Paula Mints on the Failure of Thin Films

Paula Mints is a respected member of the elite of the PV community. Her article in Solar Industry (November 2010) is not wrong, it just is not deep enough. Yes, to a great degree, thin films failed to revolutionize PV. But there are too many interesting things to discern from her data to stop there.

Perhaps Mints’ most convincing tabular evidence of the failure of thin films to revolutionize PV is her Figure 1, thin film share of total shipments. She shows that thin films’ share peaked in 1988 at 32% and plunged from there to 5% in 2004. Then it only picked up slightly to 17% by 2009. She didn’t have numbers from 2010, but given the growth of Chinese silicon, thin film share may even fall in 2010. Is this the characteristic of a revolution?

Having been part of the development of thin films, I’ll give Paula her due and say we once thought it would be a revolution. But the surprise wasn’t just about thin films – it was how good crystalline silicon could be. That’s a message I would agree with. This has to be said – crystalline silicon was better than we (the thin film community) realized; and it still seems to have plenty of potential for continued progress. There hasn’t been and there will not be any overall revolution of PV by thin films.

However, that is not “ ’nough said.”  That falls far short of ‘nough said. Because Paula’s own data shows one thing in favor of thin films and in favor of CdTe specifically – it has grown explosively, an d even more rapidly than any other technology, including crystalline silicon. And it has made a difference, along with Chinese silicon, in adding zest to PV competition, with the very desirable effect of reducing typical prices and keeping the pressure on for continuing to do so.  This is shown in Mints’ Figure 2, adapted here to show comparative CAGRs graphically:

The growth of CdTe is more than triple the growth of PV modules in general.

The lesson is thin films are not monolithic. Pick out the growth rate of amorphous and thin film silicon (purple Xs). Despite the opening made by expensive silicon, thin film silicon grew only marginally since 2004 – 38%. By comparison, CdTe (blue *) grew at 181% since 2004, three and a half times higher than the growth rate of all module shipments. This is what success looks like. To ignore the success of CdTe is to fall short in terms of understanding a crucial factor in the future of PV and to diminish unnecessarily the success of thin films to-date.

Thin films are not created equal. A bit simplistically, it’s like the old story of Goldilocks and the Three Bears. For amorphous and thin film silicon, the porridge is not hot enough. The efficiency is too low. For CIS alloys, the porridge is too hot – although CIS cells can be very efficient (20% cells!), CIS is hard to manufacture. For CdTe, the porridge is just right – efficient enough and easy to manufacture. There may be a dozen documented ways to make 10% CdTe cells.

The concept of thin films makes sense – less material, less handling through large area substrates – but it has to be executed. So far, CdTe is the only technology that has executed. But when the concept of thin films is executed, it can stand up to any competition in PV, including Chinese silicon.  Can anything else in PV say the same?

Beyond the status quo, only CIS looks like it might, someday, be a serious competitor of CdTe. And this would be good, because we need more competitors to keep the progress in PV robust and the competition lively.

There is something else to be learned about the failed thin film revolution, and this is very important, especially here in DC. A good new idea may take 30 years to pan out, and by the time it does, the status quo will have changed radically. There will very likely be no revolution, and there may not even be progress.

The technologies now in place have tremendous up-side. They will get much better and deserve the Federal support needed to keep them moving (and keep the US leading technologically). Tens of new PV ideas have been examined and dropped in comparison to them, but newcomers to PV (and I include almost everyone who comes to DC with every new Administration) make the mistake that they (1) can have a revolution in PV and (2) that we need one. We can’t, and we don’t.

For usually, when newcomers have new ideas, they are old ones – old and discarded ones that are very boring to the PV community. Plastic solar cells? Dye cells? Cells made on flexible foils? Give me a break. They exist and can be barely sold. These technologies are not terawatt-worthy. Yet often, it is just these kind of cell technologies that people new to PV fund and research when they talk of the next revolution. Someday we will have a mature PV program, when we have the patience to learn about the one we have.

So, Paula, pardon me for using your excellent story as a springboard. Your data tells a deep and important story, even beyond the necessary recognition that thin films did not revolutionize PV.

Ken Zweibel