The cell performance depends on the film morphology for a given DSSC. Nanoparticles are essential to increase surface area, and hence, amount of dye, while large particles are required to enhance absorption of red light through light scattering. It is impossible to increase surface area and light scattering simultaneously, because they oppose each other. Therefore, there must be a balance between them. Such a balance was well controlled by tuning the layer structure, and an energy conversion efficiency of 10.2% was obtained using a multilayer structure. In order to scatter the red light more efficiently, a more sophisticated multilayer structure with gradually increased particle size from the most-inner layer deserve to be constructed and tested .
Although the double layer is better than the monolayer in terms of back-scattering suppression, the light-scattering effect in the double layer is not as efficient as in structure M; the IPCEs (the incident monochromatic photon-to-electron conversion efficiency) for the double layer are lower than those of M at the wavelength greater than 620 nm .
Fig. 1. Schematic film morphologies of studied TiO2 photoelectrodes.
It is,therefore, imperative to increase the light scattering for the benefit of efficient red light harvesting. Since the path-depth length of light increases with wavelength, a better result should be obtained when the scattering centers are gradually increased. Therefore, a multilayer structure was proposed and tested .
Fig. 2 shows the action spectra of NMS and NM'MS, where the spectrum of NM is also appended for comparison. NMS adsorbed a similar amount of dye to NM, but the former structure gave higher IPCEs in the visible range. The increased amount of light-scattering particles and the scattering center gradient may account for the enhanced IPCEs. As a result of improved IPCEs, was increased by 0.4% .
Fig. 2. Photocurrent action spectra of NM, NMS, and NM'MS.
If too many large particles are incorporated into the film, the light absorption enhancement that results from light scattering can be offset by a lower dye concentration ascribed to a reduction of the total surface area.
How to balance the surface area and the light scattering is the key to controlling the cell performance. Structure NM'MS could adsorb more dye than NMS, and present both higher IPCEs and broad feature.
As a result, the DSSC with structure NM'MS yielded a higher efficiency of 9.8%. The larger surface concentration of dye and the suitable light-scattering center gradient in NM'MS may account for the higher IPCEs than those observed in NMS.
A thin anti-reflection layer was put on the cell surface to reduce the light reflection by the conducting glass. The energy conversion efficiency corresponded to 10.23% .