Towards optimisation of electron transfer processes in dye sensitised solar cells

Studies of photoinduced charge separation in supermolecular systems, such as molecular diads, have shown that achieving a high yield of long lived and energetic charge separated states requires careful optimisation of system design. Such optimisation typically requires a compromise between conflicting constraints with, for example, increased electronic coupling favouring a high charge separation yield but at the expense of lifetime of the charge separated state. In this paper we will apply such considerations to the optimisation of interfacial electron transfer dynamics in dye sensitised, nanocrystalline solar cells, focusing on the dynamics of electron injection and charge recombination between sensitiser dyes and nanocrystalline TiO2 electrodes. We will first of all review our fundamental understanding of these dynamics in terms of non-adiabatic electron transfer theory, and discuss the influence of electron trapping within the TiO2 upon these dynamics. We will then go on to address what the 'optimum' electron transfer dynamics are for efficient device function, and in particular the concept of 'kinetic redundancy' whereby it is undesirable to have unnecessarily fast forward electron transfer reactions. We consider strategies to achieve optimum electron transfer dynamics, focusing in particular on the control of these dynamics by the use of conformal metal oxide coatings. Finally the fundamental limitations to optimisation of the interfacial electron transfer dynamics are discussed, addressing in particular the potential role of interfacial energetic inhomogeneities in limiting device optimisation. (C) 2004 Elsevier B.V. All rights reserved.