The quest continues to develop the next generation of PV cells which are cheap, efficient and easy to manufacture so as to enable solar energy to play a bigger role as a renewable resource. Most agree these goals can be reached as greater power conversion efficiency levels are reached. That quest has again resulted in a number of recently announced efficiency records for CIGS, CIS and traditional monocrystalline PVs, including new claims of world record efficiency levels.
Manz AG
Reutlingen, Germany-based Manz AG announced in September 2011 it had achieved 14 percent efficiency in its copper indium gallium selenide (CIGS) thin-film module. Manz’s 100-watt thin-film solar module actually reached a never before achieved aperture efficiency of 15.1 percent, a level of efficiency that describes the photosensitive area of a module which directly corresponds to a PV efficiency of 14 percent. Reaching this world record was the result of a one-year partnership between Manz, the Stuttgart-based Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW), and solar module manufacturer Würth Solar, based in Schwäbisch Hall. ZSW had reached over 20 percent efficiency with a laboratory CIGS cell earlier this year. Manz's 600 x 1,200 mm production module is being mass-produced at Würth Solar's factory using an optimized design based on CIGS thin-film technology in which the CIGS layer is vapor deposited on the substrate. The company claims it is currently the only supplier of a production line for CIGS thin-film solar modules - their Manz CIGSfab line - that can be operated profitably today.
Roth & Rau Switzerland
Researchers at Switzerland-based Roth & Rau AG announced in September 2011 that they had achieved an output power efficiency rate of 20 percent using a 156 millimeter industrial monocrystalline silicon wafer based on heterojunction PV technology.
The company has been working with the Université de Neuchâtel to develop high-efficiency PV cells based on heterojunction technology since May 2008. This resulted in a technology transfer of the results of the Institute’s research into an industrial process that enhances the PV efficiency. The result is that for the first time Roth & Rau achieved an output efficiency rate of 20 percent using 156 mm industrial monocrystalline silicon wafers based on heterojunction technology. In a short period of time, Roth & Rau managed to bring heterojunction technology, combined with its excellent temperature coefficients (>5 percent additional energy yield per module) from the research stage to the production stage. Heterojunction technology is well known but has seldom been put to industrial use, according to Roth & Rau. Their heterojunction PV cells are based on a low-temperature production process and are characterized by “superb passivation properties,” which leads to a very high power output efficiency rate. The process is already being scaled-up for mass production at the Hohenstein-Ernstthal facility where the Roth & Rau pilot project took place.
Avancis
In September 2011 Torgau-based Avancisannounced it achieved a power conversion efficiency of 15.8 percent on their fully encapsulated CIS thin-film module, an efficiency world record that was confirmed by TÜV Rheinland. The record was achieved on a 30 x 30 cm² CIS solar module.
This new record is an improvement over Avancis’ last efficiency record of 15.5 percent at the beginning of 2011, putting the company once again in the lead in the efficiency ranking list. According to the current efficiency record tables of the August 2011 issue of Progress in Photovoltaics magazine, the Avancis result represents a new, independently certified efficiency record for a thin-film photovoltaics module with a size of 30 x 30 cm².
According to the company, “The renewed efficiency improvement can be traced back to the reduction of the layer thickness of the CIGS-Se absorber by approximately 10 percent, and an optimized in-line selenization process for the thinner absorbers. In addition, the P1 laser structuring was optimized and the structuring process between P1, P2 and P3 improved with the aim of further enlarging the active surface. The result was the achievement of an aperture efficiency of 15.8 percent.”
Silevo
PV module manufacturer start-up Silevo, based in Fremont, California, announced in mid-October 2011 that their new technology offers the best performance-to-cost ratio for solar modules, with between 20 and 21 percent power conversion efficiency on full-size substrates. One of Silevo’s main cost-cutting moves is designing its “hybrid” PV modules using of copper rather than silver, since silver paste is considered the second most expensive PV module manufacturing material after silicon.
Silevo’s proprietary “Triex” technology is a hybrid module using a combination of three materials: crystalline silicon N-type substrates, thin-film passivation layers, and a tunneling oxide layer. This hybrid design enables the Triex module to deliver high efficiency at a competitive module cost, according to the company.
Silevo maintains a research facility in Fremont and plans to build a manufacturing plant in Hangzhou, China with high-volume commercial production to begin in the first half of 2012. Silevo recently closed a $33 million financing deal to build the China facility, as well as to drive further research at its California headquarters.
Nanyang Technological University, Singapore
Researchers at Singapore’s Nanyang Technological University (NTU) and A*STAR Institute of Microelectronics (IME), announced October 12, 2011, that they have developed a highly efficient and yet cheaper thin-film silicon PV cell using advanced nanostructure technology. The nanostructure’s unique surface texture produced on amorphous silicon improves the power conversion efficiency of the thin-film PVs enough that it also increases the energy output. In fact, the researchers went as far as to say that with this new development, the cost of solar energy may be halved.
The new thin-film nano-structured silicon PV cells are designed to be made from cheaper, low grade silicon yet are able to generate electricity currents close to that produced by traditional PV cells made from more expensive, high-quality silicon. The advance was made possible by creating a texture using nanostructures thousands of times smaller than human hair on the surface of the PV cells resulting in an electric current output of 34.3mA/cm2, which is close to the 40mA/cm2 of traditional PVs. Up to this point, conventional thin-film PV cells usually could only achieve about half of the current output of traditional PVs.
According to Dr. Navab Singh, Senior Scientist of IME's NanoElectronics Program and project leader, since circuit current density is directly proportional to power conversion efficiencies, it is conceivable that efforts to improve fill factor and open circuit voltage would boost PCE of thin-film silicon PV cells greatly to match that of bulk Silicon PV cells.
Conclusion
It can be argued that the speedy expansion of solar energy around the globe is being hindered by the high cost of traditional PV panels, partially due to unstable prices of the high-grade silicon used to make those PVs. The PV module manufacturers who financially survive this stunted solar growth are those who can offer high efficiency, low cost PV modules. Professor Dim-Lee Kwong, Executive Director of IME, says the demand for thin-film PV cells is “expected to double by 2013.” The trick is getting their power conversion efficiencies to a competitive level.
While low-grade amorphous silicon thin-film PVs cost less to manufacture, their PCE has a long way to go to compete. CIGS technology is currently considered the thin-film technology with the best cost-cutting potential among competing amorphous silicon or cadmium telluride PV technologies. Though CIGS PVs have already reached an efficiency of over 20 percent in a laboratory setting, as demonstrated by the world record holder ZSW, the challenge remains to close the gap between these laboratory PVs and the PCE efficiency that is actually achieved during mass production.
