Novel industrial approaches in solar-cell production

簡評:

雷射在太陽能電池產業的用途,請多參考--本資料很有用處!

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Aart Schoonderbeek and Andreas Ostendorf

Laser technology applications are indispensable in the photovoltaic industry, allowing both enhanced energy-generating efficiency and reduced costs.

The photovoltaic industry has experienced enormous growth in recent years. However, for solar-cell technology to become competitive in the long term, both an increase in energy-generating efficiency and a reduction in production costs is required. Several laser applications for solar-cell production are shown in Figure 1, of which three—hole drilling for back-contacted solar cells, silicon dioxide (SiO2) removal for making grooves, and scribing of thin-film cells—are described here in more detail.

Hole drilling for back-contacted cells
Laser drilling is a key technology used in many new back-contact solar-cell production concepts because alternative economically feasible drilling processes are not yet available.1–4 Square 6in-wide (156×156mm2) and 250μm−thin wafers are standard in industry. Common industrial photovoltaic cells have a screen-printed contact layer on the front, which blocks 5–7% of the incoming light by shadowing. To overcome this performance degradation, many new cell concepts are being developed, usually with the emitter contact either completely or partially on the rear. This results in higher cell efficiencies for energy generation. For these emitter wrap-through (EWT) cells, laser drilling is the only suitable method to create the necessary holes from the front to the back. Typically, a drilling efficacy of 15,000 80μm-diameter holes is required.1 Figure 2 shows a typical hole. Up to several thousand holes can be drilled per second.

Figure 1. Laser applications for solar-cell production: hole drilling of back-contact solar cells, SiO2 removal for groove production, laser welding/soldering of contacts, edge isolation, wafer cutting, removal of dielectric layers for improved contact performance, texturing to increase the absorption of sunlight and therefore to enhance the efficiency of the cell, scribing of thin-film cells, and edge deletion.

Figure 2. A typical hole drilled with a pulsed-fiber laser, characterized by a burst of 20 pulses and a pulse energy of 1mJ (after etching). A human hair illustrates the small diameter of the holes.

SiO2 removal for grooves
For wafer-based solar cells, grooves are used in several novel designs.1,5 Grooves typically have a depth of up to several tens of microns. The groove is obtained in two steps. First, a barrier layer, such as SiO2 or silicon nitride (SiNx), is removed using a laser beam. Subsequently, a chemical etching process is applied to remove the laser-damaged silicon and to obtain the desired depth. The residual SiO2 or SiNx layer functions as a barrier during the etching process. Figure 3 shows a typical groove result.

Figure 3. Groove processed with an excimer laser at a wavelength of 248nm. The rectangular laser spot is shown in the processed area. The groove depth is about 20μm. The laser-processed surface is smooth after chemical etching, and does not contain any debris.

Figure 4. Cross-section of a thin-film solar cell (not to scale). The layers are typically up to several microns thick. The different layers, P1–P3 (transparent conductive oxides or absorber material), are successively deposited and scribed during the production process. The current flow is shown by the arrows.

Figure 5. Deposition and scribing for a thin-film solar cell. (1) Substrate, (2) TCO deposition, (3) P1 TCO scribing, (4) absorber deposition, (5) P2 absorber scribing, (6) TCO deposition, and (7): P3 TCO scribing.Scribing of thin-film cells

