Thin film solar cell
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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. Film thickness varies from a few nanometers to tens of micrometers.
Thin film is the rival technology to the predominant crystalline silicon solar cell and its market-share has been declining in recent years to about 9 percent of worldwide photovoltaic production in 2013.
Thin film solar panels are commercially available for installation onto the roofs of buildings, either applied onto the finished roof, or integrated into the roof covering. Advantages over traditional PV panels include lower weight, wind resistance and (limited) resistance to foot traffic. Disadvantages include increased cost and reduced efficiency. Building integrated photovoltaics (BIPV) uses semi-transparent thin film cells that can be applied as window glazing. These cells act as window tinting while also generating electricity.
Films are deposited with various deposition methods on a variety of substrates. Thin-film solar cells are usually categorized by photovoltaic material:
- Cadmium telluride solar cells (CdTe)
- Copper indium gallium selenide solar cells (CIS or CIGS)
- Amorphous silicon (a-Si) and other thin-film silicon cells.
- Emerging photovoltaics (organic, quantum dot, dye-sensitized, and perovskite solar cells)
- 1 History
- 2 Materials
- 3 Efficiencies, volumes and prices
- 4 Absorption
- 5 Production, cost and market
- 6 Installations
- 7 Awards
- 8 See also
- 9 References
- 10 Sources
- 11 External links
Thin film cells are well-known since the late 1970s, when solar calculators powerd by a small strip of amorphous silicon appeared on the market.
It is now available in very large modules used in sophisticated building-integrated installations and vehicle charging systems. GBI Research projected thin film production to grow 24% from 2009 levels and to reach 22,214 MW in 2020. "Expectations are that in the long-term, thin-film solar PV technology would surpass dominating conventional solar PV technology, thus enabling the long sought-after grid parity objective."
Thin-film technologies reduce the amount of active material in a cell. Most sandwich active material between two panes of glass. Since silicon solar panels only use one pane of glass, thin film panels are approximately twice as heavy as crystalline silicon panels, although they have a smaller ecological impact (determined from life cycle analysis). The majority of film panels have 2-3 percentage points lower conversion efficiencies than crystalline silicon. Cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si) are three thin-film technologies often used for outdoor applications. As of December 2013, CdTe cost per installed watt was $0.59 as reported by First Solar. CIGS technology laboratory demonstrations reached 20.4% as of December 2013.
Cadmium telluride (CdTe) is the predominant thin film technology. With about 5 percent of worldwide PV production, it accounts for more than half of the thin film market. The cell's lab efficiency has also increased significantly in recent years and is on a par with CIGS thin film and close to the efficiency of multi-crystalline silicon as of 2013. Also, CdTe has the lowest Energy payback time of all mass-produced PV technologies, and can be as short as eight months in favorable locations.
Although the toxicity of cadmium may not be that much of an issue and environmental concerns completely resolved with the recycling of CdTe modules at the end of their life time, there are still uncertainties and the public opinion is skeptical towards this technology.
The usage of rare materials may also be a critical issue to the economic viability of CdTe thin film technology. The abundance of tellurium—of which telluride is the anionic form—is comparable to that of platinum in the earth's crust and it contributes significantly to the module's cost.
Copper indium gallium selenide
A copper indium gallium selenide solar cell or CIGS cell uses an absorber made of copper, indium, gallium, selenide (CIGS), while gallium-free variants of the semmiconductor material are abbreviated CIS. It is one of three mainstream thin-film technologies, the other two being cadmium telluride and amorphous silicon, with a high efficiency of close to 20 percent and a share of 2 percent in the overall PV market in 2013. A prominent manufacturer of cylindrical CIGS-panels was the now-bankrupt company Solyndra in Fremont, California. Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering. In 2008, IBM and Tokyo Ohka Kogyo Co., Ltd. (TOK) announced they had developed a new, non-vacuum, solution-based manufacturing process for CIGS cells and are aiming for efficiencies of 15% and beyond.
