Function and analytical formula for nanocrystalline dye-sensitization solar cells

Based on the experimental observations that the three-phase nano-TiO2/F:SnO2/I-/I-3(-) electrolyte front contact has to have pronounced rectifying properties (reverse reaction with electrolyte suppressed) for efficient operation of the dye-sensitization solar cell and plays an active part in the generation of photoelectrochemical energy, an analytical formula is derived which allows the understanding of the relevance and involvement of a variety of kinetic and cell parameters. Essentially, the TiO2 layer is treated as a photocathode, donating electrons to a kinetically controlled front contact, with the counter-charges being. transported by the electrolyte within the pores. The formula was expanded to include photochemical kinetics of the sensitizer, for which photodegradation properties were also calculated. The branching ratio, the ratio of regeneration-rate constant of the sensitizer and of product-formation rate, turned out to be critical for long-term stability. It may have to be improved by one order of magnitude for efficient cells to reach a lifetime of 20 years. The degree of rectifying character of the nano-TiO2/F:SnO2/I-/I-3(-) electrolyte interface (electric-field-dependent charge transfer to the front contact versus recombination-rate constant with I-3(-) distinguishes between a low-efficiency ('dynamic') Galvani-type solar cell (efficiency determined by photoinduced chemical potential gradients, no rectifying contact) and a more highly efficient 'junction-type' solar cell (separation and collection of charges additionally assisted by junction potential). Several controversial subjects are addressed. The key challenges for the improvement of such cells are discussed, especially with respect to photodegradation and to solid-state devices.