used to investigate not only the power threshold and exposure times, but also the potential wavelength dependence of nucleation. First, the laser fluences and number of laser shots (exposure time) are investigated for direct crystallization. Then, working below the crystallization threshold, the samples are treated with a variant of laser fluences. The treated samples are then annealed in a conventional furnace to crystallize the silicon nucleating at the laser treated regions, while staying within the incubation period for the native nucleation sites. Data will be presented on the resultant crystallite grain sizes, and comparisons will be made to untreated a-Si:H samples which have been annealed to enable full crystallization.
4:45 PM - P2.8
Effects of Deposition Parameters on the Structure and Photovoltaic Performance of Si Thin Films by Metal-Induced Growth.
Peter Mersich 1 , Shubhranshu Verma 1 , Wayne Anderson 1 , Rossman Giese 2
1 Department of Electrical Engineering, University at Buffalo, Buffalo, New York, United States, 2 Department of Geology, University at Buffalo, Buffalo, New York, United States
Show AbstractMicrocrystalline silicon (μc-Si) is extensively researched as a material for fabrication of low cost solar cells. A metal-induced growth (MIG) process was employed to deposit thin films of μc-Si for solar cell applications. Due to different grain orientations of the crystals, the absorption coefficient of μc-Si is about 10 times higher than the absorption coefficient of single crystalline Si.The MIG process uses the fact that NiSi2 and CoSi2 have a lattice mismatch of only 0.4% and 1.3%, respectively, with Si. Therefore, Si atoms can be added on top of these silicide layers without change in volume and without appreciable strain in the lattice. The Ni and Co catalyst metals are evaporated on the substrate which is immediately taken for dc sputtering of Si. The μc-Si is deposited on the substrate by creating the plasma between the target and the substrate using 95% Ar and 5 % H2. A two-step procedure is used where for the first 45 min the power is 50 W and then it is increased to a higher level for 3 hr. When the Si atoms reach the metal surface they react to form NiSi2 or CoSi2. Once the metal is consumed, the thin film of μc-Si starts to grow on the substrate. The final quality of the μc-Si depends upon the thickness of the metal seed layer, temperature of the substrate, power, chamber pressure and the final annealing conditions.The properties of the Si film were investigated resulting from variations in several parameters. A range of Ni and Co thicknesses were examined from 7.5 nm to 60 nm including combinations of the two, while the second-step dc power was stepped up from 150 W to 225 W in 25 W increments. The deposition was also attempted at varied substrate temperatures of 625 °C and 650 °C. The structure of the resulting film was studied using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD). SEM of the film revealed that 3 hr of Si deposition at 200 W yields a film thickness of 5 µm and a maximum grain size of about 1 µm. EDS data showed that at the middle of the Si film the atomic percentage of the Si was 99.17%. AFM demonstrated that the Si surface is a combination of large and small irregular dome-shaped grains. While, XRD data showed that the dominant crystal orientation is {220}. To characterize the photovoltaic properties of the μc-Si, Schottky photodiodes were fabricated. Ni alone as the seed layer resulted in ohmic behavior. With Co only, MIG formed a rectifying contact with low barrier height. The combination of Co layered over Ni as a seed layer formed better thin films and gave Voc of 210 mV and Jsc of 3.9 mA / cm2 as a Schottky contact since the Co prevents Ni contamination of the top of the grown Si layer. This work has systematically improved the Si film grown by the MIG process. Ultimately, these films will be used for p-n junction or amorphous Si / μc-Si solar cells.
5:00 PM - P2.9
Nanoparticle Induced Crystallization of a-Si for Photovoltaic Devices.
Tae Kon Kim 1 , Prushotam Kumar 1 , Kerry Siebein 2 , Rajiv Singh 1
1 Materials science and engineering, University of Florida, Gainesville, Florida, United States, 2 2Major Analytic Instrument Center, University of Florida, Gainesville, Florida, United States
Show AbstractThe solid phase crystallization (SPC) of amorphous silicon thin films by a novel modification of nucleation step was investigated. The thin film consists of polycrystalline Si nanoparticles embedded in an amorphous matrix which can act as nuclei, resulting in lower activation energy for the nucleation. Thus, this decrease of activation energy can lead to the transition time of amorphous to polycrystalline silicon thin film and lower the processing temperature. The crystallinity of the film and the grains crystallized from nanoparticles has been extensively studied by XRD and HRTEM. The novel modified poly-Si thin film not only shows better performance but also significantly reduced the crystallization time and processing temperature as compared to those crystallized without modification. It was believed that Si nanoparticles would act as nuclei for growth of crystalline Si, thus removing the high temperature requirement for nucleation. The crystalline films induced by nanoparticles can be used for the photovoltaic devices on glass substrates at a maximum temperature of 500°C.
5:15 PM - P2.10
Optimal Crystallization Conditions of Titania Nanotubes Powders Synthesized by Anodization.
Eugen Panaitescu 1 , Suketu Patel 1 , Latika Menon 1
1 Physics, Northeastern Univ., Boston, Massachusetts, United States
Show AbstractTitanium oxide nanotubes combine the wide gap semiconductor properties of the material with the high surface area per unit volume and intrinsic 1D carriers paths offered by their structure, which allow for solar energy harvesting applications such as photocatalysis, water splitting or photovoltaics. Ultrafast synthesis of high aspect ratio titania nanotubes by anodization in chloride ions containing solutions has been reported by our group, and further optimization allowed for the rapid production of gram quantities of powders containing tightly packed nanotubes bundles. Pristine anodic titania nanotubes are amorphous, and annealing procedures are employed for their crystallization. Our group's preliminary studies regarding crystallization of the nanotubes powders revealed that annealing temperatures over 350°C and scan rates above 5°C/min are affecting the morphology at the nanoscale, causing a thickening of the tubes wall and resulting in their eventual closing and granulation, while temperatures below 300°C are too low for crystallization to occur. We carefully investigated the influence of annealing parameters in the phase transition by means of differential scanning calorimetry, coupled with spectroscopy and imaging techniques such as SEM, TEM and XRD. This allowed us to get more insight into the phase transition mechanism, and to identify optimal conditions for crystallization while keeping the nanotubular structure intact.
5:30 PM - P2.11
Unique Materials Address Stringent Process Environments for Advanced Materials Incorporated into Photovoltaic Manufacturing.
John Foggiato 1 , W. Brock Alexander 1
1 , Greene Tweed Co, Kulpsville, , Pennsylvania, United States
Show AbstractContinued introduction of new materials into advanced photovoltaic device manufacturing has resulted in the need for new materials within photovoltaic manufacturing equipment. Both higher technical performance has to be met and at lower cost leading to development of new “ceramic like” materials for high temperature processing as well as plastics for lower temperature applications.In the area of “ceramic like” materials, the component body material and surfaces need to minimize particulation and address temperature requirements. A variety of coatings have been used to extend the lifetime of components exposed to corrosive environments with new cleaning gases such as NF3 needing unique coatings. Materials for such applications are described in this paper along with elastomer seals used in equipment employing these cleaning gases. Other inorganic based ceramic materials are used in many applications especially for lower temperature processing of solar cells. These require materials and components at lower costs as components’ sizes are very large. Continued performance enhancement of photovoltaic manufacturing equipment has led to incorporation of uniquely developed plastics for various applications. Higher purity films require contamination free process environments, especially in minimizing permeability of atmospheric oxygen leading to seal materials having low oxygen permeability and low outgassing. Within the process chambers, higher cleanliness and ease of maintenance has required chamber shields made of low outgassing and plasma resistant plastic materials. Such plastics are described along with their properties and performance achieved in several applications. To attain the required properties of in-chamber inclusion of plastic based materials, a unique combination of polymers and fillers have been developed with new technologies for fabrication of parts. Plasma resistance testing with permeability and outgassing evaluation for multiple plastic formulations were performed to optimize the properties. The results of such testing and directions taken to optimize material properties are described.The use of UV based annealing and processing technology is becoming prevalent in photovoltaic manufacturing. Reducing thermal budgets is paramount in fabrication of the junctions, thus leading to development of UV annealing technology. Seals and process chamber shields of polymer based materials have to be UV resistant. In this case, unique chemical formulations of polymers are being developed along with parts fabrication technology. Optimal part molding and formation, together with surface treatments has led to polymer based materials suitable for photovoltaic manufacturing. New materials for UV resistant applications are described including extensive characterization data.Lower cost equipment systems are thus feasible with new materials development addressing the need for lower cost manufacturing of silicon based and thin film cells.
5:45 PM - P2.12
Gas Flow Hollow Cathode Sputtering for Photovoltaics.
Abe Belkind 1 2 , Gary Tompa 2
1 , Abe Belkind & Associates, Inc., North Plainfield, New Jersey, United States, 2 , Structured Materials Industriies, Inc., Piscataway, New Jersey, United States
Show AbstractThe continued evolution of the maturing photovoltaics market requires continued evolution in materials production tools. Sputtering is a cornerstone of many active layer depositions; however, methods of improving sputtering efficiency or resulting device quality are desired. These needs are for active thin films, protective coatings, and contact layers – transparent or metal. We have recently begun development of Gas Flow Hollow Cathode (GFHC) sputtering for an array of photovoltaic applications based upon the benefits GFHC has to offer, which include high deposition rates, low substrate temperatures, ability to deposit insulating, conductive and complex compound films, and the ability to control their structure. We will present our results on deposition of some metal and oxide coatings and discuss the deposition conditions and properties of the coatings.
Symposium Organizers
Bhushan Sopori National Renewable Energy Laboratory
Jeff Yang United Solar Ovonic LLC
Thomas Surek Motech Americas
Bernhard Dimmler Wurth Solar GmbH & Co. KG
P3: Hetro-junction Solar Cells, Solar Cell Processing, Polycrystalline Silicon
Session Chairs
David Carlson
Toshihiro Kinoshita
Wednesday AM, December 03, 2008
Independence E (Sheraton)
9:30 AM - **P3.1
High-Efficiency HIT Solar Cells for Excellent Power Generating Properties.
