You might mistake perovskite researchers for audiophiles the way they’ve been collecting records.
The crystalline compound has the photovoltaic industry abuzz thanks to its relatively low cost, no-fuss production, and rapid development.
Hardly a week goes by without reports of a perovskite (pronounced p’raf-skite) solar cell approach toppling another efficiency record. In the past month or so records have been set with a tandem cell, 7-cell mini module, inverted cell and the largest, most efficient flexible film-type cell. No solar technology has ever advanced this quickly.
Perovskite takes its name from a mineral discovered in Russia’s Ural Mountains in 1839. It refers to a class of material with a particular crystal structure created by two different cations, typically surrounded by oxygen (ABO3) which binds to both.
Such materials are notable for their ability to transport an electrical charge as evident in frequent features such as superconductivity, colossal magnetoresistance, ferroelectricity and spin dependent transport, among others.
Studies exploring why perovskite has proven such an efficient solar collector have zeroed in on its unique crystal structure.
When light hits a solar cell it displaces electrons, leaving a positively charged “hole” in their absence. As electrons migrate to one side of the material the different potentials of the two sides create electrical voltage. Efficiency measures how easily electrons can circumnavigate the material, which is tied to the material’s atomic structure. The better the efficiency, the more power you can cull from the light.
The tight, regular structure of silicon allows for great propagation of a charge. However, that perfect structure requires costly, painstaking processes to guard against contamination.
The crystal structure of perovskite, however, is created cheaply at room temperature and seems to allow space for the electrons to transit farther and quicker—though precisely how isn’t clearly understood yet.
“The progress has been astounding over the past five years, and exceeded any other technology by a lot,” said Mike Toney, a materials expert at Stanford, which set a record last year with a tandem solar cell working in conjunction with Arizona State University.
The first perovskite solar cell in 2009 only reached 3.8 percent efficiency. Then, in 2012 UK researcher Henry Snaith demonstrated a process for room temperature perovskite (typically formed as a lead halide), and the race was on.
Indeed, there’s been so much interest that efficiencies are beginning to approach practical limits.
“It really can’t go up at that rate any longer,” Toney explained. “It gets increasingly hard and there are fundamental limits it runs into or will run into very soon.”
Researchers are already producing perovskite solar cells near silicon’s record efficiency (26.6 percent), and it’s capable of going much higher (33 percent compared to a 29 percent theoretical limit for silicon).
It’s also easier to use. Perovskite is stable at room temperature and can be poured as a thin film or even applied via inkjet printers. In comparison, commercialized silicon’s record efficiency is 22.2 percent and most panels are around 17-18 percent.
While silicon is theoretically as abundant as silicon dioxide (i.e. beach sand) the process involves heating it to over 1000 degrees and maintaining clean rooms to prevent contamination.
“Getting the high quality silicates needed to make a solar cell is not easy. It takes a lot of energy and right now the world is not producing enough silicate at the scale that will be needed in order to have widespread implementation of solar energy,” Toney said.
“That’s the real limitation. It isn’t now but will be in 4-5 years. That’s why a lot of people see perovskites potentially replacing silicon and going into the solar farms.”
So why can’t you buy perovskite solar panels yet? Despite the success in the lab, few of the approaches are “roof-tested,” making commercialization somewhat chancy. Playing into that is fact that stability of perovskite solar cells was an early challenge—they’re particularly vulnerable to moisture, for example.
“If you put a solar cell on your roof you want it to last for 30 years,” Toney said. Right now that’s hard to promise because none of the varied perovskite approaches have been around long enough to be have faced much testing in real-world conditions. “[But] huge progress has been made over the last three years in stability.”
Toney said that it might take the adoption of perovskites in a smaller solar niche to prime the commercial pump.
“Once these cells start to become accepted then you’re going to see much more widespread acceptance,” he said. “How long it will take to be truly widespread I don’t think anyone really knows.”
While perovskite efficiencies are approaching their theoretical limit, interest in perovskites keeps growing, thanks in part to silicon’s familiarity—it’s been studied for sixty years. Perovskite offers a rare opportunity for scientists to get in on something new that’s about to revolutionize not just photovoltaics but other fields as well, such as light-emitting diodes.
“As a material it’s just fundamentally different than silicon so that drives really interesting science questions,” Toney said. “It changes the way we think about semiconductors or photovoltaic materials.”
Many groups besides Stanford are exploring tandem cells, which offer a higher theoretical efficiency limit (40 percent) than either material alone. These cells take advantage of way the materials absorb different wavelengths more efficiently. Perovskite does better with the blue end of the spectrum and silicon with the red, though there’s significant overlap that (for the moment) has limited total efficiency (27.3 percent).
Some researchers are looking at other metals such as tin to replace lead in the perovskite metal halide compound out of health concerns, though Toney suggested the risk is debatable. Others are looking for better/cheaper contact materials than gold. A lot of these issues will have as much to do price, availability, ease of use, and other factors than pure efficiency.
“There are a lot of things that go into making these that are probably going to dominate the business decision of whether you want to use this [material] or not,” Toney said. “Not necessarily how good they are.”