Researchers at the National Institute of Advanced Industrial Science & Technology (AIST) in Japan have achieved a power conversion efficiency of 12.28% in a solar cell based on an absober made of copper gallium selenide (CuGaSe2).
CuGaSe₂ is chalcogenide semiconductor belonging to the chalcopyrite family and is closely related to copper indium gallium selenide (CIGS) solar cell materials. It is is a promising material for solar cell absorber layers because it has a direct bandgap semiconductor with a bandgap of about 1.68 eV, which enables efficient absorption of visible sunlight. In addition, CuGaSe₂ has a high absorption coefficient, meaning that even very thin films can absorb a large portion of incoming solar radiation. The material also exhibits good defect tolerance, which reduces charge carrier recombination and allows the solar cell to perform well even if the crystal structure is not perfectly defect-free.
“The achieved efficiency can be regarded as the highest reported for wide-bandgap chalcogenide solar cells in the 1.65–1.75 eV range, particularly among indium-free wide-bandgap chalcopyrite, or CIGS-related, solar cells,” the research's lead author, Shogo Ishizuka, told pv magazine. “It exceeds the previously reported performance of CuGaSe₂-aluminum solar cells listed in Table 3 of the Efficiency Tables – Version 67 – published in the latest Progress in Photovoltaics.”
“The device performance was independently certified by an accredited testing laboratory, the Photovoltaic Calibration, Standards and Measurement Team at the Renewable Energy Advanced Research Center, AIST,” he went on to say.
The device builds on a previous cell design developed by AIST researchers in 2024. It incorporates aluminum (Al) in the backside region of CuGaSe₂ films, which enhances the cell’s open-circuit voltage, fill factor, and overall efficiency. This improvement is primarily due to the formation of a back-surface field (BSF), which boosts minority carrier collection.
The world-record solar cell uses a CuGaSe₂ absorber grown via a three-stage process, with Al and RbF supplied during the early first stage and additional RbF introduced in the final part of the third stage. The new design aims to increase open-circuit voltage without compromising efficiency by carefully controlling the aluminum distribution within the absorber.
The cell is built on a soda-lime glass (SLG) substrate coated with molybdenum (Mo) as a back contact. Above this sits an indium-free chalcopyrite absorber, a 150-nm cadmium sulfide (CdS) buffer layer, a zinc oxide (ZnO) window layer, and a metallic grid electrode.
Fabrication begins with sputtering the Mo back contact onto the SLG substrate. The CuGaSe₂ absorber layer is then deposited through high-temperature deposition and selenization, with Al incorporated near the backside to form the BSF. The absorber undergoes alkali post-deposition treatment to passivate defects and improve electronic properties. The CdS buffer layer is added via chemical bath deposition, forming the p–n junction, followed by sputtering of intrinsic and Al-doped ZnO window layers and the front electrode.
Optimizing the absorber with steeper Al gradients and a thicker CdS layer compared to the previous cell boosted the open-circuit voltage and reduced interfacial recombination. The device achieved an efficiency of 12.28%, an open-circuit voltage of 0.996 V, a short-circuit current of 17.90 mA/cm², and a fill factor of 68.8%.
By comparison, the 2024 device achieved an efficiency of 12.25%, an open-circuit voltage of 0.959 V, a short-circuit current of 17.64 mA/cm², and a fill factor of 72.5%.
The cell was described in “Bulk and interface engineering of 1.7 eV–bandgap chalcogenide solar cells enabling record efficiency,” published in ScienceAdvances.
“Our work focuses on the fundamental research and development of wide-bandgap devices intended for use as top cells in tandem solar cells. Fabrication of a prototype device would additionally require the development of a suitable bottom cell as well as tandem technologies. Therefore, this research is not yet at the stage where mass production can be considered,” said Ishizuka. “A detailed cost analysis has not yet been performed, as the present work is still at the stage of fundamental research.”
