HomeScience & TechOrganic photovoltaics that is sustainable and offers many advantages over traditional silicon-based...

Organic photovoltaics that is sustainable and offers many advantages over traditional silicon-based solar panels

Itaru Osaka’s story with organic photovoltaics began as a PhD student working in Hideki Shirakawa’s research group at the University of Tsukuba in Japan. In the 1970s, Shirakawa, along with American scientists Alan Heeger and Alan MacDiarmid, found a way to make plastics that could conduct electricity—a discovery that earned them the 2000 Nobel Prize in Chemistry. Currently, Osaka has its own research group at Hiroshima University working with these “conjugated polymers” to produce “organic” carbon-based photovoltaic cells. Unlike typical silicon-based cells, which are relatively bulky, heavy, rigid and opaque, organic alternatives are flexible and transparent enough to be placed where existing cells cannot, such as on building walls, greenhouse glass and even even on the sides of the tents.Significantly, they are expected to be cheaper to manufacture and use less energy than silicon-based photovoltaics.

Return of organic matter

Organic photovoltaics (OPV) research boomed between 2005 and 2015, Osaka says, but recent years have seen declining interest, especially in industry. The reasons are varied, but some factors are the lack of finance and the improved efficiency of perovskite solar cells, which can also be flexible. “I mean, go back to this area and do research on OPV systems,” Osaka says. “It’s a very important technology and directly related to carbon neutrality.”

Osaka’s lab at Hiroshima University is working with so-called π-conjugated (pi-conjugated) polymers that can be used to make solar cells that convert light into energy, similar to traditional solar cells, but made of plastic instead of silicon.

The original π-conjugated polymer was polyacetylene, discovered by Shirakawa’s team. Plastics are polymers, which means they are made up of long chains of repeating molecules. π-conjugated polymers are made of carbon atoms alternately linked by single and double bonds. Electron-containing “π orbitals” form above the double bonds and overlap each other, connecting the single-bonded carbon atoms.

At a very basic level, when an electron in a π orbital is removed by “doping”, an electron in an adjacent π orbital can move and free up space in its original orbital. This movement of electrons creates an electric current and essentially turns the plastic into a semiconductor material.

Replenishment of silicon cells

Researchers, including Osaka’s group, are making organic solar cells based on these π-conjugated polymers. Osaka explains that the intention is not to replace conventional silicon-based solar cells – which are increasingly efficient at converting sunlight into energy – but to supplement them in scenarios where they are less efficient or too cumbersome to use.

In addition, it takes a huge amount of energy and complex processing to create the purified silicon for these solar cells. To address this, much research has focused on perovskite-based solar cells. Perovskite is a crystalline material that can be printed or coated with thin films. Perovskite solar cells promise high efficiency comparable to silicon-based cells, but contain lead, which is toxic.

Therefore, other alternatives are needed, Osaka says, and OPVs made with π-conjugated polymers are one area the researchers are exploring. To make OPVs based on the π-conjugated polymer, the Osaka team uses solution processing that is similar to perovskite solar cells. Processing the solution requires significantly less energy than the heat-intensive process used to manufacture silicon solar cells. It leads to the formation of thin films of polymers that can be printed on a flexible substrate. “That’s why organic solar cells can be very flexible and light,” he explains.

The team uses π-conjugated polymers as the “p” type electron-donor material in OPVs. To create an electric current between the electrodes, they mix it with an n-type electron-accepting organic material based on a fullerene or, more recently, a non-fullerene.

But to work effectively, the “noodle-like” π-conjugated polymers must be more crystalline, Osaka explains. His team is working to find ways to straighten and stack them to provide a smooth channel for electron mobility. They also aim to make p- and n-type materials more miscible or easily miscible. “It is very difficult to achieve high crystallinity and high miscibility at the same time,” says Osaka. “But miscibility is very important to achieve high energy conversion efficiency.”

Osaka says his team is unique. The 15 graduate students and two assistants who work with him come from applied chemistry. They not only study and manipulate the molecular structures of π-conjugated polymers, but also produce OPV devices made from them. This is difficult to learn, Osaka says, but he believes it is important for his team to do both, to have a good knowledge of this area of ​​research, and also to have a positive impact on Japanese industry when some go to work outside academia.

So far, the Osaka team has achieved 16–17% efficiency with their π-conjugated polymer-based OPVs. The highest efficiency achieved by any lab is around 18%, Osaka says. One of its goals is to improve the efficiency of its OPVs in the short term and subsequently surpass the efficiency of conventional solar cells, which is currently in the range of 25%. It also wants to reduce voltage losses in OPVs based on π-conjugated polymer.

His vision is to produce cost-effective, lightweight, flexible, transparent and efficient OPVs that can be placed on the outside of structures. Energy is lost when it needs to travel long distances from the solar array to the city, Osaka says. But imagine if you could place transparent solar cells on top of agricultural greenhouses or on the sides of buildings and on windows, closer to where they are needed. Osaka further envisions OPVs that can be placed on top of emergency tents to make electricity more accessible to victims of natural disasters such as earthquakes and floods that Japan is so prone to.

Conjugated polymer-based OPVs are very promising. However, getting them to commercialization will be challenging and needs at least another five years, Osaka estimates. Still, he adds, it’s worth the wait and the potential deserves more research attention, especially from industry, to eventually realize this important technology.

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