Effect of the TiO₂ shell thickness on the dye-sensitized solar cells with ZnO-TiO₂ core-shell nanorod electrodes

Well-aligned ZnO nanorod arrays with high aspect ratio have been grown on FTO substrate by hydrothermal process. FTO substrate was pre-modified with ZnO thin film as a seed layer by sol-gel coating. TiO₂ thin shells with different thicknesses were grown on the ZnO nanorods by chemical vapor deposition method. Sample characterization was performed by X-ray diffraction (XRD), electron microscopy (FESEM, TEM, and SAD), X-ray photoelectron spectroscopy (XPS) and UV-vis Spectroscopy. XRD and TEM analyses indicated a wurtzite structured with high crystallinity and confirmed that each individual ZnO nanorod was a single crystal. The photoelectrochemical experiments were performed in a sandwich type two-electrode cell and I-V curves were obtained. Result of solar cell testing showed that addition of TiO₂ shells to ZnO nanorod significantly increased the J_SC, V_OC, fill factor and efficiency relative to devices without TiO₂ shells. Overall cell efficiency jumped from 0.45% for bare ZnO nanorod array to 0.92% for 14 nm thick TiO₂ shells on ZnO which showed a twofold increase. The fill factor increased from 0.37 to 0.60, showing a 62% improvement. For a shell with thickness of 21 nm, V_OC increased by about 150 mV to 0.68 V. The results showed that it is possible to fabricate core-shell cells of higher efficiency by using nanorod arrays and other morphologies with larger surface area.