The stereotypical image of solar cells may be of panels placed on the roofs of homes or of vast rivers of panels lined up on large swaths of land, often called mega-solar projects.
But there is a type of solar cell that is quite different: organic solar cells, which are often much thinner, lighter and more flexible.
Although the amount of energy that organic solar cells can generate from sunlight is lower than conventional cells, experts say that there is room for improvement, with many arguing they could be an effective tool for slashing carbon dioxide emissions, in part, due to their pliable nature.
“Normally, solar cells that people imagine are ones on the roof that are heavy and firm … (but) organic solar cells are very light and flexible, so they can be set up in places where it would be physically impossible to install regular solar panels,” said Itaru Osaka, a senior research scientist developing organic solar cells at the state-backed Riken institute.
Organic solar cells are made from organic compounds or polymers, while regular cells are made primarily of silicon.
A silicon-based solar panel weighs about 15 to 20 kilograms and is heated at hundreds of degrees Celsius during the manufacturing process, which actually results in greenhouse gas emissions.
Osaka said organic solar cells are made by thinly coating the polymers on various materials, such as plastic, so they are light. Also, the cells can be crafted at temperatures of less than 100 degrees, making them more environmentally friendly, he said.
And because the polymer-based cells can be coated on plastic film, they are flexible.
This way, “we can stick organic solar cells vertically or install them on weak structures, such as vinyl greenhouses,” Osaka said.
Because of this pliability, the potential usages are likely to expand in the future. Installing them on the exteriors of electric cars, so that the cars can generate electricity while driving, is just one possibility.
Also, by changing the structures of organic compounds, it is possible to alter the colors of the cells. There are even transparent ones, which can be applied to windows.
According to Osaka, research and development into polymer-based solar cells saw an explosion in the late 2000s as scientists began uncovering combinations of organic substances that increase the power conversion efficiency.
He said there are literally an infinite numbers of ways to change the structures of organic compounds, noting that discovering changes that offer high power conversion efficiency is key.
Currently, a conversion rate of roughly 10 percent seen in lab experiments is believed to be the world’s highest for organic solar cells.
Silicon-based or regular solar cells, meanwhile, boast a conversion rate of 15 to 20 percent for products currently on the market.
“Conversion efficiency needs to be higher, but it’s not that it has to beat regular solar cells, because — rather than replacing them — organic solar cells will probably be set up in different ways,” Osaka said.
For researchers, the primary goal is a power conversion rate of 15 percent in the lab, he said.
Another concern about organic solar cells has been their lack of durability, especially in high temperatures. But researchers at Riken last year developed a polymer that drastically strengthened the cells.
If solar cells are not durable, their conversion efficiency decreases over time. But the organic compound that Riken discovered was able to keep the rate steady after being exposed to 85-degree heat for 500 hours.
While the discovery of the polymer came about by chance, Osaka said that if the durability issue is resolved, it would be a major step toward the practical use of the technology.
Ultimately, though, for organic solar cells to gain traction as a viable product, it is essential that they make their market debut.
That is where Tokyo-based Mitsubishi Chemical Corp. comes in.
Mitsubishi Chemical, which also develops polymer-based solar cells, has conducted experiments using the technology, including installing transparent organic solar cells on the windows of the Sendai International Center in Miyagi Prefecture.
Mitsubishi Chemical has also been collaborating with Taisei Corp., a major Tokyo-based construction firm, to develop wall materials made with organic solar cells for buildings.
“There is still no market yet, but we are in the process of creating that now,” said Hiroaki Yamaoka, who heads the organic solar cell business team at Mitsubishi Chemical.
Because organic solar cells can be used on windows and walls, “we have received positive reactions (from potential customers and partners),” he said.
A single solar cell manufactured by Mitsubishi Chemical has an 11.7 percent conversion efficiency rate, but that plummets to about 5 percent when it is turned into a module.
The firm thinks that 5 percent is enough to make it commercially viable but still hopes to improve efficiency.
Mitsubishi Chemical plans to manufacture organic solar cells first for industrial use to increase name recognition. It then plans to make them available for all consumers, projecting that the business will take off sometime after 2020.
But for now, Mitsubishi has its work cut out for it.
Since few people know about the technology, it “is critical that we raise the profile of organic solar cells,” Yamaoka said.
This section, appearing on the second Monday of each month, features new technologies that are still under research and development but expected to hit the market in coming years.