Are simpler, cheaper solar cells on the way? Work being done to improve solar cell efficiency using a mineral called perovskite at Oxford University raises interesting possibilities.
According to a summary at SciTechDaily, from about 2009 to now, solar cells made from materials called perovskites “have reached efficiencies that other technologies took decades to achieve, but until recently no one quite knew why.”
Perovskite is a calcium titanium oxide mineral species composed of calcium titanate. It can be found in the Urals and Switzerland, as well as in Arkansas and some chondritic meteorites. And it’s quite inexpensive.
As Inhabitat reported in August, the substance “is said to be very efficient at absorbing light and uses less material to capture the same amount of energy when compared to conventional solar absorbers,” meaning it could result in “dirt cheap solar power.”
Perovskite, according to SciTechDaily, was first used in 2009 to produce 3 percent efficient photovoltaic cells. Since then, scientists pushed the technology to achieve efficiencies beyond 15 percent, which overtakes other emerging solar technologies.
There’s more exciting news. Researchers reported in Science that they have figured out the secret to perovskite’s success: It’s a property known as the diffusion length, and they also think they have a way to improve diffusion length by a factor of 10.
“The diffusion length gives us an indication of how thick the photovoltaic film can be,” Sam Stranks, who led the discovery with a group at Oxford University’s Department of Physics, wrote for Science. “If the diffusion length is too low, you can only use thin films so the cell can’t absorb much sunlight.”
So why is the diffusion length so important?
The way SciTechDaily puts it, PV cells are made from two types of material, called p-type and n-type semiconductors. The former mainly contains positively charged “holes,” and n-type materials mainly contain negatively charged electrons. They meet at a “p–n junction,” where the difference in charge creates an electric field.
Solar cells generate electricity when light particles (photons) collide with electrons, creating excited electrons and holes. The electric field of the p–n junction guides excited electrons toward the n-side and holes towards the p-side. They are picked up by metal contacts, electrodes, which enable them to flow around the circuit to create an electric current.
The diffusion length, Stranks explained, gives the average distance that charge-carriers (electrons and holes) can travel before they recombine. He wrote, “If the diffusion length is less than the thickness of the material, most charge-carriers will recombine before they reach the electrodes, so you only get low currents. You want a diffusion length that is two to three times as long as the thickness to collect almost all of the charges.”
Solar cells that are too thin don’t absorb much light, but in overly thick cells, the charge carriers inside can’t travel through. Longer diffusion lengths equals more efficient cells overall, so what scientists usually do is arrange cells into mesostructures, but they are complex, require time, and are not very commercially practical.
Or as Technology Review puts it, “Some have sought solar cells that can be made very cheaply but that have the downside of being relatively inefficient. Lately, more researchers have focused on developing very high efficiency cells, even if they require more expensive manufacturing techniques.”
Whereas before researchers could get mesostructured perovskite cells to 15 percent efficiency, using a perovskite compound with a diffusion length of around 100 nanometers (nm), by adding chloride ions to the mix, the group at Oxford achieved diffusion lengths over 1000 nm.
In addition to efficiency, the cells are cheaper and easier to produce since they don’t need all that complex structure.
The Inhabitat report indicated, “The most common form of solar cells are silicon based and cost as little as 75 cents per watt. For solar cells to be competitive with fossil fuels, the price has to drop to 50 cents per watt. Using perovskite as a stand-in could drop the price of a solar cell to only 10 to 20 cents per watt, while using less material than silicon.”
Stranks summed up the scientific breakthrough: “Being able to make 15 percent efficient cells in simple, flat structures makes a huge difference. We’ve made hundreds just for research purposes. It’s such an easy process. I expect we’ll be seeing perovskite cells in commercial use within the next few years.”
He even speculated perovskite cells hitting efficiencies of 20 to 30 percent “within the next few years.”
Still, it isn’t like silicon has just been sitting still, waiting for perovskite to blow its doors off. According to Technology Review, the costs of silicon solar cells are falling, and some analysts think they could eventually fall to as low as 25 cents per watt. This would eliminate most of the cost advantage of perovskite cells and lessen the incentive to invest in it.
But perovskite solar cells would not be difficult to manufacture. It could be “as simple as spreading a liquid over a surface or can involve vapor deposition, another large-scale manufacturing process,” according to Technology Review. But, “historically, it has taken over a decade to scale up novel solar-cell technologies, and a decade from now silicon solar cells could be too far ahead to catch.”
However, since the Oxford group was able to ramp up from 10 to 15 percent efficiency in six months, 20 percent could be in sight. Then perovskite solar cells could emerge as a real player in the cheap production of solar energy on a large scale.