Copper gallium selenide solar cells achieve an efficiency of 12.28%, Japanese team sets a new world record

(Source: Power International Exchange epintl)

Japanese researchers have achieved a photovoltaic conversion efficiency of 12.28% on copper gallium selenide solar cells, which is the highest efficiency publicly reported within the 1.65-1.75 eV bandgap range for indium-free wide-bandgap chalcogenide absorber layers. The device uses an aluminum-doped controlled thin film, combined with a back surface field and an optimized cadmium sulfide buffer layer to enhance open-circuit voltage, reduce carrier recombination, and improve overall performance.

Researchers from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan fabricated the solar cell with a copper gallium selenide (CuGaSe₂) absorber layer, achieving a conversion efficiency of 12.28%.

Copper gallium selenide is a chalcogenide semiconductor belonging to the chalcopyrite family, closely related to copper indium gallium selenide (CIGS) solar cell materials. It is an ideal material for the solar cell absorber layer because, as a direct bandgap semiconductor with a bandgap of approximately 1.68 eV, it can efficiently absorb visible light. Additionally, copper gallium selenide has a high absorption coefficient, meaning even very thin films can absorb most incident sunlight. The material also exhibits good defect tolerance, which helps reduce carrier recombination, allowing the solar cell to maintain good performance even if the crystal structure is not perfect.

The lead author of this study, Masato Ishizuka, stated to the Journal of Photovoltaics: “This efficiency can be regarded as the highest among wide-bandgap chalcogenide solar cells within the 1.65–1.75 eV bandgap range reported so far, especially in the field of indium-free wide-bandgap chalcopyrite (or CIGS-related) solar cells. It surpasses the performance data listed in Table 3 of the latest issue of Progress in Photovoltaics (Edition 67).”

He continued: “The device’s performance has been independently certified by a certified testing laboratory—the Renewable Energy Advanced Research Center, Japan Institute of Industrial Technology (AIST) Photovoltaic Calibration, Standards, and Measurement Team.”

This device was developed based on a solar cell design introduced by AIST researchers in 2024. By doping aluminum into the back region of the copper gallium selenide thin film, the open-circuit voltage, fill factor, and overall photovoltaic efficiency were effectively improved. This improvement is mainly attributed to the formation of a back surface field, which enhances minority carrier collection.

This record-breaking solar cell employs a three-step process to prepare the copper gallium selenide absorber layer, involving early doping with aluminum and fluororubidium in the first step, and additional fluororubidium treatment at the end of the third step. By precisely controlling the distribution of aluminum within the absorber layer, the design aims to increase the open-circuit voltage without sacrificing conversion efficiency.

The cell is built on a sodium-calcium glass substrate, with molybdenum as the back electrode, followed by an indium-free chalcopyrite absorber layer, a 150-nanometer-thick cadmium sulfide buffer layer, a zinc oxide window layer, and metal grid electrodes.

The manufacturing process begins with sputtering to deposit the molybdenum back electrode on the sodium-calcium glass substrate. Then, the copper gallium selenide absorber layer is prepared via high-temperature deposition and selenization, with aluminum introduced into the back region to form the back surface field. The absorber layer is subjected to alkali metal post-deposition treatment to passivate defects and improve electrochemical properties. Next, a chemical bath deposition method is used to add the cadmium sulfide buffer layer, forming a p-n junction. Finally, intrinsic zinc oxide and aluminum-doped zinc oxide window layers, as well as front electrodes, are sputtered.

Compared to previous devices, the optimization of the absorber layer by adopting a steeper aluminum concentration gradient and thickening the cadmium sulfide buffer layer successfully increased the open-circuit voltage and reduced interface recombination. The device ultimately achieved a 12.28% efficiency, 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%.

For comparison, a device fabricated in 2024 achieved an efficiency of 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%.

This research result was published in the journal Progress in Scientific Advances under the title “Achieving a New Record Efficiency for 1.7 eV Bandgap Chalcogenide Solar Cells through Synergistic Control of Bulk and Interface,”

Masato Ishizuka stated: “Our work focuses on the fundamental development of wide-bandgap devices used as top cells in tandem solar cells. Fabricating prototype devices also requires developing suitable bottom cells and corresponding stacking techniques. Therefore, this research has not yet reached the stage of large-scale production consideration. Since it is still basic research, a detailed cost analysis has not been conducted.”