Alta Devices, a start-up in Santa Clara, Calif., presented research at the 37th IEEE Photovoltaic Specialist Conference, in Seattle, this week that claims its thin-film gallium-arsenide cell can convert 27.6 percent of the sunlight striking the cell into electricity, under standardized conditions. Since the paper was submitted, the company says it has upped the efficiency to 28.2 percent. That beats the previous record of 26.4 percent for a solar cell with a single p-n junction, which was the first improvement in years over 26.1 percent. Both numbers, according to Alta, were independently confirmed by the National Renewable Energy Laboratory.
The efficiency was measured on a laboratory-made solar cell. Efficiency tends to decrease once the cells are packaged into usable modules. “We assume we will ultimately be able to achieve modules that are around 26 percent, and that’s plenty to be competitive with fossil fuels,” says Christopher Norris, CEO of Alta.
The key to achieving the record was photon recycling. When the photons in sunlight are absorbed in a photovoltaic material, they split into an electron and a hole. The electrons that pass out of the cell can be used as electricity, but many of them are lost in the semiconductor when they recombine with a hole to produce either waste heat or a new photon. By carefully growing a high-quality single crystal of gallium arsenide, the company managed to ensure that more than 99 percent of the recombinations would result in new photons. Those photons could then create a new electron-hole pair and give the electron another chance to be captured as electricity. The Alta team also improved the reflectivity of the metal contacts on the back of the solar cell, so that any photons that exited the cell would be sent back in for possible reabsorption.
The theoretical maximum conversion efficiency for a solar cell with a single junction is 33.5 percent. “We can see a path to 30 percent with our same design right now,” says Norris. Adding a second junction could also increase the energy output.
The more efficient a solar cell is, the faster it pays back the cost of manufacturing and installing it. But efficiency and cost have been at odds with each other in solar cell design. Gallium arsenide is naturally better at converting light to electricity than the chief contenders, such as silicon and cadmium telluride, but it tends to be more expensive.
The most efficient materials are single-crystalline semiconductors, but those are usually pricier. Low-cost materials, such as amorphous silicon, cadmium telluride, and copper indium gallium selenide, are less efficient; CdTe cells are around 12 percent efficient. Alta solves this problem by using only a small amount of a high-quality material—a thin film of gallium arsenide about 1 micrometer thick.
“That is the whole trick. Don’t use much gallium and don’t use much arsenic,” Norris says. He says an Alta module should cost about the same per watt as a CdTe module but produce three times the energy.
The company cut down on the material cost by using a process called epitaxial liftoff, developed by Eli Yablonovitch, an engineering professor at the University of California, Berkeley, and a cofounder of Alta. Technicians start with a GaAs wafer as a seed layer and grow a thin-film photovoltaic device structure on top of that. They peel off the thin film, attach it to a metal backing, and finish processing it into a solar cell. The process leaves the original wafer, which they can reuse for the next batch of solar cells.
Alta is working on a pilot production line to produce samples of its solar cells sometime this year and expects to have early commercial shipments by late next year, Norris says. The company has raised US $72 million to develop its production process.
The launch of the Jawaharlal Nehru National Solar Mission (JNNSM) has triggered a demand in the market for solar photovoltaic (PV) projects and products. With this, the need for solar modules has increased, driving up demand for solar cells. However, delivering low cost solar energy is the biggest challenge faced by the manufacturers. Currently, a factor that influences the cost in a big way is efficiency. Thus, the right technological choice is critical in order to ensure good financial returns, while maintaining the quality and the long life of the product.
A solar cell, also called a photovoltaic cell, is used to convert solar energy into electrical energy. Solar cells are the basic elements of a solar module or panel. These cells are made of silicon, a common semiconductor. A typical modern solar cell measures 15 cm × 15 cm. It is covered by a clear anti-reflection coating (ARC) that reduces the amount of light lost to reflection at the cell surface.
Technology behind different types of cells
There are three main types of solar cells, which are distinguished by the type of crystal used in them. They are monocrystalline, polycrystalline, and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline ingots are manufactured through C2 method from polycrystalline granules or chunks. Monocrystalline wafers and slices are extracted from these ingots. With this method, quality of silicon further improves, and cell manufactured from these wafers have higher efficiency.
Production of polycrystalline cells is more cost efficient. In this process, ingots are developed with directional solidification system furnaces. These ingots are sliced into multicrystalline wafers. During the solidification of the material, crystal structures with different direction are formed. As a result, the solar cell is less efficient.
If a silicon film is deposited on glass or another substrate material, the result is a so called amorphous or thin layer cell. The layer thickness amounts to less than 1µm—for comparison, the thickness of a human hair is 50-100 µm. The production costs of this type of solar cell are lower because of the lower material costs. However, the efficiency of amorphous cells is the lowest when compared to the other two types of solar cells.
