Solar power is one of the most widely used renewable-energy sources. Solar cells, commonly seen on rooftops and in vacant lots, use silicon semiconductors made of inorganic materials that generate electricity when exposed to sunlight. Solar cells can be installed anywhere with sunlight exposure; however, the amount of power generated depends on the weather. The high manufacturing cost, due to the use of crystalline silicon which requires high manufacturing temperatures, is also a limitation of silicon solar cells. In addition to silicon-based solar cells, research has been conducted on other types of solar cells using organic materials, such as dye-sensitized solar cells and organic thin-film solar cells.

Dye-sensitized solar cells use organic dyes as sensitizers on the surface of titanium dioxide nanoparticles; the dyes absorb light to generate electricity. Manufacturing dyesensitized solar cells, different from silicon semiconductors, can be easily performed at a low cost by applying printing technology. However, challenges remain, such as lower power generation efficiency and concerns about leakage and evaporation owing to the use of a liquid electrolytic solution.

Professor Tsutomu Miyasaka and his team at the Toin University of Yokohama discovered that light energy can be converted into electrical energy when lead methylammonium iodide (CH₃NH₃PbI₃), a compound containing iodine and lead, is used as the photovoltaic material. In 2009, they published a paper on dye-sensitized solar cells using lead methylammonium iodide instead of organic dyes. Lead methylammonium iodide is a crystal with a perovskite structure. Cells using perovskite semiconductors are called perovskite solar cells. Although the mechanism of perovskite solar cells was first identified in Japan, research into these cells has progressed significantly in overseas countries in 2012. We hope that Japan will accelerate its efforts to generate research results in this field. Atsushi Wakamiya, professor at Kyoto University, focuses on perovskite solar cells. As a researcher of dye-sensitized solar cells, Wakamiya believed in the potential of new solar cells and started the project as a team leader.

Atsushi Wakamiya, professor at Kyoto University, focuses on perovskite solar cells. As a researcher of dye-sensitized solar cells, Wakamiya believed in the potential of new solar cells and started the project as a team leader.

Lead iodide and methylammonium iodide are widely used in perovskite solar cells. When dissolved in a solvent, the constituent ions form perovskite-type crystals during the drying and evaporation processes. These crystals are soluble in polar solvents, allowing solar cells to be fabricated by applying this ink-like solution to electrodes and then drying them. However, actual solar cells fabricated in this manner have not achieved the reported power generation efficiency of 10%. The average efficiency achieved was 5%–6%, with some cells performing even worse, sometimes generating no power at all. Researchers around the world have suffered from very poor reproducibility of the expected power generation efficiency.

In rigorous organic synthesis experiments involving unstable compounds, even a small amount of water may adversely affect chemical reactions. To address this, the team ensured that all reagents were of high purity, while pretreating the instruments and other materials to remove water. However, as they succeeded in removing water and increasing the purity of the reagents, they found that less lead iodide was dissolving. This prompted them to investigate further, and they discovered that only lead iodide, one of the reagents, had been used without pretreatment. This was because the reagent was of the highest grade, with a purity of 99.999%. However, analysis of the reagent revealed that it contained as much as 2000 ppm of water. The standards for organic and inorganic materials differ, with the purity of inorganic materials depending on the main component such as trace metal basis. Once the purity of lead iodide was increased, the material became soluble in the solvent, resulting in a higher power generation efficiency for the solar cells. The issue of poor reproducibility was traced back to the purity of the reagent. The high-purity reagent developed by Wakamiya and his team is currently available on the market and is used worldwide for research on perovskite solar cells.

Perovskite solar cells are thin and lightweight.Perovskite crystals are extremely efficient at absorbing light and converting energy into electric power, allowing perovskite solar cells to generate power even when applied as a thin layer. However, the underlying mechanism was unclear at the beginning of this research. Wakamiya and his team could elucidate the physical properties that led to the identification of this mechanism, thanks to the support of Yoshihiko Kanemitsu, a professor at Kyoto University. Kanemitsu specializes in physics and has extensive knowledge of inorganic semiconductors. With this assistance, the optical properties of perovskite crystals and other materials were measured using unique spectroscopic techniques that are not available elsewhere. Their collaboration began when Kanemitsu, whose research lab is located next to Wakamiya’s, suggested “If you can get a good sample, we can analyze its optical properties to clarify the mechanism of power generation.”

The characteristics of materials that make for excellent solar cells, such as perovskite solar cells, include the ability to emit light well and stable photoexcited states. When the relationship between the intensity of the excitation light and that of the luminescence was investigated, it was found that the latter increased in proportion to the square of the excitation light intensity above a certain level of excitation energy. This phenomenon is not typically observed in excitons. This is known to occur when light emitted by the recombination of free electrons and holes. In the past, excitons were considered to contribute to power generation in perovskite solar cells. However, electrons and holes, known as free carriers, are generated when semiconductors are irradiated with light. These free carriers cause high energy conversion efficiencies of solar cells. In addition, electrons and holes usually have longer lifetimes in perovskite semiconductors than in conventional semiconductors. It has been demonstrated that perovskite semiconductors have fewer defects and higher luminescence efficiencies when electrons and holes recombine. Moreover, electrons and holes produced in semiconductors recombine to emit light, and in the case of perovskite semiconductors, the emitted light is absorbed again to produce electrons and holes repeatedly. This process is known as photon recycling. These findings demonstrate the superior performance of perovskite solar cells, and are the main characteristics that distinguish them from siliconbased solar cells.