Scribing is required to obtain a monolithic series connection of modules.6 This essential technology has both allowed economically feasible solar-cell production and enabled the strong growth market for thin-film photovoltaic applications. Thin-film solar cells can be made using different material combinations. The substrate can either be glass, plastic foil, or metal foil. Generally, the cell itself consists of a back contact, a front-contact layer, and an absorber material in between. For the contacts, transparent conductive oxides (TCOs) or metals are used. Commonly used absorber materials include silicon, cadmium telluride, copper-indium di-selenide (CIS), and other combinations with copper indium, such as CIGS or CIGSSe. The layers are typically up to several microns thick. In Figure 4, a thin-film solar-cell cross-section is shown. Common referencing to the laser processes includes ‘patterning 1’ or P1 for the first contact, P2 for the absorber, and P3 for the second contact.
Thin-film solar-cell scribing is, in essence, based on the different transmissivities of the film materials at the laser wavelengths used. The deposition and scribing processes are shown schematically in Figure 5. The first TCO layer deposited can be zinc oxide, tin dioxide, or indium-tin oxide. Usually, when glass is used as the substrate, scribing is done from the glass side. An example is shown in Figure 6. Silicon as the absorber material is usually scribed with a laser wavelength of 532nm. For this setup, TCO is transparent, and silicon absorbs radiation in a thin layer. Scribing of the third layer can be done using the same laser wavelength as for the absorber. In this case, the second absorption layer is also removed. This is not necessary, but it does not affect the solar-cell function either.

Figure 6. Example of TCO scribing of P1 with a diode-pumped solid-state laser at a wavelength of 1047nm.

In summary, the role of laser technology in the solar photovoltaic industry is gaining importance. Laser applications enable economic and technical feasibility of new design concepts. To achieve the required performance quality in acceptably short processing times, further technological system development is necessary. Both system and process development of newly emerging laser sources and their applications are the subject of future research at our institute.

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About Writer

Aart Schoonderbeek
Production and Systems Department
Technologies for Non-Metals Group
Laser Zentrum Hannover e.V.
Hannover, Germany
http://www.lzh.de/
Aart Schoonderbeek obtained his PhD at the Netherlands Center for Laser Research in 2005, supported by the chairs of Applied Laser Technology and of Laser Physics and Nonlinear Optics, both at the University of Twente (Netherlands). He was subsequently employed as a research scientist at the Laser Zentrum Hannover. He works on process technologies for nonmetals, concentrating on laser processing of glass and silicon.

Andreas Ostendorf
Managing Director
Laser Zentrum Hannover e.V.
Hannover, Germany
http://www.lzh.de/

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References:
1. P. Engelhart, A. Teppe, A. Merkle, R. Grischke, R. Meyer, N.-P. Harder, R. Brendel, The RISE-EWT solar cell: new approach towards simple high efficiency silicon solar cells, Proc. 15th PVSEC, pp. 802-803, 2005.
2. J. M. Gee, W. K. Schubert, P. A. Basore, Emitter wrap-through solar cell, Proc. 23rd IEEE Photovolt. Specialists Conf., pp. 265-270, 1993.
3. F. Clement, M. Lutsch, T. Kubera, M. Kasemann, W. Kwapil, C. Harmel, N. Mingirulli, D. Erath, H. Wirth, D. Biro, R. Preu, Processing and comprehensive characterisation of screen-printed mc-Si Metal Wrap Through (MWT) solar cells, Proc. 22nd EU-PVSEC, pp. 1399-1402, 2007.
4. I. Romijn, M. Lamers, A. Stassen, A. Mewe, M. Koppes, E. Kossen, A. Weeber, ASPIRE: a new industrial MWT cell technology enabling high efficiencies on thin and large mc-Si wafers, Proc. 22nd EU-PVSEC, pp. 1043-1049, 2007.
5. K. C. Heasman, A. Cole, M. Brown, S. Roberts, S. Devenport, I. Baistow, T. M. Bruton, Process development of laser grooved buried contact solar cells for use at concentration factors up to 100x, Proc. 22nd EU-PVSEC, pp. 1511-1512, 2007.
6. S. Haas, A. Gordijn, H. Stiebig, High speed laser processing for monolithical series connection of silicon thin-film modules, Progr. Photovolt., pp. 195-203, 2007. doi:10.1002/pip.792

A cool light bulb

簡評:
這兩個高麗棒子的研究還真不賴!
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Shawn-Yu Lin and Yong-Sung Kim



A photonic band-pass filter enclosing the filament recycles infrared emissions, reducing temperature and producing an eight-fold increase in energy efficiency.



