In nanocrystalline dye-sensitized solar cells (DSSCs) the absorption of a large fraction of the incident solar radiation is important for achieving high efficiencies. We develop a model to include both the optical process and the electrochemical process. This model allows us to calculate the performance of the different optical designs (for example the different scattering layers and the different reflecting plane). It is found that appropriate optical designs can improve the performance of DSSCs greatly.