Toshihiro Kinoshita 1 , Daisuke Ide 1 , Yukihiro Yoshimine 1 , Toshiaki Baba 1 , Mikio Taguti 1 , Hiroshi Kanno 1 , Hitoshi Sakata 1 , Eiji Maruyama 1
1 Advanced Energy Research Center, Sanyo Electric Co., Ltd., Kobe Japan
Show AbstractIn order to achieve the widespread use of HIT (Hetero-junction with Intrinsic Thin-layer) solar cells, it is important to reduce the power generating cost. There are three main approaches for reducing the cost: raising the conversion efficiency of the HIT cell, using a thinner wafer to reduce the wafer cost, and raising the open circuit voltage to obtain a better temperature coefficient. With our first approach, we have achieved one of the highest conversion efficiency values of 22.3%, confirmed by AIST, in a HIT solar cell. This cell has an open circuit voltage of 0.725 V, a short circuit current density of 38.9 mA/cm2 and a fill factor of 0.791, with a cell size of 100.5 cm2. The second approach is to use thinner Si wafers. The shortage of Si feedstock and the strong requirement of a lower sales price make it necessary for solar cell manufacturers to reduce their production cost. The wafer cost is especially dominant factor in the production cost. In order to provide low-priced, high-quality solar cells, we are trying to use thinner wafers. We obtained a conversion efficiency of 21.4% (measured by Sanyo) for a HIT solar cell with a thickness of 85 μm. Even better, there was absolutely no sagging in our HIT solar cell because of its symmetrical structure. The third approach is to raise the open circuit voltage. We obtained a remarkably higher Voc of 0.739 V with the thinner cell mentioned above because of its low surface recombination velocity. The high Voc results in good temperature properties, which allow it to generate a large amount of electricity at high temperatures.
10:00 AM - **P3.2
High Efficiency Silicon Heterojunction Solar Cells.
Qi Wang 1 , M. Page 1 , E. Iwaniczko 1 , Y. Xu 1 , L. Roybal 1 , R. Bauer 1 , H. Yuan 1 , B. To 1 , Y. Yan 1
1 , National Renewable Energy Laboratory, Golden , Colorado, United States
Show Abstract10:30 AM - **P3.3
Current Challenges and Novel Approaches for Thin-film Silicon Solar Cells.
Bernd Rech 1 , Christiane Becker 1 , Bjorn Rau 1 , Lars Korte 1 , Benjamin Gorka 1 , Frank Fenske 1 , Stefan Gall 1
1 Silicon Photovoltaics, The Helmholtz Centre Berlin, Berlin Germany
Show Abstract11:30 AM - P3.4
Hydrogen-induced Passivation of Grain-boundary Defects in Polycrystalline Silicon.
N. Nickel 1
1 , Helmholtz-Center Berlin (formerly Hahn-Meitner-Institut), Berlin Germany
Show AbstractA major drawback of polycrystalline silicon are grain-boundary defects that limit the electrical and optical properties of the material and photovoltaic devices. These defects have been detected by electron paramagnetic resonance (EPR) and identified as silicon dangling-bonds. In order to obtain device-grade poly-Si and to improve solar-cell efficiencies it is essential to passivate the grain-boundary defects by exposing the material to monatomic hydrogen. Recently, a number of solar cell concepts emerged that employ the low-temperature fabrication of fine-grained poly-Si using solid phase, metal-induced, and laser crystallization techniques. These methods have in common that the resulting poly-Si is depleted of hydrogen and hence, requires a post-hydrogenation step to minimize grain-boundary defects. While most of the research related to hydrogen in poly-Si is focused on the optimization of the hydrogenation conditions, an equally important property of H has been neglected, namely the formation of new defects. In this paper, we investigate the influence of the hydrogen content in the amorphous starting material on hydrogen bonding and defect passivation in laser crystallized poly-Si using electron-spin-resonance and hydrogen effusion measurements. After laser dehydrogenation and crystallization the specimens contain a residual H concentration of 8x1021 cm-3 to 1.5x1022 cm-3. During a vacuum anneal at least 1.5x1021 cm-3 H atoms are mobile in the lattice, however, only about 3.7x1018 cm-3 H atoms passivate Si dangling-bonds. Our results show that the annealing treatment can cause the vast majority of H atoms to accumulate in H stabilized platelets. Since defect passivation preferentially occurs at grain boundaries and platelet nucleation and growth is confined to the interior of single crystal grains, H equilibration is governed by two spatially separated processes. Moreover, our data demonstrate that the hydrogen density-of-states distribution derived from H effusion data is dynamic and changes in response to experimental parameters.
11:45 AM - P3.5
Dislocation Engineering in Multicrystalline Silicon.
Mariana Bertoni 1 3 , Katy Hartman 2 3 , Tonio Buonassisi 1 3
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Laboratory for Manufacturing Productivity, Massachussets Institute of Technology, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, Massachussets Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMulticrystalline silicon (mc-Si), which currently accounts for almost 50% of worldwide solar cell module production, offers a promising alternative energy source mainly due to its low production cost and scalability. However, in order to obtain a grid-competitive energy alternative higher efficiencies should be obtained.The major disadvantage of this material are the inhomogeneously distributed regions of low minority carrier lifetime with very high dislocation densities (104-108 cm-2) that can drastically reduce the efficiency of the overall cell since they act as “sinks” that dissipate power from good regions of the material.To improve the performance of mc-Si based solar cells, we must develop a manufacturable method to mitigate the impact of dislocations on the electrical properties of silicon. Traditionally, much effort has been focused on reducing the electrical activity of dislocations, primarily by gettering impurities or passivation. A review of these approaches will be provided. Recent evidence suggests, however, that these techniques can only achieve partial success. Even relatively “clean” dislocations with weak individual recombination activity, when present in the high concentrations typical of mc-Si underperforming regions, can have a large cumulative impact on minority carrier lifetime.We propose and demonstrate a method to remove performance-limiting dislocations from multicrystalline silicon (mc-Si) solar cell material, appropriate for wafers or bricks. Dislocation density reductions of >95% are achieved in commercial mc-Si via high temperature annealing with an impurity diffusion barrier, with controlled ambient and time-temperature profiles. We complement the experimental results with temperature-dependent and time-dependent theoretical models, adapted from the original work of Kuhlmann [D. Kuhlmann, Proc. Phys. Soc. A 64, 140, 1951] and Nes [E. Nes, Acta Metal. Mater. 43, 2189, 1995] and cell efficiency measurements.
12:00 PM - P3.6
Effect of Thermal Annealing on Characteristics of Polycrystalline Silicon used for Solar Cells.
Gou XianFang 1
1 , beijing solar energy institute, Beijing China
Show Abstract12:15 PM - P3.7
A Novel Approach for Thin-film Polycrystalline Silicon on Glass.
Mauritius van de Sanden 1 , A. Illiberi 1 , K. Sharma 1 , A. Branca 1 , M. Creatore 1
1 Applied Physics, Eindhoven University, Eindhoven Netherlands
Show AbstractThin film polycrystalline (poly-Si) solar cells are emerging as a promising PV technology, combining a large cost reduction with a large efficiency potential [1]. The grain size is a crucial factor in the production of high quality poly-Si films for PV applications because grain boundaries act as traps and recombination centers of carriers. In this framework, we are developing a new approach for the production of poly-Si thin films. The a-Si layers are deposited by using the expanding chemical vapor deposition (ETP-CVD) technique, which has previously demonstrated device grade a-Si:H at high deposition rate (7-11 nm/s) [2]. The a-Si:H layers are crystallized for 10 hours at 650C by Solid Phase Crystallization (SPC) technique. The imaginary part of the pseudo-dielectric function has been measured by means of Spectroscopic Ellipsometry (SE) to give insight into the crystallization degree of the annealed a-Si:H films [3]. The results have been confirmed by Raman diagnostic. Structural material quality of the poly-Si films has been investigated by cross-section Transmission Electron Microscopy (TEM): many crystal grains (1 μm lateral dimension) extend over the entire thickness (1 μm) of the annealed a-Si:H films on glass. This result indicates that a-Si:H films deposited in the ETP set-up are well suited in order to produce high quality poly-Si films on low cost supporting material.
12:30 PM - P3.8
Minority-Carrier Lifetime Technique Comparison for Silicon Wafers.
Steve Johnston 1 , Greg Berman 3 1 , Nathan Call 2 1 , Richard Ahrenkiel 2 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , University of Colorado, Boulder, Colorado, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractMinority-carrier lifetime may be measured in silicon by several different techniques. First, photoconductivity can be used to monitor excess carrier density. When light pulses generate excess carriers, photoconductivity transients due to carrier recombination can be fit, while mobility is assumed to remain constant. For these techniques, such as with microwave reflection, photoconductive decay follows the recombination of excess carriers. Lifetime can also be calculated based upon knowledge of the generation of carriers and their mobility in silicon. In this technique, steady-state photoconductivity is a function of carrier lifetime. Next, while photoluminescence in silicon is weak, radiative recombination does occur and is proportional to excess carriers. When surface states do not dominate total recombination rates, lifetime can be correlated to the intensity of photoluminescence. Lastly, free carrier absorption may also be used to measure excess carrier lifetime. A fast detector can monitor transients of sub-bandgap infrared light transmission when pulses of excess carriers are generated. Similar to steady-state photoconductivity, steady-state infrared absorption or emission of excess carriers can also be used to calculate lifetime. Photoluminescence imaging and infrared-based carrier density imaging are fairly recent techniques giving rapid lifetime information with sub-millimeter spatial resolution. We compare all of these techniques to show which ones have the best advantages and usefulness for various applications, material types, and stages of the silicon solar cell process.
12:45 PM - P3.9
Conducting Two-phase Silicon Oxide Layers for Thin-film Silicon Solar Cells.
Peter Buehlmann 1 , Didier Domine 1 , Julien Bailat 2 , Andrea Feltrin 1 , Christophe Ballif 1
1 , IMT, University of Neuchâtel, Neuchâtel Switzerland, 2 , Oerlikon Solar-Lab, Neuchâtel Switzerland
Show AbstractP4: Organic Solar Cells
Session Chairs
Katsumi Kushiya
Richard Swanson
Wednesday PM, December 03, 2008
Independence E (Sheraton)
2:30 PM - P4.1
Electron-transport Characteristics of High Efficiency Dye-sensitied Solar Cell based on Electrospun TiO2 Nanorod film
Byunghong Lee 1 , Sung Yeon Jang 1 , Sung Mu Jo 1 , Dong Young Kim 1
1 Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractWednesday, 12/3New Presenter P4.1 @ 1:30 PMElectron-transport Characteristics of High Efficiency Dye-sensitied Solar Cell based on Electrospun TiO2 Nanorod Film.Sung Yeon Jang
2:45 PM - P4.2
Characteristics of Zinc-octaethylporphyrin/C60 Layered-Structure Photovoltaic Cells.
