Currently, CIGS (copper, indium, gallium and diselenide) thin-film PVs have limited production volume and market share due to their low energy efficiency - around half the efficiency of their crystalline and silicon counterparts.
The current PV market is dominated by crystalline silicon solar cells, which account for about 90 percent of the market. But the CIGS PV market appears to be poised for a comeback and it has been conservatively forecasted that the market share of thin-film PVs will increase from 10 to 35 percent in the next 20 years.
CIGS PVs are made by depositing a thin, multilayer of semiconductor material of 1/200 mm in thickness on an inexpensive substrate such as glass, plastic or metal foil. One of the advantages of flexible, light-weight CIGS PVs is the potential of not only lower manufacturing costs, but also lower costs for transportation and installation. This is because CIGS PVs are as much as 50 times thinner – and so lighter - and the fabrication costs nearly 50 percent less then conventional silicon PVs.
Manufacturing CIGS PVs
Thin-film technology aims to develop silicon-replacing processes by using alternative semiconductor materials. In the case of CIGS, the semiconductor materials of copper, indium, gallium and diselenide are deposited as a thin film on the substrate material.
Tucson, Arizona-basedGlobal Solar is the only manufacturer in large-scale commercial production of CIGS PVs. Global Solar’s CIGS PowerFlex BIPV achieves an output of up to 300 watts with an efficiency of up to 12.6 percent, according to the company. Global Solar deposits CIGS on a flexible, stainless-steel substrate using a roll-to-roll process, meaning that a steel film is unrolled, coated with the CIGS film, and rolled up again. The production process Global Solar applies at its Tucson and Berlin-Adlershof facilities includes several steps for turning these raw materials into CIGS thin-film PVs.
First, a molybdenum layer is deposited onto the stainless steel film using a “sputtering” process. This layer is the PV’s rear contact. The copper, indium, gallium and diselenium raw materials that convert sunlight into electricity enter the machine and are vapor-deposited onto the molybdenum-coated steel band using a proprietary process, followed by a very thin buffer layer.
The top layer of the PV requires a transparent but conductive oxide layer (TCO) which is also sputtered on like the molybdenum layer.
A special printing machine unrolls the stainless steel band, prints it with a grid of silver paste to conduct electricity, and then rolls up the band again. That stainless steel band is actually a single, large PV with a voltage of only about 0.5 V. To increase the power output, the steel band with dimensions of 700 m to 1000 m x 0.3 m is cut into individual cells measuring 10 cm x 21 cm which are electrically connected in rows typically 1.85 m long and 21 cm wide. At this point their electrical properties are tested and classified in categories before they are shipped to be further processed at any number of solar module factories worldwide.
CIGS Research Advances
In May 2011, the Center for Solar Energy and Hydrogen Research in Stuttgart, Germany, announced a breakthrough with the potential to improve the thin-film manufacturing process for making CIGS PVs more efficient using an "efficient web coating of thin film solar modules made of CIGS on plastic film." ZSW claims this has the potential to "create a new generation of affordable, flexible photovoltaic modules.”
The ZSW researchers had previously developed a technique where all production steps were completed in one continuously running system, but this new development allows the CIGS deposition process to be completed in one stage. The CIGS research team’s “cathode sputtering” coating process is carried out in a vacuum chamber so "the interfaces will not be contaminated by oxygen or atmospheric moisture." The timeframe for commercial application of this technology remains unclear, though the Stuttgart team has indicated it may be between five and ten years.
Also in May 2011, researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) announced they were able to increase the conversion efficiency of flexible CIGS PVs to a world record of 18.7 percent. The 18.7 percent, which was certified by Germany’s Fraunhofer Institute for Solar Energy Systems, surpasses the same team’s previous record of 17.6 percent achieved in June 2010. Empa researchers have partnered with FLISOM to refine the manufacturing process for commercial production.
Meanwhile, researchers at theChinese University of Hong Kong announced in early June 2011 that they had also developed high efficiency CIGS PVs that can be made inexpensively. Led by professors’ of physics Xudong Xiao and Quan Li, the researchers say they achieved a conversion efficiency of 17 percent using a physical vapor deposition process to produce the CIGS layer. Careful optimization of each functional layer of the PV cell was critical to achieving the high efficiency, according to the researchers who are also working on developing production equipment to make the CIGS PVs. The two-year research project is being conducted in cooperation with the Shenzhen Institute of Advanced Technology and the Chinese Academy of Sciences.
