Lighter and more flexible solar cells achieve world’s highest efficiency

(Press-News.org) The Korea Institute of Energy Research (President Yi Chang-keun, hereinafter referred to as “KIER”) has successfully developed ultra-lightweight flexible perovskite/CIGS tandem solar cells and achieved a power conversion efficiency of 23.64%, which is the world’s highest efficiency of the flexible perovskite/CIGS tandem solar cells reported to date. The solar cells developed by the research team are extremely lightweight and can be attached to curved surfaces, making it a promising candidate for future applications in buildings, vehicles, aircraft, and more.

Crystalline silicon-based single-junction solar cells are predominantly used in solar power generation due to their low production cost and suitability for mass manufacturing. However, as the efficiency of single-junction solar cells approaches their theoretical limit, tandem solar cells— which combine silicon with perovskite* solar cells to enhance efficiency—are gaining increasing attention.

* Perovskite: A material with a crystal structure similar to calcium titanate (CaTiO₃), known for its excellent light absorption properties, making it a promising next-generation solar cell material.

Currently, perovskite/silicon tandem solar cells have achieved efficiencies as high as 34.6%. However, their heavy weight and susceptibility to physical damage limit their application in fields where lightness and adaptability are critical, such as in automobiles, aircraft, and satellites.

To overcome these limitations, flexible thin-film perovskite/CIGS* tandem solar cells are being developed. CIGS-based thin-film solar cells are extremely lightweight and flexible, making them well-suited for use on curved surfaces such as buildings, vehicles, and aircraft. However, their lower efficiency and greater manufacturing complexity compared to perovskite/silicon tandem solar cells have posed barriers to commercialization.

*CIGS: A compound semiconductor made of copper, indium, gallium, and selenium (CuIn₁₋ₓ GaₓSe₂), known for its excellent photoelectric properties. It is used as a light-absorbing layer in thin-film solar cells. In particular, it can be fabricated on highly flexible substrates such as polyimide or metal films, giving it exceptional flexibility.

To enhance the manufacturability, flexibility, and lightness of tandem solar cells, the KIER research team developed a simple lift-off process and identified the underlying mechanisms behind the performance improvement. As a result, the fabricated perovskite/CIGS tandem solar cell achieved a power conversion efficiency of 23.64%, representing the highest recorded efficiency among flexible perovskite/CIGS tandem solar cells reported to date.

The lift-off process developed by the research team involves coating a polyimide layer onto a glass substrate, fabricating the perovskite/CIGS tandem solar cell on top of it, and then separating it from the glass. Unlike conventional methods that use flexible polyimide film directly as the substrate, this approach utilizes rigid glass as a supporting base, allowing for more stable fabrication of the solar cells. The use of a flat, rigid glass substrate also ensures uniform layer deposition, leading to improved device performance and higher reproducibility.

The research team also identified a method to enhance performance by reducing defects in the solar cell. During the fabrication process, alkali metal elements such as potassium diffuse from the glass substrate into the CIGS light-absorbing layer. Excessive diffusion of potassium can create defects within the absorber layer that hinder charge transport, ultimately degrading the performances of the solar cells. However, until now, no technology had been reported that effectively suppresses the diffusion of potassium to an optimal level.

Using computational science, the research team predicted that the polyimide layer coated on the glass substrate could suppress potassium diffusion. When applied to the solar cell fabrication process, this approach effectively reduced defects in the CIGS light-absorbing layer. As a result, the fabricated device achieved a power conversion efficiency of 23.64%, significantly exceeding the previous record of 18.1% for flexible perovskite/CIGS tandem solar cells.

In addition, to verify the durability of the fabricated CIGS solar cells, the research team measured the mechanical properties of the materials and analyzed the stress applied during bending through simulations. After conducting 100,000 bending cycles, the solar cells maintained 97.7% of their initial efficiency, demonstrating excellent durability.

Dr. Inyoung Jeong, who led the study, stated, “This research is a key achievement that demonstrates the commercial potential of next-generation high-efficiency solar cell technology with flexibility and lightness,” adding, “It serves as an important milestone toward realizing ultralight, flexible solar cells with 30% efficiency in the future.”

Dr. Kihwan Kim, principal investigator of the study, stated, “The power-to-weight ratio of the fabricated solar cell is approximately 10 times higher than that of perovskite/silicon tandem solar cells, making it highly promising for applications in fields that require ultralight solar modules, such as building exteriors, vehicles, and aerospace.” He added, “By advancing large-area fabrication processes and improving stability, we aim to strengthen the competitiveness of related industries and significantly contribute to the expansion of renewable energy adoption.”

Meanwhile, this research was published in the March issue of Joule (Impact Factor: 38.6), a prestigious international journal in the field of energy and materials. The study was conducted with support from the Research and Development Program of the Korea Institute of Energy Research, in collaboration with Professor Tae Kyung Lee of Gyeongsang National University and Professor Hae-Jin Kim of Yonsei University.

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