11.88% Efficient Flexible Ag-Free CZTSSe Solar Cell: Spontaneously Tailoring the Alkali Metal Level.

Alkali metal is the requirement for highly efficient CuZnSn(S, Se) (CZTSSe) solar cells, thus it is crucial to additionally incorporate alkali metal into the absorber layer for flexible solar cells. However, the efficiency of flexible CZTSSe devices reported to date, based on the conventional alkali incorporation strategies, still lags behind those made on rigid substrates. One of the main issues is the inability to control the alkali content and distribution in the absorber layer. Here, a facile alkaline incorporation approach is proposed, effectively regulating the content and distribution of alkali metals in the film. Such a method can spontaneously tailor the alkali metal content to a proper level, thus leading to the suppression of non-radiative recombination and a better carrier transport through the enhanced film quality and the optimized band binding structure. Finally, a champion flexible CZTSSe solar cell with an efficiency of 11.88% is achieved, the highest reported efficiency for a CZTSSe solar cell without noble Ag doping. This study affords an innovative spontaneous alkali-doping design for the preparation of high-performance flexible CZTSSe solar cells and provides a deeper insight into the extent of alkali metal doping.