Henry Ji Transmill Technologies, Inc.
Venkatesen Mannivanan Colorado State University
Binxian Ren Hebei University of Technology
Loucas Tsakalakos General Electric
General Electric - Global Research
Transmill Technologies Inc
C1: Nanostructure-Based Photovoltaics & Multi-Component Thin Film PV Manufacturing I
Tuesday AM, April 26, 2011
Room 2000 (Moscone West)
9:00 AM - **C1.1
Waferless High Efficiency Photovoltaics Based on Flexible Si Wire Arrays.
Harry Atwater 1 Show Abstract
1 Applied Physics, California Institute of Technology, Pasadena, California, United States
Rapid progress in silicon wire array solar cells has enabled cells with high open circuit voltage (>600 mV) and high (>90%) quantum efficiency, in wire arrays grown by a metal-catalyzed vapor-liquid-solid growth process. Following growth on a crystalline (111) Si wafer, Si wire arrays are embedded in a polymethyldisiloxane (PDMS) film and can be peeled off the growth template substrate, yielding an unusual photovoltaic material: a flexible, bendable, wafer-thickness Si absorber. Following wire array peel off, the original growth template substrate can be reused for subsequent array growth without further lithography. In this paper, I will report progress on large-area (> 10cm x 10cm) peel-off of Si wire arrays and directions for high efficiency cell and module fabrication that can enable the < $1/W module manufacturing goal.
9:30 AM - C1.2
High-efficiency Ordered Silicon Nano-conical-frustum Array Solar Cells by Self-powered Parallel Electron Lithography.
Yuerui Lu 1 , Amit Lal 1 Show Abstract
1 , Cornell University, Ithaca, New York, United States
Nanostructured silicon thin film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top-down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance of ∼99% (upper bound) over wavelengths 400-1000 nm by a thickness of only 5 μm, is realized by our recently reported technique self-powered parallel electron lithography that has high-throughput and sub-35-nm high resolution. Moreover, high-efficiency (up to 10.8%) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays. These related fabrication techniques can also be transferred to low-cost substrate solar energy harvesting device applications. The fabrication of large-area ordered controllable Si nanostructure arrays needs top-down planar lithography with both high throughput and high resolution. Conventional optical lithography has high throughput, but its critical dimension (CD) is limited to a fraction of the wavelength. Traditional electron beam lithography (EBL) has the highest resolution <10 nm, but EBL suffers from high cost and low throughput due to the required electron beam raster scanning serial exposure. Nanoimprint lithography could be used to achieve nanostructured arrays, but the prospect of mask mechanical contact to substrate leads to a large number of defects and short mask life. Our recently reported technique self-powered parallel electron lithography (SPEL), using large-area planar radioactive beta electron thin film emitters to parallel expose e-beam resist through a stencil mask, demonstrated sub-35-nm resolution. Using naturally emitted high-energy beta particles, the SPEL system can be compact as the electron focusing column needed in EBL systems is no longer needed. Elimination of vacuum in SPEL will significantly simplify the overall lithography system and greatly reduce the cost, while enabling large area massively parallel high-throughput electron lithography with high resolution. Therefore, SPEL is a very promising way for large-area ordered nanostructure array fabrication, especially for solar cells applications.
9:45 AM - C1.3
Atomic Layer Deposition and Chemical Vapor Deposition of Copper Sulfide for Nanostructured Solar Cells.
Ian Carbone 1 3 , Glenn Alers 3 1 , Anna Bezryadina 3 1 , Frank Bridges 1 , Scott Medling 1 , Timothy Ohno 2 , Jonathan Kintnerr 2 Show Abstract
1 Physics, University of California, Santa Cruz, Santa Cruz, California, United States, 3 Advanced Studies Laboratories, NASA Ames Research Center, Moffett Field, California, United States, 2 Physics, Colorado School of Mines, Golden, Colorado, United States
Atomic layer deposition (ALD) is a gas-phase deposition process that can penetrate into pores less than 5nm in diameter , making it a promising tool for the fabrication of nanostructured heterojunctions in extremely thin absorber solar cells. ALD and chemical vapor deposition (CVD) of Cu-rich CuxS has been performed on planar ZnO and nano-porous TiO2 using a new precursor (KI5) and H2S. Copper sulfides occur in five stable crystal phases ranging from Cu-rich Cu2S to Cu-poor CuS. The semiconducting Cu2S phase is a promising solar cell material consisting entirely of non-toxic and earth abundant materials. Cross-sectional SEM images taken of nanoporous TiO2 films with and without ALD treatment show backfilling and uniform coverage at penetration depths of over 200nm. X-ray absorption fine structure (EXAFS) data indicates that film crystal structures are disordered and dominated by Cu-rich phases for films deposited in the temperature range 150-400C. X-ray photoelectron spectroscopy was used to isolate the composition at the surfaces of the ALD-deposited films. Results are consistent with the Cu2S crystal phase. Optical absorption was measured using photothermal deflection spectroscopy for a wide range of CVD-deposited film thicknesses. The initial film growth (<100nm) shows high absorption at low photon energies, a characteristic of metallic, Cu-poor CuxS. As thickness increases, distinguishable direct and indirect band gaps appear in the ranges 1.11-1.15eV (indirect) and 1.81-2.03eV (direct). These values are consistent with accepted Cu2S values . The sheet resistances of ALD and CVD-deposited CuxS films on planar ZnO do not scale linearly with thickness, indicating the presence of a Cu-poor material preferentially forming at the substrate/CuxS boundary. This Cu-poor region at the ZnO and TiO2 interface effectively shorts out the semiconducting Cu2S phase.  L. Reijnen, B. Feddes, A. M. Vredenberg, J. Schoonman, and A. Goossens, The Journal of Physical Chemistry B 108, 9133-9137 (2004).  O. Madelung, Semiconductors - Basic Data, 2nd ed. (Springer, 1996).
10:00 AM - C1.4
Resonating Mode Enhanced Optical Absorption in Si Hollow Nanospheres.
Yan Yao 1 , Jie Yao 1 , Zhichao Ruan 2 , Jia Zhu 2 , Ching-Mei Hsu 1 , Shanhui Fan 2 , Yi Cui 1 Show Abstract
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Department of Electrical Engineering, Stanford University, Stanford, California, United States
Amorphous silicon is widely used in thin film solar cells, however, a large portion of incident light is reflected back from flat surface of a-Si due to its high refractive index. Certain nanostructures have been demonstrated for broadband reflection suppression. Here we report on the design and fabrication of a-Si hollow nanospheres using a wafer-scale Langmuir-Blodgett assembly technique and chemical vapor deposition. We investigated light absorption and reflection properties of these a-Si nanostructures for solar cell applications. These Si hollow nanospheres display greatly enhanced absorption to the flat control film, particularly in the spectral range from 550 nm to 800 nm. Full-wave electromagnetic simulation of the absorption in the active a-Si:H layer agrees well with experimental results. We show strong absorption peak due to resonance mode in the shell of Si nanosphere.
10:15 AM - C1.5
Performance of Ultra-Thin Film Mo/CdTe/AZO Schottky Diode Photovoltaics in a Substrate Configuration.