Companies Ramping-up CIGS Production
California-based XsunX announced in mid-July 2011 it had reached a CIGS energy conversion efficiency of 16.36 percent.XsunX had the record confirmed by the National Renewable Energy Laboratory (NREL) for a 125 mm substrate which was divided into four pieces that produced an efficiency range from 15.3 to 16.36 percent, with an overall average of 15.91 percent efficiency.
The patent-pending processing technology, which XsunX calls “CIGSolar,” focuses on the mass production of individual thin-film CIGS PVs utilizing co-evaporation for rapid deposition of final product sized PV cells to control the CIGS layer deposition process. XsunX begins and ends this process using individual substrates sized to match equivalent sized silicon PV cells. According to the company, this not only allows for a smaller and more accurate deposition process, it also helps avoid any performance losses that can happen when PVs are either cut from rolls of CIGS material or mismatched electrically in monolithic assemblies.
San Jose, California-based SoloPower announced on August 19, 2011, it had received a $197 million U.S. Department of Energy loan guarantee for the construction and operation of three facilities that are expected to produce approximately 400 MW of CIGS modules annually. SoloPower plans to expand its existing operation in California, as well as build two new facilities in Portland, Oregon starting as early as September 2011.
Portlandcity officials wooed SoloPower to their area with a $17.9 million incentive package, while state loan and tax credits will give the company another $40 million. Together, the two new facilities in Portland and the expanded California plant will create 450 permanent jobs, as well as 270 construction jobs, according to the company.
In February 2011, Solar Frontier began commercial production at its large-scale CIGS Kunitomi factory located in Miyazaki, Japan. Launched only 16 months after breaking ground in September 2009, the factory will reach full capacity by fall of this year, according to the company. The company has already obtained certifications by Japanese and European standards organizations, and U.S. certifications are expected soon.
Nexcis Photovoltaic Technology, a CIGS PV maker based in Rousset, France, recently acquired Ultrasonic Systems’high-performance ultrasonic PV-480 spray coating equipment for the application of a sodium fluoride solution, a process the company says improves the overall efficiency of CIGS cells. The PV-480 uses USI’s proprietary nozzle-less ultrasonic spray head technology for a more uniform and repeatable coating, as opposed to conventional air atomizing spray nozzles, conventional ultrasonic nozzles, or conventional roll-on coating technology.
Challenges for CIGS Technology
The continued demand for higher conversion efficiency PVs will present challenges to thin-film CIGS makers. Solar firms are currently focused on rooftop systems which demand PVs with higher conversion efficiency to gain the most power on limited rooftop spaces. This means PVs with lower conversion efficiencies, such as thin-film CIGS, will continue to have a tough time competing with mainstream silicon PV modules. Because of this, flexible CIGS technology is still considered an “emerging technology.”
But the recent improvements in CIGS efficiency in research labs and pilot plants are contributing to performance improvements to overcome these barriers. In fact, recently developed processes that have resulted XsunX conversion efficiencies above 16.5 percent, and the new Empa CIGS record of 18.7 percent, nearly closes the efficiency gap with PVs based on polycrystalline silicon wafers. The incentive is that flexible CIGS is seen as a $13 billion dollar market opportunity, targeting flexible, thin-film PV applications ranging from roofs and other BIPV products, to being integrated into consumer products such as handbags, vests, tents and backpacks for charging personal electronic products.
Nevertheless, there have been casualties.
Massachusetts-based Veeco Instruments Inc.announced on June 28, 2011, it is ceasing their production of manufacturing equipment for CIGS thin-film PVs. Veeco had received over $60 million in DOE “SunShot” funding since February 2011, but has been operating its solar business at a significant loss due to price declines in silicon PVs and lower-than-expected consumer acceptance of CIGS, according to a statement released by John R. Peeler, Veeco's Chief Executive Officer.
"While CIGS remains an important thin-film solar technology, we have determined that the timeframe and cost to successful commercialization are not acceptable to Veeco," said Peeler.
Instead, Veeco intends to transfer their R&D facility, pilot line, technology and key personnel to the College of Nanoscale Science and Engineering at the University at Albany, State University of New York (SUNY) in Albany, New York. It is a move designed to support the school’s recent formation of the Photovoltaic Manufacturing Consortium (PVMC).
“We believe the PVMC is much-needed to drive CIGS industry roadmaps, collaboration, market acceptance and commercialization,” said Veeco’s Peeler.