Charge Transport and Transfer Mechanisms for Solid State Metal Complex Systems

Abstract

Recently there have been excited reports that copper-based molecular redox couples can be effective hole collection and transport materials when dissolved in liquid as well as in the solid state. The mechanism of charge transport in the solid state system is not understood, however, which inhibits further improvements or the development of design rules for intentionally made solid state DSSCs utilizing molecular redox complexes. In this talk, a change of diffusion mechanism for copper redox complexes will be presented during the solidification by an in-situ electrochemical analysis. The modified Dahms-Luff equation, which was partially substituted by the viscosity term, was used to evaluate the overall mechanism, and it was confirmed that the calculated self-exchange rate constant varies with phase and the viscosity-dependent value. Eventually, as the solidification progressed, the transport mechanism dominating was changed from ionic to electrical diffusion, which led to an increase in the apparent diffusion coefficient, indicating a high conductivity and a high self-exchange rate constant. However, in the case of cobalt complexes, which are often used in the liquid state, the transport mechanism continued to be dominated by ionic diffusion during solidification with a decrease in apparent diffusion and device performance. In addition to charge transport, the solidification affects rates of heterogeneous charge transfer and ultimately the performance in DSSCs, which will also be described. Finally, the results will be interpreted in design rules for metal complexes as solid state hole transport materials.