Chris France 1 2 , Hector Romo 3 , Sue Carter 1 2 , Glenn Alers 2 1 Show Abstract
1 Physics Department, University of California-Santa Cruz, Santa Cruz, California, United States, 2 Advanced Studies Laboratories, NASA Ames Research Center, Moffett Field, California, United States, 3 Electrical Engineering Department, University of California-Santa Cruz, Santa Cruz, California, United States
We fabricate ultra-thin Cadmium Telluride (CdTe) Schottky diode photovoltaics in a substrate geometry. Devices were comprised of a 350nm thick CdTe microcrystalline layer deposited by spin-casting and sintering colloidal nanorods onto RF sputtered Molybdenum films on glass. A transparent conducting Aluminum doped Zinc Oxide (AZO) top contact was then RF sputtered on the CdTe. A typical device has a 0.2 mA/cm2 short-circut current density (Jsc), a 100mV open-circuit voltage (Voc) and a 25% fill factor under AM1.5G, 100mW/cm2, illumination. We compare these results to our best superstrate devices with 5% power conversion efficiency on pre-patterned ITO with a structure of ITO/CdTe/Al and a 22mA/cm2 Jsc, a 520mV Voc and a 43% fill factor. Our substrate device performance is limited by the high series resistance of our RF sputtered AZO films and heat damage to the CdTe during the RF sputtering process. To alleviate this problem moving to a low-temperature solution-deposited transparent electrode is required. This work highlights the possibility of successfully fabricating a CdTe Schottky diode solar cell on Molybdenum metal foils.
11:00 AM - **C1.6
Scaling CdTe PV from Pilot Production to High Volume Manufacturing.
Fred Seymour 1 Show Abstract
1 , PrimeStar Solar, Inc, Arvada, Colorado, United States
As PrimeStar Solar transitions from CdTe PV module pilot production to high volume manufacturing we are focused on minimizing production variability and costs while maximizing yield, performance and reliability. Steps being taken include a factory design with in-line metrology and characterization to enhance real time monitoring and process control feedback; distributed flow to optimize equipment capacity and facilitate statistical process control; and careful automation to minimize product handling defects. Our equipment design is a balance between minimizing capital cost, maximizing availability and throughput as well as preserving flexibility for anticipated future process enhancements. We are also tightening source material specifications to minimize product variability. A number of the opportunities and challenges with this transition are discussed.
11:30 AM - **C1.7
Ion Beam Texturing and Reactive Deposition for Photovoltaic Materials.
Bruce Clemens 1 , J. Groves 1 , Vardaan Chawla 1 , Joel Li 2 , Garrett Hayes 1 , Charles Teplin 3 Show Abstract
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
Successful deployment of photovoltaic power generation at the terawatt level will require synthesis techniques that can produce low-cost, large-area devices with performance that rivals single crystal materials. We report here on two physical vapor deposition approaches that have large-scale potential. The first approach utilizes ion beam assisted deposition (IBAD) to produce biaxial texture in thin films. The ion beam is incident on the substrate concurrent with deposition and at an inclined angle corresponding to a channeling direction in the growing film. This produces in-plane and out-of-plane alignment of the crystals in the film. We have developed a seeded epitaxy technique that uses a seed layer that is easily aligned with IBAD, upon which the semiconductor of interest can be grown. Here we report on Si films grown on IBAD deposited CaF2. We have used a variety of approaches for Si film growth including evaporation, sputtering and hot wire CVD. We will report on structural characterization as well as electronic properties. The second approach utilizes reactive sputter deposition to produce sulfide absorber layers. The material Cu2ZnSnS4 (CZTS) has many advantages for application in photovoltaic devices, including self-doping, favorable band-gap and no expensive or rare constituents. However, conventional growth approaches require an anneal to react with form the desired sulfide phase. Since the metal species are the fast diffusing species, formation of the sulfide is accompanied by incorporation of Kirkendal voids and defects. Our approach is to incorporate the sulfur in the growth process and directly form the desired phase. Here we report on material quality and device performance.
12:00 PM - C1.8
Synthesis of CuInSe2 Absorbers from Bilayer Compound Precursors.
Rangarajan Krishnan 1 , David Wood 1 , Vaibhav Chaudhari 1 , Andrew Payzant 2 , Rommel Noufi 3 , Timothy Anderson 1 Show Abstract
1 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
The chalcopyrite solid solution Cu(InxGa1-x)Se2 (CIGS) is a commercially emerging thin film absorber material based on the promise of low manufacturing cost and high conversion efficiency (champion cell now exceeds 20%). The primary challenge in achieving low processing cost is increasing the synthesis rate of CIGS at lower temperature. Recognizing this challenge the national solar technology roadmap calls for decreasing the absorber synthesis time to 2 min by 2015 while retaining high efficiency. To assist in indentifying improved synthesis routes, we have been using in-situ high temperature X-ray diffraction to better understand reaction pathways and determine rate constants1. These studies have suggested that synthesis of Cu(InxGa1-x)Se2 is diffusion limited for most precursor structures. This suggests that pathways that include a liquid phase or involve an interstitial or high vacancy concentration diffusion mechanism would be good candidates.Time-resolved selenization is used to study the formation of CuInSe2 (CIS) from novel compound bilayer precursors. Specifically the bilayer structures glass/Mo/γ-In2Se3/CuSey (sample A) and glass/Mo/γ-In2Se3/β-Cu2Se (sample B) were investigated. The structures were deposited by thermal evaporation on sputtered Mo/thin sodium-free glass substrates. ICP analysis indicated both samples were copper-rich with a Cu/In ratio = 0.97 for sample A and 1.18 for Sample B. Intermediate liquid phases are expected for Cu-rich and high Se activity conditions. Initial temperature ramp experiments using the glass/Mo/γ-In2Se3/CuSey sample revealed the reaction sequence of formation of β-CuSe, selenization to CuSe2, decomposition of this compound to γ-CuSe and γ-In2Se3 to InSe, and final synthesis of CIS. The sequence for the glass/Mo/γ-In2Se3/β-Cu2Se precursor showed β-Cu2Se reacts with Se to form CuSe2, then melts peritectically giving L + γ-CuSe, and γ-In2Se3 disproportionates yielding InSe and Se, followed by final synthesis of CIS. Isothermal experiments were performed to quantitatively extract kinetic parameters using the Avrami and parabolic growth models. SEM images revealed significant grain growth for both temperature ramp annealed samples. Based on these results, these precursor structures were annealed for 2 min using rapid thermal annealing in a Se atmosphere to test the feasibility of this precursor. Interestingly the reaction was complete in 2 minutes at a very low temperature (390 οC for samples A and 370 οC for sample B) while showing large grain growth. Additionally, TEM was performed to provide compositional and structural support for the indentified pathways.W. Kim, S. Kim, E. Payzant, S. Speakman, S. Yoon, R. Kaczynski, R. Acher, T. Anderson, O. Crisalle, and S. Li, Journal of Physics and Chemistry of Solids 66/11 (2005) 1915.
12:15 PM - C1.9
In-line Control Quality of Chacolpyrite Based Solar Cells by Advance Raman Spectroscopy.
Victor Izquierdo Roca 3 , Xavier Fontane 2 , Edgardo Saucedo 2 , Jesus Salvador Jaime Ferrer 1 , Jacobo Alvarez 3 , Alejandro Perez Rodriguez 2 , Juan Ramon Morante 2 , Veronica Bermudez 1 Show Abstract
3 , IN2UB, Barcelone Spain, 2 , IREC, Barcelone Spain, 1 , NEXCIS, Rousset France
The development of Raman scattering based strategies for process monitoring in chalcopyrite based photovoltaic thin film technologies is reported. Raman spectra measured at different process steps during the fabrication of the absorbers are very sensitive to features related to their crystalline quality, presence of secondary phases and polytypes and alloy composition. All these are features that have a significant impact on the characteristics of the final solar cells. New strategies based in the use of quasi-resonant Raman measurements are described for the non destructive assessment of the composition of quaternary alloys. The methodology developed can be used to monitor the fabrication of CIGS absorbers by using several techniques, such as sputtering, solution based processes, electrodeposition, .... In particular, in this work the implementation of Raman scattering for monitoring of electrodeposition processes used in the fabrication of low cost electrochemical based CuIn(S,Se)2 solar cells is reported at both on-line and in-situ levels due to the strong interest for the development of technologies with low fabrication costs of the single step electrodeposition of CuInSe2 precursors followed by a Rapid Thermal Process (RTP) sulphurisation process. The potential of using RS as a quality assessment and monitoring tool in the production of chalcopyrite absorbers is described, with a particular emphasis on the most relevant structures: CuInSe2 (CISe), CuInS2 (CIS), CuGaS2 (CGS) and CuGaSe2 (CGSe). We will discuss the most important information that can be inferred from the analysis of the Raman spectra to process monitoring, including crystalline quality, crystallographic structure, chemical composition in the case of quaternary alloys, and the presence of secondary phases. One remarkable advantage of the developed methology is the identification of Ordered Vacancy Compounds (OVCs) in CISe based absorbers. These phases arise as a result of a deficiency of Cu during the film formation, leading to the introduction of randomly distributed In[Cu] antisite defects in the chalcopyrite lattice, which are electrically compensated by Se vacancies, and which are difficult to be identified in line with other monitoring techniques. Several OVCs with different stoichiometries have been studied in this work, including CuIn2Se3.5, CuIn3Se5, and CuIn5Se8.The advantages of quasi-resonant measurements can be achieved by selecting an excitation wavelength close enough to the band-gap of the alloy is also discussed. This determines a strong increase of the intensity of the Raman modes, which allows for a significant decrease of the measuring time, improving the potentiality the implementation of this technique as an in-line in-site quality control technique.