In order to provide suitable voltages and outputs for different applications, solar cells are connected together to form larger units. Cells connected in series have a higher voltage, while those connected in parallel produce more current. The interconnected solar cells are usually embedded in transparent ethylene vinyl acetate (EVA), fitted with an aluminium or stainless steel frame, and covered with transparent glass on the front and with tedlar on the backside to make a solar module.
Crystalline silicon cell: About 80 per cent of the manufacturers in the world use crystalline silicon (C-Si) cell based technology. “The current installed capacity of solar cells based on this technology stands at 600 MW and that of modules ranges between 900 MW to 1 GW in India,” informs Dr VK Kaul, general manager, Solar Photovoltaic Group, Central Electronics Ltd (CEL). CEL is among the pioneers in solar technology, and manufacturing crystalline silicon cells.
C-Si technology has been time tested and thus manufacturers rely on it a lot more. “One of the scoring factors for C-Si cells is its high efficiency. Two decades back, crystalline silicon cell efficiency was 8-10 per cent. Today, we are getting 16-17 per cent efficiency,” says Sunil Goel, vice president, Maharishi Solar Technology Pvt Ltd.
Monocrystalline cell: Websol Energy Systems Ltd manufactures monocrystalline (mono Si) cells, and has recently introduced two solar modules—W2300 and W2800. “Both the modules are made with 156 mm x 156 mm mono Si cells. A W2300 module is a configuration of 60 cells connected in series and giving an output of 235/240/245Wp with module efficiency of more than 14.3 per cent. A W2800 module uses 72 cells in a 9cm x 8cm configuration to give an output of 280/285/290Wp with a module efficiency of 14.5 per cent,” explains S Vasanthi, director, technical and marketing, Websol Energy Systems Ltd.
Bharat Heavy Electricals Ltd, Bharat Electronics Ltd, Euro Multivision, Jupiter Electronics, Maharishi Solar Technology Pvt Ltd, Tata BP Solar India Ltd, Indosolar Ltd and Websol Energy Systems Ltd are some of the major players that have a strong foothold in this field of technology.
Amorphous silicon cell: Due to the inflated price of solar silicon, the PV industry introduced a cost efficient technology—the amorphous silicon (a-Si) cells or thin film technology, which was a big breakthrough in the industry. Its biggest advantage is that it uses a much smaller quantity of silicon than C-Si, which is expensive. There are three main thin film technologies to date—copper indium gallium selenide (CIGS), cadmium telluride (CdTe) and amorphous silicon.
The disadvantage of thin film technology is its lower efficiency compared to C-Si but there are solutions to overcome this limitation. Thus, low cost and high efficient thin film is not far from becoming a reality. Another concern about this technology has been regarding its high tendency to break. But today, the technology has been developed to have a non-breakable flexible material as the substrate. This technology performs better in low light and works well in both diffused and direct sunlight, which has a definite cost reduction potential.
The market for thin film PV is huge in countries like Germany and USA. In India, the deployment of this technology has not yet picked up, though the potential seems high. Some of the major players in this field are Moser Baer Solar Ltd, Hind High Vacuum Company Pvt Ltd (HHV) and Shurjo Energy Pvt Ltd.
Thin film technology has some advantages over crystalline silicon as it requires less silicon (it has a thin layer with a thickness of few micrometers), whereas the silicon thickness in multi-crystalline and mono-crystalline solar cells is thicker than 200 micrometers. Another advantage is that it produces panels that show a much lower temperature coefficient than crystalline modules, which results in a higher yield in the field. In hot countries, this advantage results in a 5 to 10 per cent higher output per installed watt compared to crystalline silicon. Currently, cost reduction is the main focus of the PV industry and thin film technology emerges as the right choice for the industry.
“Data shows that the thin film solar modules in India are generating 5-8 per cent more energy than their installed capacity,” informs Vivek Chaturvedi, global head, sales and marketing, Moser Baer Solar Ltd. The company’s total installed capacity of 90 MW of C-Si cells, 100 MW C-Si modules and 50 MW of thin film panels has resulted in it taking a lead in the industry.
Bengaluru based Hind High Vacuum Company Pvt Ltd, with its turnkey production lines, has opened up a new chapter in large scale manufacturing that is based on amorphous silicon technology. With modifications and improvements in a-Si micromorph technology, the company claims to derive an efficiency between 10 per cent to 12.5 per cent.
Kolkata based Shurjo Energy Pvt Ltd is also in the business of CIGS thin film technology. In case of this technology, silicon is not used as the active PV semiconductor and, therefore, production is not hampered by any raw material supply problem. In addition, using only minute quantities of active material means the energy payback is far superior to conventional crystalline products. Studies show that CIGS material actually yields more energy per KW installed, compared to traditional crystalline products, due to its better performance in low light conditions.
