The photocurrent density-voltage curves of DSSCs based on the photoanodes of bare ZnO nanorods, ZnO/Nb
2O
5(1) core/shell nanorod (0.135 g NbCl
5), and ZnO/Nb
2O
5(2) core/shell nanorod (0.270 g NbCl
5) are shown in Fig. 6. The photovoltaic performances of the three devices are listed in Table 1. The device based on the photoanode of bare ZnO nanorods showed poor performances with short-circuit current density (
Jsc), open-circuit voltage (
Voc), fill factor (
FF), and power conversion efficiency (PCE) of 4.46 mA·cm
-2, 537 mV, 0.357, and 0.856%, respectively. This efficiency was similar to the efficiencies of DSSCs based on bare ZnO nanorod arrays reported by other groups [
21,
22]. The device based on ZnO/Nb
2O
5(1) core/shell nanorod exhibited improved performances with
Jsc,
Voc,
FF, and PCE of 5.52 mA·cm
-2, 569 mV, 0.512, and 1.609%, respectively. The increased
Jsc is mainly attributed to the increased surface area of ZnO/Nb
2O
5 core/shell nanorod, which was caused by the small Nb
2O
5 nanoparticles wrapped on the ZnO nanorod. The increased
Voc is attributed to the fact that the Fermi level of ZnO/Nb
2O
5 core/shell nanorod is higher than that of bare ZnO nanorod, since Nb
2O
5 has a higher conduction band than ZnO. This increases the
Voc value because the maximum
Voc of DSSC is mainly decided by the difference between the Fermi level of the photoanode and the redox potential of I
-/I
3-. In addition to the increased
Jsc and
Voc, the device based on ZnO/Nb
2O
5(1) core/shell nanorod exhibited a higher
FF of 0.512 than the device based on ZnO nanorod (0.357). This indicates that the Nb
2O
5 shell provided another charge-transporting channel other than the ZnO nanorod, resulting in decreased series resistance. As for the device based on ZnO/Nb
2O
5(2) core/shell nanorod, the
Jsc,
FF, and PCE were further increased to 5.95 mA·cm
-2, 0.592, and 1.995%, respectively. Compared with the ZnO/Nb
2O
5(1) core/shell nanorod, the ZnO/Nb
2O
5(2) core/shell nanorod had a thicker diameter and had additional Nb
2O
5 particles on top of the nanorod. Such a core/shell nanorod structure makes the ZnO/Nb
2O
5 nanorod photoanode advantageous in the following two aspects. On the one hand, since the protons that are released from the dye molecules in the ethanolic solution dissolved ZnO to generate Zn
2+-dye aggregates, the structure of the ZnO crystals was easily destroyed after loading the Ru-complex dyes. However, after being coated with Nb
2O
5, the recombination was suppressed by passivating its centers on the ZnO nanostructure surface. On the other hand, the electrolyte was suppressed presumably due to the energy barrier formed at the ZnO/ Nb
2O
5 interface. Thus,
Jsc and
Voc of the ZnO/ Nb
2O
5 nanorod photoanode are much higher than that of the bare ZnO photoanode.