12:30 PM - **C1.10
Rapid Printing of High-efficiency Monolithically Integrated CIGS Photovoltaic Modules.
Louay Eldada 1 , Baosheng Sang 1 , Dingyuan Lu 1 , Peter Hersh 1 , Casey Martinez 1 , Billy Stanbery 1 Show Abstract
1 , HelioVolt Corporation, Austin, Texas, United States
We describe the design, development and manufacture of monolithically integrated photovoltaic modules based on high-quality high-uniformity copper indium gallium selenide (CIGS) thin films produced with the unique combination of ink based and physical vapor deposition (PVD) based nanoengineered precursor thin films, and a reactive transfer printing method. Reactive transfer is a two-stage process relying on chemical reaction between two separate precursor films to form CIGS, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage, these precursors are brought in proximity and rapidly reacted under pressure while heat is applied. The use of two independent thin films provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the synthesis of CIGS. When atmospheric deposition of inks is utilized, the approach provides lower energy consumption, higher throughput, and reduced capital equipment cost with higher uptime. High quality CIGS with large grains on the order of several microns, and of preferred crystallographic orientation, are formed in under a minute based on compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 14% and module efficiencies of 12% have been achieved using this method. HelioVolt commercialized the reactive transfer process on a 20 MW pilot line, and is in the process of scaling the process on multiple 125 MW lines in a mass production GW-scale factory.
C2: Solution-Based Processes I & Transparent Conductors and Coatings I
Tuesday PM, April 26, 2011
Room 2000 (Moscone West)
2:30 PM - **C2.1
Molecular Precursor and Nanocrystal-ink Based Routes to CIGS and CZTS Solar Cells.
Hugh Hillhouse 1 Show Abstract
1 Chemical Engineering, University of Washington, Seattle, Washington, United States
The development of colloidal inks that can be used to yield high quality semiconductor layers are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photoabsorbing layer. Chalcopyrite structure copper indium gallium diselenide (CIGSe) and stannite or kesterite copper zinc tin sulfides (CZTS) are key photoabsorbing materials for thin film solar cells due to their near ideal band gap and their serendipitous defect chemistry (CIGSe) and Earth abundance (CZTS). Due to their unique defect chemistry, high quality layers of these materials can be formed from solution phases processing techniques. The presentation will focus on our recent advances in the development of molecular precursor and nanocrystal ink routes to thin film photovoltaic devices . In particular, we have recently reported the solution-phase synthesis of stoichiometric chalcopyrite structured CuInSe2 nanocrystals , Cu(In,Ga)S2 , and the very first synthesis of Cu2ZnSnS4 nanocrystals . The syntheses proceeds rapidly from elemental and halide reagents via a simple batch reaction without “hot injection” in a single component coordinating solvent. We have demonstrated the use of these nanocrystals for low-cost solar cells by fabricating devices without using any oxygen-free techniques (after NC synthesis) and employing a scalable roll coating method. The nanocrystal inks are first coated on a back contact (Mo coated sodalime glass in this case). The nanocrystal layer is then easily consolidated into large crystalline domains by a brief thermal treatment in a selenium rich atmosphere to prevent selenium loss or to replace sulfur with selenium. The fabricated cells are robust and increase in efficiency with time, exhibiting similar serendipitous defect chemistry as layers formed by vacuum co-evaporation. We have fabricated solar cells by roll coating CIGS or CZTS nanocrystal-inks over large areas. CIGS devices fabricated by roll coating over large areas with a device architecture of Mo/CIGSSe/CdS/i-ZnO/ITO/Ni/Al are (at the time of the abstract submission) 12.0% efficient under standard AM1.5G illumination while CZTS devices are now at 7.2%. The presentation will focus on the key aspects of the nanocrystal synthesis, ink coating, nanocrystal consolidation, and device fabrication and characterization for both CIGS and CZTS solar cells. Hillhouse H.W. & Beard M.C., Current Opinion in Colloid & Interface Science, 14, 245 (2009). Guo, Q.J., Kim, S.J., Kar, M., Shafarman, W.N., Birkmire, R.W., Stach, E.A., Agrawal, R., Hillhouse, H.W.,Nano Letters 8, 9, 2982 (2008). Guo, Q.J., Ford, G.M., Hillhouse, H.W., Agrawal, R., Nano Lett. 9, 8 3060 (2009). Guo, Q.J., Hillhouse, H.W., Agrawal, R., J. Am. Chem. Soc. 131, 11672 (2009).
3:00 PM - C2.2
Cu2ZnSn(S,Se)4 Thin Film Solar Cells from Binary and Ternary Chalcogenide Nanoparticles.
Yanyan Cao 1 , Michael Denny 1 , Jonathan Caspar 1 , Alex Ionkin 1 , Lynda Johnson 1 , Meijun Lu 1 , Irina Malajovich 1 , Daniela Radu 1 , H. Rosenfeld 1 Show Abstract
1 , DuPont CR&D, Wilmington, Delaware, United States
Cu2ZnSn(S,Se)4 (CZTS) is attracting rapidly growing attention as a direct gap semiconductor for use in thin film photovoltaic devices. In particular, the fact that CZTS relies only upon relatively earth abundant elements recommends it as a sustainable material which can potentially play a significant role in satisfying future energy demand. We report here a novel synthetic method employing mixtures of binary and ternary chalcogenide nanoparticles which has been used to synthesize CZTS thin films suitable for use in photovoltaic devices. Two strategies will be presented in detail. First, Cu2SnS3 and ZnS nanoparticles can be synthesized separately, formulated into a precursor ink, coated on a substrate and then converted into a CZTS thin film in an annealing step. An alternative strategy relies upon only binary sulfide nanoparticles (e.g. CuS, SnS and ZnS) which are formulated to form an ink and then cast into a film, which can again be converted into a CZTS thin film upon annealing. The CZTS thin films formed by this method are characterized via powder x-ray diffraction, x-ray absorption spectroscopy, scanning electron microscopy etc. This new method offers key advantages over alternative synthetic methods. The size of the nanoparticle precursors ensures the formation of dense precursor films with good film smoothness. Further, the Cu/Sn/Zn ratio in the final film can be easily adjusted by changing the precursor ratio in the ink. CZTS films with large grain size can be achieved with this method presumably due to reactive sintering. Photovoltaic devices have been successfully fabricated from these films. Device performance is correlated to the Cu/Sn/Zn ratio among other factors.
3:15 PM - C2.3
Microwave-enhanced Synthesis of Copper Zinc Tin Sulfide Colloidal Nanoparticle Inks.