The process involves ① oxidation, in which cumene is oxidized with air to produce CMHP; ② epoxidation, in which PO and cumyl alcohol are obtained from CMHP and propylene; and ③ hydrogenation, in which cumyl alcohol is hydrogenated to produce cumene. A notable feature of this method is cumene recycling, where cumene serves as a reaction mediator and an oxygen carrier.

Once the challenges related to the materials were solved and the power-generating mechanisms were identified, the possibility of achieving a 20% power-generation efficiency, which is equivalent to that of silicon solar cells, became a realistic goal. One significant advantage of perovskite solar cells is that they can be applied as inks by dissolving the materials in a solvent. In this process, the solution is dropped onto the substrate and spread by high-speed rotation, during which toluene is added. Since toluene does not easily dissolve the materials, it forms an intermediate film. When this intermediate film is heated and dried, it transforms into a film of black perovskite-type crystals.

This method produces high-quality cells that are suitable for research purposes. However, the maximum size of the squares produced using this method is only 25 mm, which is insufficient for practical applications. The limitations arise from the time required for the materials to dissolve in the solvent and the precise reaction conditions required, such as the timing of the toluene drop and the brief few seconds for the solution to dry. To increase the size of the cells, the team reviewed the materials and reaction processes, thoroughly checking the conditions. Eventually, a new material was developed, namely a DMF complex of lead halide perovskite (lead methylammonium iodide: CH₃NH₃PbI₃). This material dissolves faster in solvents and at higher concentrations. Factors such as solvent and temperature were revisited to increase the drying time. Consequently, it became easier to control the reactions and thus apply the solvent to a larger area. This innovation paved the way for practical applications. Taking advantage of a support program that had just been established at Kyoto University, Wakamiya started a start-up company, EneCoat Technologies, in 2018, together with Naoya Kato, who was a classmate during his university days.

Launching a start-up does not necessarily guarantee the practical application of the new technology. Transferring university research results to a commercial company can be challenging, often leading to impeded technological development and slower-than-expected progress. Fortunately, this is not the case with EneCoat Technologies. Tamotsu Horiuchi, an engineer with extensive knowledge and experience in solar cells, joined the company and facilitated the technology transfer.

The goal of EneCoat Technologies is to increase the size of perovskite cells while maintaining good and stable energy conversion efficiency. Currently, the company can produce perovskite solar cells with dimensions of 75 mm and 300 mm, with efforts underway to produce even larger cells. Larger cells can be cut into smaller cells, allowing mass production and cost reduction.

Once larger perovskite solar cells are available, the next challenge is to ensure homogeneous quality. Practical applications would be difficult if cell quality differed between locations, such as the center and edges. To address this, the company is developing equipment that automatically applies the material to the film and checks its quality in real time.

In June 2022, the power-generation efficiency of perovskite solar cells reached 25.7%, which is almost equivalent to that of silicon solar cells. In addition, perovskite solar cells are extremely thin, approximately 1 µm. Of that 1 µm, the perovskite semiconductor layer constitutes only 500 to 600 nm, roughly one hundredth the thickness of a hair. Although highly advanced technology is required to fabricate such thin films, it does not require many materials. They also consume low levels of energy because the materials react at low temperatures, leading to reduced carbon dioxide emissions. Meanwhile, EneCoat Technologies is working towards increasing device durability. They are expected to last for 10 years, but the company believes that it is possible to extend device lifespan by improving their design. Moreover, because perovskite semiconductors are soluble in water, they can be easily recovered for recycling, which is another technology currently under development.

One concern to be addressed is the potential impact of the lead used in semiconductors on human health and the environment. To address this issue, Wakamiya et al. developed a semiconductor using tin instead of lead. Although this is a challenging task, and the power generation efficiency is only approximately 15%, the team aims to achieve an efficiency comparable to that of lead-based cells, so that lead-free cells can eventually penetrate the market.

The film-type perovskite solar cells are very light, weighing only 0.2 kg/m2, which is ~1/50th the weight of silicon solar cells. These thin, lightweight, film-like, bendable solar cells can be installed in locations unsuitable for conventional silicon solar cells, such as building walls, windows, and car bodies. Other potential installation sites include soundproof walls along highways and roofs that cannot support heavy objects. Because perovskite solar cells can generate electricity even on rainy days or indoors, they are expected to be useful in a variety of applications, ranging from cell phone chargers to emergency power sources.

As the development race intensifies worldwide, the commercialization of perovskite solar cells is imminent. Prof. Kanemitsu reflects that their research has advanced thanks to the fruitful collaboration of experts and engineers in the fields of chemistry and physics. EneCoat Technologies is not resting on its past accomplishments but is stepping up its efforts to strengthen its research and development to keep up with the competition and achieve its ultimate goals.

Compared to silicon solar cells, which are currently widely used, perovskite solar cells offer higher power-generation efficiency and an easier manufacturing process. As such they are receiving increasing attention and are expected to play a leading role in the next generation of energy. As summarized in EneCoat Technologies’ catchphrase, “Dokodemo Dengen® (power supply anywhere),” it will not be long before power can be generated with perovskite solar cells anywhere in the world.