Interface optimization and defects suppression via NaF introduction enable efficient flexible Sb₂Se₃ thin-film solar cells

Sb2Se3 with unique one-dimensional (1D) crystal structure exhibits exceptional deformation tolerance, demonstrating great application potential in flexible devices. However, the power conversion efficiency (PCE) of flexible Sb2Se3 photovoltaic devices is temporarily limited by the complicated intrinsic defects and the undesirable contact interfaces. Herein, a high-quality Sb2Se3 absorber layer with large crystal grains and benign [hk1] growth orientation can be first prepared on a Mo foil substrate. Then NaF intermediate layer is introduced between Mo and Sb2Se3, which can further optimize the growth of Sb2Se3 thin film. Moreover, positive Na ion diffusion enables it to dramatically lower barrier height at the back contact interface and passivate harmful defects at both bulk and heterojunction. As a result, the champion substrate structured Mo-foil/Mo/NaF/Sb2Se3/CdS/ITO/Ag flexible thin-film solar cell delivers an obviously higher efficiency of 8.03% and a record open-circuit voltage (V-OC) of 0.492 V. This flexible Sb2Se3 device also exhibits excellent stability and flexibility to stand large bending radius and multiple bending times, as well as superior weak light photo-response with derived efficiency of 12.60%. This work presents an effective strategy to enhance the flexible Sb2Se3 device performance and expand its potential photovoltaic applications.