Sou Ryuzaki 1 , Toshihiro Kai 1 , Toshiaki Nishii 2 , Jun Onoe 1 3
1 Department of Nuclear Engineering , Tokyo Institute of Technology, Tokyo Japan, 2 , Electric Power Development Co., Ltd., Kanagawa Japan, 3 Resarch Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo Japan
Show AbstractOrganic thin-film photovoltaic (O-PV) cells consisting of electron- donor and acceptor molecules have been intensively investigated as a candidate for next-generation PV cells. However, their photo-energy conversion efficiency (ηE) is still low at most 6 % under a standard illumination condition (100 mW/cm2, AM1.5). Metal porphyrin and C60 have often been used as an electron- donor and acceptor material, respectively. In particular, the interaction between octaethylporphyrins (OEPs) and C60 is strong. In addition, the ηE of a Schottkey-type PV cell using OEP [Al/H2OEP/Ag] became lager by ten times than that using Pc [Al/MgPc/Ag].[1] This suggests that OEP is better than Pc as an electron-donor for fabrication of high-performance O-PV cells.We recently examined the crystalliniy and molecular orientation of Zinc-octaethylporphyrin [Zn(OEP)] on SiO2/Si and ITO substrates, because Zn(OEP) has a large absorbance in visible region and forms a multi-layered structure with C60.[2] It was found that Zn(OEP) films with a thickness of more than 200 nm have two kinds of grains (“grain 1” with an interplane distance of d = 1.12 nm, “grain 2” with that of d = 1.24 nm). When the Zn(OEP) film is used for O-PV cells, a film thickness of less than 50 nm, which is comparable to the exciton diffusion length in the film, is suitable.[3] We also found that a 20 nm-thick Zn(OEP) film is amorphous, while a 40 nm-thick one has only “grain 1”. In addition, the grain size (20 nm) was independent of film thickness and substrate temperature.In the present study, we have fabricated two kinds of O-PV cells consisting of 20 nm or 40 nm-thick Zn(OEP) and 30 nm-thick C60 films [ITO/Zn(OEP)/C60/Al] and measured their external quantum efficiency (EQE). Both of them showed a spectral response corresponding to their individual photo-absorption spectra. The EQE of the cell using the 40 nm-thick Zn(OEP) film was obtained to be 13% at 400 nm, 6.6% at 545 nm, and 7.5% at 590 nm. These wavelength values correspond to the photo-absorption peaks of Zn(OEP). On the other hand, the EQE of the cell using the 20 nm-thick Zn(OEP) significantly increased to be 36% at 400 nm, 12% at 545 nm, and 14% at 590 nm. These results suggest that the grain boundaries inhibit the diffusion of excitons or free carriers for the former cell.[1] F. Kampas, M. Gouterman, J. Phys. Chem. 81. 690 (1977).[2] S. Ryuzaki, T. Ishii, J. Onoe, Jpn, J. Appl. Phys. 46. 5363 (2007).[3] P. Peumans, A. Yakimov, S. R. Forrest, Appl. Phys. Rev. 93. 3693 (2003).
3:00 PM - P4.3
Rational Design of Photoactive Materials for Organic Solar Cells.
Liping Huang 1 2 3 , Dario Rocca 4 , Stefano Baroni 5 , Keith Gubbins 1 , Marco Buongiorno Nardelli 2 6
1 Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Physics, North Carolina State University, Raleigh, North Carolina, United States, 3 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Chemistry, University of California at Davis, Davis, California, United States, 5 , SISSA and CNR-INFM DEMOCRITOS National Simulation Center, Trieste Italy, 6 CSMD, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractOrganic semiconductors are promising candidates for making low-cost solar cells because of their easiness to process. However, they usually have limited response to the solar spectrum and very low charge carriers mobility. We carried out first-principles calculations at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) levels to understand the effect of molecular size, functional groups and molecular packing on the optical absorption spectrum and charge carrier motility of acenes. Our calculations demonstrate that these properties not only depend on the molecular identity but also on the molecular packing. By designing the interaction between metal substrates and the first layer of acene molecules, they can be packed in a cofacial (face to face) fashion instead of the conventional herringbone (face to edge) arrangement. Acenes packed in the cofacial way show improved response to the solar spectrum and high charge carriers mobility. This could open the door to a new route of materials design for cost-effective organic solar cells.
3:30 PM - P4.5
Characterization of Inkjet Printed, Macro-Porous Titanium Dioxide Coatings Obtained using Emulsion Templating Method for Dye Sensitized Solar Cells.
Sarika Phadke 1 , Sau Pei Lee 1 , Mike Oreilly 1 , Dunbar Birnie,III 1
1 Materials Science & Engineering, Rutgers University, Piscataway, New Jersey, United States
Show Abstract3:45 PM - P4.6
Extremely-Thin Absorber Solid-State Solar Cells Made from Hyperbranched Polymeric Phthalocyanines.
Yong Li 1 , Qi Wang 1 , Xingzhong Yan 1 , Michael Ropp 1 , David Galipeau 1
1 Advanced Photovoltaics & Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota, United States
Show AbstractPhotovoltaic solar energy is very attractive as it is sustainable and environment-friendly. Hybrid solar cell, consisting of penetrating conjugated polymers within cheap TiO2 aerogel frames, has been recently considered as one of the most promising and cost-effective approaches for photovoltaic technologies. In this work, hyperbranched polymeric phthalocyanines have been synthesized and implanted as the strong absorber layers into an extremely-thin absorber solar cell structure. The polymeric phthalocyanines were prepared by cyclo-tetramerization of 1,3-bis(3,4-dicyanophenoxy)benzene with or without the presence of titanium (IV) butoxide under the catalysis of 1,8-diazabicyclo[5.4.0]undec-7-ene in n-pentanol solution. These polymeric phthalocyanines have been implanted into a cell structure of ITO/TiO2-polymeric phthalocyanine-CuSCN/Carbon by in-situ cyclo-tetramerization of the monomer on the TiO2 nanoparticles or spin-coating of the hyperbranched polymeric phthalocyanine on nanostructured TiO2 films. The hyperbranched polymeric phthalocyanines in the cell structure act as sunlight absorber, electron donor and hole-transporter. Photocurrent measurement has demonstrated a successful organic-inorganic hybrid cell design to replace the expensive chalcogenides in conventional extremely-thin absorber solar cells by these polymeric phthalocyanines. A wet chemistry technique has been established for the fabrication of new type extremely-thin absorber solar cells in stead of the conventional vacuum deposition.
4:30 PM - P4.7
Low-Temperature PECVD of Silico-based Films using Cyclohexasilane.
Konstantin Pokhodnya 1 2 , Joseph Sandstrom 1 , Xuliang Dai 1 , Philip Boudjouk 1 , Douglas Schulz 1 3
1 Center of Nanoscale Science and Engineering, North Dakota State University, Fargo, North Dakota, United States, 2 Physics, North Dakota State University, Fargo, North Dakota, United States, 3 Mechanical Engineering and Applied Mechanics, North Dakota State University, fargo, North Dakota, United States
Show AbstractThe development of lower-temperature PECVD routes to silicon-based electronic materials is of interest given the promise of producing flexible solar cells on cost-effective substrates such as polyethylene terephthalate. In addition, the demonstration of higher-growth rates for PECVD silicon may address a bottleneck in some existing photovoltaic manufacturing technologies. We have used a new silicon precursor cyclohexasilane, (Si6H12 or CHS), in the PECVD growth of amorphous and polycrystalline silicon thin films. FTIR, Raman and UV/Visible spectroscopies along with SEM and resistivity measurements were used to determine the optimal deposition conditions and post-deposition treatment for both maximizing the growth rate and minimizing the growth temperature. Preliminary results for the growth of PECVD dielectrics such as silicon nitride and silicon oxide from CHS may also be presented. Acknowledgements. This material is based on research sponsored by the National Science Foundation through ND EPSCoR grant EP-0447679 and the Defense Microelectronics Activity under agreement number H94003-06-2-0601. The United States Government is authorized to reproduce and distribute reprints for government purposes, notwithstanding any copyright notation thereon.
4:45 PM - P4.8
A Novel Solution Processable Electron Acceptor, C60(CN)2, for Bulk Heterojunction Photovoltaic Applications.
Vaishali Koppolu 1 , Mool Gupta 1 , Chunying Shu 2 , Harry Gibson 2 , Harry Dorn 2
1 Electrical & Computer Engr., University of Virginia, Charolttesville, Virginia, United States, 2 Chemistry, Virginia Polytechnic Institute and State University , Blacksburg, Virginia, United States
Show AbstractPhotovoltaic devices based on soluble conjugated polymers have gained great interest in recent years because of the potential low cost of production and ease of fabrication by spin casting processing. PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) , a soluble fullerene derivative, has been extensively investigated as a solution processable electron acceptor for bulk-heterojunction photovoltaic devices blended with conjugate polymers like P3HT (Poly (3-Hexylthiophene)). Here, we investigated a novel solution processable organic semiconductor, C60(CN)2, as a potential electron acceptor for bulk heterojunction photovoltaic applications. Optical and electrical properties of C60(CN)2 are studied and compared with PCBM. Blend devices with P3HT and C60(CN)2 have been fabricated and compared with P3HT-PCBM devices. Photoluminescence studies of P3HT and C60(CN)2 blends have shown significant quenching indicating efficient charge transfer between P3HT and C60(CN)2. The effect of the two acceptors on the open circuit voltage and short circuit current density is studied. Effect of thermal annealing on the morphology and hence the device performance is also studied
5:00 PM - P4.9
Internal Electrical Polarization Effects on Excited States in Organic Semiconducting Materials.
Tho Nguyen 1 , Anping Li 1 , Bin Hu 1
1 , University of Tennessee, Knoxville, Tennessee, United States
Show AbstractInternal electrical polarization can inevitably occur in organic semiconducting materials based on delocalized pi electrons. Particularly, internal electric polarization can influence Coulombic interaction between electron and hole in excited states and consequently change optoelectronic processes in organic semiconducting materials. We selected an organic material composite that contains two types of molecules: Alq3 (tris-(8-hydroxyquinoline) aluminum) and DCM2 (DCM2([2-methyl-6-[2-(2,3,6,7-tetrahydro-1H, 5H-benzo[i,j] quinolizin-9-yl)-ethenyl]-4H-pyran-4-ylidene] propane-dinitrile) dispersed in inert PMMA (Poly(methyl methacrylate)) matrix. Particularly, this organic composite offers tunable internal electrical polarization based on inter-molecular dipole-dipole interaction formed between Alq3 and DCM2 molecules. By using photoluminescence dynamics and magnetic field dependence of electroluminescence we found that the internal electrical polarization can significantly change the exciton formation, dissociation, and exciton-charge reaction. This presentation will discuss how internal electric polarization affects gain and loss excited processes in organic semiconducting materials.
5:15 PM - P4.10
Mesoporous TiO2 Thin Films for Photovoltaic Applications.
