Taking a look at the National Renewable Energy Laboratory’s (NREL) data on various solar energy technologies’ efficiencies, we may wonder why crystalline silicon cells, with stagnant efficiencies in the mid-20% range, are the norm. There are technologies out there flirting with 50% efficiency, so why haven’t these ousted their profligate silicon counterparts?
The answer has to do with the gap between laboratory and real-world conditions. Take Sharp’s concentrator triple-junction compound solar cell, which clocked 44.4% efficiency in April 2013.
The results were obtained with a 0.165 cm2 cell. Unfortunately, maintaining the efficiency of a photovoltaic system as you increase its surface area is tricky. Bernie Bulkin, former Chair of the UK Office for Renewable Energy, told the BBC that scaling CIGS, another solar technology, up to a usable size cost about a billion dollars.
Surroundings are also an issue. On a solar farm in the real world—where clouds, sand, hail, lightning, bird droppings and the occasional eclipse intervene—it is difficult to match the sterile predictability of the Fraunhofer Institute, where the test was conducted.
Finally, there is the cost. Ultra high-efficiency cells have been put to use on Mars rovers, but most of us do not have NASA’s budget. Despite their underwhelming efficiency, silicon cells are affordable and work on earth under naturally occurring conditions, so we are stuck with them for now.
That is, unless a new technology proves to be more effective than silicon cells without being prohibitively expensive. Some in the industry think we have a candidate in perovskites, hybrid organic-inorganic compounds that exhibit a particular crystalline structure. Since 2009, the efficiency of perovskite solar cells has soared from 3.8% to over 20%. While the best silicon cells still beat that by four or five percentage points, the clip at which perovskite cells are improving is unprecedented.
More importantly, they may be much cheaper to manufacture than silicon cells. In a 2012 article in Science, Oxford University’s Henry Snaith predicted that perovskites could eventually cut the cost of generating a watt of solar energy by three quarters, making solar truly competitive with fossil fuels.
The company Snaith co-founded, Oxford PV, announced in March 2015 that perovskites would be on the market by 2017. But not everyone is convinced. The barriers between perovskites and commercialization are substantial.
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On the NREL chart above, perovskites have the dubious distinction of being labeled “not stabilized.” Keith Emery, who curates the NREL’s solar cell efficiency data, explains why perovskites need a disclaimer: “The samples degrade very quickly to zero. They degrade fast enough that it has prevented intercomparing results among groups or even having an independent efficiency measurement.” Light, air and water are all kryptonite to perovskites. Silicon cells are far more durable.
According to Yuanyuan Zhou, PhD candidate at Brown University’s School of Engineering, perovskites’ instability issues are “intrinsic.” Given what they know now, the best researchers can hope for in overcoming this problem is “good encapsulation technology.” The economics of that technology may not currently be feasible, however; according to Emery, “the packaging requirements are too costly.”
Unstable though they are, there may still be applications for perovskites. “Unstable but low cost materials may have a niche market in consumer electronics where the stability demands are less severe,” says Emery. Since smartphones are replaced much more frequently than rooftops, the solar cells that charge them do not need to last as long as their silicon counterparts.
For this application, perovskites have the advantage of being transparent. An opaque blue solar cell wouldn’t do you much good if it were superimposed on your screen, but a pale yellow one might not be so bad. Zhou and his colleagues at Brown’s Padture Lab have developed a technique for making ultra-thin perovskite cells at room temperature, cutting expense and time out of the normal manufacturing process, which requires heating. The cells are fairly efficient, at 15%, and larger—though still only a few square centimeters.
Nitin Padture, Director of the Institute for Molecular and Nanoscale Innovation, thinks these cells could be used to make power-generating windows. They can be produced in a range of colors, so they could even be “decorative.”
It is not clear how manufacturers will overcome the perovskites’ instability to achieve this application. Still, a few are pretty gung ho about the idea. Oxford PV—the same startup that hopes to go to market in 2017—would like to coat office buildings in perovskite cells. The company estimates that a 35-story building in London could generate 60% of the electricity it uses.
Perhaps it is too early to give up hope. Keith Emery describes the gains in stability that have been made for another solar technology: “in the early days of organic PV they were as unstable as perovskite cells (>50% loss after a few hours in air). At that time they were shipped to our lab in ultra-high vacuum containers filled with inert N2. Now they are shipped via express mail in plastic box[es].”
On the other hand, organic solar cells haven’t displaced crystalline silicon cells, either, and “it is still not clear if a low cost organic module can be made that will survive the qualification test,” a rigorous ordeal that subjects prototypes to swings in temperature and high humidity.
The most effective application could be combining perovskites with silicon to create a “multijunction cell.” The most tantalizingly efficient prototypes on the NREL’s chart are multijunction cells, since a combination of materials captures a wider spectrum of sunlight.
A team at Stanford has increased a low-quality silicon cell’s pitiable 11.4% efficiency to 17%—an almost 50% boost—by combining it with 12.7% efficient perovskites. But Michael McGehee, co-author of the study, echoed the usual caveat: “We have a ways to go to show that perovskite solar cells are stable enough to last 25 years.”
The Bottom Line
Silicon remains the market leader in photovoltaic systems, and for good reason. Extreme heat, extreme cold, wind, rain and snow don’t faze it. By contrast, there is little that won’t render a perovskite cell useless. The compound degrades in water, air and light. Even so, rapid improvements have been made in terms of perovskites’ efficiency, the cost of manufacture, and the range of sizes, thicknesses and colors they come in.
Startups hoping to market emerging technologies will always be optimistic, veterans who have seen these startups come and go will always be skeptical. Which camp is right—or more right—remains to be seen, but investors interested in the future of renewable energy should look out for more news about perovskites.