Brendan Flynn 1 , Gregory Herman 1 Show Abstract
1 School of Chemical, Biological and Environmental Engineering, Oregon State University, Corvallis, Oregon, United States
A variety of colloidal nanoparticle materials are currently under consideration for the low cost manufacture of photovoltaic devices. Initial efforts are primarily focused on toxic and expensive nanomaterials that are unlikely to be widely adopted for commercial photovoltaic modules. A promising approach is using solution-based methods for the deposition of earth-abundant absorber layers to reduce manufacturing costs while maintaining good cell performance. Copper zinc tin sulfide/selenide (CZTS) absorber layers are considered a leading candidate in this area and good power conversion efficiencies have recently been reported. Our efforts focus on the microwave-enhanced synthesis of binary, ternary and quaternary colloidal nanoparticle materials from the CZTS system. We have found significant control in size, shape, and crystalline structure of the binary nanoparticles by modifying the metal salt precursor, sulfur source, solvent, temperature, pH, and reaction time. Considerable care is taken to optimize the chemistries on the binary materials so that they can be directly applied to the ternary and quaternary systems. We found that we are able to synthesize the quaternary systems using environmentally benign precursors and solvents, and can form stable colloidal nanoparticle dispersions are designed to minimize the incorporation of impurities into the resulting films. Thin films are formed onto glass substrates by spin-coating nanoparticle dispersions, and we have performed annealing experiments to determine the optimal conditions to obtain high quality CZTS absorber layers. The nanoparticle materials and associated films are characterized using x-ray diffraction, scanning electron microscopy, transmission electron microscopy, secondary ion mass spectroscopy, and UV-vis spectroscopy.
3:30 PM - C2.4
Fabrication and Performance of Dual Back-contact Thin Film Photovoltaic Devices.
Daniel Josell 1 , Carlos Hangarter 1 , Behrang Hamadani 2 , Suyong Jung 2 , Jonathan Guyer 1 Show Abstract
1 Metallurgy Division, NIST, Gaithersburg, Maryland, United States, 2 CNST, NIST, Gaithersburg, Maryland, United States
I will describe the fabrication and performance of photovoltaic devices with a dual back contact geometry created by depositing semiconductors onto two interdigitated comb-electrodes that have been previously patterned lithographically on insulating substrates. Widths and spacings of the individual wires in the combs are on the order of one micron, with the interdigitated comb electrodes in the 4 mm square area of the test devices each comprising thousands of parallel wires. The active devices are fabricated by electrochemical deposition of a semiconductor on one of the electrodes, independent control of the applied potentials on the electrodes permitting selective deposition, followed by deposition of a second semiconductor over the entire device. I will describe homojunction CdTe devices fabricated from a single electrolyte. I will also describe heterojnction devices, including CdS/CdTe devices with CdS electrodeposited on one electrode followed by CdTe electrodeposited over both the coated and uncoated electrode through impingement of the CdTe deposits to create active devices. I will detail the performance of devices, including i-V and external quantum efficiency, including the impact of electrode geometry, device processing, and electrode(contact) material. I will also present modeling results that explain the observed device performance. Finally, I will detail the positive aspects of the geometry and process, including electrode-selectivity enabled by electrodeposition in the first step and process generality for subsequent deposition steps, elimination of light-blocking front contact metals and UV-light blocking conducting oxides and the inherent absorption enhancing surface contouring as well as materials related issues arising from the requirement that all processing be done with both electrodes present.D. Josell, C. R. Beauchamp, S. Jung, B.H. Hamadani, A. Motayed, L.J. Richter, M. Williams, J.E. Bonevich, A. Shapiro, N. Zhitenev, T.P. Moffat, Three-Dimensionally Structured CdTe Thin Film Photovoltaic Devices with Self-Aligned Back-Contacts: Electrodeposition on Interdigitated Electrodes, Journal of the Electrochemical Society 156(8), H654-H660 (2009).D. Josell, C. R. Beauchamp, B.H. Hamadani, S. Jung, J.E. Guyer, A. Motayed, C. Hangarter, N. Gergel-Hackett, H. Xu, N. Zhitenev, Three-Dimensionally Structured Thin Film Heterojunction Photovoltaic Devices on Self-Aligned Back-Contacts, Transactions of the Electrochemical Society, Vol. 28(2), 521-532 (2010).
3:45 PM - C2.5
Analysis and Control of Plating Baths in the Electrodeposition of Copper Indium Gallium Selenide (CIGS) Films with Ion Chromatography.
Joseph Duimstra 1 , Sarah Lastella 1 , Muharrem Kunduraci 1 , Tuncay Cetiner 1 , Serdar Aksu 1 , Mustafa Pinarbasi 1 Show Abstract
1 , SoloPower Inc, San Jose, California, United States
Cu(In,Ga)Se2 (CIGS) is one of the most advanced absorber materials for thin film solar cells due to its direct bandgap, high absorption coefficient, and ability to yield good quality devices. CIGS-based solar cells have yielded the highest conversion efficiencies of all thin film solar cells, reaching up to about 20%. One of the techniques used to form CIGS layers is a two-stage approach, which involves deposition of a precursor layer on a substrate followed by a high temperature activation step that converts the precursor layer into solar cell grade CIGS. Although various techniques such as evaporation and sputtering have been employed to prepare precursor layers, electrodeposition is especially attractive due to its low cost, efficient utilization of raw materials and scalability to high-volume manufacturing. Electrodeposition of high-quality CIGS layers in a high-throughput manufacturing environment requires a strict control of chemical composition of the plating baths. In the present study, we demonstrate the use of ion chromatography (IC) for the quantitative analysis of all the chemical constituents in our novel one-step aqueous alkaline CIGS electroplating solutions. With the selection of appropriate complexing agents, Cu, In and Ga ions were solubilized at high pH and their reduction potentials became closer to that of the Se reduction potential in these solutions. Since no complexation occurs between Se and the complexing agents, the Se reduction potential could be independently controlled by the amount of dissolved Se. In the formulation of the plating baths, multiple complexing agents with different affinities were included to promote selective complex formation and thereby regulate plating potential of each metal ion separately. We determined that ethylenediaminetetraacetic acid (EDTA), tartaric acid, and citric acid are suitable complexing agents for the chelation of the metal ions in solution. Ion chromatography was selected due to its ability to measure the concentrations of both anions and cations in the electrolyte solution. During the analysis, main cations such as Cu, In and Ga were separated through a cation exchange column and detected with a UV-visible spectrometer detector. Similarly, anions were separated through an anion exchange column and detected with a conductivity detector. The use of this method may be further extended to detect the organic plating additives and their decomposition products. With the inclusion of a remote solution delivery system, the measurements can be carried out in an in-line fashion to adjust the dosing levels to maintain the desired chemical constitution of the plating baths.
4:30 PM - C2.6
Durable Anti-reflective Coatings for PV Module Cover Glass.
Sudip Mukhopadhyay 1 , Renato DeTorres 1 , Hai Bien 1 , Boris Korolev 1 , Ahila Krishnamoorthy 1 , VaraPrasad Desaraju 1 Show Abstract
1 , Honeywell, Sunnyvale, California, United States
Challenges remain to achieve affordable solar power technologies; yet, gradual improvement of solar efficiency and cost by rapid technological advances certainly deserves recognition. Tendency to incorporate antireflective coatings (ARC) in production line for solar cells and solar cover glass to improve efficiency is gaining momentum because of the benefits these coatings bring in managing light efficiently by minimizing reflectance loss from the air-glass interface. It has been demonstrated that a thin coating of an antireflective coating on one side of a module cover glass effectively increase absolute transmittance to 2-3% in the visible region, which in turn improve the power efficiency. However requirements for low refractive index (RI~1.22), specific thickness (thickness~100-150 nm), and demand for at least 20 year life time (must pass standard accelerated durability tests) of these materials left one with only a few material choice. Although porous silica meets some of the requirements to be used as ARC, improvements needed on the adhesion and durability areas. Therefore, there is incentive to find durable ARC materials with improved adhesion and optical properties. Here we present results of optical and durability tests of some of the new ARC materials for solar cover glass that have been developed at Honeywell’s California R&D laboratory. A detail simulation results over a wide range of solar spectrum and optimization of thickness and RI of these materials will be discussed. Effect of different process parameters on durability will also be discussed.