Jennifer Dewalque 1 , Rudi Cloots 1 , Catherine Henrist 1
1 Chemistry, University of Liege, Liege Belgium
Show AbstractThis study aims at developing thin films of nanocrystalline, mesostructured titanium dioxide in order to build low cost and efficient photovoltaic devices.The synthesis is based on the Evaporation Induced-Self Assembly method using titanium isopropoxide as inorganic source, block copolymers as structuring and porogeneous agents and ethanol as solvent. The films were obtained by dip-coating various substrates in the solution.The influence of different experimental parameters, such as dip-coating and ageing relative humidity, withdrawal speed, surfactant:Ti ratio, substrate, was studied. The post-deposition thermal treatment had to be accurately adjusted in order to maximise the crystallisation of the inorganic network while avoiding the collapse of the porous mesostructure. The final structure obtained is discussed in the light of the XRD results combined with TEM analysis. Moreover the cell performance is limited by the film thickness which is mainly responsible of the small amount of absorbed light. Therefore a multilayer deposition process was studied and the as-obtained mesostructure was characterized by TEM, RBS and environmental ellipsometry.
5:30 PM - P4.11
Porous TiO2 Electrode for Dye-sensitized Solar Cells Fabricated by Excimer Laser Annealing.
Heng Pan 1 , Seunghwan Ko 1 , Costas Grigoropoulos 1
1 , UC-Berkeley, Albany, California, United States
Show AbstractWe report a rapid and low-cost method to fabricate porous TiO2 electrodes for dye-sensitized solar cells through incorporating an excimer laser annealing step. Excimer laser annealing is used to induce TiO2 nanoparticle coalescence and form porous structure optimized for light harvesting and electron transportation. The cells show a high open-circuit voltage and enhanced overall photoelectric conversion efficiency under an illumination of AM1.5 sun.
P5: Poster Session: Photovoltaic Materials, Solar Cells, Characterization
Session Chairs
Thursday AM, December 04, 2008
Exhibition Hall D (Hynes)
9:00 PM - P5.1
Grain Size-Dependent Dielectric Function of CdTe Micro- and Nanocrystals Prepared by Langmuir-Blodgett Technique and Ion Implantation.
Peter Petrik 1 , Miklos Fried 1 , Andras Deak 1 , Nguyen Quoc Khanh 1 , Jian Li 2 , Robert Collins 2 , Tivadar Lohner 1
1 Photonics, Research Institute for Technical Physics and Materials Science, Budapest Hungary, 2 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Show AbstractWe investigated different parameterizations of the dielectric function of CdTe micro- and nanocrystals. The fitted critical point (CP) parameters can in turn be used for the indirect optical measurement of the grain size. The samples of systematically changing grain/nanocrystal size were prepared using two techniques. The first method utilizes the Langmuir Blodgett (LB) method to create monolayers of CdTe colloidal particles of well-defined sizes. The second method creates grains of decreasing size using ion implantation of Bi at an energy of 175 keV with increasing doses from 4×1013 cm-2 to 3×1014 cm-2. The reason for the use of the high atomic mass Bi ions was that previous studies using lighter ions revealed damage at a very low level, even for doses several times higher than the amorphization level estimated by simulation [P. Petrik et al., phys. stat. sol. (c) 5, 1358 (2008)]. Bi ions create damage high enough to investigate the change of CP features in a broader range of structures from single-crystalline to highly damaged. The CP features can be described by numerous methods from the generalized CP model [B. Johs et al., Thin Solid Films 313-314, 137 (1998)] through Adachi’s parameterization [S. Adachi et al., J. Appl. Phys. 74, 3435 (1993)] to the standard critical point model [P. Lautenschlager et al., Phys. Rev. B 36, 4821 (1987)]. The latter has been proved to be a reliable approach for photovoltaic CdTe characterization [Li et al. phys. stat. sol. (a) 205, 901 (2008)]. In this work we developed optical models for the ellipsometric characterization of Bi-implanted CdTe and LB-deposited micro- and nanocrystals to describe both the vertical sample structure and the dielectric function of CdTe of different grain sizes, verified by backscattering spectrometry and electron microscopy.
9:00 PM - P5.10
Geology Meets Photovoltaics: Silicon-Bearing Compounds for Solar Cell Manufacturing.
Sarah Bernardis 1 3 , Steve Hudelson 2 3 , Tonio Buonassisi 2 3
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 3 Laboratory for Manufacturing Productivity, MIT, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractSilicon is the second most abundant element of the Earth’s crust. As such it is contained in a variety of different minerals (e.g., quartz, silicates) and rocks (e.g., granite), primarily in an oxidized chemical state. High purity quartz is primarily used as raw material for both electronic grade (EG) and solar grade (SoG) silicon industries, as this raw material contains relatively low concentrations of the most deleterious impurity species. As industrial demand for raw materials is quickly increasing, and defect engineering technology evolves an arsenal of tools to mitigate the impact of impurities, a wider range of potential sources of silicon becomes of interest.Over the past few decades, bulk analysis techniques, such as inductively-coupled plasma mass spectroscopy (ICP-MS) and glow discharge mass spectroscopy (GDMS), have made great strides, detecting dissolved impurity concentrations below parts per billion. However, such techniques have yielded little information about the chemical states of these impurities, their evolution during the course of solar cell processing, and their impact on device performance on a micron or sub-micron level.In this study, we start with an assessment of the Si-bearing compounds in the Earth’s crust, with attention towards their geographical distribution and impurity concentrations. Si-bearing materials with different chemical states are considered and evaluated as future possible raw material sources. Then, to understand the microscopic distribution of impurities and their chemical states, we perform synchrotron-based x-ray microprobe investigations on geographically and geologically diverse Si-bearing raw materials. By assessing the different chemical states of impurities, we discuss their potential evolution during the solar cell production processes, and their impact on device performance.
9:00 PM - P5.11
Fiber Drawing Nanomanufacturing of Micromirror Array and Microlens Array for Future Photovoltaic Applications.
Zeyu Ma 1 , Yan Hong 1 , Ming Su 1
1 MMAE, University of Central Florida, Orlando, Florida, United States
Show AbstractWe have developed a new method for the large scale productions of glass micromirror arrays and polymeric microlens arrays by combining fiber drawing nanomanufacturing (FDN) and selective chemical etching. Briefly, a glass rod is inserted into a glass tube with matching properties for fiber drawing. The two glasses have similar softening point and coefficients of thermal expansion, but have dissimilar chemical resistance. The obtained fiber is cut into short pieces of equal length, and stacked to form a glass bundle for next drawing. The textural features (i.e. size, length and spacing) can be controlled by repeating the draw-cut-stack process. The fibers from the last process are stacked to form a glass rod, which is cut and polished in the cross-section direction. The subsequent chemical etching will produce ordered arrays of glass micromirrors or microwells. The glass micromirror array shows a great capability in deflecting normally incident light in the range of 400 nm to 900 nm. The microwell structures have been used to make polymeric microlens array, which can focus incoming light. Compared to the anisotropic etching of silicon, this method is ideal to make structure-adjustable textured surface over a large range at high yield. After depositing transparent conductive oxide films, these structures have been used as substrates for photovoltaic applications.
9:00 PM - P5.12
Influence of CdCl2 Treatment on Electronic Properties of CdTe/CdS Structure.
I. Riech 1 , J. Peña 2 , J. Mendoza 3 , E. Arceo 1
1 Facultad de Ingenieria, Universidad Autonoma de Yucatan, Merida Mexico, 2 Departamento de Física Aplicada, CINVESTAV-IPN , Mérida Mexico, 3 Departamento de Física, CINVESTAV-IPN , Mexico Mexico
Show AbstractIn order to improve efficiency in thin film solar cells, we need to have a better understanding of the interaction between CdCl2 treatment, impurities and interdiffusion. In particular, for CdS/CdTe thin films solar cells, incomplete collection of the photogenerated carriers is determined by recombination at the interface. In this work, CdS and CdTe films were grown by CBD and CSVT techniques respectively. The samples were subject to post-growth treatment, consisted of evaporating CdCl2 layer. The aim of this research was to explaining the CdCl2 annealing effect in the electronic properties of CdTe/CdS structure. We have studied photoacoustic (PA) signal phase in order to determine surface recombination velocity at CdTe/CdS interface as a function of different CdCl2 treatments. Also, we have used low temperature photoluminescence (PL) to study the emission bands of the same structures and determine S diffusion into the CdTe as well as the changes in the impurity distribution in CdTe layer. The results are discussed correlating the interface quality to nonradiative recombination at the interface, quantified by PA method.
9:00 PM - P5.13
Firing Behavior and Elemental Depth Profile of CuInSe2 Melt Pellets.
Deuk Ho Yeon 1 , Bhaskar Mohanty 1 , Yeon Hwa Jo 1 , Ik Jin Choi 1 , Yong Soo Cho 1
1 Materials science and engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractThe present work aims at preparation of low cost, well densified, and phase pure CuInSe2 (CIS) targets for growing thin films, for example in sputtering or pulsed laser deposition. CIS ingots were obtained from direct reaction of stoichiometric amounts of high purity Cu, In and Se powders in an evacuated quartz ampoule. Ingots, obtained from directly quenching the melt from ~1100 oC into water, were crushed and ball-milled for 24 hours. Analysis of XRD patterns of the powder showed the formation of single phase chalcopyrite CIS. Pellets of typical diameter of 8 mm were obtained by uniaxial cold pressing (~7 X 10^6 Pa) of this fine powder. In order to densify, and thus making suitable for use as target material, the pellets were fired at different temperatures ranging from 300-800 oC in N2. XRD, SEM, and EDX were used to study the phase purity, microstructure and elemental compositions of the pellets respectively. Elemental composition variation along depth was studied using dynamic secondary ion mass spectrometry depth profiling. Although it was possible to prepare more compact and dense targets, substantial amount of Se loss as a consequence of firing at temperatures above 600 oC was observed. Interrelation of Se loss with firing temperature and duration has been elucidated.
9:00 PM - P5.14
Fabrication of AZO/ SiO2/p-Si Based Violet Solar Cell.
Z. Ma 1
1 Physics, Shanghai University, Shanghai China
Show Abstract9:00 PM - P5.17
Optical Properties of Nanostructured CdTe Thin Films.
Juan Pena 1 , Jose A. Flores-Livas 2 , R. Castro-Rodríguez 1 , I. Riech 3 , Eduardo Perez-Tijerina 2
1 Fisica Aplicada, CINVESTAV-IPN Merida, Merida, Yucatan, Mexico, 2 Laboratorio de Nanociencias y Nanotecnología, FCFM/UANL, Monterrey, Nuevo Leon, Mexico, 3 Facultad de Ingeniera, UADY, Merida, Yucatan, Mexico
Show Abstract9:00 PM - P5.18
Effects of Growth Parameters on Surface-morphological, Structural and Electrical Properties of Mo Films by RF Magnetron Sputtering.
