Fossil energy stakeholders have been ginning up opposition to large-scale solar farms, but they should have been watching their backs. The latest development in photovoltaic technology involves low-cost perovskite solar cells created in the form of a solution which can be applied to a thin film or coated onto almost any surface. The opposition will need to figure out a new angle of attack when the world is suddenly plastered with perovskites.

The Promise Of Perovskite Solar Cells

Perovskite research has been a regular feature on the pages of CleanTechnica for a number of years, so the idea of paint-able solar cells is not particularly new or shocking. The promise of a low-cost, high-performing perovskite solar ink that can turn any surface into a power plant has been a long time coming, though.

One key challenge to overcome is durability. Unlike conventional silicon solar material, perovskites are lab-grown crystals that require tailoring to withstand exposure to air.

Another challenge is energy efficiency. The naturally occurring mineral perovskite was discovered in the 19th century, but growing perovskite-type crystals and applying them to solar cells is a 21st-century invention. Initial experiments back in 2009 yielded a solar conversion efficiency of just around 3%, far below the goalpost for commercial solar cells. However, the superior optoelectronic properties of perovskites sparked a torrent of follow-on research, and solar conversion efficiencies topping 20% are common today (see more perovskite background here).

More Than 27% Solar Conversion Efficiency For New Perovskite Solar Cells

Oxford University is among the hotspots for perovskite research. They are eager to share the results of their latest paint-on solar cell efforts, a solar conversion efficiency of more than 27%.

That’s a significant jump up from the 22% the school cites as the conversion efficiency for silicon solar panels.

While perovskite-based solar paint won’t replace silicon at scale any time soon, it does open up a wide new range of potential sites for solar energy harvesting.

“By using new materials which can be applied as a coating, we’ve shown we can replicate and out- perform silicon whilst also gaining flexibility,” explains Dr. Junke Wang, who holds the position of Marie Sk?odowska Curie Actions Postdoc Fellow at Oxford University Physics.

“This is important because it promises more solar power without the need for silicon-based panels or specially-built solar farms,” he elaborates.

The Many Benefits Of Lightweight, Flexible Solar Cells

Paint-on solar cells will need some sort of power take-off system in order to produce usable electricity, so it’s not a simple matter of coating any random surface with light-harvesting material. Still, perovskite-based solutions and other thin-film solar technologies have some significant advantages over silicon. They are much lighter, thinner, far more flexible, and more amenable to low-cost manufacturing processes.

“We can envisage perovskite coatings being applied to broader types of surface to generate cheap solar power, such as the roof of cars and buildings and even the backs of mobile phones,” explains Dr. Wang.

Perovskite is not the only ingredient in the new solar material. In a press release dated August 9, Oxford explains that it has a multi-junction format, meaning that it is composed of more than one light-harvesting material. It is also extremely thin. “At just over one micron thick, it is almost 150 times thinner than a silicon wafer,” Oxford emphasizes.

I’ve reached out to Oxford for more details. In the meantime, Dr. Junke’s past efforts provide some insights into the challenges of fabricating multi-junction perovskite solar cells.

“Stacking various perovskite materials that can absorb different colors in one device, namely a perovskite multijunction solar cell, offers an exciting route to break the efficiency limit at a low fabrication cost,” Junke explains on the Oxford website.

“Manufacturing such multijunction solar cells is a non-trivial task, given that different material layers need to be deposited on top of each other while also being compatible with each other,” he elaborates. “Each perovskite material requires a certain film formation strategy to achieve high performance, which imposes tremendous processing challenges.”

In a 2020 paper Junke and his team described their work on a streamlined, two-step process aimed at resolving those challenges. “Based on the development of robust and low-resistivity interconnecting layers, we achieve power conversion efficiencies of above 19% for monolithic all-perovskite tandem solar cells with limited loss of potential energy and fill factor,” the team reported.

“In a combination of 1.73?eV, 1.57?eV, and 1.23?eV perovskite sub-cells, we further demonstrate a power conversion efficiency of 16.8% for monolithic all-perovskite triple-junction solar cells,” they added.

Breaking The 20% Barrier For Multi-Junction Perovskite Solar Cells

More recently, in March of 2023 Junke and his team published a paper that describes an improved triple-junction solar cell deploying rubidium and caesium along with perovskite. “Using an approximately 2.0-electron-volt rubidium/caesium mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3?per cent (23.3?per cent certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21?volts,” they reported.

“This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells,” they added.

The Rise Of Tandem Solar Cells

If the world has to wait a while for truly paint-on solar cells, it does not have to wait for perovskites to make an appearance in thin-film solar technology that could be applied to buildings and other surfaces.

One emerging option is a tandem solar cell that combines the low cost of perovskites with the durability and efficiency of silicon. It’s no surprise to find the UK startup Oxford PV front and center and nearing commercial production of tandem perovskite-silicon modules, considering that the company spun out from research conducted at Oxford University.

In June, Oxford PV reported “unprecedented” solar conversion efficiency of 26.9% for its new modules. Sized for the residential rooftop market, the modules are currently being manufactured at the company’s first-of-its-kind factory near Berlin, Germany.

If you’re wondering why not the UK, Oxford PV co-founder and CSO Professor Henry Snaith has something to say about that.

“We originally looked at UK sites to start manufacturing but the government has yet to match the fiscal and commercial incentives on offer in other parts of Europe and the United States,” he explains, using the occasion of the Oxford University press release to put in a plug for perovskites.

Professor Snaith’s comment also indicates that cleantech stakeholders around the world have been paying attention to supportive federal policies in the US under the presidency of Joe Biden, highlighted by the 2021 Bipartisan Infrastructure Law and the 2022 Inflation Reduction Act.

Whether or not those policies continue past Biden’s term in office is up to the voting public. If you are a US citizen and eligible to vote, now would be a good time to register if you haven’t done so already.