Thin Film Solar Cell

Thin film solar cell




A Thin-Film Solar Cell (TFSC), also called a Thin-Film Photovoltaic Cell (TFPV), is a solar cell that is made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers.

Many different photovoltaic materials are deposited with various deposition methods on a variety of substrates. Thin Film Solar Cells are usually categorized according to the photovoltaic material used. The following catgories exist:

• Cadmium Telluride (CdTe)

• Copper indium gallium selenide (CIS or CIGS)

• Dye-sensitized solar cell (DSC)

• Organic solar cell

• Thin-film silicon (TF-Si)

The TFSC types that appear in bold letters are currently in mass production while the rest are still in the development or pilot-plant phase. As you can see, in most cases the TFSC are named after the photovoltaic material that they use, although there seems to be some confusion regarding some TFSC types because manufacturers and researchers use different names to describe the same technologies.

Contents

• 1 Time Award

• 2 Thin-film silicon

o 2.1 Design / Fabrication

o 2.2 Micromorphous silicon

o 2.3 Efficiency

o 2.4 BIPV

• 3 Organic solar cells

• 4 Efficiency

• 5 Cost and Market

• 6 Installations

• 7 See also

• 8 References

o 8.1 Sources

• 9 External link




Thin-film silicon

A silicon thin-film cell is a thin-film cell that uses amorphous (a-Si or a-Si:H), protocrystalline, nanocrystalline (nc-Si or nc-Si:H) or black silicon

Thin-film silicon is opposed to wafer silicon (also called bulky or crystalline silicon).

Design / Fabrication

The silicon is mainly deposited by chemical vapor deposition, typically plasma-enhanced (PE-CVD), from silane gas and hydrogen gas. Other deposition techniques being investigated include sputtering and hot wire techniques.

The silicon is deposited on glass, plastic or metal which has been coated with a layer of transparent conducting oxide (TCO).

A p-i-n structure is usually used, as opposed to an n-i-p structure. This is because the mobility of electrons in a-Si:H is roughly 1 or 2 orders of magnitude larger than that of holes, and thus the collection rate of electrons moving from the p- to n-type contact is better than holes moving from p- to n-type contact. Therefore, the p-type layer should be placed at the top where the light intensity is stronger, so that the majority of the charge carriers crossing the junction would be electrons[2].

Micromorphous silicon

Micromorphous silicon module technology combines two different types of silicon, amorphous and microcrystalline, in a top and a bottom photovoltaic cell. Use of protocrystalline silicon for the intrinsic layer has shown to optimize the open circuit voltage of an a-Si photovoltaic cell. [3]

Efficiency

These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails). The solar cells made from these materials tend to have lower energy conversion efficiency than bulk silicon (also called crystalline or wafer silicon), but are also less expensive to produce. The quantum efficiency of thin-film solar cells is also lower due to reduced number of collected charge carriers per incident photon.

Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it absorbs the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. As nc-Si has about the same bandgap as c-Si, the nc-Si and a-Si can advantageously be combined in thin layers, creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si.

Recently, solutions to overcome the limitations of thin-film silicon have been developed. Light trapping schemes where the incoming light is obliquely coupled into the silicon and the light traverses the film several times enhance the absorption of sunlight in the films. Thermal processing techniques enhance the crystallinity of the silicon and pacify electronic defects.[citation needed]

BIPV

A silicon thin film technology is being developed for building integrated photovoltaics (BIPV) in the form of semitransparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

Organic solar cells

The Organic solar cell is another alternative to the more conventional materials used to make photovoltaics. Although a very novel technology it is promising since it offers a very low cost solution.

Efficiency

Efficiency is anticipated to rise from a current 6%–12% to 10%–15% in the coming years, with a potential of more than 20% in the longer term. [4]

Cost and Market

Main articles: List of photovoltaics companies and Low-cost photovoltaic cell

Scaling factors, efficiency gains and the new production technologies are expected to reduce thin-film module manufacturing costs to €1/Wp (and below) in the near future. The thin-film PV market, showing a spectacular annual growth rate of 126% in 2007[5]

In recent years, the manufacturers of thin-film solar modules are bringing costs down and gaining in competitive strength through advanced thin film technology. However, the traditional crystalline silicon technologies will not give up their market positions until the next few years later because they still hold considerable development potential in terms of the cost. Efficiency of thin film solar is considerably lower and thin film solar manufacturing equipment suppliers intend to score costs of below USD 1/MW, and claimed by Anwell Technologies Limited that they intend to bring it down further to USD 0.5/MW. [6] Those equipment suppliers have been doing R&D for micro-morphous silicon modules since 2008. This technology represents a development based on the thin-film panels made of ordinary amorphous silicon marketed at present that brings higher cell efficiency by depositing an additional absorber layer made of micro crystalline silicon on the amorphous layer. Some equipment suppliers even claim that there will be machinery in market to manufacture these new modules at USD 0.70. [7] With such potential of further development of thin film solar technology, the European Photovoltaic Industry Association (EPIA) expects that manufacturing capacities for these technologies will double to over 4GW by 2010 representing a market share of around 20%. [8]

