N-doped hierarchical hollow mesoporous carbon as metal-free cathode for dye-sensitized solar cells

In this study, novel nitrogen (N)-doped hollow core-mesoporous shell carbon (N-HCMSC) capsules with high surface area are synthesized by simple three-step processes and explored as metal-free counter electrode (CE) in comparison with undoped HCMSC in dye-sensitized solar cells (DSSCs). It is shown that N-HCMSC significantly improves the catalytic activity toward the reduction of both Co(bpy)₃³⁺ and I₃^- due to much lower charge-transfer resistance (R_CT) at the CE/electrolyte interface. This is attributed not only to excellent surface properties including three-dimensionally interconnected hierarchical marco/mesoporosity with high surface area but also to N doping in N-HCMSC framework. Among various N species in the N-HCMSC framework, pyridinic and quaternary N species are considered to contribute significantly to the catalytic activity. In addition, N-HCMSC CE exhibits much better electrochemical stability than Pt CE toward both Co(bpy)₃^2+/3+ and I^-/I₃^- redox reactions. The N-HCMSC enhances the cell efficiency and fill factor up to 8.76 and 77.6%, respectively, at one sun illumination and is readily applicable as a potential alternative of costly Pt-based CE for DSSCs with Y123-sensitized photoanodes.