ZSW (Zentrum für Sonnenenergie- und Wasserstoff-Forschung — or Center for Solar Energy and Hydrogen Research — Baden-Württemberg) in Stuttgart, Germany has set a world record for thin-film photovoltaic cell energy conversion efficiency for the fifth time, with 22.6% for its latest copper indium gallium diselenide (CIGS) PV cell, as confirmed by Fraunhofer Institute for Solar Energy Systems ISE. This surpasses ZSW's prior world record of 21.7% (set in September 2014), its European record of 22% (announced at the end of March), and the most recent world record of 22.3% set by a Japanese-made cell, as the pace of advances accelerates: the efficiency of CIGS-based thin-film solar cells has risen more in the last three years than in the previous 15 years.
ZSW's latest record-setting cell has an area of about 0.5cm2 (a standard size for test cells), and again was produced in a laboratory coating machine using the co-evaporation method. ZSW achieved the boost in performance by improving the manufacturing process at several points, including post-deposition treatment of the CIGS surface (with alkaline metal compounds being incorporated into this layer).
Momentum has been building in thin-film photovoltaics in recent years, notes ZSW. Records were few and far between from 1998 to 2013, with cells holding the best mark for as long as two or three years. Efficiency increased by just 0.1 percentage points per year on average. However, in the last three years the world record has been eclipsed every six months or so, with an average increase of 0.7 percentage points per year.
With these latest advances in R&D, thin-film cells could soon be a serious contender for the silicon-based solutions that have dominated the PV market for years, reckons ZSW. The lead of multi-crystalline silicon cells is now just 1.3 percentage points.
ZSW researchers still see plenty of untapped potential in the technology behind CIGS solar cells. "We can achieve up to 25% efficiency in the years ahead," expects professor Michael Powalla, ZSW board member and head of the Photovoltaics division.
Thin-film cells are also making strides in other areas, as evidenced by key performance indicators: CIGS PV technology is catching up to multi-crystalline silicon regarding efficiency of the entire module (the larger module is always slightly less efficient than the smaller cell). The two technologies are not very apart these days, with efficiencies of 15% and 17% respectively. At US$0.40 per watt, the cost of manufacturing CIGS modules has also fallen to a level on a par with that of silicon technology. However, researchers are confident that module efficiency could reach 18% or more, with production costs dropping to about US$0.25 per watt. Also, whereas the price of silicon PV products depends largely on economies of scale, CIGS modules can be manufactured at low cost even on a relatively small scale, notes ZSW.
In the months ahead, ZSW will team up with its industry partner Manz AG of Reutlingen, Germany (a mechanical engineering firm that offers turnkey manufacturing lines for CIGS thin-film solar modules) to transition the latest advance from the lab into the factory.
