Investigation of TiO2 nanotubes/nanoparticles stacking sequences to improve power conversion efficiency of dye-sensitized solar cells

In this study, dye-sensitized solar cells (DSSCs) were fabricated using a hybrid photoanode with freestanding TiO2 nanotube (TN) membranes and TiO2 nanoparticles (TPs). To form the hybrid structures, up to 20 layers of TPs were screen-printed on to fluorine-doped tin oxide (FTO) glass, and the TNs, which were grown separately, were placed on top of the TPs. To investigate the effects of TNs on the energy conversion efficiency of these cells, TNs were placed on and sandwiched between TPs. Vertically oriented and smooth TN surfaces were obtained using a two-step anodization process. To obtain crystallized TNs, heat treatment was performed and anatase crystal structure was confirmed with XRD. For the anodized-TN-based solar cells, incident light was scattered by the three-dimensional topography at the bottom of the highly ordered TNs. This effect increased the optical path length in the photoanode, which allowed more light to be recycled by the dye molecules for additional photocurrent generation. This process also yielded a significantly higher power conversion efficiency (7.0%) than other geometries of photoanodes, which was further increased to 7.5% and charge transfer resistance decreased 14.5 Ω to 5.51 Ω when the TNs were sandwiched between the TPs, which was analyzed by EIS simulation.