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A solar cell, also called a photovoltaic cell, or PV, absorbs sunlight and then uses that energy to generate electricity. When put together as a solar panel, these cells can create enough electricity to power a home, school or office, or distribute power directly into the electricity grid.
Although silicon is the most common material used in solar cells, it’s not the only material. In fact, other materials have the potential to offer higher efficiency, more versatility and better cost-effectiveness. Take for instance perovskites, a material type that has the same crystal structure as the mineral composed of calcium titanate.
“When used as an absorber material, perovskites have proven capable of producing highly efficient cells, almost matching the efficiency of traditional silicon cells,” says Henry Snaith, a professor from Oxford University.
But how can perovskite cells compete with the already efficient silicon cells, which also benefit from being produced at scale? The answer, according to Snaith, is to go for higher efficiency – which is exactly what the EU-funded PERTPV project aims to do.
Stacking cells for increased voltage – and efficiency
What perovskite cells have that their silicon cousins don’t is versatility. “You can change the composition of perovskites to absorb different bands of light,” notes Snaith. “What this means is that instead of absorbing all the light in a single material, as is the case with silicon, you can stack two or more cells on top of each other and absorb different bands of sunlight.”
Snaith goes on to explain that this feature is important because different bands of sunlight essentially carry different levels of energy. “A solar cell can only produce as much voltage as the band of light it is able to absorb,” he says. “By stacking cells, you can increase the band range and, in doing so, increase both voltage and efficiency.”
Currently Oxford PV, a spin-off company of Oxford University, is in the process of stacking perovskites on top of silicon, with a commercial product set to hit the market next year. The PERTPV project is taking this one step further and stacking perovskite cells on top of perovskite cells.
“Ultimately, our goal is to demonstrate high efficiency on both wide and narrow band gap perovskite cells,” remarks Snaith. “This in turn will allow us to deliver 30 % efficient tandem cells, which is perovskite stacked on perovskite.”
More work to be done
The project is already seeing some promising results. For instance, its wide band gap perovskite, which goes on top of the stack and gets the sunlight first, is just short of its targeted efficiency. Researchers are also seeing relatively good stability in the low band gap material, which is comprised of a mixture of tin and lead.
According to Snaith, the challenge is with the efficiency of the low band gap perovskite. “This is still in the 18–19 % efficiency range, and we really need to get this up to 23 % to be able to deliver the 30 % efficient tandem cell,” he explains. “With the efficiencies we have right now, we should be able to deliver a 25 % efficient cell, but we need to pump that number up some more.”
In addition to developing the technology itself, the project is also tackling the issue of how to best manufacture the tandem perovskite cells. “All the pieces are in place, we just need to put them together and deliver,” adds Snaith. “But I am confident that by the time the project is over, we will have delivered a high-efficiency tandem perovskite cell.”
Although extremely difficult to predict timelines, Snaith says he can see multi-junction perovskite cells hitting the market as a viable – and sustainable – alternative to silicon PV cells within the next 5 years.
“The development of perovskite cells is an opportunity to make an already sustainable industry even better, which makes funding research like the PERTPV project so important,” concludes Snaith.