In a study published in Nature Energy, a research team led by MENG Qingbo from the Institute of Physics of the Chinese Academy of Sciences investigated the roles of alkali metals in flexible Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells, and clarified the distinct effects of sodium (Na) and lithium (Li) during crystallization by combining experimental characterization and theoretical analysis.

Flexible solar cells are promising for lightweight and portable energy applications. Among them, kesterite CZTSSe has attracted considerable attention due to its low cost, non-toxicity and earth-abundant elements. However, flexible CZTSSe devices still lag behind rigid counterparts in efficiency, largely because of difficulties in controlling crystallization and defect formation during film growth.

Alkali-metal incorporation has been widely adopted as an effective strategy to improve device performance in chalcogenide solar cells. In CZTSSe, Na is known to enhance crystal growth and improve film morphology, leading to notable efficiency gains. However, Na simultaneously induces phase segregation and generates secondary phases, which limits device performance improvement.

In this study, the researchers found that Na incorporation promoted selenium enrichment in the system, which facilitated crystal growth and improved the microstructure of CZTSSe films. However, at the same time, the excess selenium drove the formation of SnSe_x secondary phases and led to large-scale phase segregation. These effects hindered further improvement in device performance.

To address this issue, the researchers introduced Li into the system. They discovered that Li could reshape the free-energy landscape of Cu-related phases, promoting the formation of Cu_xSe. This process consumed excess selenium and suppressed the growth of SnSe_x phases, thereby reducing phase segregation and enabling more uniform crystallization.

The researchers developed high-quality CZTSSe films which achieved reduced defect density and improved charge transport properties. As a result, the flexible solar cells reached a power conversion efficiency of 14.5% (certified 14.2%), while flexible modules achieved 12.7% efficiency (certified 12.0%).

This study offers insights into alkali-metal regulation in complex materials, and provides a strategy for controlling phase evolution in multinary systems.