Wei-Ting Lin 1 , Shung-Cheng Hu 2 , Yong-Tian Lu 2 , Shou-Yi Kuo 1 , Ming-Jer Jeng 1 , Liann-Be Chang 1
1 Department of Electronic Engineering, Chang Gung University, Tao-Yuan Taiwan, 2 Chemical Systems Research Division, Chung-Sung institute of Science & Technology, Tao-Yuao Taiwan
Show AbstractThis work reports on the fabrication and characterization of Mo thin films on soda-lime glass substrate grown by reactive RF magnetron sputtering. Film thickness was measured by α-step surface profiler. The structural property and surface morphology were analyzed by x-ray diffraction (XRD), atomic force microscope (AFM) and scanning electron microscopy (SEM). Electrical properties were measured by four-point probe. It was found that the growth parameters, such as argon flow rate, RF power, film thickness, have significant influences on properties of Mo films. As the argon flow rate was increased, the resistivity and adhesion of Mo films increase as well. Higher RF power leads to a decrease on resistivity, and the optimal thickness and resistivity are 1 μm and 0.2 Ω/sq. Furthermore, we have also investigated the electrical, mechanical and the diffuse of Na ion properties of double Mo films sputtered onto soda-lime glass. The mechanisms therein will be discussed in detail. Our experiment results could lead to better understanding for improving further CIGS-based photovoltaic devices.
9:00 PM - P5.19
Efficient Organic Photovoltaic Cell with Nano-domain Control of the Interpenetrating Network Morphology Through Post Thermal Treatment.
Namkyoon Kim 1 , Minseok Kim 1 , Cheol Eui Lee 1
1 Department of Physics and Institute for Nano Science, Korea University, Seoul Korea (the Republic of)
Show AbstractRegioregular poly(3-hexylthiophene) (rr-P3HT) is a promising candidate for polymer photovoltaic research due to its stability and absorption in the red region. We obtained more efficient bulk heterojunction organic photovoltaic cells, using blend films of regioregular poly(3-hexylthiophene)(P3HT) and 1-(3-methoxycarbonyl)-propyl-l-phenyl-[6,6]-C61-butyric acid methyl ester(PCBM) that are subjected to a post thermal treatment process. Blend films (P3HT:PCBM = 1:0.8 weight-ratio) were prepared using chloroform. Through a thermal annealing at temperatures approaching the glass transition temperature, we examined the formation of nanodomains within the matrix. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron-collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the organic photovoltaic cells.
9:00 PM - P5.2
A Photovoltaic Textile Material Based on SiC Microwires and Poly(3-alkylthiophenes).
Bettina Friedel 1 , Marc Zoeller 2 , Siegmund Greulich-Weber 2
1 Physics, University of Cambridge, Cambridge United Kingdom, 2 Physics, University of Paderborn, Paderborn Germany
Show Abstract9:00 PM - P5.20
CdTe Films on Mo/Glass Substrates: Preparation and Properties.
Vello Valdna 1 , Maarja Grossberg 1 , Jaan Hiie 1 , Urve Kallavus 2 , Valdek Mikli 2 , Rainer Traksmaa 2 , Mart Viljus 2
1 Department of Materials Science, Tallinn University of Technology, Tallinn Estonia, 2 Centre for Materials Research, Tallinn University of Technology, Tallinn Estonia
Show Abstract9:00 PM - P5.21
Synthesis and Optical Characterization of CuInGaSe2 Nanoparticles by Pulsed Laser Ablation.
A Reum Jeong 1 , William Jo 1
1 Physics Department, Ewha Womans University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - P5.22
Understanding and Controlling Photovoltaic Effects in Ferroelectric Oxide Thin Films.
Thomas Conry 1 2 , L. Martin 1 2 , S. Basu 1 2 , V. Bal 2 , S. Byrnes 1 , J. Ager 1 , R. Ramesh 1 2
1 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Materials Science and Engineering, The University of California, Berkeley, Berkeley, California, United States
Show Abstract9:00 PM - P5.23
Surface Modification of As-prepared Multi-walled Carbon Nanotubes and Their Applications in Counter Electrodes for Dye-sensitized Solar Cells.
Hee Jung Choi 1 , Gi Won Lee 2 , Nam Kyu Park 2 , Kyung Kon Kim 2 , Sung Chul Hong 1
1 Department of Nano Science and Technology, Sejong University, Seoul Korea (the Republic of), 2 Materials Science and Technology Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractWell-defined block copolymers having controlled number of functional groups can be used as surface modifiers for nano-objects. In this presentation, well-defined polymeric surface modifiers for as-prepared multi-walled carbon nanotubes (MWCNT) were prepared through nitroxide mediated polymerization (NMP) technique. Poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene)s with controlled molecular weights and molecular weight distributions were synthesized, followed by functionalization with pyrene through imidization reactions. Pyrene groups were introduced to the block copolymers for non-destructive modification of MWCNT. P-acetoxystyrene repeating units were then converted to p-hydroxystyrene repeating units through hydrolysis reactions to increase the solubility of MWCNT in alcohol based pastes. The fabrications of counter electrodes for dye–sensitized solar cells (DSSC) were then attempted using the pastes containing modified MWCNTs, and their properties were investigated. Improved morphological homogeneity was observed in the presence of the surface modifiers, which resulted in improved electrical properties and solar cell efficiencies.
9:00 PM - P5.24
Method Of Fast Hydrogen Passivation To Solar Cell Made Of Crystalline Silicon
Wen-Ching Sun 1 , Chih-Wei Wang 2 , Jia-De Lin 2 , Chwung-Shan Kou 2 , Jian-You Lin 3 , Sheng-Wei Chen 3 , Jenn-Chang Hwang 3 , Jon-Yiew Gan 3 , Jian-Hong Lin 1 , Wei-Lun Chang 1
1 Photovoltaics Technology Center, Industrial Technology Research Institute, Hsinchu Taiwan, 2 Department of Physics, National Tsing Hua University, Hsinchu Taiwan, 3 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Show Abstract9:00 PM - P5.4
Rational Synthesis of Earth Abundant Materials for High Efficiency Heterojunction Thin Films Solar Cells.
Yun Seog Lee 1 2 , Mariana Bertoni 1 2 , Maria Chan 3 , Gerbrand Ceder 3 , Tonio Buonassisi 1 2
1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractPhotovoltaics is perhaps the most promising renewable energy source that can meet the global energy demand while keeping low concentrations of CO2 in the atmosphere. To become a major energy source, module costs should be reduced significantly and energy conversion efficiencies should be increased. In terms of reducing module cost, thin film solar cells are viable option because of lower material usage and the potential of very inexpensive manufacturing. However, current materials for thin film solar cells such as CdTe and CIGS suffer from concerns over resource scarcity (e.g., Tellurium and Indium) and toxicity (e.g., Cadmium) respectively and are therefore limited to sub-terawatts deployment. In this study, we analyze material abundance requirement for thin film solar cells that can meet tens of terawatts level deployment potential. Also, we study the requirements of electrical and mechanical material properties from modeling a tandem structure for high-efficiency energy conversion. To meet these goals, we derived a list of low-cost elements based on their abundance in Earth’s crust and annual production capacity. From these elements, over 500 hundred semiconductor compounds were found from a combinatorial search. We are performing large-scale band gap calculations to complement our literature reviews, which have reduced the list to tens of candidate semiconductor compounds. As the first selection for thin film solar cells materials, cuprous oxide (Cu2O) was investigated. We deposited a thin film with a DC magnetron sputtering system and characterized its material properties. The most challenging issue of this material is to improve electrical conductivity without sacrificing optical properties. We tried to tune the electrical properties with two ways: (1) Controlling grain structure and (2) adding impurities that enhance electron transport. We improved the grain structure by controlling reactive sputtering parameters as well as increasing the substrate temperature up to 850°C. Additionally, the effects of introducing various impurities to enhance electrical conductivity were investigated both theoretically and experimentally.
9:00 PM - P5.5
ZnO Patterning using the Defined Polymer Template by Nanoimprint Lithography.
Mi-Hee Jung 1 , Hyoyoung Lee* 1
1 National Creative Research Initiative Center for Smart Molecule Memory, , Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractIn the present communication, step and flash nanoimprint lithography was used for ZnO patterning on the substrate. ZnO has been widely investigated in applications such as ultraviolet nanolaser sources, gas sensors, solar cells, and filed emission display devices because it has a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV. Most of these applications require a high degree of precision alignment, position, and size of the rods. So we used the polymer mask which was made by nanoimprint lithography to pattern the ZnO. One of the very important advantages of our procedure was entirely integrated with the nanoimprint lithography. In our first approach for fabricating nanopatterned ZnO, the polymer islands act as masks preventing access to the underlying substrate for adsorbing or etching species. The polymer mask is then subjected to a brief oxygen plasma step (50 W, 30 mtorr, 22 s) to remove the intermediate layer between the bumps and expose the silicon oxide surface. The power and duration of the plasma exposure chosen such that it resulted in removal of approximately 20 nm of the polymer only. This is sufficient to remove the continuous thin intermediate layer of polymer and formation of nanometer-scale polymer masks with an average height of 50 nm and periodicity same as that of the original polymer mask. The subsequent exposure of this surface to octadecyltrichlorosilane vapors in selective silanization on the silicon oxide exposed region and masking areas under the polymer mask. The polymer masks were lifted off by acetone immersion. For ZnO patterning on the substrate, a substrate was coated with a droplet of 0.01 M zinc acetate dehydrate [Zn(CH3COO)2 2H2O] aqueous solution with the spin coating method at 3000 rpm for 30 s. Then, the substrate coated with a film of zinc acetate crystallites is heated to 350 oC in air atmosphere for 30 min to yield layers of ZnO on the substrate surface. The patterning of ZnO nano array was successfully conducted on Si wafers. Although we have shown ZnO pattering process on the Si wafer substrate, we expected that this pattering process can be easily applied to synthesize ZnO nanorod array on other substrate, such as transparent glass, and flexible polymer substrate, in an aqueous solution under ambient conditions.
9:00 PM - P5.6
Electrodeposition of Cu2ZnSnS4 Thin Films Using Ionic Liquids.
