Osaka University develops transparent solar cells that absorb infrared light: Expectations for use in window glass

A research group led by Professor Masanori Sakamoto, who studies photochemistry at the Institute of Scientific and Industrial Research at Osaka University, is developing transparent solar cells that generate electricity by absorbing infrared light, which accounts for nearly half of the light energy that comes from the sun. This solar cell is expected to be used for window glass, where existing black solar cells cannot be installed. After solving the issues of conversion efficiency and enlargement, their goal is to produce a prototype palm-sized battery by 2025 and to ship samples of window glass that generates electricity by 2030.

Extracting electrons from nanoparticles with invisible light

The basic structure of a solar cell includes a layer that absorbs light, a layer that takes out electrons, and a layer that receives electrons, allowing light energy to be converted into electrical energy. Currently, the most widely used solar cell is the silicon solar cell, which utilizes silicon semiconductors. These cells generate electricity from visible light. Commercially available solar cells exhibit conversion efficiencies of more than 20% for extracting electrons from light. We often see power plants with the large-scale installation of big black panels.

Perovskite solar cells are being rapidly developed as a potential alternative to silicon-based solar cells. The materials for the solar cell can be made by coating or printing them on a film, etc., and its low-cost production is expected. Other promising technologies with low environmental impact and low cost include organic thin-film solar cells, which use a thin film of organic semiconductors as the power-generating layer, and dye-sensitized solar cells, which take advantage of flexibility and translucency.

Meanwhile, Sakamoto and his team are developing solar cells that use "localized surface plasmon resonance (LSPR)," by which electrons in nanoparticles vibrate collectively when exposed to light. Invisible infrared light, which has a longer wavelength than visible light, is focused on specially doped semiconductor nanoparticles to extract electrons.

Diversion from the world's most efficient photocatalyst

Sakamoto, who moved to Osaka University in April this year, has been researching infrared energy conversion since 2012 when he arrived at the Institute for Chemical Research at Kyoto University. Infrared light accounts for 42%−46% of the light that comes from the sun and has great potential as an energy resource. Because heat rays cause global warming, "the use of this technology will help prevent global warming," he says.

Sakamoto and his colleagues have discovered that when metal sulfide nanoparticles exhibiting LSPR are exposed to infrared light with a wavelength of 1.1 micrometers, hydrogen is produced with the world's highest efficiency, which act as a photocatalyst at that time.

Hearing a student say in a photocatalyst experiment that "it is difficult to see even after the irradiation with infrared light," Sakamoto thought, "invisibility means transparency. So, if the photocatalyst we have discovered is converted to solar cells, we can differentiate them from (black) silicon-based solar cells." In 2019, he announced that transparent solar cells could be made by applying tin-doped indium oxide nanoparticles, which also exhibit LSPR, to light absorbing materials.

Heat-shielding sheets to go on advance sale this summer

To commercialize the transparent solar cells made of LSPR materials, Sakamoto and his team established a Kyoto University venture company, OPTMASS (Uji City, Kyoto Prefecture), in 2021. Conversion efficiencies of up to 4.4% were reported in 2022 when nanoparticles containing cadmium and sulfur, or copper and sulfur were used as LSPR materials and exposed to 1.1 micrometer infrared light.

Currently, the conversion efficiency has increased to approximately 6% after improvements were made to the materials. However, when exposed to sunlight, which also includes visible light and ultraviolet light, the efficiency drops to approximately 1%, implying that there is much more room for improvement.

In addition, for practical use, solar cells must have a large area, such as the size of a windowpane. Because even few pinholes can significantly reduce the amount of power generated, a technology that can arrange material nanoparticles uniformly in a wide area is also required to achieve large-area solar cells.

Furthermore, LSPR materials prevent shielding heat rays from entering the room from outside. Therefore, OPTMASS is developing a heat-shielding sheet made of LSPR materials for advance sale this summer. The prototype sheet is transparent but has a slightly green color. The green color reflects OPTMASS' goal of "turning the city into a forest".