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 JSC, VOC, 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, VOC 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.