Chung Pui Chan 1 , Hong Lam 1 , Charles Surya 1
1 Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong China
Show AbstractCopper-Zinc-Tin-Sulfide, Cu2ZnSnS4, (CZTS) is a p-type semiconductor, which has great potential to become a new generation of solar cell materials. This is due to the high absorption coefficient of the material, the abundance and the non-toxicity of the constituent elements. This makes large-scale deployment of the device feasible. Compared to the well known photovoltaic materials such as Chalcopyrite (CIGS) and Cadmium Telluride (CdTe), CZTS received little attention in the literature. In the present study, we report on the electrodeposition of CZTS thin films from metal salts using non-aqueous choline chloride based ionic liquids. The ionic liquids were prepared by mixing a ratio of one mole of choline chloride (C5H14ONCl, from Sigma-Aldrich) to two mole of urea ((NH2)2CO, from Aldrich) or two mole of ethylene glycol (C2H6O2, from Riedel-de Haën). The mixtures were obtained by stirring the two chemicals together at 80°C until a colourless, homogenous liquid formed. Cyclic voltammetry was conducted to determine the reduction potentials of the metal salts using an Autolab PGSTAT 302N potentiostat controlled with GPES programme. A three-electrode system was used with a silver/silver chloride as the reference electrode and a platinum foil as a counter electrode. The substrates were cleaned in acetone using an ultrasonic bath and rinsed thoroughly with de-ionized water. The anhydrous chloride salts of CuCl2, SnCl2 and ZnCl2 are dissolved in ionic liquid to provide the ions of interest and the films are deposited at constant potential mode. Typical deposited film thicknesses are in the order of 1 μm. A post-deposition thermal annealing process was performed in sulfur vapor with objective to sulfurizating the films and improving the crystallinity. The annealing process is carried out in a quartz furnace tube at 450°C for 2-3 hours in sulfur vapor using Argon as the carrier gas. The technique does not involve the use toxic gas such as H2S. Absorption measurements show that the material offers promising photovoltaic properties and bandgap energy of the films is around 1.5eV, which is close to the theoretical optimum value for a single-junction solar cell. The microstructure and morphology of the sulfurized CZTS material are investigated by X-ray diffraction and scanning electron microscopy. Optical and electrical characterizations were performed to obtain the absorption coefficient, conductivity, Hall mobility etc. The success of this study would open up a low-cost and environmentally friendly technique to prepare large area CZTS films.
9:00 PM - P5.7
Characteristics of Flexible Indium Tin Oxide Electrode Grown by Continuous Roll-to-roll Sputtering Process for Flexible Organic Solar Cells.
Han-Ki Kim 1 , Kwang-Hyuk Choi 1 , Jin-A Jeong 1 , Jan-Wook Kang 2 , Dong-Yu Kim 3
1 Information and Nano Materials Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk, Korea (the Republic of), 2 , Korea Institute of Materials and Science, Changwon Korea (the Republic of), 3 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)
Show Abstract9:00 PM - P5.8
Thickness Dependent Effects of Thermal Annealing and Solvent Vapor Treatment of Poly (3-hexylthiophene) and Fullerene Bulk Heterojunction Photovoltaics.
Lynn Rice 1 , Zhouying Zhao 1 , Harry Efstathiadis 1 , Pradeep Haldar 1
1 , College of Nanoscale Science and Engineering, University at Albany, State University of New York , Albany, New York, United States
Show AbstractRoom temperature solvent vapor treatment followed by thermal annealing was carried out on bulk heterojunction (BHJ) photovoltaic devices based on blends of poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) of varied active layer thickness. The morphological and photovoltaic performance characteristics of the cells subject to these treatments were found to be dependent on active layer thickness. The devices were characterized using, atomic force microscopy (AFM), ultraviolet visible absorption (UV-Vis) spectrometry and opto-electrical and external quantum efficiency measurements in order to analyze the mechanism behind the observed trend. Performance indicators including fill factor, short circuit current and power conversion efficiency were tabulated and correlated to the ordering of device active layers. The maximum power conversion efficiency achieved was 4.1 %.
9:00 PM - P5.9
Annealing Effects on Crystallinity and Hall Measurements onExcimer Laser Ablated CuIn1-xGaxSe2 Thin Films.
Yeon Hwa Jo 1 , Bhaskar Mohanty 1 , Deuk Ho Yeon 1 , Ik Jin Choi 1 , Yong Soo Cho 1
1 Materials science and engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractCuIn1-xGaxSe2 (CIGS) is the leading material for fabrication of solar cells because of its demonstrated highest efficiency and flexibility in process parameters for fabrication. Many techniques including two/three stage elemental evaporation, co-sputtering, electrodeposition, have been used to prepare CIGS thin films. Remarkably, reports on CIGS thin films grown by PLD are very scarce in spite of the superiority of the technique in reproduction of stoichiometry of many-element-compounds. In the present study we have grown these films using excimer laser (KrF; wavelength of 248 nm; pulse width 20 nm; repetition rate of 5 Hz) ablation from a CuGa0.3In0.7Se2 target. Numerous growth conditions such as laser energy (20-40 mJ on target surface), target to substrate distance (20-60 mm) and deposition time (15-60 min) were varied during the growth process and their influences are presented. The as-deposited films were amorphous, highly resistive (~105 Ωcm) and exhibited p-type conduction independent of the variations in the growth parameters. The as-deposited films were subsequently given heat treatment via rapid thermal annealing at 200-500 oC for 5-60 s in N2 atmosphere. X-ray diffraction patterns of the films greatly changed during annealing with evolution of peaks indicating crystalline nature of the films. The annealing effects on microstructure, resistivity, hole concentration and mobility of the films has also been presented.
Symposium Organizers
Bhushan Sopori National Renewable Energy Laboratory
Jeff Yang United Solar Ovonic LLC
Thomas Surek Motech Americas
Bernhard Dimmler Wurth Solar GmbH & Co. KG
P6/F7: Joint Session: Solution Processed Photovoltaic Materials
Session Chairs
Thursday AM, December 04, 2008
Room 208 (Hynes)
9:30 AM - **P6.1/F7.1
FASST® Reactive Transfer Printing for Morphology and Structural Control of Liquid Precursor Based Inorganic Reactants.
Billy Stanbery 1 , M. Taylor 1 , M. van Hest 2 , J. Nekuda 2 , A. Miedaner 2 , C. Curtis 2 , J. Leisch 2 , P. Hersh 1 , D. Ginley 2 , R. Oswald 1 , L. Eldada 1
1 , HelioVolt Corporation, Austin, Texas, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractSoluble inorganic precursors to binary Cu-Se, In-Se, and Ga-Se materials have been developed and processed by rapid thermal processing to form solid multinary reactant films for subsequent Copper Indium Gallium Selenide (CIGS) synthesis using the reactive transfer printing method designated by the acronym FASST (Field-Assisted Simultaneous Synthesis and Transfer). The FASST method is a two-stage technique which separates the deposition of two precursors in the first stage from their reactive transformation into the final material layer in the second stage. This separate deposition of the precursors, one onto the final product film’s substrate and the other onto a reusable printing plate, allows independent optimization of their corresponding deposition methods whilst eliminating their pre-reaction. This flexibility has proven immensely valuable as will be demonstrated by comparing the results of depositing these two reactant films by various combinations of low-cost solution-based and conventional vacuum-based physical vapor deposition techniques. High-performance CIGS is characterized by relatively large grain sizes and the formation within individual grains of a nanoscale interpenetrating network of copper-rich and copper-deficient domains which form a percolation network for electrons and holes respectively. Conventional high temperature co-evaporation methods have been used to synthesize all of the world record thin film CIGS devices for more than two decades, and the multi-step deposition sequences developed to achieve this performance always involve the topotactic transformation of a large-grain precursor into CIGS rather than the direct synthesis of CIGS from condensation of elemental vapors as in molecular beam deposition. This same process characteristic is enabled by the two-stage FASST method’s separation of solid multinary reactant film deposition onto two different surfaces in its first stage. This will be demonstrated by comparing the unprecedented large-grain structure of FASST-synthesized CIGS with the results of its direct synthesis from the same soluble inorganic precursors by rapid thermal processing. This unique combination of low-cost solution-based precursor deposition and FASST reactive transfer printing methods provides reduced capital costs compared to vacuum deposition methods, low thermal budget, high throughput, control of CIGS crystallographic orientation, and very high device quality CIGS.
10:00 AM - P6.2/F7.2
Use of Direct Write Methods for Low Cost Photovoltaics.
Maikel van Hest 1 , Jennifer Nekuda 2 , Jennifer Leisch 1 , Peter Hersh 1 , Alex Miedaner 1 , Calvin Curtis 1 , Ken Steirer 2 , Ryan O'Hayre 2 , Reuben Collins 2 , David Ginley 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Show Abstract10:15 AM - P6.3/F7.3
CuIn(Se,S)2 Absorbers Processed using a Hydrazine-Based Solution Approach.
Wei Liu 1 , David Mitzi 1 , Min Yuan 1 , Andrew Kellock 2 , S. Jay Chey 1
1 , IBM TJ Watson Research Center, Yorktown Heights, New York, United States, 2 , IBM Almaden Research Center, San Jose, California, United States
Show AbstractWith tunable bandgap and demonstrated high efficiency, the chalcopyrite CuInSe2 and its alloys have shown great potential as absorbers for single and multi-junction solar cells. However, the current deposition techniques mostly rely on expensive vacuum-based processing or involve complicated precursor solution preparation. These high-cost absorber preparation processes make it difficult to commercialize this technology. In this work, CuInSe2-xSx (CIS) absorbers are deposited using a simple hydrazine-based solution process. Precursor solutions were prepared by dissolving the component metal chalcogenides and chalcogen in hydrazine, forming homogeneous solutions containing adjustable concentrations of desired elements mixed on a molecular level. These precursor solutions are then spin coated on substrates followed by a heat treatment in an inert environment to produce high quality CIS thin films. Significantly, no post deposition selenization process is required using this technique. Laboratory scale devices with conventional glass/Mo/CIS/CdS/i-ZnO/ITO structure have been fabricated using CIS absorbers deposited via this process. For the baseline low-bandgap CIS system with no Ga added (to compare with our previously reported results with Ga incorporated), AM1.5 conversion efficiency of as high as ~8% has been achieved for devices with 0.45cm2 effective area.
10:30 AM - **P6.4/F7.4
All-chemically Deposited Thin Film Solar Cells.
P. Karunakaran Nair 1 , Harumi Moreno 1 , Sarah Messina 1 , David Avellaneda 1 , Oscar Gomezdaza 1 , M. T. Santhamma Nair 1
1 Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco, Morelos, Mexico
Show Abstract11:30 AM - **P6.5/F7.5
Solution Routes to Synthesis of Cu(In,Ga)(S,Se)2 Chalcopyrite Solar Cells.
Jean-Francois Guillemoles 1
1 IRDEP, CNRS, Chatou France
Show AbstractLarge scale developement of photovoltaics requires large area coating of semiconductors, something solution based processes can provide. From Cu2S to CdTe or Cu(In,Ga)(S,Se)2, not to mention TiO2 in dye cells, exemples are numerous of efficient (>10%) photovoltaic devices whose semiconductors have been synthesized in aqueous solutions.This presentation will focus on growth of chalcogenides, and more specifically CuInSe2 (CIS), for which (one step)electrodeposition is a method of choice. CIS, and related compounds, are somewhat more complex to prepare than other chalcogenides due to a richer chemistry, as evidenced by the number of phases that can be formed. As grown semiconductors, when prepared at low temperatures are generally unfit for photovoltaic applications: photovoltaic application is very demanding because it is specially sensitive to material defects, already in the ppm range. On the pathway to truly functionnal materials, additionnal steps are therefore required. These involve generally annealings but also surface/interface treatments.The presentation will discuss these different aspects through the exemple of the realisation of electrodeposited CIS solar cells with efficiencies above 10%.
