Dominant defect and microstructure transformation engineering for highly efficient low-bandgap stannite solar cells

Abstract

To meet the requirements of high-efficiency multi-junction tandem, it is imperative to seek cost-effective low-bandgap bottom subcell candidates. The bandgap of Cu₂ZnSn(S,Se)₄ (CZTSSe) can be effectively narrowed by Cd alloying with the lowest achievable bandgap of 0.87 eV after 60 % Zn substitution by Cd. However, the low-bandgap Cu₂CdZnSn(S,Se)₄ (CCZTSSe) is not efficient, majorly owing to the poor crystallinity of thin films and severe non-radiative recombination. Herein, we report an effective approach to enhance the crystallinity of CCZTSSe via in situ Na-addition in the bottom precursor. Sodium attracts Se within the bottom layers, facilitating the grain growth. Electrical properties are promoted accordingly, with the reduced interface defect densities, modified grain boundaries and superior Ohmic contact at the back interface. The most significant improvement is the dominant defect transformation, from detrimental deep Cu_Cd to benign shallow V_Cu. These advances enable non-radiative recombination to be greatly suppressed both at the interface and in the bulk. Eventually, an efficiency of 10.48 % has been achieved, which is the highest reported efficiency for stannite solar cells. This is a significant step forward on the road to applying CCZTSSe in future tandem technologies.