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Organic PV reality check: NREL’s Keith Emery cautions against irrational exuberance
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Although encouraged by the rapid rate of improvement in organic photovoltaics, the National Renewable Energy Lab's Keith Emery still sees a long road ahead for organics before they become widely commercial, especially if they want to move outside the consumer space into larger-scale power generation applications. I had contacted the industry veteran for his take on Solarmer’s latest record-setting efficiency ratings, and ended up getting a rambling combination of tutorial, history lesson, and commentary on OPV materials testing and characterization.
Organics have come a long way over the past few years, he told me, the devices are less delicate and reasonably stable, they don’t need to be hand-delivered and can now survive normal shipping channels on their way to the lab. But what they can’t survive—and in Emery’s opinion won’t for years to come—are the rigors of one of the key tests that all PV devices must pass before they will be accepted for mainstream markets: IEC 61646.
“The biggest problem for these organic devices is efficiency and stability, at least from my perspective,” the manager of NREL’s PV cell and module performance and characterization team told me during our phone chat. “The lifetime and reliability quals they have to hit, in my opinion, are the IEC 61646 qualification tests. If they can’t pass that test, they’re not gonna be able to sell in a lot of markets and a lot of people won’t buy a product that doesn’t pass that test. Passing that test is gonna be their first hurdle, and it’s not a small hurdle.”
“I’ve yet to see an organic PV pass a thousand-hour test of damp heat, at 85°C and 85% RH. Passing that test, at least with silicon, means that the cell degraded less than 10%....The organic guys are gonna have to pass that test, and if they don’t, they’ll be relegated to consumer applications."
Emery doesn’t want to hear any excuses from the OPV camp either. “They are not gonna be able to [argue] that 85°C is too hot for their technology. That’s not gonna fly. It’s already been discussed at the standards meetings, and nobody’s budging.”
“What does it mean to say that a device will last a thousand hours?” he asks. “Those kinds of words are very fuzzy in this business. That could mean did it drop 50% after 1000 hours, or 5%? Unless they give that qualifier, you have no way of knowing.”
“Passing that test is still no assurance of a 25-year lifetime—it’s pure speculation at this point that passing the test correlates to that. But it’s like a minimum durability standard. If they can’t pass that test, they won’t last 10 or 20 years.”
Keep in mind that even 10-year lifetimes are well down the road(maps) of most OPV module developers.
With nearly 30 years of PV experience, Emery has reason to be wary of untested, unproven PV technologies getting commercialized before they’re ready. He’s gone on record in the past, concerned about the lack of independent verification of supposed OPV conversion efficiencies and the difficulties of testing the organic materials.
He was a cosignatory to an editorial titled “The Value of Values” in the November 2007 issue of Materials Today and was quoted as a source in http://spectrum.ieee.org/energy/renewables/solarcell-squabble/0">“Solar Cell Squabble,” a very informative feature about organics testing and verification—or the lack thereof—in the April 2008 edition of IEEE Spectrum.
“My concern is that if you put too much product out there too early and it fails, it may give the technology a black eye that it may not deserve,” he says. “I’ve seen it before with other thin films that were pushed out to market too soon.”
“I claim organic PV technologies have multiple problems in all of these areas. That said, CIS and CadTel had these kinds of problems for years and years, much longer than organics have been around, so it doesn’t bother me that they’re having these problems, it’s to be expected. If you look at history, it will take another 5 to 10 years before we can expect them to pass those tests, if they go at the same rate that the other material systems did."
Device stability is another sticking point for OPV, he believes. “The other test, at the very end, is the stabilization test, and some of the current organic PV will never pass that test. You keep doing [the test routine] until the power doesn’t change, and [the device] remains stable. With the current organics technology, every time you do that test, their power will drop a bit."
“I would not expect to see any OPV pass that test in the next couple of years,” he continues. “There are too many stops where they’re not even close. They’ve got some work to do. In the meantime, they can sell to the consumer market to their heart’s content.”
Emery believes OPV’s extreme sensitivity to moisture presents another hurdle. “You can’t package your way out of a moisture sensitivity problem easily, short of putting it between two pieces of glass, and even that may not be good enough…. The water vapor transmission rates are just not there yet."
“Until they can come up with a design that’s less sensitive to moisture, they’re gonna have a hard time passing the test," he explains. "State-of-the-art packaging is not good enough, in my opinion. The ideal solution to for someone to come up with stable materials that are less sensitive to moisture, so you don’t have to have as tight a packaging requirement.”
“There are solutions to all these problems,” notes the optimistic realist, “but there are also some interesting testing problems associated with the organic technologies. They’re often quite nonlinear, which means they have to pay more attention to what the response looks like at low light levels versus one-sun conditions. Some researchers use the current from their quantum efficiency as a check, but where the systems are very nonlinear, that number can be all over the map. So it may not be a check.”
But Emery’s beliefs and observations certainly offer an OPV reality check.
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Emery is one of the great and most experienced characters in PV, so please check back with him often. Ultimately, all these new thin films are aimed at lower device cost, but face up hill efficiency and stability battles, while their cost reductions don't change the inactive material in a module and BOS, which are very significant drivers. low efficiency is very hard to overcome.
Useful comments, but can Emery explain why NREL agreed to test the 0.1 cm2 cell from Solarmer and, even worse, masked the area down to 0.047 cm2. This, I thought NREL had agreed was an unrealistic cell size. They have not helped the OPV community by doing this.