4:45 PM - C2.7
Deposition of Aluminum Doped Zinc Oxide Using an Atmospheric, Low Temperature Plasma Deposition Process.
Mirjam Theelen 1 , Hans Winands 1 , Frank Grob 1 , Sandra Kouijzer 1 , Hans Goverde 1 , Joop van Deelen 1 , Ariel de Graaf 1 , Paul Poodt 1 Show Abstract
1 , TNO, Eindhoven Netherlands
Atmospheric pressure deposition techniques like APCVD are an asset toward reducing the production costs of e.g. solar cells. However, APCVD often requires high deposition temperatures, while next generation flexible solar cells often require low temperature processing. One route towards low temperature deposition is Atmospheric Pressure Plasma Enhanced CVD (AP-PECVD). At TNO, we have developed this technology for the deposition of (Aluminum doped) Zinc Oxide (AZO). AZO is used as transparent conductive oxide in CIGS solar cells and is used in turn key solutions for the amorphous silicon solar cell industry. Using zinc acetyl acetonate and aluminum acetyl acetonate as well as dimethyl zinc and thimethyl aluminum as precursors, AP-PECVD was used to deposit zinc oxide on aluminum and polymer foils, as well as on glass and silicon to allow simple analysis. Typical deposition temperatures adopted were <200°C. Deposition rates up to 0.7 nm/sec were obtained over a deposition area of ~1.5 x 8 cm2. XPS and EDX measurements showed that the deposited layers are largely carbon free and that the Zn:O ratio is close to stochiometry, indicating adequate precursor dissociation. SEM imaging revealed that the material is deposited in the form of nano-crystals, with typical grain diameters <200 nm. The resulting high grain-boundary density is likely to be responsible for the observed high resistivity of the films.To reduce the resistivity, atmospheric pressure, low temperature plasma annealing treatments have been investigated. Improvements, up to an order of magnitude, have been obtained, the exact value depending on treatment time, gas conditions, initial resistivity and plasma intensity. Another path to increase the quality of the deposited layer is by using a seed layer. Deposition on a substrate with a thin zinc oxide layer seemed to result in an increased conductivity. This approach will be further explored. To enhance the functionality of ZnO as a transparent conductor, it can be combined with metal grids to ensure high conductivity as well as transparency. For this purpose, ZnO with such grids were applied on polymer films, which in principle can be used as substrate for thin film solar cells.
5:00 PM - C2.8
Investigation on the Discharge Formation Mechanisms and Surface Analysis of SiO2-like Layers on Polymers Synthesized Using DBD Assisted CVD at Atmospheric Pressure.
Antony Peter 1 3 , Richard van de Sanden 1 , Sergey Starostin 2 , Hindrik de Vries 2 , Mariadriana Creatore 1 Show Abstract
1 , Eindhoven University of Technology, Eindhoven Netherlands, 3 , Materials innovstion institute (M2i), Delft Netherlands, 2 , FUJIFILM Manufacturing Europe B.V, , Tilburg Netherlands
The dielectric barrier discharge is recognized as a promising tool for PECVD of thin films at atmospheric pressure. Emerging applications including encapsulation of flexible solar cells and flexible displays requires low costs production of transparent uniform and dense layers with low level of coating defects. Among the two discharges Townsend like discharge (TD) and glow like discharge (GD) the latter offers more flexibility for the high growth rates in plasma enhanced deposition. In this investigation we demonstrate the utilization of glow like discharge in, He free, industrially relevant gas mixture comprising Ar/N2/O2/HMDSO for the deposition of high quality silica like films on large area polymeric substrates (PET or PEN) in a roll-to-roll configuration. While the discharge physics exhibiting the glow like behaviour is investigated via fast ICCD camera, voltage-current waveforms and optical emission spectroscopy, the deposited silica like films is comprehensively analyzed using AFM, SEM, XPS, SE and FTIR. The time evolution of the diffuse atmospheric discharge showed several phases starting from the initial ignition of the low current Townsend-like mode followed by the transition to glow like discharge which then undergoes lateral expansion providing uniform treatment of the whole substrate width. As a generic characteristic of the developed technology, it is observed that, irrespective of precursors (TEOS or HMDSO) and process gases (Ar, N2 or air) employed, the films are smooth, both locally and globally, and of near stoichiometric silica with very low carbon content (< 2%). Detailed AFM morphology description and surface statistical analysis on SiO2 dynamics showed that no film roughening in growth front and lateral directions observed and the synthesized layers (~ 350 nm) grow in a self-similar fashion following the topology of the substrate. The films are uniform with no defects or particle being incorporated during the deposition process and exhibit excellent barrier performances towards O2 and H2O permeation.
5:15 PM - C2.9
Recent Progress in Transparent Conducting Materials by Use of Metallic Grids on Metaloxides.
Joop van Deelen 1 , Henk Rendering 1 , Bert Huis in het Veld 1 , Mirjam Theelen 1 , Paul Poodt 1 , Arjan Hovestad 1 Show Abstract
1 Materials Technology, TNO, Eindhoven Netherlands
Due to many new application fields, research into transparent conductive materials has increased rapidly in the last ten years. The classic material used as a transparent conductor has been so-called transparent conductive oxides (TCOs). In this field, new amorphous materials have been discovered which have excellent characteristics and research is conducted in search of explanations. Furthermore, materials a graphene and carbon nanotubes have emerged. A comparison between the different materials will be presented, evaluating their performance. In addition, to these single material approaches, it has been discovered that the use of a combination of different materials can enhance the performance beyond the possibilities of a single material. To illustrate this, thin film photovoltaics was used as as application case. Modeling shows that the addition of metallic grids to a TCO can reduce the optical and electrical losses of a thin film solar cell by as much as 26%. Through this modeling, optimum grid dimensions were determined. Experimental work focused on patterned electrochemical deposition of metallic grids on TCOs. Various patterning methods such as laser, nano-imprint have been used to create a surface on which selective deposition was possible. By application of metallic grids on ITO on PET, the conductivity could be enhanced by more than two orders of magnitude, at the expense of only a few percent in transmittance.Lowering the coverage, while keeping the other dimensions of the grid the same, results in an improved transmittance and an excellent sheet resistance on the order of 0.1 Ohm/sq at a transmittance of over 80%. These results are a great step forward compared to reported work on grids by printing techniques. A few advantages of electrochemical deposition over printing are:1.high conductivity of the material, which is continuous material, rather than printed particles that have percolation conductivity2.No need for special post deposition treatments that characterize printing processes, which are needed to enhance the conductivity or remove the solvent or binder materials from the printing ink3.Strongly reduced feature size. In fact, the limit of patterned electrochemical deposition lies in the patterning step, rather than the deposition method itself.Electrochemical deposition can also be used in combination with printing, thereby enabling higher conductivity of printed materials. A comparison between outcomes of modeling and experimental work will be presented as well as results on different application methods. Furthermore, results on issues as adhesion, selectivity, deposition rate and relation to process parameters will de discussed.
5:30 PM - C2.10
Scalable Transparent Electrodes with Electrospun Nanowires.
