Development of alternative counter electrode materials for the replacement of platinum in dye-sensitised solar cell applications

Dye-sensitised solar cells (DSSCs) are one of the emerging photovoltaic technologies, known for low cost, easy fabrication procedures and high efficiency, attracting the attention of researchers worldwide. Unlike the conventional silicon based solar cells, a typical DSSC consists of a dye absorbed on a semiconducting working electrode, a counter electrode and an electrolyte containing a redox couple (usually I-/I3-). One of the most important focuses in the current development of DSSCs is to reduce the cost of the devices. Conventionally, the counter electrode (CE) is constructed of fluorine-doped tin oxide (FTO) glass coated with nano-dimensional platinum, which has good electrocatalytic properties. However, as Pt is a noble and rare material it accounts for over 40% of the device cost, together with the conducting substrate. In addition, conventional platinum-coated cathodes show unsatisfactory electrocatalytic activity towards non-iodide based redox reactions. Therefore, there is a need to develop alternative, more economical platinum-free catalysts. In the early stages of this project, several composite CEs were developed by electrodeposition of highly electrocatalytic poly(3,4-ethylenedioxythiophene) (PEDOT) into mesoporous hard template layers constructed using nanoparticles such as TiO2 or TiC, designed to combine the benefits of high electrode surface area with mechanical robustness. The results showed that the conductivity and porosity of the template layer can influence the EDOT electropolymerization and also the final device performance. The most promising results were achieved with the TiC-PEDOT composite electrodes, which showed equivalent DSSC performance to Pt-coated electrodes with the I-/I3- electrolyte. The electrocatalytic and photovoltaic properties of the developed CEs were also compared to other electrocatalysts for a [Co(bpy)3]2+/3+ redox system. The results showed that TiC-PEDOT and TiN-PEDOT nanocomposites exhibit much faster charge transfer kinetics, obtaining higher photovoltaic conversion efficiencies compared to cells with pristine PEDOT or Pt coated electrodes. This highlights the advantages of nanocomposites having both highly catalytic PEDOT and also nanoparticles with large surface area. In the later stages of the thesis, research investigated the use of two novel carbonaceous materials, nitrogen doped graphene (N-graphene) and carbon nanocages (CNC), as electrocatalysts for both I-/I3- and Co2+/Co3+ based electrolytes. With optimized heating conditions and film thickness, higher efficiency was obtained with the CNC electrode for the I-/I3- system. However, the N-graphene electrodes exhibited much lower charge transfer resistance in the Co2+/Co3+ system, leading to a higher efficiency. To further reduce the weight and cost of the electrodes, a subsequent focus was fabrication of alternative CEs on either a flexible or a non-conducting substrate. Two low-temperature methods for preparing TiC-PEDOT composites on ITO-PEN substrates were compared. The electrodeposited TiC-PEDOT electrode, which had a uniform microstructure and polymer distribution, exhibited better performance in both I-/I3- and Co(bpy)32+/3+ electrolytes than the TiC/PEDOT:PSS composite prepared by mechanical mixing. Finally, several low cost materials were investigated to simultaneously replace the Pt and the expensive transparent conductive oxide (TCO) layer on glass substrates. A mixture of graphite and commercial organic conductive paste (Orgacon) was used to produce a film with relatively low sheet resistance and intermediate thickness. Finally, the electrocatalytic activity of this composite electrode was further improved by the electrodeposition of additional PEDOT.