12:00 PM - **P6.6/F7.6
Chalcogenide Solar Cells by Printing and Solution (non-electrodeposition) Based Methods.
Ayodhya Tiwari 1
1 Thin Film Physics Group, ETH , Zurich Switzerland
Show Abstract12:30 PM - P6.7/F7.7
Fabrication and Performance of Highly Textured Electrodeposited ZnO Back Reflector for nc-Si Solar Cells.
Dinesh Attygalle 1 , Qi Hua Fan 1 , Shibin Zhang 1 , William Ingler 1 , Xunming Deng 1
1 Department of Physics and Astronomy, The University of Toledo, Toledo, Ohio, United States
Show AbstractTextured back reflector (BR) is an essential component used in substrate type solar cells for light trapping, which enhances the long wavelength absorption. Most commonly used BR consists of a reflecting metal layer(s) of Ag and/or Al and a transparent conducting oxide (TCO) layer such as ZnO. This type of BR, if properly textured, can lead to about 20% increase in the short-circuit current and cell efficiency. A widely used technique for producing the BR is sputtering due to its simplicity and easy operation for large area thin film solar cell applications. The TCO layer needs to be thick enough (>500 nm) to reach a textured structure and to prevent the metal in the BR from diffusing into the solar cell layers. Thus, the ZnO deposition becomes the bottleneck in the BR process. Significant efforts have been putting on developing novel techniques that can produce ZnO coatings with better texture and high deposition rate. In this paper, we report a novel electrodeposition procedure to fabricate uniformly textured ZnO films at high deposition rate. We show that ZnO films prepared by this novel technique have much larger surface roughness than sputter deposited films, as evidenced by AFM images and optical reflectance measurements. Thin film nanocrystalline silicon (nc-Si) solar cells are fabricated on the electrodeposited BR and sputtering deposited BR. I-V characteristics and quantum efficiency of these solar cells are compared, which show that the solar cells based on the electrodeposited ZnO BR have 2 mA/cm2 higher current density. The improvement is observed in long wavelength region as well as short wavelength region of the spectrum. Possible mechanisms accounting for this improvement is discussed.
12:45 PM - P6.8/F7.8
Zinc Oxide/Cadmium Sulfide Nanocomposites for Solar Energy Conversion.
Erik Spoerke 1 , Matthew T. Lloyd Lloyd 2 , Erica Martin 1 , David Scrymgeour 2 , Dana Olson 2 4 , Paul Clem 3 , Julia Hsu 2
1 Electronic and Nanostructured Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Microsystem Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractContinued development of designer inorganic materials for modern electronics applications has produced an exciting array of materials with engineered properties such as band gaps, electrical conductivities, optical properties and morphology. While many of these materials are of interest in their own right, combining these materials into composites can produce new, functional behavior and enhanced optoelectronic performance. In the present work, we focus on a specific example of this idea, employing a simple, solution-phase process to create nanostructured composites of cadmium sulfide (CdS) and zinc oxide (ZnO). This process relies on the careful selection of reaction precursors to produce controlled, selective growth of CdS on ZnO. The selectivity and solution-phase nature of the growth process allow us to apply this process to a diverse set of ZnO nanostructured films ranging from planar films to complex extended films of branched ZnO architectures and patterned nanorods. While CdS and ZnO are wide band gap semiconductors commonly found in photovoltaic systems such as CIGS solar cells, our simple, solution phase growth process has allowed us to incorporate this composite material into less conventional hybrid organic/inorganic solar cells with promising results. In particular, we have observed improvements in critical metrics such as open circuit voltage, fill factor, and overall device efficiency when compared to control devices employing ZnO alone. Finally, the chemical flexibility of ZnO and CdS and the solution-phase growth enable us to explore further tailoring of this functional system by doping, alloying, or capping each component to modify overall composite properties. This exciting composite system represents a promising application of simple, solution-based chemistry to produce functional materials for the next generation of optoelectronic applications. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.
P7: Thin Film Materials, Thin Film Solar Cells, Processing
Session Chairs
Mowafak Al-Jassim
Qi Wang
Thursday PM, December 04, 2008
Independence E (Sheraton)
2:30 PM - **P7.1
Improvement of the Interface Quality Between Zn(O,S,OH)x Buffer and Cu(InGa)(SeS)2 Surface Layers to Enhance the Fill Factor over 0.700.
Katsumi Kushiya 1 , Yoshiaki Tanaka 1 , Hideki Hakuma 1 , Yuri Goushi 1 , Shunsuke Kijima 1 , Tetsuya Aramoto 1 , Yousuke Fujiwara 1 , Ayako Tanaka 1 , Yoshiyuki Chiba 1 , Hiroki Sugimoto 1 , Yuka Kawaguchi 1 , Kazuki Kakegawa 1
1 New Business Development Div., Showa Shell Sekiyu K.K., Atsugi, Kanagawa, Japan
Show Abstract3:00 PM - P7.2
MOCVD Growth of High Hole Concentration (〉2x1019 cm-3) P-Type InGaN.
Hongbo Yu 1 , Andrew Melton 1 , Omkar Jani 1 , Balu Jampana 2 , Shen-Jie Wang 1 , Shalini Gupta 1 , John Buchanan 1 , William Fenwick 1 , Ian Ferguson 1
1 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States
Show AbstractInGaN alloys can cover a wide bandgap range from 0.70 eV up to 3.4 eV which is ideal for high efficiency solar cell applications. P-type activity in Mg-doped InGaN is a significant step toward achieving a solar cell using this material. Although there exist many reports on the structural and optical properties of InGaN films, there are only a few studies on the p-type electrical properties [1, 2]. In this paper, we report on the MOCVD growth and electrical studies of p-type InGaN layers with indium molar fractions of approximately 20% for solar cell applications. Samples used in this study were grown on c-plane (0001) sapphire substrates by low pressure MOCVD. First, 2 μm thick undoped GaN film was grown at 1050°C on a 25 nm GaN nucleation layer, which was deposited at 500°C. Next, a 100 nm thick Mg-doped InGaN layer was grown on the undoped GaN film at 750°C using N2 as a carrier gas. X-ray diffraction (XRD) was carried out to determine the indium mole fraction in the InGaN layer. After growth, the samples were activated using rapid thermal annealing (RTA) in N2 ambient. The electrical properties of p-type InGaN were studied by Hall measurement (standard Van Der Pauw). After thermal annealing, p-type conduction was verified by Hall measurement. The influence of growth conditions on p-type conductivity was investigated. It was found that the electrical characteristics of Mg-type InGaN layers depend strongly on the V/III ratio. As the V/III ratio increased from 2.36x105 to 4.72x105, the Mg-doped InGaN layer changed from n-type conduction to p-type conduction with hole concentration of 2.15x1019 cm-3. A relatively high V/III ratio was needed to improve the p-type conductivity of the Mg-doped InGaN layer, possibly due to the suppression of nitrogen vacancy formation under high V/III ratio. It was also found that the Mg incorporation has to exceed the background of undoped InGaN. To achieve p-type conduction, Cp2Mg/(TEGa+TMIn) ratio must be higher than 7.5x10-3. The effect of annealing temperature on the conductivity of p-type InGaN was also studied. The temperature was varied from 500 to 750 °C. The optimum thermal activation process for Mg-doped InGaN was 550°C for 15 minutes. The resistivity of p-type InGaN increased with increasing annealing temperature when the temperature was higher than 550°C. The optimized annealing temperature was lower than that of Mg-doped GaN (750 °C for 15 min). After MOCVD growth and thermal annealing optimization, p-InGaN with hole concentration greater than 2x1019 cm-3 at room temperature was achieved. The lowest resistivity of p-type InGaN film was measured to be 0.1 Ω-cm, which is about 10 times lower than that of normal p-type GaN film. The performance of InGaN solar cell using this p-type InGaN will also be demonstrated in this paper.
3:15 PM - P7.3
Optical Optimization of a Thin-film Wide-bandgap CuGaSe2 Solar Cell for Tandem Applications.
Martina Schmid 1 , Janez Krc 3 , Reiner Klenk 2 , Martha Ch. Lux-Steiner 1 2 , Marko Topic 3
1 SE2, Freie Universität Berlin, c/o Helmholtz-Zentrum Berlin, Berlin Germany, 3 Faculty of Electrical Engineering, University of Ljubljana, Ljubljana Slovenia, 2 SE2, Helmholtz-Zentrum Berlin, Berlin Germany
Show Abstract3:30 PM - P7.4
The Effect of Interfaces on Charge Transport in Cu(In,Ga)Se2 Thin Film Solar Cells.
Gustaf Ostberg 1 , Krister Svensson 2 , Eva Olsson 1
1 Microscopy and microanalysis, Applied physics, Goteborg Sweden, 2 , Physics and electrical engineering, Karlstad Sweden
Show AbstractThe multilayer structure and multicrystallinity of Cu(In,Ga)Se2 (CIGS) thin film solar cells inevitably lead to a number of interfaces within the device. Interfaces, such as grain boundaries, directed along the charge transport path are not necessarily detrimental to the device performance. Indeed, it has been shown, theoretically as well as experimentally, that a doping type inversion can occur at the CIGS grain boundaries such that holes are repelled and recombination is suppressed [1-3]. However, interfaces between different layers intercept the path of charge transport and may contain dangling bonds and other defects which can act as recombination centers. The charge transport is thereby hampered and the device performance is impaired.In this study, effects of interfaces on charge carrier transport between different layers in a CIGS thin film solar cell were investigated. This was done by correlating interfacial atomic structure to electrical properties.The cells were prepared by depositing a 1.5 µm CIGS absorbing layer mainly by sputtering on a SLG substrate covered with a 400 nm layer of Mo, used as back contact. On top of the CIGS, a 50 nm CdS buffer layer was grown by chemical bath deposition (CBD) and a 400 nm ZnO layer followed by a transparent top contact of indium tin oxide (ITO) were deposited by sputtering. Conductivity measurements were made in situ with a scanning tunneling microscope (STM) operated inside a scanning electron microscope (SEM). The measurements were performed along separate layers and across the layer interfaces of interest by sweeping an STM probe along cross-sectional samples of the solar cell. The atomic structure and chemistry of the interfaces were examined by transmission electron microscopy (TEM), high resolution TEM (HRTEM), electron energy loss spectroscopy (EELS) and nanoprobe energy dispersive x-ray spectroscopy (EDX). Cross-sectional thin foil TEM samples were prepared by in situ lift-out in an FEI FIB-SEM workstation and were attached to a supporting grid. Specimens for the in situ STM measurements were produced in the same way, but in order to control the path of charge transport, slits were cut in the foil. The cuts were made in such a way that the charge transport was directed along the film or across the interface of interest. In order to remove surface oxides and to reduce the surface damage and Ga implantation from the FIB-SEM preparation, the foil was ion polished for 60 seconds in a Gatan PIPS system using ±10° beam angle at 2.0 keV. References1.Persson, C. and A. Zunger. Physical Review Letters, 2003. 91(26): p. 266401(1-4).2.Hetzer, M.J., Y.M. Strzhemechny, M. Gao, M.A. Contreras, A. Zunger, and L.J. Brillson. Applied Physics Letters, 2005. 86(16): p. 162105(1-3).3.Romero, M.J., C.-S. Jiang, R. Noufi, and M.M. Al-Jassim. Applied Physics Letters, 2005. 87(17): p. 172106(1-3).