Hui Wu 1 , Liangbing Hu 1 , Yi Cui 1 Show Abstract
1 , Department of Materials Science and Engineering, Stanford Univ., Palo Alto, California, United States
Thin conducting films consisting of one-dimensional nanostructures are currently of great technological interest, in particular as transparent electrodes for thin-film solar cells, light-emitting diodes, display technologies and many other optoelectronic applications. Herein, a template-catalyst-free method for the preparation of conductive and transparent metal nanowire mesh is reported. Metal nanowires with length over 1 cm were prepared by electrospinning. Due to the continuous 1D nanostructure with extremely high aspect ratio, the nanowire web exhibited metallic conductivity over large areas, high transparency, and flexibility. Further, we fabricated oriented metal nanowire arrays and patterned nanowire grids by employing a modified fiber collector during electrospinning. Anisotropic electrical conductivities were obtained from this metal nanowire arrays. This is the first demonstration of transparent electrode with directional conductivities using scalable process. This work suggests that electrospun metal nanowire webs can be a highly scalable and low cost solution for high performance photovoltaics, touch screen and other optoelectronic devices.
5:45 PM - C2.11
Formation of Self Assembled MgO Nano-facet: Toward the Yablonovitch Limit for the Light Trapping in Thin Silicon Solar Cell.
Hak Ki Yu 1 2 , Jong-Lam Lee 1 2 , Gwan Ho Jung 1 2 , Wan Jae Dong 1 2 , Kihyon Hong 1 2 , Sungjun Kim 1 2 Show Abstract
1 Material Science & Engineering, POSTECH, Pohang, Kyungbuk, Korea (the Republic of), 2 Graduate Institute of Advanced Materials Science, POSTECH, Pohang, Kyungbuk, Korea (the Republic of)
Currently crystalline silicon (c-Si) yields the highest efficiency in single junction solar cells and takes up ~80 % of the solar cell market. A challenge in c-Si solar cells is that, due to its poor absorption, c-Si wafers are typically 100-300 μm thick and account for ~40% of the total module cost. If c-Si with a thickness of a few micrometers can be made to absorb as efficiently as thick Si wafers, a significant cost reduction is expected. The ideal limit in increase of optical path length could be defined as Yablonovitch limit as 4n^2/sin2θ, where n is the refractive index of the film, and θ is the acceptance angle. For Si with refractive index ~3.5 the Yablonovitch limit can increase by a factor of ~50. If this limit can be reached, the c-Si wafer thickness can be reduced from 300 um to 6 um, significantly reducing the material cost. Various schemes have been studied to enhance optical absorption in Si thin film including nanorod arrays, surface plasmon, and diffraction grating. Although these methods can increase the optical path length, they have several limitations, such as poor stability, complex processing techniques, and high cost fabrication procedures.In this work, we demonstrate a novel way of enhancing the light trapping in thin silicon solar cell by using MgO nano-facet structure. Formation of MgO nano-facet is simple and low cost process without additional lithography or patterning process because it is formed spontaneously due to material anisotropic characteristics of MgO between crystal orientations. The MgO facet is formed due to anisotropic properties between (111) and other main planes of MgO; (200) and (220). The (111) orientation of MgO with alternating array of Mg cation and O anion is very unstable because of dipole energy accumulation induced by polarity (Mg2+ plane and O2- plane). The polar (111) plane has 3 dangling bonds per atom whereas neutral (100) plane has only 1 dangling bond per atom. A lot of dangling bonds are needed to be stabilized energetically by adsorbing adatom dominantly, resulting in increase of sticking probability of adatom. Moreover, adatom on polar surface could not move laterally because a lot of dangling bonds cause large activation energy in lateral movement. This led the crystal growth with a preferred orientation of MgO (111). However, MgO (111) surface has large surface energy than other planes and is needed to be stabilized by surface reconstruction (2×2) reconstruction with octo-pole structure). So, MgO (111) films tend to grow with surface termination by (200) and their family plane to acquire the most stable atomic arrangement, resulting in facet structure. From the roughended MgO facet surface, light could be trapped largely in thin silicon solar cell, resulting in high efficiecy solar cell.
C3: Poster Session: Advanced Processes and Manufacturing for Photovoltaics
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
6:00 PM - C3.1
Low Cost ZnO/Cu2O Solar Cells Produced by Atmospheric ALD.
Andrew Marin 1 , David Munoz-Rojas 1 , Diana Iza 1 , Judith MacManus-Driscoll 1 Show Abstract
1 Materials Science & Metallurgy, University of Cambridge, Cambridge United Kingdom
Atmospheric Atomic Layer Deposition (AALD) is a fast and scalable method of producing high-quality films of transition metal oxides without the use of vacuum conditions. Eliminating vacuum significantly reduces the cost of producing these films and makes this technique attractive for low-cost photovoltaic production. AALD can be used to grow cuprous oxide (Cu2O), which is a p-type semiconductor that absorbs visible light (band gap ~2.1eV). Additionally, its atmospheric stability and low toxicity make Cu2O an attractive candidate for photovoltaic devices. Zinc Oxide (ZnO) is a wide band gap n-type material that is also easily grown via AALD at temperatures as low as 150oC. Earlier work by this group has demonstrated the potential utility of ZnO/Cu2O solar cells grown from other scalable methods (i.e. electrochemical deposition), however significant recombination at interface traps limited the device performance. This investigation illustrates the potential of AALD to deliver improved interface and charge carrier properties in ZnO/Cu2O devices compared with other low-temperature solution-processing methods. Impedance spectroscopy was used to analyze electrical properties and validate illuminated solar cell performance.
6:00 PM - C3.10
Rapid Growth of Large Transparent Films of Chemically Converted Graphene, Graphite Oxide, and Aligned Single-walled Carbon Nanotubes.
Julio D'Arcy 1 2 , Richard Kaner 1 2 3 , Yang Yang 3 2 1 Show Abstract
1 Chemistry and Biochemistry , University of California, Los Angeles, Los Angeles, California, United States, 2 California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California, United States, 3 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States
Carbon is ubiquitous on earth and is a fascinating element possessing morphological anisotropy at the nanoscale. Dimensionality in structure affords tunable electronic properties in these inorganic materials and makes them attractive nanoscale semiconductors. Chemical processing typically involves casting thin films using methods such as spin-coating or dip-coating. These methods however, suffer from a low material utilization yield, lack of scalability, and are therefore not cost-effective. Here we show rapid film growth of large transparent films of carbon nanostructures, deposited via solution processing at the interface of two immiscible liquids. This protocol can coat non-activated flexible substrates, affords layer-by-layer deposition for controlling film transparency, and leads to the growth of a transparent film comprised of carbon nanostructures. Solution chemistry leads to 2D graphite oxide, graphene sheets, and functionalized single-walled carbon nanotubes. These amphiphiles disperse in a plethora of solvents, possess different dimensionalities, decrease the surface tension of fluids, and serve as stabilizers in Pickering emulsions. Mechanical agitation of carbon nanostructures, water, and oil leads to droplets stabilized by interfacially trapped solids. Anisotropic carbon nanostructures are both colloidal and molecular amphiphiles solvated by both water and oil. The liquid/liquid interface provides a chemical environment for molecular interactions to occurs such as hydrogen bonding between water, and the functional groups on carbon amphiphiles. These interactions are controlled via pH by tuning the degree of protonation of functional groups and emulsion stability. When droplets containing carbon stabilizers coalesce, their interfacial surface area decreases expelling excess oil and interfacially adsorbed nanostructures. A coating of single-walled carbon nanotubes possesses a transparency higher than 90% in the visible spectrum, and a sheet resistance of 1 kΩ/square. Chemically converted graphene films are comprised of single sheets that stack side-by-side, and large scale graphite oxide films deposit in seconds. Alignment of one-dimensional nanostructures such as carbon nanotubes is demonstrated in the micrometer scale on hydrophilic surfaces.
6:00 PM - C3.11
Fabrication of Improved p-AgGaSe2/n-Si Heterojunction Solar Cells on Optimum Quality Thermally Evaporated AgGaSe2 Thin Films.
