A study on flexible polycrystalline Cu2ZnSn(S,Se)4 thin film solar cells

Abstract

Polycrystalline Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have advantages such as high thermal stability, using low-cost elements and being both lightweight and flexible. Many studies are being conducted by various companies and laboratories to surpass the performance of Si solar cells. Especially, most of the leading groups of Cu(In,Ga)(S,Se)2 solar cells with power conversion efficiencies of more than 20% are using alkali post-deposition and alkali-containing film to improve performance. In addition, alkali doping is essential when flexible substrates are employed in place of the more commonly used rigid soda-lime glass due to Na and K diffusion effects. Generally, it is known that alkali elements avoid becoming recombination centers, form benign secondary phases, help minority carrier transportation through grain-boundary (GB) passivation, and prevent the formation of deep-level defects. Although mainstream principles have been elucidated, there is still much debate on how alkali doping leads to performance improvement. This work showed a CZTSSe solar cell with a FF of over 60% on a flexible metal substrate (for the first time as far as we are aware) and used it to analyze the effects of Na doping. From the results, we postulate that the carrier transport path depends on the amount of Na dopant, which affects the electrical resistance and FF of the polycrystalline CZTSSe solar cells.