Since the time of Thomas Edison, incandescent light bulbs have been the dominant light source for illumination. Today, incandescent bulbs still hold crucial advantages. They produce a warm white light and can be dimmed easily using inexpensive controls. They have a relatively inexpensive first-cost per lumen, a long-established infrastructure, and do not contain hazardous materials such as mercury. However, because of their relatively low energy efficiency, incandescent bulbs are being replaced rapidly in many areas. It is time to re-visit the fundamental limit of the incandescent bulb and to improve its efficiency.
Incandescent bulbs emit light in a manner closely resembling Plank's law of blackbody radiation. The law describes how a body capable of absorbing all radiation contacting it (a blackbody) will emit at a given range of wavelengths dependant on its temperature. The inefficiency inherent in an incandescent bulb is due to the fact that it emits both infrared and visible light at temperatures between 2000 and 3000K. Specifically, the infrared portion of the radiation consumes about 88% of the input electric energy and becomes wasted heat (see Figure 1). Hence, recycling infrared light into useful visible light would improve incandescent efficiency.



Figure 1. A blackbody radiation curve at T=2800K, which is a typical operating temperature of a 100W incandescent bulb. Approximately 88% of the light is emitted in the infrared region. BB: blackbody.



Recycling processes have previously been developed in the form of reflecting envelopes using either a dielectric metal film stack1,2 or a dielectric multi-layered film.3 However, for both structures the reflectance in the near-infrared region is not high enough to bounce back all the infrared light. To overcome these limitations, we employed a two-dimensional metallic photonic band gap (PBG) filter architecture to enclose the incandescent filament. The filter acts as a perfect transmitter for the useful visible light and a perfect reflector for the undesirable infrared light. The reflected light is re-absorbed which, in turn, helps to heat up the filament. This infrared recycling process has two major energy consequences. First, it reduces the amount of electricity required to maintain a hot filament and thus improves electric-to-optical conversion efficiency. Second, it reduces the thermal radiation of the bulb as infrared photons cannot escape. With this approach, the energy efficacy of an incandescent light bulb can be improved by as much as eight times. Accordingly, the cost of a million-lumen-hour is reduced to $1.00–$2.00. We used silver as the metallic material because it has a low intrinsic absorption in the visible and near infrared wavelengths. The low absorption of silver is key to simultaneously achieving a high transmittance in the visible and a high reflectance in the infrared regions. A metallic PBG filter is also more practical to use as it is robust against thermal stress at high temperatures.



Figure 2. (a) The photon recycling scheme. (b) Schematic of the 2D metallic photonic crystal where 'a' is the pitch, 'd' is the size of the air opening, 'w' is the bar width, and 'h' is the thickness. Silver is used as the metal due to its low absorption in the visible and near infrared wavelengths. rf: radius of the filter. rb: radius of the blackbody filament, here a sphere.



To illustrate the validity of our approach, we have employed an ideal system that has a spherical blackbody filament enclosed by the filter: see Figure 2(a). The maximum luminous efficacy reaches 125lm/W. The details of the calculation were reported recently.4 For general purpose illumination, not only the efficiency but also the color quality is important in evaluating a bulb. The color quality of a bulb is commonly characterized by the correlated color temperature (CCT), used to categorize color tone, and the color rendering index (CRI), which measures the ability of a bulb to reproduce the true color of objects. If the CCT is lower than 3300K the color is categorized as a warm tone, whereas if the CCT is higher than 5300K the color is categorized as a cool tone. The CCT of our incandescent bulb did not exceed 3500K, indicating the filtered light is in the desired warm range. The CRI has a range between 0 and 100, with 0 being the minimum and 100 being the maximum color rendering capability. The color rendering index of our new light bulb is calculated to be between 68 and 90, better than that of a standard fluorescent lamp with a CRI of approximately 60.
Photon recycling via a metallic PBG filter is a promising new route to creating a ‘cool’ light bulb. Our next step is to study a cylindrical filter geometry that is comparable to the commonly used tungsten-filament configuration.