Amorphous silicon (a-Si) is a non-crystalline, allotropic form of silicon and the most well-developed thin film technology to-date.
This type of thin-film cell is mostly fabricated by a technique called plasma-enhanced chemical vapor deposition. It uses a gaseous mixture of silane (SiH4) and hydrogen to deposit a very thin layer of silicon on a substrate, such as glass, plastic or metal, that has already been coated with a layer of transparent conducting oxide. Other methods used to deposit amorphous silicon on a substrate include sputtering and hot wire techniques. Thin-film silicon is an alternative to conventional wafer (or bulk) silicon (monocrystalline or polycrystalline).
a-Si is attractive as a solar cell material because it is abundant and non-toxic (unlike its CdTe counterpart) and requires a low processing temperature, enabling production of devices to occur on flexible and low-cost substrates. As the amorphous structure has a higher absorption rate than crystalline cells, the complete light spectrum can be absorbed with a very thin layer of photo-electrically active material. A film only 1 micrometer thick can absorb 90% of the usable solar energy.
Amorphous silicon cells suffer from significant output loss (in the range 15-35%) from sun exposure due to the Staebler-Wronski effect. Thinner layers increase the electric field strength across the material, reducing degradation but also light absorption/efficiency. A laser scribing sequence enables the front and back of adjacent cells to be directly interconnected in series, without solder connection between cells, increasing voltage. a-Si production is scaleable, due to its reduced material requirement and because large-area production systems are available.
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 higher energy infrared portion of the spectrum.
Its basic electronic structure is the p-i-n junction. A-Si's amorphous structure implies high inherent disorder and dangling bonds, making it a bad conductor for charge carriers. These dangling bonds act as recombination centers that severely reduce carrier lifetime and pin the Fermi level so that doping the material to n- or p- type is not possible.
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 n- to p-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 are electrons.
A layer of amorphous silicon can be combined with layers of other allotropic forms of silicon to produce a multijunction photovoltaic cell. When only two layers (or p-n junctios) are combined, it is called a tandem-cell. By stacking these layers on top of one other, a broader range of the light spectra is absorbed, more photons are captured, improving the cell's overall efficiency.
In micromorphous silicon, a layer of a-Si is combined with a layer of nanocrystalline silicon creating a tandem cell. The top amorphous silicon layer absorbs the visible light, leaving the infrared part to the bottom nanocrystalline silicon layer.
Because all layers are made of silicon, they can be manufactured using PECVD. The band gap of a-Si is 1.7 eV and that of c-Si is 1.1 eV. The c-Si layer can absorb red and infrared light. The best efficiency can be achieved at transition between a-Si and c-Si. As nanocrystalline silicon (nc-Si) has about the same bandgap as c-Si, nc-Si can replace c-Si.
Amorphous silicon can also be combined with protocrystalline silicon into a tandem-cell. Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open-circuit voltage. 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).
Black silicon ia another material used in thin-film technology.
The National Renewable Energy Laboratory (NREL) classifies a number of thin-film technologies as emerging photovoltaics—most of them have not yet been commercially applied and are still in the research or development phase. Many use organic materials, often organometallic compounds as well as inorganic substances. Despite the fact that their efficiencies had been low and the stability of the absorber material was often too short for commercial applications, there is a lot of research invested into these technologies as they promise to achieve the goal of producing low-cost, high-efficient solar cells.
Emerging photovoltaics include:
- Organic solar cell
- Polymer solar cell
- Quantum dot solar cell
- Dye-sensitized solar cell, also known as the Grätzel cell,
- Copper zinc tin sulfide solar cell (CZTS), and derivates CZTSe and CZTSSe
- Perovskite in photovoltaics
Especially the achivements in the research of perovskites have received tremendous attention in the public, as their research efficiencies soared in recent years to almost 20 percent. They also offer a wide spectrum of low-cost applications.