Krishna Mandal 1 , Sandip Das 1 Show Abstract
1 Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina, United States
Optimum quality polycrystalline AgGaSe2 thin films were deposited on H-terminated n-Si substrates by controlled thermal evaporation method. The film deposition conditions were varied to optimize the structure and optoelectronic properties of AgGaSe2 thin films. X-ray diffraction (XRD) studies showed that all AgGaSe2 films were of chalcopyrite structure and while the films were deposited at room temperature (300 K) had random grain orientation, the films deposited at higher substrate temperature (≥450 K) showed preferred (112) orientation. The composition of the films was analyzed by energy dispersive x-ray analysis (EDAX) and by x-ray photoelectron spectroscopy (XPS) with and without argon ion etching. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.66 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited at 300 and 650 K were ~5 x 103 and ~200 Ω.cm respectively. The electrical activation energies of the films were also determined and are discussed in terms of the possible defect energy levels present. p-AgGaSe2/n-Si heterojunction solar cells, having an active area of 0.18 cm2 and without any antireflection coating, were fabricated. It was observed that the films deposited at 650 K produced junctions with significantly improved photovoltaic properties. Under solar simulator AM1 illumination, the improved junction exhibited an efficiency of 5.2% whereas the AgGaSe2 films deposited at 300 K showed an efficiency of 2.1%. The evidence of the barrier height modifications have been provided by C-V measurements and an energy band diagram of the p-AgGaSe2/n-Si heterojunction solar cells has been proposed.
6:00 PM - C3.12
Application of a Dual-spectral-range, Expanded-beam Spectroscopic Ellipsometer for Mapping Large-area, Laterally-inhomogeneous, Photovoltaic Multilayers.
Miklos Fried 1 , G. Juhasz 1 , C. Major 1 , A. Nemeth 2 1 , P. Petrik 1 , P. Polgar 1 , C. Salupo 2 , R. Collins 2 Show Abstract
1 Photonics, MFA, Budapest Hungary, 2 PVIC, University of Toledo, Toledo, Ohio, United States
We have developed a prototype spectroscopic ellipsometer for imaging/mapping purposes requiring only one measurement cycle (one rotation period of a polarizer or analyzer) for the acquisition of a two-dimensional array of data points . Our new measurement technique serves as a novel form of imaging ellipsometry, using an uncollimated (non-parallel, diffuse) source system and a detection system consisting of an angle-of-incidence-sensitive pinhole camera . By adding multicolor supplements, the instrument provides full high-resolution spectra along a line image. Information on multilayer photovoltaics stacks can be obtained over large areas (several dm2) at high speed . The technique can be expanded to even larger areas by scaling-up the geometry. The lateral resolution is limited by the minimum resolved-angle as determined by the detection system. Small-aperture polarizers (25 mm diameter) are incorporated into the instrument. The near-ultraviolet-to-visible (nuv-vis) range limits photovoltaics applications; as a result, an extension into the near-infrared (nir) region is desired to probe below the band gap of absorber layers in order to measure their thicknesses. Thus, with a broadened spectral range, it becomes possible to characterize a wider variety of layers and structures. Unfortunately, because of the uncollimated beam, expanded beam ellipsometers must be employ film polarizers, which exhibit a limited spectral range. Thus, it is impossible to operate the ellipsometer over the full nuv-nir range using one polarizer-analyzer pair. A dual spectral range capability is a convenient solution whereby the optical elements (polarizer-analyzer pairs, optical grating) are automatically interchangeable, and the entire nuv-nir spectra for a line image is detecable in two steps with one CCD camera. The prototype is designed to enable in situ imaging/mapping within a cluster tool chamber at the Center for Photovoltaic Innovation and Commercialization (PVIC) of the University of Toledo (Ohio). Demonstration mapping measurements have been performed on intentionally non-uniform multilayer samples including 100-1000 nm thick hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) layers and 100-500 nm thick transparent conducting ZnO:Al layers on opaque silver. Measurements on both rigid glass and roll-to-roll polymer will be possible. Alternative commercial instruments for SE mapping must translate the sample in two dimensions. Even a 15x15 cm2 sample with cm-resolution requires >200 measurements and at least 15 min. By imaging along one dimension in parallel, the expanded-beam system can measure with similar resolution in < 2 min.References Patent pending: P0700366, PCT/HU2008/000058 G. Juhasz et al, Phys Stat Sol C v.5, 1081 (2008) C. Major et al, Phys Stat Sol C v.5 1077 (2008) M. Fried et al, "Expanded beam (macro-imaging) ellipsometry", accepted for publication in Thin Solid Films (2010)
6:00 PM - C3.13
CdS, CdSe Nanoparticles in Silica Matrix by Sol Gel Method.
Nilima Hullavarad 1 , Shiva Hullavarad 1 Show Abstract
1 Advanced Materials Group, Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, Alaska, United States
‘Quantum dots’ or nanoparticles of diverse semiconductor materials are extensively studied because of their interesting size dependent properties. It is further interesting to organize the quantum dots in the form of superlattices thin films, monolithics, ordered arrays for fruitful applications. Various applications such as sensors, displays, recordings, communications etc. require condensed organic or inorganic tunable material in the ultra violet to visible range. This work discusses the synthesis of CdS and CdSe nanoparticles in a silica matrix. The UV absorption measurements of CdS nanoparticles indicated sharp absorption at 260 and 350 nm for different precursor compositions. The particle size of CdS nanoparticles was estimated to be 1.5-2 nm. Silica gel containing the CdS nanoparticles was spin coated onto substrates to form thin film samples. Scanning Electron Microscope (SEM) measurements revealed formation of CdS nanoparticles within the branches of gel network. Depending upon the mole ratio of additives and drying method, fibrous or monolithic tablets of CdS nanoparticles embedded in silica matrix could be produced. The paper discusses the effect of precursors in obtaining the single size distribution of CdS nanoparticles in the silica gel matrix. The matrix consisting of monodispersed CdS nanoparticles in a silica gel matrix has potential applications in sensors, tunable waveguides, and sensitive photon counting systems, luminescent displays, and laser micro cavities.
6:00 PM - C3.16
Low-temperature Atomic Layer Deposition of ZnO, TiO2, and Al2O3 Thin Films for Application to Transparent Conducting Oxide.
Woo-Hee Kim 1 2 , Jae-Min Kim 1 , Min-Kyu Kim 1 , Hyungjun Kim 1 Show Abstract
1 School of Electrical & Electronic Engineering, Yonsei, Seoul Korea (the Republic of), 2 Department of Material Science and Engineering, POSTECH , Pohang Korea (the Republic of)
We have investigated low temperature (< 80 °C) atomic layer deposition (LT-ALD) of ZnO, TiO2 and Al2O3 thin films for the application to transparent conducting oxide (TCO). For LT-ALD, diethylzinc (DEZ), tetrakisdimethylaminotitanium (TDMAT), and trimethylaluminum (TMA) precursors were used as Zn, Ti, and Al precursors, respectively and water was used as a reactant. The saturated growth rates were observed for the ZnO, TiO2 and Al2O3 films, suggesting that the inherent growth characteristics of ALD are achieved for the LT-ALD processes. In addition, for all the films, cross sectional images by field emission scanning electron microscopy (FE-SEM) exhibited excellent conformality in nanoscale via hole patterns with the aspect ratio of 5:1. Based on x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) results for the structural and chemical compositional analysis, current ALD process can produce thin films with high purity even at low growth temperature down to room temperature. Optical and electrical properties of TCO films prepared by these oxides were investigated. The current LT-ALD processes is one of the viable options for advanced large-scale interconnect, solar cells, and flexible displays.
6:00 PM - C3.17
Atomistic Simulations of the Silicon Surface Structure at the Interface of Silver Thick Film Contacts on n-Type Silicon.
