Predict the lifetime of solar cells installed in the early stages of their popularization

Solar PV power generation has become increasingly popular as a leading renewable energy source. By the end of 2021, the cumulative amount of solar PV in operation worldwide reached 942 gigawatts (GW), which is 5% of the global electricity demand, a 23% increase from the end of the previous year. Since the beginning of the 21^st century, Japan has seen a rapid expansion of PV, in part due to the Feed-in-Tariff (FIT) system, which allows electric power companies to purchase electricity generated from renewable energy sources at a price set by the national government. At the end of 2021, the cumulative amount of installed solar capacity was 78.2 GW, 8.3% of the world's total and 9.4% of the domestic electricity demand. This is equivalent to the energy demands of Japan's total population, about 100 million, every person owning two solar cell modules with an output of 400W each. About 97% of them are silicon solar cells mainly made of crystalline silicon.

Now that solar cells that were installed in the initial years of gaining popularity are approaching 20 years since their installation, there is concern about their deterioration over time. Takashi Minemoto, Professor of Electrical and Electronic Engineering at the College of Science and Engineering at Ritsumeikan University, is working on a wide range of research topics related to PV power generation, including the prediction of solar cells' lifetimes. "There is no clear definition of the lifetime of a solar cell," explains Minemoto. "It is very unusual for a solar cell to suddenly stop producing electricity, but its output decreases every year, and it eventually becomes unable to produce the expected amount of electricity." If the power generation capacity decreases, the profit made by selling electricity also decreases. Therefore, the estimated lifetime is important when considering the warranty of solar panels.

One of the causes of reduced power output is the corrosion of metal electrodes by acids. The structure of silicon-based solar cell modules is made up of multiple components: the solar cell that consists of silicon, which is the power-generating part, and metal, which is the power-extracting part, and is protected by a resin seal with glass on the surface and an insulator sheet on the back. The silicon is virtually non-degradable, but the ethylene-vinyl acetate (EVA) resin used in the seal can degrade and decompose under heat, ultraviolet rays, and moisture, and the resulting acetic acid corrodes the electrodes. Minemoto speaks of this problem as follows: "We know that EVA resin contributes to the shortening of electrode life. However, it is easy-to-process, inexpensive, and has a proven track record as an industrial product. We are looking for sealants that can replace EVA and electrode materials that are resistant to corrosion in order to keep material and manufacturing costs from increasing."

Promote recycling and upcycling to use up the exhausted solar cells

Even with extended lifetime, disposal will eventually come as an unavoidable problem. If recycling is not actively promoted and landfills account for the majority of waste, we will eventually face a critical shortage of disposal site capacity. "The market for exhausted solar panels is not very active because the appraised value tends to be low. This is due to the difficulty of ensuring their quality in the absence of an established method that predicts their life expectancies," says Minemoto. "It is possible to technically estimate the remaining power generation capacity, but, at present, it is difficult to secure economic benefits because inspections are often very expensive." Based on this awareness, Minemoto and his team are working to assess the value of solar panels so they can be used to their full potential, while also promoting upcycling, in which panels with remaining power generation capacity can be re-sold with added value. "Under current standards, panels that are submerged in water due to climatic hazards like typhoons can be replaced with new ones under insurance. However, these panels are actually not fully destroyed and still have power generation capacity. We are considering using these panels as independent power sources for lighting and digital displays."

Perovskite solar cells that challenge green innovation

While silicon-based solar cell modules are heavy, weighing approximately 10 kg per square meter, perovskite solar cells are receiving attention due to their lighter weight and high-power generation efficiencies comparable to that of silicon-based modules. Perovskite is the name of a crystal structure. To form a power-generating layer with a perovskite structure, a solution of multiple elements such as lead, iodine, and carbon are mixed under certain conditions and deposited as a film on a substrate.

Although perovskite cells must be tightly sealed to prevent their reaction and dissolution in water, they are lightweight and inexpensive to manufacture, and their power generation efficiency increases with shorter wavelengths of light when compared with silicon-based cells. This difference in power generation efficiencies at different wavelengths can be used to create tandem-type solar cells by combining perovskite and silicon systems to compensate for their power generation abilities in weak wavelength bands. While silicon-based solar cells have relied on imports of Chinese products, perovskite is a technology that has been developed in Japan. If the lifetime of these cells can be extended, they are expected to offer a wide range of applications as well as economic benefits.

"I want to achieve the world's best result in solar cells in Japan," says Minemoto passionately as he continues his research. However, he also senses a crisis in the current state of solar cells and says, "Although they seem to have become widespread, they have not yet been fully accepted by society." The knowledge required for solar cell research and business is diverse, and many companies are unsure where to begin when entering the market. Efforts to recycle and reuse are making progress. In 2019, Minemoto established "Scholars Co., Ltd" at Ritsumeikan University as a venture company through which Ritsumeikan researchers support research and development related to solar PV and reusable energy, and started advising companies and undertaking entrusted research. He also continues to share his knowledge about PV on his YouTube channel "PhotoVoltaics University." "The spread of solar PV power generation will continue to advance in the future. With academic collaboration among researchers around the world, I would like to promote research and development that utilizes Japan's strength through industry-government-academia collaboration," concludes Minemoto.