A research team based at the National Institute of Standards and Technology has determined that an economical process for cranking out thin film solar cells can get this close to hitting the “sweet spot” of 10% conversion efficiency. The process yields thin film solar cells that clock in at just over 9.5% and the NIST team appears to be confident that a bit more tinkering will bring it up past the 10% mark.
If 10% conversion efficiency sounds a bit on the unambitious side, it is. It’s not even close to conventional silicon and it’s not exactly stellar when measured against next-generation thin film results.
So, what’s the big deal?
What’s So Great About 10% Conversion Efficiency?
If you think of the mass market for solar power as a balance between multiple factors including conversion efficiency, manufacturing costs, supply chain reliability, lifecyle issues, the potential for high volume production, and range of application, then a 10% bar actually gets you places.
Research team member He Yan of the Hong Kong University of Science and Technology explains:
“Efficient roll-to-roll fabrication is key to achieving the low-cost, high-volume production that would enable photovoltaics to scale to a significant fraction of global energy production.”
Here’s another explainer:
[Polymer solar cells] can be produced using extremely high-throughput roll-to-roll printing methods similar to those used to print newspapers7. PSCs also offer several other advantages: vacuum processing and high-temperature sintering are not needed, and no toxic materials are used in the end product…
Thin film solar cells are made of special polymer (aka plastic) films, and like any other kind of film (think anything from Saran Wrap to newspapers) they can be produced with standard roll-to-roll fabrication methods.
The trick is to get the process down to a level of efficiency that translates into a low enough cost, enabling the product to compete in the clean energy market.
The team looked at spin-coating, which is already in common usage in the photovoltaic field, but they determined that the process involves too much waste to be economical. In addition, spin-coating does not particularly lend itself to continuous, roll to roll fabrication.
Thin Film The Size Of A Large Virus
The team finally settled on something called blade-coating, which is already in wide use commercially. It’s a bit like buttering a slice of bread, except that you hold your knife perfectly still while the bread moves.
Substitute a polymer for the butter and a glass or plastic substrate for the bread, and you’re in business.
The choice of blade-coating dovetailed with the team’s decision to use the polymer PffBT4T-2OD for their solar cell (the name is intimidating but it’s basically just a fluorinated polymer combined with a buckyball).
PffBT4T-2OD has already been used in spin-coating to produce thin film solar cells that top 11% conversion efficiency in the lab, but that involves a small batch method that does not lend itself to commercial application.
The team decided to try blade-coating on PffBT4T-2OD because it “can be applied in relatively thick layers of 250 nanometers and more, or roughly the size of a large virus.”
So, there’s your large virus.
They then moved on to another commercially process called slot-die coating and achieved nearly identical results.
The Large Virus Strategy Pays Off
Aside from coming out with a relatively decent achievement in terms of conversion efficiency, the research team describes an important difference between spin-coating and blade-coating:
Surprisingly, at the nanometer level, the end products differed significantly from the spin-coated “champion” devices made in the lab. Detailed real-time measurements during both blade-coating and spin-coating revealed the different structures arose from the rapid cooling during spin-coating versus the constant temperature during blade-coating.
In other words, you can take the same polymer and get different results in terms of the final structure of the thin film, depending on what fabrication method you use.
According to the NIST team, that finding kind of blows up the conventional way of thinking about how to do high-volume thin film solar cell production:
“The ‘rule of thumb’ has been that high-volume polymer solar cells should look just like those made in the lab in terms of structure, organization and shape at the nanometer scale…Our experiments indicate that the requirements are much more flexible than assumed, allowing for greater structural variability without significantly sacrificing conversion efficiency.”
The team also found that in their initial tests, using PffBT4T-2OD on a glass substrate, the final structure varied depending on the temperature under which the blade-coating process was performed, and also the temperature at which the film was dried.
Those finding provide R&D teams with more variables to play around with. The end goal is to design a material that out-performs PffBT4T-2OD in terms of conversion efficiency, while lending itself to the least costly, highest volume fabrication process.
For the record, aside from NIST and Hong Kong University, the team also drew from Saudi Arabia’s King Abdullah University of Science and Technology and North Carolina State University.
How Low Can Solar Go?
You can catch all the details in the journal Energy & Environmental Science under the title, “Morphology changes upon scaling a high-efficiency, solution-processed solar cell.”
The abstract leads with this throwdown:
Solution processing via roll-to-roll (R2R) coating promises a low cost, low thermal budget, sustainable revolution for the production of solar cells.
This is an important election year in the US, and a recent public opinion poll has indicated that solar power could be a significant factor for some voters in key states, so let’s have a word from Republican presidential candidate and former reality show star Donald Trump, speaking earlier this month:
“It’s so expensive,” Trump said of alternative energy at a rally in Pennsylvania.
“And honestly, it’s not working so good. I know a lot about solar. I love solar. But the payback is what, 18 years? Oh great, let me do it. Eighteen years…”
Trump seems to have overlooked the trendline for the cost of solar energy. The National Renewable Energy Laboratory has determined that the payback period for the typical rooftop solar array currently ranges from two to four years, depending on the type of solar cell:
As indicated by the chart above, NREL anticipates a payback period of less than one year for thin film solar.
Democratic nominee and former Secretary of State Hillary Clinton has offered a somewhat more detailed take on the issue, which you can find on her campaign website. For those of you on the go, here’s a hot take that appears to leverage thin film’s potential for high volume production and rapid payback:
The United States will have more than half a billion solar panels installed across the country by the end of Hillary Clinton’s first term.
Photo (cropped): “A demonstration solar park based on polymer solar cells at the Technical University of Denmark in Roskilde, Denmark,” via DTU Energy.
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