As we aim to reduce carbon dioxide emissions to achieve a carbon-neutral society, there are growing expectations for the widespread use of renewable energy. Interest in photovoltaic solar energy is particularly high, and a variety of solar cells that differ from conventional crystalline silicon types have recently been proposed. Among them, tandem solar cells are expected to have even higher photoelectric efficiency than conventional solar cells, however until now, they have been limited to special applications, such as space use, due to the reliance on expensive materials such as III-V compounds. Therefore, to make tandem solar cells widely available for civilian use, the development of materials and devices that balance both cost and performance is a key challenge. Research and development of materials and devices that enable the realization of inexpensive, high-performance tandem solar cells is being conducted worldwide. Recently, there has been significant activity in the research and development of metal-halide perovskite-type compounds. On the other hand, CIS-type compounds are also promising and have long been the focus of attention as materials that enable the fabrication of lightweight and flexible solar cells that take advantage of the characteristics of thin films. CIS solar cells with a relatively narrow bandgap energy of about 1.1 eV have already been commercialized. However, it is challenging to achieve high photovoltaic efficiency in CIS solar cells with a wide bandgap energy of 1.6 eV or greater (specialized for short-wavelength light absorption), which can be used as top cells, and technological development to achieve this has been required.

A researcher at AIST has developed a technology to improve the photovoltaic efficiency of wide-bandgap CIS-type thin-film solar cells that do not contain the rare metal indium.

For tandem-type solar cells, which are expected to achieve even higher performance than the solar cells currently in widespread use, the development of top cell materials for short-wavelength light absorption that satisfy all the elements of "low cost," "high performance," and "high reliability (stability)" has been a challenge. In this study, we developed a technology to improve the quality of photo-absorber layers with a wide bandgap energy, which is particularly suitable as a top cell among CIS-type compounds, a promising material group that satisfies these elements. The newly developed photo-absorber layer has excellent stability and is expected to be applied to inexpensive, high-performance, and flexible tandem-type solar cells in the future.

Journal: ACS Applied Materials & Interfaces
Title of paper: Photovoltaic Efficiency Enhancement of Indium-Free Wide-Bandgap Chalcopyrite Solar Cells via an Aluminum-Induced Back-Surface Field Effect
Authors: Shogo Ishizuka, Jiro Nishinaga, Yukiko Kamikawa, Takeshi Nishida, and Paul J. Fons
DOI: https://doi.org/10.1021/acsami.4c12891