In the segment 'A Look Around Innovation,' we introduce research and development (R&D) sites that have led to social implementation. In the 28th installment of this series, JST News interviewed Professor Yutaka Ie of SANKEN (the Institute of Scientific and Industrial Research) at the University of Osaka who is working on the development of sheet-type organic solar cells for installation in agricultural greenhouses. His wavelength-selective approach absorbs only green light, which contributes little to photosynthesis, and is environmentally friendly. He aims for "solar matching," which creates harmony between agriculture and power generation. Verification trials are underway on farmland across Japan in collaboration with multiple companies.
Thin, lightweight, and ideal for greenhouse installation — Conventional types face challenges of weight and toxic materials
Adjacent to the Expo '70 Commemorative Park and boasting a vast site, the University of Osaka's Suita Campus is home to SANKEN (the Institute of Scientific and Industrial Research). In Ie's laboratory at this institute, JST News was shown an organic photovoltaic cell (OPV) as thin and light as a sheet (Figure 1). He says it is ideal for installation on the roofs and ceilings of agricultural greenhouses.
Agrivoltaics, which generate solar power on farmland, and is known in Japan as "solar sharing," is nothing new. However, Ie points out several problems with inorganic solar cells such as silicon panels, which are widely used today, and perovskite, which is expected to become more prevalent in the future.
Silicon panels are heavy and require dedicated mounting frames, which drives up installation costs and makes disposal difficult. Perovskite cells offer high power generation efficiency, but their toxic lead content is a concern for agricultural use. Furthermore, silicon panels block light entirely, casting shade on the area below them, while perovskite cells block the blue and red wavelengths needed for photosynthesis. Because insufficient sunlight hinders crop growth, these panels must be placed in areas without crops. For all these reasons, they are mostly installed on large farms with ample space, and there is a view that the electricity is not so much used for farming as sold to improve business profitability. Ie says that the farmer's perspective is crucial when developing solar cells for agricultural land. "For farmers, it's not just about good power generation efficiency. What matters is having a reliable harvest of safe, high-quality crops." That is the driving force behind his development of a solar cell that allows crops to grow beneath it while also generating electricity.
Using only the green wavelength for power generation — Enabling "locally produced, locally consumed" electricity
In an OPV, a thin film blending donor and acceptor materials (through which electrons move) serves as the power-generating layer. When light is absorbed, excitons are generated within the donor or acceptor; when these reach the interface between the two, charge separation occurs and an electric current flows. As a first step, Ie chose an inexpensive, commercially available donor molecule that absorbs only green-wavelength light. Since photosynthesis, which is essential for crop growth, primarily requires blue and red wavelengths, he reasoned that if only the green wavelength, which contributes little to photosynthesis, were used for solar power generation, crop growth would not be significantly affected (Figure 2).
Through the JST Mirai Program, Ie independently developed an acceptor molecule optimally matched to a donor molecule that absorbs only green-wavelength light (Figure 3). The resulting wavelength-selective OPV—which generates electricity primarily using green-wavelength light—is lightweight and flexible, with the advantage of being installable on curved surfaces of agricultural greenhouses. Because it transmits the blue and red wavelengths needed for photosynthesis, its impact on crop growth is minimal.
Ie explained the distinctive advantages of wavelength-selective OPVs: "By making full use of the greenhouse ceiling, they can be installed even on small plots of farmland. Integrating them into the structure simplifies the equipment, and it also enables 'locally produced, locally consumed energy'—for instance, using the electricity generated to maintain the temperature inside the greenhouse." Unlike solar sharing, which aims to sell electricity externally, solar matching is different in that the electricity is consumed on-site.
To install wavelength-selective OPVs in agricultural greenhouses, modules must be manufactured in meter-scale widths to match the greenhouse dimensions. With this in mind, Professor Ie applied to the Feasibility Verification (Entrepreneurship Challenge) part of JST's University-based New Industry Creation Fund Program, repeatedly verified the path toward modularization, and, with the cooperation of agribusiness companies, installed wavelength-selective OPVs in actual agricultural greenhouses to examine the effects on growth and crop yield. For some vegetables, using wavelength-selective OPVs made it easier to control the indoor temperature, and results showing improved crop yields have emerged. "We are continuing verification with crops such as strawberries and tomatoes. If we can confirm that yields increase, we expect that adopting wavelength-selective OPVs will become a clear benefit for farmers."
Scaling up toward practical use: An "integrated system" is called for
The research has now been selected as a project by the New Energy and Industrial Technology Development Organization (NEDO), and work is underway to scale up toward practical implementation. "We are collaborating with three private companies to advance discussions on improving the performance of modularized wavelength-selective OPVs and achieving mass production," said Ie. Many farmers are paying close attention, and requests for demonstration experiments show no sign of letting up.
For practical use, he says there is a need not only to improve power generation efficiency and verify durability, but also to make the modules even larger. "Japan's domestic manufacturing equipment limits us to a width of 40 to 50 centimeters, whereas Europe already has facilities that can produce a width of two meters in a single long sheet. Significant capital investment is needed to upgrade the manufacturing environment, so government support is also necessary." Looking further ahead, he has his sights set on building an integrated system that incorporates not just the solar cell itself, but also wiring, charging devices, sensors, and communication equipment.
As the scale of the research and development framework and verification activities grows, Professor Ie is finding it increasingly difficult to manage both research and business operations at the same time. "While I intend to remain focused on driving research and development, I am also considering entrusting the business side to someone who can comprehensively understand the wavelength-selective OPV technology and manage the whole operation," he said. With an eye toward starting a venture, he is steadily taking steps toward real-world implementation.
(Article: Shosuke Shimada, Photography: Hiroyuki Obayashi)
