Photo credit: Sun Film
Copper-Indium-Gallium-Selenide (CIGS) solar cells have long been one of the more exciting thin film possibilities. Unfortunately, they’ve also been the most elusive of thin film technologies. CIGS excel in conversion efficiency, reaching levels as high as 19.5% in the laboratory environment. But the problem is finding a way to efficiently manufacture CIGS solar cells while preserving that very competitive conversion efficiency.
Relative to other thin film semiconductor materials —amorphous silicon (a-Si) and cadmium telluride (CdTe)—CIGS is seen by many as destined to dominate the thin film market, assuming production hurdles are overcome. Amorphous silicon is the most popular at present because it is essentially a pared down version of the traditional silicon solar cell that dominates the solar market as a whole at this time. Therefore a-Si is much easier to produce because it takes little advancement in current, in-place technology to do so, despite being the least efficient of thin film technologies. CdTe, made popular by thin film leader First Solar, reaches efficiencies close to 10 percent and is simpler to produce than CIGS.
Yet recent breakthroughs in CIGS manufacturing are quickly shredding any barriers to wide-scale distribution. More on that in a moment, but first let us quickly have a look at:
How CIGS solar cells work
The copper, indium, gallium and selenide are carefully combined to form a semiconductor in the solar cell that absorbs solar heat, exciting the electrons that produce the electrical current. But the semiconductor cannot do it alone, and if you picture the solar cell as a sort of sandwich, beneath the CIGS layer is a substrate that acts both as a base and electrode. There are two substrates used in CIGS solar cells: metal foil and glass. The metal foil acts itself as an electrode while the glass requires a layer of molybdenum to create an effective electrode.
On the top side of the CIGS layer an independent layer of Zinc Oxide (ZnO) makes up the other electrode, with a thin layer of Cadmium Sulfide (CdS) in between as a buffer. The CIGS and ZnO make up the p-type and n-type (respectively) sides of the junction that actually creates the electric current. The top layers of ZnO and CdS have wide band gaps to minimize absorption so that the solar radiation can reach the CIGS semiconductor with little interference.
One of the biggest breakthroughs for has been using foil as a substrate. San Jose-based Nanosolar has led the way in this regard by actually developing a sort of semiconductor “ink” which they can with relative ease print onto the metal foil substrate, greatly reducing the complexity and cost of CIGS solar module production. Not to limit the more traditional (if that’s possible for such a new technology) glass -based CIGS solar cell.
The first ever tandem junction, CIGS power plant just opened in Germany by SunFilm AG. Tandem junction cells greatly increase absorption rates although actual conversion efficiencies still lag behind laboratory testing. SunFilm’s plant will start with 8% efficient cells, hoping to move up into double digits within the next few years.