Solar energy is one of the most promising renewable energy sources in recent years. While placing a wind turbine in a residential backyard is impractical, installing photovoltaic panels on the roof is entirely feasible. Currently, a four-panel solar power system in Spain costs approximately €4,500 on average, covering both labor and equipment expenses. However, this data is for reference only, as the energy consumption of homes varies from one to another, and the power-generating capacity of different panel types also differs.

Solar panels are primarily composed of photovoltaic cells—thin semiconductor wafers (typically made of silicon) that generate a voltage when exposed to light. However, there are many different types of silicon materials used in the manufacture of solar cells. Among them, monocrystalline silicon is the most popular. Although its production cost is higher, it generally boasts higher efficiency, typically ranging from 15% to 22%. In addition, there are polycrystalline silicon (which is more affordable) and amorphous silicon (which has lower production costs but significantly lower efficiency).

Last year, the University of New South Wales in Australia published the 66th edition of the “Solar Cell Efficiency Tables” in the journal Progress in Photovoltaics. The table featured a world-record efficiency of 27.81% achieved by the Chinese manufacturer LONGi Green Energy using a back-contact heterojunction (HIBC) crystalline silicon solar cell. Today, scientists at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) have set a new efficiency record for copper gallium selenide (CuGaSe₂) solar cells. In contrast, the team’s device efficiency in 2024 was 12.25%. In this study, however, by employing a copper indium gallium selenide solar cell, the research team successfully increased the power conversion efficiency to 12.28%.

CuGaSe₂ is a chalcogenide chalcopyrite semiconductor closely related to copper–indium–gallium–selenium (CIGS) materials. Its direct bandgap of approximately 1.68 eV enables efficient absorption of visible light, thereby facilitating high-efficiency solar energy conversion. Therefore, CuGaSe₂ is regarded as one of the leading candidate materials for next-generation indium-free solar cells. Furthermore, this material exhibits excellent defect tolerance, which helps to reduce the recombination rate of charge carriers. As a result, the solar cell can still operate efficiently even if the crystal structure is not entirely defect-free.

The lead author of the study, Shogo Ishizuka, told pv magazine: “The efficiency achieved can be regarded as the highest reported value for wide-bandgap chalcogenide solar cells in the 1.65 to 1.75 eV range, particularly among indium-free wide-bandgap chalcopyrite or CIGS-related solar cells. This surpasses the previously reported performance of a CuGaSe₂–aluminum cell, which was listed in the latest edition of Progress in Photovoltaics, Volume 67, Efficiency Table.” The researcher added, “The device’s performance has been certified by an independently accredited testing laboratory—the Photovoltaic Metrology Calibration Group at AIST’s Advanced Energy Research Center.”

The device is based on an earlier battery design developed by AIST researchers in 2024, with aluminum dopants incorporated into the back region of the CuGaSe₂ thin film. This modification has improved the open-circuit voltage, fill factor, and overall efficiency of the cell. This world-record-breaking solar cell employs a CuGaSe₂ absorber layer grown via a three-stage process: aluminum (Al) and rubidium fluoride (RbF) are supplied during the first, initial stage, and additional RbF is introduced at the end of the third stage. The new design aims to increase the open-circuit voltage without compromising efficiency by precisely controlling the aluminum distribution within the absorber layer.

The device achieved an efficiency of 12.28%, with an open-circuit voltage of 0.996 V, a short-circuit current density of 17.90 mA/cm², and a fill factor of 68.8%. In contrast, the device efficiency in 2024 was 12.25%, with an open-circuit voltage of 0.959 V, a short-circuit current density of 17.64 mA/cm², and a fill factor of 72.5%.