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The physical and chemical processes taking place in dye-sensitized solar cells (DSC) are reviewed. In order to achieve high power efficiencies under solar irradiation, it is essential to incorporate kinetic asymmetry into the design of DSCs. Electron transfer to the redox mediator should be slow from the nanocrystalline oxide and fast from the regenerating electrode. This asymmetry can be achieved by the choice of redox electrolyte as well as by selective use of electrocatalysis to accelerate electron transfer at one contact and suitable blocking layers to retard electron transfer at the other interfaces. In the discussion of the processes that determine the efficiency of DSCs, particular emphasis is placed on the role of electron trapping in determining the rates of electron transport and interfacial electron transfer. The prospects for improving the performance of DSCs are discussed in a quantitative framework based on current understanding of the cells. (c) 2006 Elsevier B.V. All rights reserved.