We would like to acknowledge the financial support of DOE-BES under grant number DE-FG02-06ER46347
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About Writer
Shawn-Yu Lin, Yong-Sung Kim
Physics
Rensselaer Polytechnic Institute
Troy, NY
Shawn-Yu Lin is an institute constellation professor and professor of physics at Rensselaer Polytechnic Institute. His expertise is in the interaction of light with hierarchy nanostructure. He is a fellow of the American Physical Society, a fellow of the Optical Society of America, and a distinguished member-of-technical-staff at Sandia National Laboratories
Yong Sung Kim specializes in electromagnetic wave modeling of three-dimensional photonic crystal structures including finite difference time domain, dispersion calculation and transfer matrix methods.
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References:
1. J. Brett, R. Fontana, P. Walsh, S. Spura, L. Parascandola, Development of high energy-conserving incandescent lamps, J., J. Illuminating Eng. Soc. 214, pp. 93, 1980.
2. R. Fontanta, I. Goldstein, L. Thorington, R. Howson, The design, construction and performance of an incandescent light source with a transparent heat mirror, Lighting Tech. 18, pp. 93, 1986.
3. R. Bergman, T. Parham, Application of thin film reflecting coating technology to tungsten filament lamps, IEE Proceedings-A 140, pp. 418, 1993.
4. Y. S. Kim, S. Y. Lin, A. Chang, J. H. Lee, K. M. Ho, Analysis of photon recycling using metallic photonic crystal, J. Appl. Phys. 102, pp. 063107, 2007.doi:10.1063/1.2779271

Self-Assembling Crystals Could Produce Better Optical Materials

Posted on: Friday, 27 June 2008, 06:02 CDT
By Shelley, Suzanne
MATERIALS Chemical engineers have developed a "self-assembling" method that could allow optical devices to be made less expensively than conventional processes, which require complex etching and other techniques common in the semiconductor industry.
The method, developed at Purdue Univ., works by positioning tiny particles onto a silicon template containing precisely spaced holes that are about one one-hundredth the width of a human hair. To produce the singlelayer structure, the engineers used Langmuir- Blodgett monolayer deposition, a standard technique used in physical chemistry, primarily to create lipid membranes for research.
The template is immersed in water in a trough-like vessel where a layer of particles has formed at the surface. As the template is pulled vertically out of the trough, the partides are pushed into the template holes by capillary force, the same phenomenon that causes water to rise to a higher level in a tube placed in a pool of water. It is critical for the particles to be spaced properly prior to the Langmuir-Blodgett deposition so that water can draw the particles into the holes in the template using capillary force, explains You-Yeon Won, an assistant professor of chemical engineering.
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The researchers have used the technique to create a "nearly perfect two-dimensional colloidal crystal," or a precisely ordered layer of particles, which is a critical step toward growing three- dimensional crystals for use in optical technologies.
"Making the first layer is very difficult, so we have taken an important step in the right direction," Won says. "Creating three- dimensional structures poses a big challenge, but I think it's feasible."
The single-layer structures might be used to form micro lenses to improve the performance of optical equipment, such as cameras and scientific instruments, or to control the color and other optical properties of materials for consumer products. More importantly, the technique could be used to create "omni-directional photonic band- gap materials," which would dramatically improve the performance of optical fibers, the researchers say. Omni-directional coatings would increase the amount of light transmitted by fiber-optics, and could possibly be used in future sensor technology and in optical computers and circuits that use light instead of electronic signals to process information.
The Purdue engineers are now investigating the creation of three- dimensional crystals from the two-dimensional structures. Currently, omni-directional materials are prohibitively expensive to manufacture.
A scanning electron microscopy photo shows a side-by-side comparison between Purdue's structure (right) and a structure that results when a template is not used. Photo courtesy of Y. Won and J. Hur.
Copyright American Institute of Chemical Engineers Jun 2008
(c) 2008 Chemical Engineering Progress. Provided by ProQuest Information and Learning. All rights Reserved.
Source: Chemical Engineering Progress

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