Thin film’s original advantage over traditional PV was the result of innovation, and there is still plenty of scope for improvement. But not everyone agrees further lab findings will help.
The University of Notre Dame’s ‘solar paint’ announcement probably drew more energy from journalists than the substance is ever likely to capture from the sun.
According to a paper in IEEE Spectrum Energywise, the paint, made from semiconducting nano-particles, registered a 1% conversion efficiency in artificial sunlight, more or less dispelling any concerns of it becoming a worthwhile challenger to PV.
It did, however, focus attention on solar technology innovation at a time when thin film could arguably do with a renewed sense invention to stay in the game. Innovation is central to the thin film story, of course.
High prices of silicon have shifted the demand for solar cells towards thin film PV cells. Initially dominated by a-Si thin film PV cells, the market is now shifting towards CdTe and CIS/CIGS based thin film PV cells. Higher efficiency of CdTe and CIS/CIGS cells has resulted for the demand at global level, according to 6Wresearch.
Only a couple of years ago, the technology advantages of newly developed thin film variants such as copper indium gallium selenide (CIGS or CIS) made them look like serious competitors to mono- and polycrystalline silicon PV.
That advantage has been eroded in the face of plummeting silicon prices.
But it does not mean there is not still plenty of room for further improvements in thin film technology, says Dr Franz Karg, managing director and chief technology officer of the German CIS module-maker Avancis.
“If we start with the substrate, a modified glass matrix could increase the selenisation temperature and improve the crystal quality and efficiency,” he says. “We expect significant progress on this in two to three years’ time.”
Other improvements could be made to the current back electrode, he adds, to make it thinner, reducing cost, and more reflective, to improve efficiency. And another major avenue for future development is increasing the band-gap of the absorber.
Transparent conducting oxides
Finally, he notes, there is also room for gains in the buffer layers and transparent conducting oxides (TCOs). “Here we have seen improvements in the lab, looking at alternatives like amorphous TCOs,” he says.
All this sounds like it could add up to a whole load of extra benefit for thin film.
The question is how much thin film developers can afford to tinker in the lab while their market share continues to diminish, particularly given that many of the advances Karg is contemplating are still two or three years off at least.
MJ Shiao, a solar analyst for GTM Research, believes most thin-film makers are currently putting research efforts on the backburner, in order to focus on commercialising the technologies they already have.
“I am not sure there's going to be any drastic innovations in either CdTe or CIGS, or amorphous silicon for that matter,” he says. “Certainly, individual manufacturers might make a leap in their own company's or their own technology's efficiency.
“But most of the average efficiencies at this point are incremental. It is about process. Thin film in general has moved towards ‘now we've got a workable efficiency, we need scale in order to lower cost, to meet the price point that we need to compete with crystalline silicon’.”
Stefan de Haan, principal photovoltaic analyst at IHS iSuppli, shares that view. “I think the speed of innovation will not be significantly ahead of crystalline,” he says. “I don't see any disruptive change that will change the game.
“The technologies that are competitive today will remain competitive in the mid-term. The low-efficiency technologies are dropping out. People are always thinking about revolutionary technologies, but the technology is there.
“It is sufficient to replace existing energy sources right now. Now it's a question of scaling it, bringing it to market and launching the technology,” says De Haan.
Does that mean the end of innovation in thin film for good? Not necessarily, says Shiao. “In a couple of years we might see other technologies, like a multi-junction multi-layer CIGS technology,” he says.
Similarly, other thin film novelties, such as products made with gallium arsenide, are still some way down the road, Shiao says: “I think that’s two or three years from now, but certainly not in the next year and certainly not at anything like commercial levels.”
But even if big discoveries are not likely to be forthcoming in the next 12 months, the scene could be set for plenty of incremental research and development (R&D).
For example, Shiao says: “If there's any silver lining at all, if your utilization rates are low, it does give your engineers more time to experiment with the lines. Hopefully when the market improves you can build the improvements into your lines and have a more competitive product.”
Assuming thin-film makers can keep up the current rate of R&D activity then Karg believes CIGS at least could achieve a 15% module efficiency over the next four years.
“We are planning to be on a par with polycrystalline silicon, and that’s quite an achievement,” he says. But he accepts that it depends on there being “market pull through”. Which is a pretty big ‘if’ in the current climate.