Influence of Ge layer location on performance of flexible CZTSSe thin film solar cell

Sputtered Ge layer was placed between Mo/substrate as a transition layer or between CZTSSe/Mo as a doping layer for CZTSSe/Mo/substrate. The effects of these two Ge layers on the structural, mechanical and photo-electrical properties of CZTSSe thin film and solar cells were studied. It was found that the residual stress of CZTSSe thin film decreased from -2.49 to -0.27 GPa after inserting a Ge transition layer, leading an enhancement in adhesion property. Besides, XPS analysis demonstrated that the diffusion behavior of transitional Ge into CZTSSe absorber through Mo layer during annealing process was nonexistent. The inserted Ge doping layer not only could adjust the band gap of absorber but also improve the band mismatching of CZTSSe/CdS interface. Moreover, the oxidation from Se4+ to Sn2+ could be suppressed, which could decrease some deep defects in CZTSSe absorber. Ge transition layer and doping layer both made contributions to the increased crystalline quality. After inserting double Ge layers, a relative efficiency enhancement of 48% was achieved due to the reduced residual stress, improved crystalline quality, increased CBO and the alleviated band mismatching of CZTSSe/CdS interface.