3:45 PM - P7.5
Growth and Characterization of InxGa1-xN (0.25 ≤ x ≤ 0.63) Alloys.
Bed Pantha 1 , Jingyu Lin 2 , Hongxing Jiang 2
1 Physics, Kansas State University, Manhattan, Kansas, United States, 2 Nano Tech Center and ECE, Texas Tech University , Lubbock, Texas, United States
Show Abstract InGaN ternary alloy is of great interest because of its ability to tune the direct band gap from the near infrared region ~ 0.7 eV (InN) to the near UV region ~3.4 eV (GaN). In particular, high quality In-rich InGaN alloys offer potential applications in many important areas: (1) high efficiency and radiation hard multijunction solar cells, (2) high efficiency photoelectronchemical (PEC) cells, and (3) high brightness III-nitride green and yellow light emitting diodes (LEDs) and laser diodes (LDs). Moreover, our recent study shows that high In-content InGaN alloy is potentially important thermoelectric (TE) material for power generation and/or solid-state cooling. This finding has further extended the application of ternary InGaN alloy into areas different from the traditional photonics/electronics field. However, almost all previous experimental studies have shown that growth of high quality In-rich InGaN alloys is extremely challenging due to the solid phase miscibility gap between InN and GaN. Here we present the growth and characterization of single phase InxGa1-xN alloys with x ranging from 0.25 to 0.63 by metal organic chemical vapor deposition (MOCVD) on AlN/Al2O3 and/or GaN/Al2O3 templates which cover previously thought miscibility gap region. X-ray diffraction (XRD), Hall-effect measurements, and atomic force microscopy (AFM), and photoluminescence (PL) were employed to characterize the grown films. Single peak of XRD theta-2theta scans of (002) InGaN alloys confirms that there is no phase separation. Hall-effect measurements show that both mobility and electron concentration increase as In-content increases. AFM characterization reveals that the surface roughness varies between 1.5 and 4.0 nm and free from In droplets. PL studies of these InGaN alloys will be presented in the talk.
4:30 PM - P7.6
Cu-doped Nanostructured CdS Thin Films.
Pathiyamattom Sebastian 1 3 , Luis Ixtlilco 2 , Joel Pantoja 3
1 Solar Materials, Energy Research Center-UNAM, Temixco, Morelos, Mexico, 3 CAES, UP Chiapas, Tuxtla Gutierrez, Chiapas, Mexico, 2 CIICAP, UAEM, Cuernavaca, Morelos, Mexico
Show Abstract5:00 PM - P7.8
Fixed Negative Charge in Nanoscale Al2O3 and its Role in the Surface Passivation of c-Si.
Bram Hoex 1 , Joost Gielis 1 , Richard van de Sanden 1 , Erwin Kessels 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractAl2O3 is a versatile high-K dielectric that has good interface or surface passivation properties which are vital for devices such as nanocrystal or wafer-based light emitting diodes, photodetectors, and high-efficiency solar cells. The level of passivation by a surface passivation layer is determined by both the bulk and interface electrical properties of the layer. In this contribution we will mainly focus on the fixed charge density in Al2O3 films and discuss its role on the level of passivation achieved for c-Si surfaces. Nanoscale Al2O3 films (6 - 30 nm) were synthesized by plasma-assisted atomic layer deposition and it was demonstrated these films yield an excellent level of surface passivation on various c-Si surfaces after a post-deposition anneal.[1,2] The presence of a high density of fixed negative charge (up to 1x1013 cm-2) in these Al2O3 films was confirmed by employing three different techniques. Carrier lifetime spectroscopy demonstrated unambiguously the important role of the fixed negative charge density on the level of surface passivation. Capacitance-voltage measurements and contactless optical second-harmonic generation experiments revealed that the majority of these fixed negative charges are formed during the post-deposition annealing step. In addition to the fixed negative charge, also the role of the reduction of the interface defect states by the post-deposition annealing and the formation of an interfacial oxide between the Al2O3 and c-Si will be addressed. Finally, it will be demonstrated that the excellent surface passivation by Al2O3 on c-Si has recently resulted in high-efficiency diffused emitter n-type c-Si solar cells fabricated at Fraunhofer ISE with an efficiency of over 23%.[3]
5:15 PM - P7.9
Ultrafast Laser Textured Silicon Solar Cells.
Barada Nayak 1 , Mool Gupta 1
1 Electrical & Computer Engineering, University of Virginia, Charlottesville, Virginia, United States
Show AbstractA novel ultrafast laser texturing method has been developed to produce arrays of nano/micro surface textures in silicon. Laser processing conditions have been optimized for achieving appropriate optical and electronic properties for photovoltaic applications. Textured silicon surfaces absorb greater than 99% of incident light over the entire solar spectrum and the material appears complete black to bare eye. Textured materials are characterized for surface morphology, optical properties, laser induced defects and carrier lifetime. Chemical etching and thermal annealing steps have been performed to remove any surface defects. Photovoltaic devices have been fabricated and their quantum efficiency and total efficiency (1 Sun illumination, 1.5 AM) have been measured. Photovoltaic device performance for control and textured surfaces has been compared.
5:30 PM - P7.10
Light Induced Passivation of Si by Iodine Ethanol Solution.
Bhushan Sopori 1 , Przemyslaw Rupnowski 1 , Jesse Appel 1 , Steve Johnston 1 , LaTecia Anderson-Jackson 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , North Carolina Agricultural and Technical State University, Greensboro, North Carolina, United States
Show AbstractMeasurement of the bulk minority-carrier lifetime (τb) by optical methods, such as photocurrent decay or QSSPC, is strongly influenced by surface recombination, and requires effective surface passivation to get the correct values. The two most common approaches to passivate Si wafer surfaces involve deposition of a thin silicon nitride (SiN:H) or immersion in iodine-ethanol (I-E) solution. The iodine-based passivation is popular because it is nondestructive, inexpensive, easy to apply, and is expected to produce better passivation than SiN:H deposited by PECVD. However, in practice, it is difficult to obtain τb-values reproducibly, particularly when the wafer lifetime is long. We have determined that this problem arises from two sources: (i) improper wafer cleaning, and (ii) instability of I-E solution when in contact with a Si wafer. Recently NREL proposed a new technique for wafer preparation before I-E passivation, which reproducibly yields the correct values of bulk lifetime as high as 1 ms. Our procedure consists of a sequential optical oxidation and chemical cleaning, which also removes about 200 Å from the wafer surfaces. After cleaning, a sample is placed in a polyethylene bag and covered with I-E solution (in the molarity range of 0.1). This cleaning procedure has also led to observation of light-induced passivation of I-E solution in some wafers. We have found out that an exposure of the wafer, immersed in I-E solution, to high intensity light (such as a solar simulator), for 5-10 minutes, can cause τb to increase by an order of magnitude. This light-induced passivation is manifested as an increase in lifetime with each measurement because of the exposure to the light from a flash lamp or a laser used as light source. This effect can be quite pronounced in long lifetime (> few hundred microseconds) wafers. The mechanism behind the passivation illumination effect has not been well understood yet. This paper will present new experimental results leading to the explanation of the light induced passivation. Our experiments are performed on single crystal CZ and FZ wafers of various resistivities, dopant types, and different thicknesses. To verify the reproducibility, both Sinton’s tester and the laser PCD machine developed at NREL are used to measure τb as a function of injection level. The lifetime measurements are made with and without passivating illumination. These experimental results suggest passivation resulting from formation of a photo-induced surface charge. We will discuss experimental results and present a detailed model of surface passivation.
5:45 PM - P7.11
Contact Firing of Screen-Printed Si Solar Cells for Maximum Cell Performance: Studies on Fundamental Mechanisms.
Bhushan Sopori 1 , Vishal Mehta 1 , Peter Rupnowski 1 , Helio Moutinho 1 , Aziz Shaikh 2 , Chandra Khadilkar 2 , Murray Bennett 3 , David Carlson 3
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Ferro Electronic Materials, Vista, California, United States, 3 , BP Solar, Frederick, Maryland, United States
Show AbstractIn commercial Si solar cell fabrication, contact formation is typically done through co-firing of screen-printed front (Ag-based) and back (Al-based) contacts. This process, also called fire-though contact metallization, performs several functions: (i) On the front contact, the glass frit dissolves (SiN:H) antireflection coating underneath and the solvent metal reacts with Ag to lower the eutectic point of Si-Ag alloy. Various Ag particles fuse to produce a laterally conducting bus, and formation of a Si-Ag alloy at the interface. To minimize shunting and series resistance, one must understand these reactions and control them; (ii) Diffuse hydrogen from the interface into the bulk of the cell for impurity and defect passivation; and (iii) Produce an interaction of Si and Al to form a deep back surface field (BSF). Formation of a BSF must also result in a very effective impurity gettering. The reactions involved in co-firing are quite complex and occur in a very short time ( <10 s at the peak temperature). Currently, the firing conditions that yield the best cell efficiency are determined empirically, which may not represent the highest achievable cell performance. Our studies are aimed at developing a coherent theory into the physics of various mechanisms involved in a fire-through process by establishing process conditions that individually maximize each of the functions. Once these data are obtained, the cell performance can be maximized by a judicious trade off between these functions. We will present experimental results aimed at examining process dependence of each of the above mentioned functions. This work was done on mc-Si solar cells that were screen-printed and fired under different processing conditions in a static optical furnace, and their solar cell parameters were measured. We determined other characteristics, such as sheet rho of the bus bar, and shunt resistance of the front contact. We studied correlations between firing profiles and: (a) cell parameters, (b) degree of H passivation, (c) depth of BSF, and (d) depth of Ag penetration into the n-region of the cell. We will also present a hypothesis for the formation of the front Si-Ag contact, invoking interactions of Ag with solvent-metal-doped glass frit through ion-exchange mechanism.