The latest breakthrough in cell technology is the introduction of dye or organic cells. An organic solar cell is flexible, lightweight and affordable (it is about 1/3 cheaper than an inorganic solar cell). The main drawback of organic cells is their lower efficiency in converting sunlight into electricity, when compared to conventional solar cells. In general, organic cells have around a 1-3 per cent efficiency rate, while silicon based solar cells have an energy efficiency of about 15-20 per cent. However, the advantages of this technology over conventional solar cells are its low impact on the environment and easy manufacturing process. Also, because these cells can be attached to flexible materials, they can be put on many surfaces.
“This technology is at a very nascent stage and it will take some time to replace the current single crystal silicon solar energy solutions. Rather, they are seen as an opportunity in their own niche market. As time progresses and the focus of the research concerning these cells broadens, the dye/ organic solar cell might just become a contender in the current solar panel market,” opines Dr Kaul.
Challenges buyers face
A buyer faces the dilemma of keeping up with the pace of changes in the solar power industry. The most obvious criterion for buyers is the choice of technology—its viability, track record, future prospects and the total cost of the project in relation to the revenue generated. This is the most critical aspect as the success of the whole project depends on the option the buyer chooses. While both crystalline silicon and thin film have their own benefits and shortcomings, it is the end usage of the technology that will decide the success of a particular project.
“The most important criterion is not the upfront cost, but the lifetime cost compared to the product’s value over its entire life time. So a buyer should basically evaluate the end use of the technology. For a grid market, a high efficiency module is required. The total profitability of the project depends on the energy generated and evacuated to the grid,” says Sunil Goel.
Thin film has not yet been proven in India as the technology has not yet been deployed on a large scale. Since its fairly recent entry into the industry in 2002-03, the time frame seems less for testing a technology with decades-old and tested crystalline silicon technology. However, experts feel this technology is good for reducing grid parity.
Apart from the choice of technology, buyers should be aware about the efficiency of cells while making a purchase decision. Efficiency of cells depends on the quality of raw material used. The buyer should always ensure the efficiency aspect, and if possible, should check the performance of solar PV power plants by ensuring the energy generated (kWh).
Another factor that comes into the picture is the cost. “Since India is a price sensitive market, the buyer should also look at this aspect. Besides, buyers should also look for warranty periods that ensure quality and efficiency of the cells. Product certification, and environment and safety certification is equally important for a buyer, as in the long run, these play a huge role in the viability of the project,” informs S Vasanthi.
Latest developments towards increasing cell efficiency
Globally, manufacturers are moving towards developing new methods of increasing cell efficiency and reducing the Kerf loss in wafering. US based company Solaicx, a solar cell ingot maker, has developed a manufacturing process for wafers used in solar cells, which it claims results in higher efficiency. It has been designed keeping in mind the special needs of the solar cell industry. Similarly, Japan’s New Energy and Industrial Technology Development Organisation (NEDO) launched a research initiative to develop innovative solar cells. This led to achieving the world’s highest solar cell conversion efficiency of 35.8 per cent.
Although India is not in contention in the higher efficiencies race as yet, the efforts made by the domestic manufacturers are commendable. The enegy efficiency achieved today ranges between 16-18 per cent. Websol Energy Systems Ltd claims to have achieved an efficiency of up to 18.2 per cent. “Manufacturers are reducing the thickness of silicon, which reduces the usage of silicon and eventually helps in cost reduction,” says Dr Kaul.
Other initiatives to enable high performance solar cells while maintaining or even reducing manufacturing costs include the use of silver solar paste with high solids, and using techniques such as hot melt and double printing. This further helps in increasing the performance of the cells. The hot melt technique reduces the overall capital cost of equipment by eliminating dryers.
Key solar cell manufacturers
* Bharat Electronics Ltd
* Bharat Heavy Electricals Ltd
* Central Electronics Ltd
* Euro Multivision Ltd
* Hind High Vacuum Company
* Pvt Ltd
* Indosolar Ltd
* Jupiter Electronics
* Maharishi Solar Technology Pvt Ltd
* Moser Baer Solar Ltd
* Shurjo Energy Pvt Ltd
* Tata BP Solar India Ltd
* Websol Energy Systems Ltd
(Note: The names of the companies are in alphabetical order)
* Check the credibility and track record of the manufacturer
* If possible, verify the performance of solar PV power plants that have installed the solar PV modules that you’re considering
* Check the quality of raw material used
* Pay attention to the consistency of product performance
* Ask for product certification
* Look for quality management systems certification
* Check environment and safety certification
* Ask for product warranty