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What is Copper Indium Gallium Selenide (CIGS)?
Copper Indium Gallium Selenide (CIGS) solar cell is a thin-film solar cell, which is used for converting sunlight into electricity. CIGS is made through deposition a thin layer of copper, indium, gallium, and selenium on glass or plastic, and with electrodes on the front and back to collect electricity.
CIGS modules have the highest level of efficiency (22.8%) compared to crystalline silicon (c-Si) wafer-based solar cells. For producing 1000 MW/yr with 15% module efficiency, the cost of CIGS module production is estimated to be $0.34/W.
Thin-film solar cells that are based on copper indium gallium selenide are earning the attention of the solar power generation industry all over the globe. This is due to its higher efficiency that have achieved 22.8 percent as compared to the efficiency of crystalline silicon (c-Si) wafer-based solar cells.
For a production capacity of 1000 MW y−1 with 15 percent module efficiency, the cost of CIGS module production is estimated to be $0.34 W−1. While CIGS cells with a glass layer substrate, a graded bandgap high-temperature deposition process has been established. But this deposition process is not yet been established for CIGS over flexible polymer substrates and was maintained under the low-temperature disposition process.
As to small area devices, the main focus is the precise control over CIGS film efficiency. For industrial production, low-cost, production, high-throughput and tolerance process are the main focus in commercializing solar thin-film technology. Due to the complexity process, the production of the CIGS module was delayed behind the cadmium telluride (CdTe) modules.
What is Copper Indium Gallium Selenide (CIGS) solar cell?
Copper Indium Gallium Selenide (CIGS) solar cell is being manufactured by depositing thin semiconductor layers of copper, indium, gallium, and selenium on plastic or glass with electrodes on the front and back to collect electricity. It is used in generating electricity by converting sunlight. A much thinner film is required instead of those semiconductor materials because the material strongly absorbs sunlight and it has a high absorption coefficient.
Furthermore, CIGS thin films act as the direct semiconductor bandgap and form a heterojunction when the bandgaps of the two materials are not equal. The thin film is being deposited onto a substrate, it can be a polyamide film, soda-lime glass or metal in forming the rear surface contact. On the other hand, if a nonconductive material is used for the substrate then a metal like molybdenum will be used as the conductor.
Whereas, materials like doped zinc oxide, indium tin oxide and other advanced organic films which are nano-engineered-carbon-based are being used to provide ohmic contact. These are required to enable the front surface contact to conduct electricity and to be transparent so it will allow the light to reach the cell. The process occurred when the electron-hole pairs have been formed and the so-called area “depletion region” is being formed at the p– and n-type heterojunction materials of the cadmium-doped surface of the CIGS cell. Thus, separating the electrons from the holes and allowing them to generate an electrical current.
In addition, CIGS solar cell is considered as one of the three mainstream thin-film PV technologies in Solar cells industry, while the other two thin-film PV devices are the cadmium telluride and amorphous silicon. Similarly, CIGS layers are also made up of a thin layer that is flexible enough to be deposited easily and firmly on flexible substrates. Since it can be manufactured on flexible substrates, it becomes more ideal for a variety of applications like styling and designing for architects unlike current crystalline photovoltaics and other thin-films solar devices that are fixed and hard-bounded.
Also, CIGS solar cells can be manufactured without glass to be shatter-resistant and these are just a fraction of the weight of silicon cells. They can be integrated into vehicles from small ones like cars to large tractor-trailers, and even aircraft. Since their low profile can minimize air resistance and they are not significantly heavy-weight.
CIGS solar cells are also considered in the early stages of large-scale commercialization, as these solar cells can be manufactured by using a process that has the capacity to reduce the cost of photovoltaic devices production. Moreover, as the uniformity, reliability and performance of CIGS products develop, the bigger the potential of the technology to expand significantly its market share and this can eventually become prominent in the near future. Furthermore, given the usage and hazards of cadmium extraction, CIGS solar cells contribute only a few health factors and environmental concerns compared to the cadmium telluride solar cells.
Copper Indium Gallium Selenide (CIGS) Efficiency
CIGS is commonly used in the form of polycrystalline thin films. Its modules have the highest level of efficiency which is estimated at about 22.8 percent in comparison to wafer-based crystalline silicon solar cells. CIGS surface was modified to make it almost a look-alike of CIS by one of the team at the National Renewable Energy Laboratory and it achieved almost 20 percent efficiency record at the time. Furthermore, the U.S. National Renewable Energy Laboratory affirmed that a large meter-square area production panel managed to obtain 13.8 percent module efficiency and 13 percent total-area efficiency with other production modules.
While in September 2012, the German Manz AG introduced a CIGS solar module having an efficiency of 14.6 percent on the total module surface and 15.9 percent on the aperture, which has been manufactured on a mass production facility. On the other hand, MiaSolé managed to obtain a certified aperture-area efficiency of 15.7 percent on a 1m2 production module while Solar Frontier claimed that they gain a 17.8 efficiency on a 900 cm2 module. In 2013, CIGS solar cells on flexible polymer foils that were developed by the scientists at the Swiss Federal Laboratories for Materials Science and Technology were able to attain a new record efficiency of 20.4 percent. This record displays both the highest efficiency and greatest flexibility of CIGS.
Higher efficiencies, around 30 percent can only be obtained by utilizing optics to concentrate the incident light. Additionally, the use of gallium can increase the optical bandgap of the CIGS layer than the pure CIS, the increase of the optical bandgap will also increase the open-circuit voltage.
Whereas, the module production cost of the CIGS is estimated to be at $0.34 per watt for producing 1000 MW per year with 15 percent module efficiency. Moreover, laboratory experiments presented an efficiency record of a CIGS cell with a modified surface structure of about 23.2 percent, in the year 2014. However, most commercial CIGS cells have lower efficiencies, with some modules obtaining about 14 percent conversion.