Installations

First Solar, the CdTe thin-film manufacturer stated that "at the end of 2007, over 300 MW of First Solar PV modules had been installed worldwide." [9] Below is a list of several recent installations:[10]

1. Since 16 October 2008, Germany's largest thin-film pitched roof system, constructed by Riedel Recycling, has been in operation and producing solar power in Moers near Duisburg. Over eleven thousand cadmium telluride modules, from First Solar, deliver a total of 837 kW [11].

2. First Solar recently completed a 2.4 MW rooftop installation as part of Southern California Edison program to install 250 MW of rooftop solar panels throughout Southern California over by 2013. [12]

3. First Solar announced a 7.5 MW system to be installed in Blythe, CA, where the California Public Utilities Commission has accepted a 12 ¢/kWh power purchase agreement with First Solar (after the application of all incentives). [13]

4. Construction of a 10 MW plant in the Nevada desert began in July 2008. [2] [3] First Solar is partnering with Sempra Generation, which will own and operate the PV power-plant, being built next to their natural gas plant.

5. Riedel Recycling (837 kW and will deliver around 750 MWh per year)

6. Stadtwerke Trier (SWT) in Trier, Germany (the plant is expected to produce over 9 GWh annually)

7. A 40 MW system is being installed by juwi group in Waldpolenz Solar Park, Germany. At the time of its announcement, it was both the largest planned and lowest cost PV system in the world. The price of 3.25 euros translated then (when the euro was equal to US$1.3) to $4.2 per installed watt. [14]

On the other hand, the solar energy experts at Denver-based Conergy Americas and officials at California's South San Joaquin Irrigation District (SSJID)[15] have installed what is believed to be the world's first single-axis solar tracking system featuring thin-film photovoltaic cells. [16]

Flexible CdTe cells with a record efficiency of 12.4%

EMPA, the Swiss Federal Laboratories for Materials Testing and Research in Dubendorf, Switzerland, has improved the efficiency of flexible CdTe thin film solar cells to 12.4%.



To improve the stability of CdTe/CdS solar cells, a new back contacting process was developed. According to the laboratory, new materials for the buffer layer and the metallisation yield stable cells. Accelerated testing under aggravated conditions (1 sun irradiation @ 80°C cell temperature) show the stability of these cells corresponding to 70 years in the field, it added.



The Laboratory for Thin Films and Photovoltaics, which is working on both CdTe and CuIn1-xGaxSe2, attached flexible CdTe thin-film solar cells on a lightweight polyimide film by using low temperatures (lower than 450°C) vacuum evaporation process to grow CdS/CdTe layers. The researchers used zinc oxide doped on aluminium (ZnO:Al) as a transparent electric contact instead of the expensive indium tin oxide (ITO) layer used in earlier 11.4% solar cells. In addition to being cheaper, the ZnO/ZnO:Al bi-layer improved process yield and reproducibility of high efficiency solar cells, according to EMPA.



The 12.4% efficiency of the flexible thin-film solar cells was measured under standard AMI.5 illumination condition. The parameters were Voc = 823 mV, Jsc = 19.6 mA.cm-2, FF = 76.5%.

References

1. ^ http://www.time.com/time/specials/packages/article/0,28804,1852747_1854195_1854153,00.html

2. ^ Amorphes Silizium für Solarzellen[1]

3. ^ J. M. Pearce, N. Podraza, R. W. Collins, M.M. Al-Jassim, K.M. Jones, J. Deng, and C. R. Wronski "Optimization of Open-Circuit Voltage in Amorphous Silicon Solar Cells with Mixed Phase (Amorphous + Nanocrystalline) p-Type Contacts of Low Nanocrystalline Content", Journal of Applied Physics, 101(11), 114301, 2007.http://me.queensu.ca/people/pearce/publications/documents/t14.pdf

4. ^ http://www.renewableenergyworld.com/rea/news/article/2009/03/utility-scale-thin-film-three-new-plants-in-germany-total-almost-50-mw?cmpid=WNL-Friday-March13-2009

5. ^ http://www.renewableenergyworld.com/rea/news/article/2009/03/utility-scale-thin-film-three-new-plants-in-germany-total-almost-50-mw?cmpid=WNL-Friday-March13-2009

6. ^ "ANWELL produces its first solar panel" (html). NextInsight. 2008-09-02. http://www.nextinsight.com.sg/content/view/1480/60/.

7. ^ "Photovoltaics: Thin-film technology about to make its breakthrough" (html). Solar server. 2008-08-07. http://www.solarserver.de/solarmagazin/index-e.html.

