2023 Outstanding Master Student Award － Yosuke Abe (Yamada-Miyajima Laboratory) －

Renewable energies are being introduced to reduce greenhouse gas emissions, and photovoltaic power generation is expected to become a key power source.

In our laboratory, we are conducting research on Cu(In,Ga)Se₂ (CIGS) solar cells, which require only about 1/100th of the thickness compared to the mainstream Si solar cells. CIGSe solar cells, known for their top-tier conversion efficiency in thin-film technology, are anticipated as the next-generation solar cells due to their low cost and superior long-term reliability. Moreover, by varying the composition ratio of In and Ga, it is possible to continuously control the bandgap of CIGSe. Currently, high efficiency is obtained under In-rich conditions with a band gap of about 1.1 eV, but there is a problem that the efficiency deteriorates when the band gap is 1.4 eV, which is the theoretical limit efficiency of solar cells. To address this issue, research has been conducted on adding Ag to the CIGSe layer. The addition of Ag leads to an increase in grain size, a decrease in defect density, and enables control of the conduction and valence bands while achieving a wider bandgap. Taking advantage of these benefits, this study systematically evaluated the effects of Ag addition to CIGSe on CIGSe bulk and interface properties. As a result, it was confirmed that the open-circuit voltage of the solar cell increased by 20 mV due to the increase in the band gap near the CIGSe surface and the improvement in film quality caused by the Ag addition.

Furthermore, the back contact characteristics of CIGSe solar cells have not received much attention so far, and the carrier transport mechanisms have not been thoroughly analyzed. In this regard, we constructed a device model for CIGSe solar cells based on experimental data and conducted simulations to analyze them. As a result, it was demonstrated that a very thin MoSe₂ layer of about 5 nm, which is naturally formed at the interface between CIGSe and the back electrode Mo plays a crucial role in the carrier transport of CIGSe solar cells. Moreover, by advancing the simulations, it was revealed that inserting a layer with a high acceptor density at the Mo/CIGSe interface improves the back contact characteristics. This finding provides guideline for further enhancing the efficiency of CIGSe solar cells.