Stefan Kontermann 1 2 , Alexander Ruf 1 , Ralf Preu 1 Show Abstract
1 , Fraunhofer Institute for Solar Energy Systems, Freiburg Germany, 2 , Fraunhofer Heinrich Hertz Institute , Goslar Germany
We present results from an extensive study of the nanostructure of silver thick film contact interfaces on n-type Si-(100) and Si-(111). Nanoscale silver crystals are found at the interface of such contacts. The silver crystals carry the current across the contact and therefore control the contact resistance, which is a main performance limiting parameter for semiconductor devices. The silver crystals are located in pits at the silicon surface. During contact formation, these pits form before the silver crystals and hence determine their size and shape. Consequently, the pits with the crystals influence the contact resistance. We investigate these pits experimentally by scanning electron microscopy. Then we simulate the mechanism of pit formation by considering the removal probability of silicon surface atoms. For this purpose, an existing model, which was originally designed to describe the mechanism of wet chemical etching of silicon, is modified to match our interface conditions. Our model leads to a consistent and quantitative correct description of all experimental data. It enables to predict pit formation for arbitrary process parameters like temperature and duration for silver thick film contact formation on n-type silicon.
6:00 PM - C3.18
Surface Passivation of Pre-oxidized Silicon by Atomic Layer Deposited Al2O3.
Wei-Cheng Lai 1 , Jenn-Chang Hwang 1 , Chien-Hsun Chen 2 , Hung-Jen Yang 2 , Wei-Yu Chen 2 Show Abstract
1 , National Tsing Hua University, Hsinchu Taiwan, 2 , Industrial Technology Research Institute., Hsinchu Taiwan
Surface passivation becomes more important when the trend of crystalline silicon based solar cell toward thinner wafer and higher efficiencies. Ethanol/iodine solution is the well known passivation material. Its passivation characteristics was considered superior to thermally grown high quality passivating oxides. Al2O3 is also an excellent surface passivation material for c-Si, which exhibits low surface recombination velocity. The negative fixed charge accumulated within Al2O3 layer provide an effective field-effect passivation is usually grown on c-Si at low temperature (~200°C) using atomic layer deposition (ALD) in either thermal or plasma mode. In this article, we present the enhancement of effective carrier lifetime due to the field-effect passivation of Al2O3 deposited on a pre-oxidized 3 Ωcm n-Si(100) substrate . Effective carrier lifetime increases greatly from 35 to 400 μs by two major steps. One is the formation of an amorphous chemical pre-oxidized SiO2 layer prior to Al2O3 deposition. The other is the annealing in nitrogen at 600°C for 30 min. Comparing with the wafer passivated by Ethanol/iodine solution, we demonstrate the passivation characteristic of Al2O3 deposited on a pre-oxidized is superior to Ethanol/iodine solution. Finally, Al 2p spectra support that Al diffuses into the amorphous pre-oxidized SiO2 layer and a new Al-related chemical bond environment induced after annealing in nitrogen at 600°C for 30 min. Fixed charge density increases from1.8 x 10<12/sup> to 6.2 x 10<12/SUP> cm<-2/sup> in the Al2O3 layer near the Al2O3/SiO2 interface, based on capacitance-voltage data. And the TEM image directly shows the change of the thickness of pre-oxide layer after annealing.
6:00 PM - C3.19
Enhanced Optical Absorption of Medium-temperature PECVD Grown Si-rich Si1-xCx.
Chaio-Ti Lee 1 , Gong-Ru Lin 1 Show Abstract
1 Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, Taipei Taiwan
Amorphous silicon carbide (a-Si1-xCx) with wide bandgap and high thermal conductivity makes it an excellent material for applications in light emission, solar energy transformer and transistors, etc. The optical absorption of low-temperature grown a-Si1-xCx has been comprehensively investigated, which covers a broadband solar spectrum owing to the SiC alloy with different phase structure to be a potential candidate other than the bulk or single crystalline Si for solar-energy transferring device. In this work, the enhanced optical absorption of Si-rich SiC is demonstrated by arising the substrate temperature during plasma enhanced chemical vapor deposition (PECVD) growth. The solar device based on the high-temperature grown Si1-xCx material is fabricated to shown the improved internal/external quantum efficiency (IQE and EQE).The a-Si1-xCx films were grown on ITO glass and quartz by PECVD with reactance gas of Ar-diluted silane (SiH4) and pure methane (CH4) at RF plasmas power of 20 W (power density about 22mW/cm2) and the chamber pressure of 0.18 Torr. The fluence ratio ([CH4]/[SiH4]) is fixed by setting Ar-diluted SiH4 (8%) at 75 sccm and pure methane at 9 sccm. The substrate temperature during deposition is detuned from 450oC to 650oC with increment of 100oC for the 20-min deposition. The SEM analysis reveals that the SiC thickness is linearly decreased from 400 nm to 360 nm with increasing substrate temperature. The p-i-n junction device of n-SiC/SiC/p-SiC with on ITO glass is grown by setting 5% [PH3]/[SiH4]+[CH4] doping ratio during n-type SiC deposition.At different substrate temperatures during PECVD growth, the optical absorption coefficients of all Si-rich SiC samples show at least 104 cm-1 or higher at photon energy >2.33 eV. The near-infrared optical absorption coefficient is enhanced by increasing the substrate temperature up to 650oC, which effectively promotes the absorbance in the visible region (400-600nm) to reache 105 at photon energy >2.33 eV. In particular, the optical absorption coefficient at 600-800 nm is greatly enlarged by two orders of magnitude at least when increasing the substrate temperature to 650oC. With a surface reflectance of about 0.2, the external quantum efficiency (EQE) and internal quantum efficiency (IQE) are increasing from 10% to 30% as the photon wavelength red-shifts from 300 to 800 nm. The oscillation fringe caused by the multilayer interference of the p-i-n junction structure is also observed. The electrical property is degraded by the doping density of the n-/p-SiC layer and the interfacial defect induced strain/dislocation. The EQE is slightly degraded by the carrier confinement in the high-barrier p-i-n junction with large series resistance, or by the large lattice mismatch (almost 57%) and the distinguished thermal expansion between Si1-xCx (0.4358 nm) and ITO (1.0118 nm).
6:00 PM - C3.2
Optimizing Materials Processing and Device Geometry for Scalable ZnO/Cu2O Solar Cell Fabrication.
Talia Gershon 1 , Kevin Musselman 2 , Andrew Marin 1 , Judith MacManus-Driscoll 1 Show Abstract
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 2 Department of Physics, University of Cambridge, Cambridge United Kingdom
Transition metal oxides are attractive candidates for low-cost photovoltaic (PV) applications because of their good stability, low toxicity, relative abundance, and ease of processing. Electochemically-deposited ZnO/Cu2O PV devices, however, fall short of achieving their theoretical limits and are plagued by low open-circuit voltages (Voc) and short-circuit current densities (Jsc). Studies have shown that low Jsc in ZnO/Cu2O solar cells can be attributed in part to the low minority carrier diffusion length in Cu2O resulting from the high defect density and poor quality of electrodeposited films. This can be mitigated by controlling the electrodeposition conditions, including temperature and pH, which determine film characteristics such as charge carrier concentrations, grain size, defect density, and film roughness. Studies have also shown that a reduction in Voc is often caused by recombination inside of the depletion region near the p-n junction; the factors responsible for reducing Voc must be identified and controlled to optimize device performance. This investigation will discuss how selecting ideal processing conditions and layer thicknesses and incorporating semiconducting polymers can help overcome some of these problems.
6:00 PM - C3.20
Optical Layers and Materials for Next Generation Solar Cells.
Ping Lee 1 , Jason Shank 1 , Mikael Marra 1 , Yeona Kang 1 , C. Fortmann 1 2 Show Abstract
1 , Stony Brook University, Stony Brook, New York, United States, 2 , Idalia Solar Technologies LLC, New York, New York, United States
Every component of present era solar cells is superbly optimized. Nonetheless, solar electric power generation represents less than 1% of the electric power consumed globally. Photovoltaic solar cells must increase the power