Electrodynamically Sprayed Thin Films of Aqueous Dispersible Graphene Nanosheets: Highly Efficient Cathodes for Dye-Sensitized Solar Cells

Abstract

Highly efficient cathodes for dye-sensitized solar cells (DSSCs) were developed using thin films of graphene nanosheets (GNS), which were fabricated by the electrospray method (e-spray) using aqueous dispersions of chemically driven GNS. The e-sprayed GNS films had the appropriate properties to be an efficient counter electrode (CE) for DSSCs; sufficient electrocatalytic activity for I(-)/I3(-) redox couples and low charge transfer resistance (RCT) at the CE/electrolyte interface as characterized by cyclic voltammetry and electrochemical impedance analysis. The performance of the GNS film based CEs was optimized by manipulating the density of surface chemical functional groups and plane conjugation of GNS via post thermal annealing (TA). Upon TA, the oxygen-containing surface functional groups, which have been shown to improve electrocatalytic activity of carbon based materials, were significantly reduced, while the electrical conductivity was enhanced by ∼40 times. The improvement of electrocatalytic activity and fill factor (FF) with reduced RCT of DSSCs after TA was primarily attributed to the increased charge transport within the GNS films, while the chemically prepared GNS typically contained sufficient defects, edges and surface functional groups for electrocatalysis. The performance of the DSSCs using our GNS-CEs was nearly identical (>95%) to the DSSCs using the state-of-the-art CE, thermolytically prepared Pt crystals. Our e-sprayed GNS-CE based DSSCs had a higher FF (69.7%) and cell efficiency (6.93%) when compared previously reported graphene based CEs for DSSCs, demonstrating the outstanding properties of graphene as the electrodes in electrochemical devices.