8. ^ "EPIA projects a rosy picture for the thin film industry" (html). Thin film today. http://social.thinfilmtoday.com/news/epia-projects-rosy-picture-thin-film-industry.

9. ^ http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=324202

10. ^ http://www.renewableenergyworld.com/rea/news/article/2009/03/utility-scale-thin-film-three-new-plants-in-germany-total-almost-50-mw?cmpid=WNL-Friday-March13-2009

11. ^ http://www.pv-tech.org/news/_a/germanys_largest_thin_film_pitched_roof_system_begins_production/

12. ^ "California Utility to Install 250MW of Roof-Top Solar". SustainableBusiness.com. 2008-03-27. http://www.sustainablebusiness.com/index.cfm/go/news.display/id/15670.

13. ^ "First Solar announces two solar projects with Southern California Edison". Semiconductor-Today.com. 2008-07-17. http://www.semiconductor-today.com/news_items/2008/JULY/FIRSTSOLAR_170708.htm.

14. ^ Report at juwi.dePDF (401 KB)

15. ^ http://www.ssjid.com

16. ^ http://www.renewableenergyworld.com/rea/partner/conergy-inc-1210/news/article/2009/04/conergy-brings-worlds-first-known-thin-film-solar-energy-tracking-system-and-400000-in-annual-utility-bill-savings-to-californias-south-san-joaquin-irrigation-district?cmpid=WNL-Wednesday-April22-2009

Sources

• Grama, S. “A Survey of Thin-Film Solar Photovoltaic Industry & Technologies.” Massachusetts Institute of Technology, 2008.

• Green, Martin A. “Consolidation of thin-film photovoltaic technology: the coming decade of opportunity.” Progress in Photovoltaics: Research and Applications 14, no. 5 (2006): 383-392.

• Green, M. A. “Recent developments in photovoltaics.” Solar Energy 76, no. 1-3 (2004): 3-8.

• Beaucarne, Guy. “Silicon Thin-Film Solar Cells.” Advances in OptoElectronics 2007 (August 2007): 12.

• Ullal, H. S., and B. von Roedern. “Thin Film CIGS and CdTe Photovoltaic Technologies: Commercialization, Critical Issues, and Applications; Preprint” (2007).

• Hegedus, S. “Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers.” Progress in Photovoltaics: Research and Applications 14, no. 5 (2006): 393-411.

• Poortmans, J., and V. Arkhipov. Thin Film Solar Cells: Fabrication, Characterization and Applications. Wiley, 2006.

• Wronski, C.R., B. Von Roedern, and A. Kolodziej. “Thin-film Si:H-based solar cells.” Vacuum 82, no. 10 (June 3, 2008): 1145-1150.

• Chopra, K. L., P. D. Paulson, and V. Dutta. “Thin-film solar cells: an overview.” Progress in Photovoltaics: Research and Applications 12, no. 2-3 (2004): 69-92.

• Hamakawa, Y. Thin-Film Solar Cells: Next Generation Photovoltaics and Its Applications. Springer, 2004.

• Green, Martin. “Thin-film solar cells: review of materials, technologies and commercial status.” Journal of Materials Science: Materials in Electronics 18, no. 0 (October 1, 2007): 15-19.







Flexible electronics





An Olympus Stylus camera without the case, showing the flex circuit assembly.

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide and PEEK Film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use. Flexible substrates have several advantages in many application:

• Tightly assembled electronic packages, where electrical connections are required in 3 axes, such as cameras (static application).

• Electrical connections where the assembly is required to flex during its normal use, such as folding cell phones (dynamic application).

• Electrical connections between sub-assemblies to replace wire harnesses, which are heavier and bulkier, such as in rockets and satellites.

• Electrical connections where board thickness or space constraints are driving factors.

Contents

• 1 Applications

o 1.1 Photovoltaic cells

• 2 See also

• 3 References


Applications

Flex circuits are often used as connectors in various applications where flexibility, space savings, or production constraints limit the serviceability of rigid circuit boards or hand wiring. In addition to cameras, a common application of flex circuits is in computer keyboard manufacturing; most keyboards made today use flex circuits for the switch matrix.

In LCD fabrication, glass is used as a substrate. If thin flexible plastic or metal foil is used as the substrate instead, the entire system can be flexible, as the film deposited on top of the substrate is usually very thin, on the order of a few micrometres.

OLEDs are normally used instead of a back-light for flexible displays, making a flexible organic light-emitting diode display.

Photovoltaic cells

Flexible solar cells have been developed for powering satellites. These cells are lightweight, can be rolled up for launch, and are easily deployable, making them a good match for the application.

On the other hand, Copper indium gallium diselenide (CIGS) solar cells are lightweight, flexible, and durable, which make them ideal for portable power (including solar jackets). CIGS is 1.5 to 2x greater in performance than comparable thin film flexible solar materials.