Organic photovoltaics: the path to lightweight, flexible and transparent solar cells

Itaru Osaka’s story with organic photovoltaics began as a PhD student working in the research group of Hideki Shirakawa 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 can conduct electricity — a discovery that won them the Nobel Prize in Chemistry in 2000.

These days Osaka has his own research group at Hiroshima University that is working with these ‘conjugated polymers’ to make carbon-based, ‘organic’ photovoltaic cells. In contrast to typical silicon-based cells, which are relatively bulky, heavy, rigid and opaque, the organic alternatives are flexible and transparent enough to be placed where existing cells cannot, such as on the walls of buildings, the glass of greenhouses and even on the sides of tents.

Significantly, their fabrication is expected to be cheaper and consume less energy than silicon-based photovoltaics.

Return of organics

Research on organic photovoltaics (OPV) boomed between 2005 and 2015, says Osaka, but recent years have seen waning interest, especially in industry. The reasons are varied, but some factors are a lack of funding, and the improved efficiency of perovskite solar cells, which can also be flexible.

“I want to say come back to this area and do research on OPV systems,” says Osaka. “This is a very important technology and is directly related to carbon neutrality.”

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

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

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

Complementing 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 complement them in scenarios where they are less effective, or too cumbersome to use.

Furthermore, huge amounts of energy and complex processing are needed to create the purified silicon for these solar cells. To address this, much research has focussed on perovskite-based solar cells. Perovskite is a crystalline material that can be printed or coated in thin films. Perovskite solar cells promise high efficiencies comparable to silicon-based cells, but they contain lead, which is toxic.

Therefore, further alternatives are needed, says Osaka, and OPVs made with π-conjugated polymers are one of the areas researchers are investigating.

To make their π-conjugated polymer-based OPVs, Osaka’s team uses solution-processing, which is similar to perovskite solar cells. Solution processing requires considerably less energy than the heat-intensive process used to make silicon solar cells. It leads to the formation of thin films of polymers that can be printed onto a flexible substrate. “That is why organic solar cells can be very flexible and lightweight,” he explains.

The team uses π-conjugated polymers as the ‘p-type’ electron-donating material in an OPV. For an electric current to form between electrodes, they mix it with an organic, fullerene-based or recently nonfullerene, ‘n-type’ electron-accepting material.

But to work efficiently, the “noodle-like” π-conjugated polymers need to be more crystalline, explains Osaka. His team works on finding ways to straighten them out and stack them to provide a smooth channel for electron mobility. They also focus on making the p- and n-type materials more miscible, or easy to mix.

“It is very difficult to achieve high crystallinity and high miscibility at the same time,” says Osaka. “But miscibility is very important for achieving high power-conversion efficiency.”

Surprising applications

Osaka says his team is unique. The 15 graduate students and two assistant professors who work with him come from an applied chemistry background. Not only do they study and manipulate the molecular structures of π-conjugated polymers, but they also fabricate OPV devices made from them. This is difficult to learn, says Osaka, but he believes it is important for his team to do both in order to have a strong grasp of this research field, and to also have a positive impact on Japanese industry when some move on to work outside academia.

So far, Osaka’s team has achieved 16–17% efficiency with their π-conjugated polymer-based OPVs. The highest efficiency achieved by any lab is around 18%, says Osaka. One of his goals is to improve the efficiency of his OPVs in the short-term, and then eventually exceed that of conventional solar cells, which is currently in the range of 25%. He also wants to reduce voltage loss in π-conjugated polymer-based OPVs.

His vision is to fabricate 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 a solar field into a city, Osaka says. But imagine being able to 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 emergency tents to make electricity more accessible for victims of natural disasters, such as the earthquakes and floods to which Japan is so prone.

Conjugated polymer-based OPVs show much promise. Getting them to commercialization will be challenging, however, and needs at least another five years, Osaka estimates. Nevertheless, he adds, it will be worth the wait and the potential deserves more research attention, especially from industry, to eventually realize this important technology.