Highly conductive and stable graphene/PEDOT:PSS composite as a metal free cathode for organic dye-sensitized solar cells

Platinum (Pt) as the counter electrode (CE) in dye-sensitized solar cells (DSSCs) is expensive and non-optimal for cobalt(ii/iii) redox couples, which can tune and improve the performance of DSSCs, thus motivating the search for replacements of the Pt CE. Graphene nanoplatelets (GnPs) are possible alternatives to Pt CEs but they are mechanically unstable as CEs due to their poor substrate adhesion. Here we report a new type of PEDOT:PSS (PP)/GnP (PPG) composite that maintains the catalytic performance of GnPs with enhanced adhesion to the substrate via a conductive PEDOT matrix. The resultant PPG exhibited extremely low charge-transfer resistance (R_ct) compared to Pt in its role as an electrocatalyst toward a Co(bpy)₃^2+/3+ (bpy = 2,2′-bipyridine) redox couple, and displayed extremely high electrochemical stability for Co(bpy)₃³⁺ reduction even after 1000 cycles. The inter-stacking of GnP layers between PEDOT and PSS was confirmed by XPS and Raman spectra. It helps to delocalize charges in the PEDOT backbone and rapidly transfers electrons from the external circuit to Co(bpy)₃³⁺. This reduces the R_ct and ultimately improves the photovoltaic performance. The DSSC based on Y123 sensitizer and PPG-CE showed a higher photovoltaic performance of 8.33% than its Pt counterpart does (7.99%) under the optimized conditions.