Tiny Shells Yield “Remarkable” Increase In Organic Solar Cell Conversion Efficiency

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The cost of solar power has been dropping like a rock, due in part to cost-cutting improvements in silicon solar cell technology. Still, silicon remains relatively expensive. If some inexpensive new material could come along to boost the affordability factor, the pace of solar adoption could pick up and spread from rooftops and ground-mounted arrays to cover all sorts of surfaces, including windows, greenhouses, and other elements of the built environment.  Thin, transparent organic solar cells could do the trick, if only they were more efficient and durable. Oh, wait…

The Long Road To The Organic Solar Cell Of The Future

Despite the name “organic,” most solar cells that fall under the organic umbrella are not made of crunchy granola. The “organic” in an organic solar cell refers to carbon, and that is typically sourced from petrochemicals. That’s why organic photovoltaic technology is often referred to as polymer or just plain old plastic.

Though the use of petrochemicals is problematic (more on that in a second), the polymer formula has a smaller environmental footprint than silicon during the manufacturing and application stages. In contrast to silicon, organic solar cells are processed in a solution that can be painted, sprayed, or printed onto a different kinds of surfaces, including flexible ones. In addition to cutting costs, that opens up a whole new range of applications from fabric-embedded solar devices to see-through solar windows.

On the down side, organic solar cells are not nearly as efficient and durable as their silicon counterparts. Although lab results appear promising, real-world performance falls short.

The state of affairs looked gloomy as recently as 2021, when Dr. Alexander Gillett, of the Cavendish Laboratory at Cambridge University, noted that organic solar cells “can do lots of things that inorganic solar cells can’t, but their commercial development has plateaued in recent years, in part due to their inferior efficiency.”

“A typical silicon-based solar cell can reach efficiencies as high as 20 to 25%, while organic solar cells can reach efficiencies of around 19% under laboratory conditions, and real-world efficiencies of about 10 to 12%,” he added.

Tiny Cups & The “Remarkable” Organic Solar Cell Of The Future

Dr. Gillett was not the only one to take notice of the resistance to commercial development. CleanTechnica covered the organic solar cell field fairly often up until about four years ago, and then the subject dropped off our radar as well (our complete archive is here).

On the bright side, back in 2021 Gillett and his research team were already hot on the trail of a strategy for developing new, more durable materials. Last April, they presented new details about a key factor inhibiting solar conversion efficiency.

More recently, another solution emerged last November, when the University of Southern Denmark reported that Technical Faculty member Vida Engmann was co-author of new study titled, “Vitamin C for Photo-Stable Non-fullerene-acceptor-Based Organic Solar Cells,” along with Sambathkumar Balasubramanian of the school’s Centre for Advanced Photonics and Energy.

The school noted that vitamin C, aka ascorbic acid, protects human cells from damage caused by ultraviolet radiation, among other risks.

“It turns out that C-vitamins can also have a similar protective effect on organic solar cells. Organic solar cells are carbon-based materials that can degrade over time, especially when exposed to sunlight and other external factors. This degradation is a significant reason why the lifespan of organic solar cells has been limited,” the school explained, citing Engmann.

“The results of her research are remarkable and open the door to more durable and reliable organic solar cell technologies,” the school emphasized.

That’s not the only remarkable development to cross the CleanTechnica radar in recent months.

Other new research has been cropping up thick and fast. Just last week, a particularly interesting study appeared in the SPIE Journal of Photonics for Energy under the title, “Hemispherical-shell-shaped organic photovoltaic cells for absorption enhancement and improved angular coverage.”

The study, spearheaded by Professor Dooyoung Hah of Abdullah Gül University in Türkiye, achieved “remarkable” results by deploying a shell-studded surface instead of a conventional flat surface for the active layer of an organic solar cell.

Don’t get too excited just yet. The study describes the results of a computer research method called three-dimensional finite element analysis, which breaks down large, complicated problems into smaller, less complicated chunks.

That’s not exactly the same thing as fabricating and testing an actual solar cell, but the new study does indicate that a purpose-built surface layer makes a significant difference in solar conversion efficiency.

“The FEA [finite element analysis] results reported are remarkable. When subjected to transverse electric (TE)-polarized light, the hemispherical shell structure exhibited a remarkable 66 percent increase in light absorption compared to flat-structured devices,” Abdullah Gül University observed.

“Similarly, for transverse magnetic (TM)-polarized light, a notable 36 percent improvement was observed,” the school added.

The study also indicates that the shape of the shell makes a difference. Previous studies deployed semi-cylinder shells to improve solar conversion efficiency. The new cup-like, hemispherical shells yielded a 13% increase for TE and a 21% increase for TM compared to the semicylinders.

The new shells also capture light from a wide range of angles, making them especially suitable for applications on the move, such as wearable solar cells.

“With the improved absorption and omnidirectionality characteristics, the proposed hemispherical-shell-shaped active layers will be found beneficial in various application areas of organic solar cells, such as biomedical devices, as well as applications such as power-generation windows and greenhouses, internet-of-things, and so on,” explained Professor Hah.

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Here Comes The Non-Plastic Organic Solar Cell Of The Future

As for the use of petrochemicals, that sticking point may be moot in the future. Last December a multi-school research team from Linköping University and KTH Royal Institute of Technology in Sweden reported a method for creating organic solar cells from kraft lignin, a common form of industrial wood pulp, instead of deploying plastic from petrochemicals.

The school also notes that chemically altered wood-based materials have already been used to stabilize perovskite solar cells as well as organic solar cells. The new research deploys raw lignin directly, with the aim of developing a more environmentally friendly supply chain as well as improving the stability of the solar cell.

“We have created a material, or composite, from kraft lignin which is to constitute the cathode interface layer. It turned out that this made the solar cell more stable. The advantage of kraft lignin is that it has the ability to create many hydrogen bonds, which helps to stabilise the solar cell,” explained Qilun Zhang, principal research engineer at the Linköping University Laboratory of Organic Electronics.

Don’t get too excited about that just yet, either. The cathode interface layer is just a small part of the solar cell. However, the researchers have affirmed a long term goal of close to 100% wood-derived material for an organic solar cell, so stay tuned for more on that.

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Image: “hemispherical-shell-shaped organic active layer for photovoltaic application, to improve energy efficiency and angular coverage; (left bottom) spatial distribution of electric field norms” by Hah via Eurekalert.