Under the hood of the latest 21.7% CIGS world record

Earlier this week, German research group ZSW announced that it had wrestled back the conversion efficiency for CIGS technology. Its record 21.7%, on a very small CIGS cell, is certainly a striking result, and one that once again points to the potential of the technology. pv magazine spoke to ZSW head Michael Powalla about what’s going on under the hood to achieve such a milestone.

pv magazine: The 21.7% CIGS world record conversion efficiency was achieved on a 0.5cm squared cell. How challenging will it be for manufacturers to approach similar efficiencies at a module level – with the difficulties in achieving deposition uniformity with CIGS absorber layers known to be a feature of the technology?

Michael Powalla: The 21.7% was reached on a lot of cells, so we already demonstrated the robustness of the process. However, if we go to large area production, there are losses due to interconnection of cells and statics. The new process seems to be very tolerant and could be scaled up. On the other hand, we see more room for improvement on the cell level. I would assume that we could see large area products in the range from 17% to 19% conversion efficiency.

Why do you believe the co-evaporation deposition method delivers good results for CIGS?

The co-evaporation method gives us more freedom for the optimization of the CIGS material. We can control things including beneficial material gradings by controlling the rates of each component. Also, we need only one machine doing deposition and crystallization at the same time. And importantly, it can be done at high speed. There was the problem in the past that large area equipment especially evaporation sources and reaction chambers were not available.

This has changed now. In summary, I believe it is also demonstrated that co-evaporation delivers higher efficiency, higher throughput, and most importantly at the end, lower cost.

That you were able to achieve 21% efficiencies on a number of cells is a positive sign for your CIGS processes. What were some of the variables that the ZSW team was able to tweak to achieve the world record result?

We don’t comment about that.

You collaborate with (equipment supplier) Manz on CIGS technology. Why do you believe in the future for CIGS is bright when c-Si continues to be the predominant technology in the industry?

This is very complex. The situation 2014 is characterized by overcapacities, mainly equipped with processes that are no more competitive. The PV producer must invest in new and cheaper, more productive, equipment. The only chance to bring down cost is to enhance efficiency and to go to highly automated thin-film technology. Polycrystalline Si will be replaced in the future by thin-film CIGS and a maybe also CdTe. The lab record efficiency of poly Si has not changed in more than 15 years!

At the EU PVSEC trade show pv magazine was a part of discussions in which CIGS was mentioned in variations on "stack cell" techniques as a next generation PV technology. Do you think there is strong potential for CIGS as a part of a semiconductor stack?

Yes, but people should be looking at the technology carefully. Due to the complexity of CIGS technology, only the carefully proven technologies are sustainable. Second movers should rely on proven technologies, otherwise it could take them years before commercialization.