The Dutch Radboud University Nijmegen made a single junction GaAs cell that reached 25.8% in August 2008, using a 4 µm thick GaAs layer transferred from a wafer base to glass or plastic film. Recently, this increased to 28.8%. The efficiency was attributed to high quality GaAs epitaxial growth, surface passivation by the AlGaAs and the promotion of photon recycling. The lab efficiency of GaAs/GaAs concentrator solar cell topped 28%.
Efficiencies, volumes and prices
Since the invention of the first modern silicon solar cell in 1954, incremental improvements have resulted in modules capable of converting 12 to 18 percent of solar radiation into electricity.
Cells made from these materials tend to be less efficient than bulk silicon, but are less expensive to produce. Their quantum efficiency is also lower due to reduced number of collected charge carriers per incident photon.
The performance and potential of thin-film materials are high, reaching cell efficiencies of 12–20%; prototype module efficiencies of 7–13%; and production modules in the range of 9%. The thin film cell prototype with the best efficiency yields 20.4% (First Solar), comparable to the best conventional solar cell prototype efficiency of 25.6% from Panasonic.
Multiple techniques have been employed to increase the amount of light that enters the cell and reduce the amount that escapes without absorption. The most obvious technique is to minimizing the top contact coverage of the cell surface, reducing the area that blocks light from reaching the cell.
Anti-reflective coatings can create destructive interference within the cell. This can be done by modulating the refractive index of the surface coating. Destructive interference eliminates the reflective wave and thus all incident light enters the cell.
Surface texturing is another option for increasing absorption, but increases costs. By applying a texture to the active material's surface, the reflected light can be refracted into striking the surface again, thus reducing reflectance. A textured backreflector can prevent light from escaping through the rear of the cell.
Thermal processing techniques can significantly enhance the crystal quality of the silicon and thereby increase efficiency.
Further advancement into geometric considerations can exploit nanomaterial dimensionality. Large, parallel nanowire arrays enable long absorption lengths along the length of the wire while maintaining short minority carrier diffusion lengths along the radial direction. Adding nanoparticles between the nanowires allows conduction. The natural geometry of these arrays forms a textured surface that traps more light.
Production, cost and market
- CI(G)S: 0.7 GW (2.0%)
- a-Si: 0.7 GW (2.0%)
- CdTe: 1.8 GW (5.1%)
- Mono-Si: 12.6 GW (36.0%)
- Mulit-Si: 19.2 GW (54.9%)
Crystalline silicon technologies continue to dominate the photovoltaic market, because of superior efficiency. In July 2013 the price of thin-film a-Si dropped to €0.38/Wp, about $0.50. using a micro crystalline silicon layer above the amorphous layer. In 2011, GE announced plans to spend $600 million on a new CdTe solar cell plant and enter this market.
Thin film manufacturers faced price competition from Chinese refiners of silicon and manufacturers of conventional solar panels. As of January, 2013, some thin film firms together with their patents were sold to Chinese firms below cost.
In 2013 thin-film technologies account for about 9 percent of worldwide deployment, while 91 percent being crystalline silicon.
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- A 2.4 MW First Solar rooftop installation was part of Southern California Edison program to install 250 MW of rooftop solar panels throughout Southern California by 2013.
- First Solar announced a 7.5 MW system to be installed in Blythe, CA, where the California Public Utilities Commission signed a 12 ¢/kWh power purchase agreement with First Solar (after incentives).
- Construction of a 10 MW plant in the Nevada desert began in July 2008. First Solar is partnering with Sempra Generation, which will own and operate the PV power-plant, being built next to their natural gas plant.
- Stadtwerke Trier (SWT) in Trier, Germany was expected to produce over 9 GWh annually.
- A system by Juwi in Waldpolenz Solar Park, Germany was at the time of its announcement, both the largest and lowest cost PV system in the world at 40 MW. The price of 3.25 euros translated then (when the euro was equal to US$1.3) to $4.2 per installed watt.
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