Cost-efficient platinum-free DSCs using colloidal graphite counter electrodes combined with D35 organic dye and cobalt (II/III) redox couple

Abstract

Aqueous dispersions of colloidal graphite (CG) were deposited by spin coating on fluorine doped tin oxide (FTO) substrates and the resulting electrodes were characterized by a variety of experimental microscopic and spectroscopic techniques. The performed analysis has shown that the optical and electronic properties of the CG electrodes depend on the colloidal graphite’s concentration and the number of the deposited layers. Tafel plots confirmed that the constructed CG electrodes show excellent electrocatalytic activity towards the Co(bpy)33+ reduction. Robust dye-sensitized solar cells (DSCs) based on nanostructured titania film electrodes sensitized with D35 organic dye have been prepared employing the Co(bpy)3(PF6)2/Co(bpy)3(PF6)3 −based redox electrolyte. Colloidal graphite films, showing sub-micron plate-like morphology with excessive edge graphitic planes exposed, are excellent counter electrodes (CE) in DSCs, surpassing in efficiency the commonly used very expensive platinum (Pt) analogues and considerably reducing the cell fabrication costs. Electrochemical impedance spectroscopy (EIS) confirmed that the higher performance of the CG electrodes is due to lower electron transfer resistance at the corresponding electrolyte/counter electrode interface, in agreement with cyclic voltammetry (CV) experiments showing high stability and faster charge-transfer rate for carbon electrodes than that obtained for Pt. Using the CV data, the Fermi energy of the TiO2 photoelectrode under open circuit (OC) conditions was determined and the corresponding energy diagram in the DSC device was constructed. Additional EIS analysis on complete DSCs in the dark and under illumination certifies that the cobalt (II/III) couple works as a very effective electronic shuttle, enabling fast charge transfer at the CG counter electrode and also efficiently reducing the oxidized dye on the surface of the sensitized photoelectrode.