I-III-VI Chalcopyrite compound semiconductor, represented by CIGS(CuInGaSe2) has direct band gaps and light absorption coefficient of 1x105 cm-1 which is the highest among semiconductors. Therefore solar cells can be made of 1~2 μm thick thin film and is a metrial with high electro-optical reliability.
CIGS thin film solar cell was first studied as a lightweight, high efficient space solar cell and as alternative to mono crystalline solar cell by the American company Boeing. Being safe and efficient, at unijunction its power to weight ratio is 100W/kg superior to other materials e.g. silicon of GaAS solar cells which have that of 20~40 W/kg.
From the late 1980s, low price and highly efficient land generating thin film solar cells have been developed by the developed countries. With 1.2 eV band gap and reached the highest energy conversion efficiency of unijunction CuInGaSe2, 20.1%, approaching the original multi silicon wafer’s 19.8%.
Picture 1. CIGS Structure of the thin film solar cell
Currently CIGS solar cells are studied of commercialized in Germany, China, Japan, United States, Taiwan and South Korea. Among them, Germany and United States are the two unparalleled in that 7 and 6 private businesses respectively are actually producing and commercializing CIGS solar cells.
In manufacturing such cells, many technologies are used to form CIGS layers including Co-Evaporation, Sputtering, Electroplating, Nanoparticle Ink. This column will show major companies that use Co-Evaporation and its process.
Co-evaporation is a process that Cu, In, Ga, Se are co-evaporated by using thermal evaporator or Knudsen cell and form thin film on a high temperature board<Picture 2>.
This was first developed by Boeing(U.S) in 1982. Here controlling each element is easy since evaporators are used independently, and Ga, the doping material’s best preparation ratio Ga/(In+Ga)=0.3 is also easy to achieve.
However it has many disadvantages since basically the evaporators are point source and thin film has to be formed on thousands-of-square-meter board. So, line evaporation source is of great need to manufacture a large area thin film. For a mass production of large area thin film, the board needs to move above the line evaporation source in-line and horizontally, forming the thin film. In this case preparation control is much harder than in a stationary condition which is being an obstacle increasing its efficiency.
Picture 2. Co-evaporation technology mimetic diagram
Picture 3. In-line evaporation technology mimetic diagram
In the industry in-line evaporation which is more appropriate to large area film has been used<picture 3>. It had been critisized for its low material usage rate and that large area films were hard to be formed. Also, the evaporation source’s fabric limited the usage to only in-line. However with the development of in-line evaporation source, it could overcome these weaknesses.
Co-Evaporation is a widely used technology and has an efficiency of 7.8~11.7%.
• Developing large, downward, high temperature linear source
• Controlling Cu, Ga, In Rate at a Se atmosphere
• Erosion of metal components because of Se at 550℃ (Part durability)
• Transferring glass heated to 550℃ into the line
• Transforming glass by high temperature glass heating
• Throughput problem: Decreased efficiency in fast deposition
Picture4. Thermal history curve of co-evaporation and process of CIGS formation
2.2. CIGS Business Trend
• Global Solar(USA)
It makes CIGS solar cells of stainless steel foil and the whole production consists of roll-to-roll process.
In a high vacuum chamber, Cu, Ga, In and Se are co-deposited forming CIGS film. To improve the crystalline growth, the deposition is processed in a high temperature over 550 ℃.
After taking out the roll which has the CIGS layer from the vacuum chamber, it goes to the equipment that forms CdS film. Deposition source from Veeco is used and the deposition equipment they use is developed on their own.
Picture 5. Panoramic views of CIGS module production lines of Global Solar
Using thin stainless steel foil increases the portability since it is light, flexible and unbreakable. Also, cheap boards can be used and when used as BIPV or Rooftops, regardless of the buildings’ shape, it can be installed. It’s lightness also decreases the installation fee.
Different from glass boards, using flexible boards has limits in scribing technology which makes it difficult to manufacture monolithic modules. Global Solar, borrowing the concept of crystalline silicon module, connects a number of unit cells (210x100 mm2) in series to manufacture modules. In this case, you can pick the poorly efficient part by dividing the roll into several unit cells which allows efficient yield management. The products are used as military, exploring, hiking and portable purpose and recently expanding its market for BIPV usage touting its lightness.
Currently Global Solar is concentrating on constructing mass production system by increasing productivity and efficiency. 1% per year, it is planning to reach 14% of efficiency in 5 years. It built a 40MW production line as Tucson and plans to build a 30MW one at Berlin in 2010. By then it also plans to expand Tucson’s factory to a 100MW line.
Picture 6. Production of Global Solar and applied production
(glass board applied module)
Picture 7. Production of Global Solar and applied production
(Applied flexible BIPV)
WurthSolar is a leading Germany CIGS manufacturer which first opened the market using Co-Evaporation technology. It built a 15MW line CIGS factory in 2006. From July, 2007 it started to produce on full scale and with its new 15MW line in July, 2008 its production line was of 30MW by 2009. In 2010 it built another 10MW line which adds up to 40MW of its net production capacity. It has its own technology by developing deposition source by itself and not selling. Its deposition chamber is manufactured by an East German company Von Ardenne.
Looking into the manufacturing process, the Mo layer is formed by sputtering DC and CIGS is formed through simultaneous Cu, In, Ga, Se thermal evaporation. Here the board is heated to about 550 ℃ for the compound to be appropriately made. As for buffer layer CdS is formed by CBD. Afterwards, first form i-ZnO layer and deposit AZO, a transparent electrode layer on which 2% of Al203 is doped.
Picture 8. Structure of Wurth Solar cell and shape of module
Picture 9. CIGS Thin Film Module of Wurth Solar
CIGS developed and manufactured by Solyndra is similar to fluorescent light in its form. CIGS solar cell is produced on the surface of the glass tube. The manufacturing process uses thick film technology and makes one panel using 40 solar cells including tube cells produced in this process. Such form, which is well shown in the picture below, can absorb lights from all sides with its cylindrical form. Therefore it does not have to be installed so it could face the sun, and is not affected by air resistance which reduces the installation fee by 50% and the takes only 2/3 of time needed to install the original ones.
There is a reflecting board at the back of the cell which reflects the light that comes in between the cells. In the manufacturing process, Cu, In, Ga, Se are deposited through Co-Evaporation on sodalime glass and CdS, the buffer layer, is deposited with CBD technology. The 1820Ⅹ1080 mm modules made by the process have efficiency of 7.63~10.02%.
Solyndra was found in 2005, started sales in July, 2008. It also installed a self-made 50KW system on its roof. $1.2 billion worth products, same amount to a 5 year-sales-product, have been already ordered with 500 employees.
Picture 10. Structure of Solyndra solar cell and fundamental of photovoltaic absorption
Picture 11. Advantages of Solyndra CIGS module installation
We have looked into Co-Evaporation using CIGS thin film manufacturers, their technology and the current trend. In the 2nd and 3rd columns, we will view two other CIGS manufacturing process, Sputtering and Electroplating using manufacturers and their trends.