Edge-carboxylated graphene nanoplatelets as oxygen-rich metal-free cathodes for organic dye-sensitized solar cells

Edge-carboxylated graphene nanoplatelets (ECGnPs) were synthesized by the simple, efficient and eco-friendly ball-milling of graphite in the presence of dry ice and used as oxygen-rich metal-free counter electrodes (CEs) in organic dye-sensitized solar cells (DSSCs), for the first time. The resultant ECGnPs are soluble in many polar solvents including 2-propanol due to the polar nature of numerous carboxylic acids at edges, allowing an electrostatic spray (e-spray) to be deposited on fluorine-doped SnO₂ (FTO)/glass substrates. The ECGnP-CE exhibited profound improvements in the electrochemical stability for the Co(bpy)₃^2+/3+ redox couple compared to the platinum (Pt)-CE. The charge transfer resistance (R_CT), related to the interface between an electrolyte and a CE, was significantly reduced to 0.87 Ω cm², much lower than those of (Pt)-CE (2.19 Ω cm ²), PEDOT:PSS-CE (2.63 Ω cm²) and reduced graphene oxide (rGO)-CE (1.21 Ω cm²). The DSSC based on the JK-303-sensitizer and ECGnP-CE displayed a higher photovoltaic performance (FF, J_sc, and η, 74.4%, 14.07 mA cm^-2 and 9.31%) than those with the Pt-CE (71.6%, 13.69 mA cm^-2 and 8.67%), PEDOT:PSS (68.7%, 13.68 mA cm^-2 and 8.25%) and rGO-CE (72.9%, 13.88 mA cm^-2 and 8.94%).