  Flexible Cu2ZnSn(S, Se)4 (CZTSSe) solar cells take the advantages of
environmental friendliness, low cost, and multi-scenario applications, and have
drawn extensive attention in recent years. Compared with rigid devices, the
lack of alkali metal elements in the flexible substrate is the main factor
limiting the performance of flexible CZTSSe solar cells. This work proposes a
Rb ion additive strategy to simultaneously regulate the CZTSSe film surface
properties and the CdS chemical bath deposition (CBD) processes. Material and
chemical characterization reveals that Rb ions can passivate the detrimental
Se0 cluster defect and additionally provide a more active surface for the CdS
epitaxial growth. Furthermore, Rb can also coordinate with thiourea (TU) in the
CBD solution and improve the ion-by-ion deposition of the CdS layer. Finally,
the flexible CZTSSe cell fabricated by this strategy has reached a high
total-area efficiency of 12.63% (active-area efficiency of 13.2%), with its VOC
and FF reaching 538 mV and 0.70, respectively. This work enriches the alkali
metal passivation strategies and provides new ideas for further improving
flexible CZTSSe solar cells in the future.