However, the best performance normally comes from cells deposited on glass substrate, as all of these solar technologies generally use high-temperature deposition techniques, and although some advances in low-temperature deposition still CIGS cells have erased much of this performance in other substrates.
The most widely recognized vacuum-based procedure is to co-evaporate or co-sputter indium, gallium and copper onto a substrate at room temperature at that point, toughen the subsequent film with a selenide vapour. An elective procedure is to co-vanish copper, gallium, indium and selenium onto a warmed substrate.
The alternative way of a non-vacuum-based stores nano-particles of the antecedent materials on the substrate and then it was being sintered in situ. Electroplating is another minimal effort choice to apply the CIGS layer.
Co-evaporation, or co-deposition, is the most common CIGS fabrication strategy. Boeing’s co-evaporation procedure stores bilayers of CIGS with a variety of stoichiometry onto a warmed substrate and enables them to intermix.
NREL built up another procedure that includes three deposition steps and created the efficiency of the current CIGS at around 20.3 percent. The initial phase in NREL’s strategy is the co-deposition of indium, gallium, and selenide. Then followed by copper and selenide kept at a higher temperature to allow the dispersion and intermixing of the components. For the last step of the process, indium, selenide and gallium are being deposited again stored to make the overall composition of copper deficient.
Compound Vapour Deposition
Chemical vapour deposition (CVD) has been executed in numerous ways for the deposition of CIGS. Procedures involve plasma-enhanced CVD (PECVD, atmosphere pressure metal-organic CVD (AP-MOCVD), aerosol assisted MOCVD (AA-MOCVD) and low-pressure MOCVD (LP-MOCVD). Research is planning to change from double source forerunners to single-source precursors. Numerous source precursors must be homogeneously blended and the precursor’s flow rate must be kept at the proper stoichiometry. Single-source precursor techniques are not experiencing these disadvantages and the film composition is held in better control.
CVD was not utilized for business CIGS synthesis since 2014. The film produced by the CVD has low efficiency and a low VOC, which is a halfway result of a high defect concentration. Also, film surfaces are commonly rough which further diminishes the VOC. However, the essential lack of copper has been accomplished by utilizing AA-MOCVD alongside a (112) crystal orientation.
The temperatures of CVD deposition are significantly lower as compared to the processes of co-evaporation and selenizing of metallic precursors.
Hence, CVD has a lower thermal budget and lower costs. Potential assembling issues incorporate some difficulties in changing over CVD to an inline procedure just as the cost of dealing with volatile precursors.
Electrospray deposition can produce CIS films. This method includes the electric field-assisted spraying of ink directly which contains CIS nanoparticles onto the substrate and then sintering in an inactive environment. The main advantage of this method is that the procedure occurs at room temperature and some continuous or larger-scale production systems like roll-to-roll production mechanisms can be possible to attach to this process.
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DuPont will later this year try to show off a tool that can help get rid of glass.
The chemical giant-one of the biggest suppliers of chemicals andfilms to the solar industry-will be able to demonstrate a prototype ofa tool later this year that effectively will let manufacturers ofcopper indium gallium selenide (CIGS) solar cells and OLED displaysprotect their products with thin layers of ceramic and polymer materialinstead of glass. The prototype is being developed under a Departmentof Energy grant. Ideally, the material could move towardcommercialization in a few years.
“The CIGS sensitivity to moisture is quite severe,” said Marc Doyle,global business director, DuPont Photovoltaic Solutions. “It is easy tomake a good moisture barrier with glass, but what the industry needs isa transparent, flexible barrier with electrical properties.”
To date, moisture has been a major stumbling block for both OLEDand CIGS manufacturers: water vapor seeps in and defects andmalfunctions often follow. The moisture problem for OLEDs is probably aharder one to fix, said Doyle. The OLED industry, though, only has toproduce displays and bulbs that will last five to ten years. Customerswho buy CIGS solar panels will want twenty year warranties and expectthe products to last for 30 years.
A handful of companies-Nanosolar, Ascent Solar Technologies-have begun to produce flexible CIGS modules while start-up Palioswants to commercialize a barrier film it is acquiring in a bankruptcysale. Still, all of these companies will have to pass through rigoroustests before sales can zoom.
Besides protecting against moisture, a ceramic/polymer barrier couldcut costs. Glass for solar cells can cost $10 per square meter or more,he said, which can translate roughly to a cost of ten cents per watt ina solar module. Ideally, ceramic/polymer barriers would cost less oncein mass production. These sheets would also reduce the overall weight,which would cut shipping costs.
And of course, the ceramic/polymer solar modules would be flexible,which would expand the options for installation. These sorts of modulescould be integrated, potentially, into membrane roofing.
Potentially, the market might begin to see lighter and/or flexible crystalline silicon modulesbased around similar barriers and substrates. Manufacturers are workingto reduce the thickness of crystalline wafers to 160 microns. Thethinner wafers become, the more flexible crystalline will become.
DuPont isn’t a name many think of when contemplating the solar industry, but neither are 3M and Dow Chemical for that matter. Still, all three play crucial background roles in the industry. DuPont has produced encapsulants and metal pastes for the past several decades to PV makers.Tedlar, the material that serves as a back sheet for crystalline solarcells, is a de facto standard. DuPont, in fact, recently committed$295 million to expand its Tedlar manufacturing capacity.
Although solar companies continually tweak their formulas, onechange that may not come about as quickly is a switch from silver tocopper paste for wiring the solar cells inside modules. Copper costsless, but the raw material costs could be outweighed by the changesrequired in processing. Suntech Power Holdings employs copper in itsPluto panels, and start-up 1366 Technologies has promoted copper. It’sone of those issues to keep an eye on
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