ZnO/Nb₂O₅ core/shell nanorod array photoanode for dye-sensitized solar cells

Abstract

In this paper, ZnO/Nb₂O₅ core/shell nanorod arrays were synthesized and used as photoanodes for dye-sensitized solar cells (DSSCs). We first synthesized ZnO nanorod array on fluorine-doped tin oxide (FTO) glasses by a hydrothermal method, and then ZnO/Nb₂O₅ core/shell nanorod array was directly obtained via solvothermal reaction in NbCl₅ solution. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images revealed that the ZnO nanorods were uniformly wrapped by Nb₂O₅ shell layers with a thickness of 30–40 nm. Photovoltaic characterization showed that the device based on ZnO/Nb₂O₅ core/shell nanorod photoanode exhibited an improved efficiency of 1.995%, which was much higher than the efficiency of 0.856% for the DSSC based on bare ZnO nanorod photoanode. This proved that the photovoltaic performance of ZnO nanorods could be improved by wrapping with Nb₂O₅ shells.

Keywords

ZnO / Nb ₂O ₅ / core/shell nanorods / solvothermal / dye-sensitized solar cell (DSSC)

Introduction

As a promising alternative to conventional silicon-based solar cell, dye-sensitized solar cells (DSSCs) have attracted considerable attention in the last two decades, because of their low manufacturing costs and relatively high energy-conversion efficiencies [¹–⁴]. In DSSC components, the photoanode is a key part that plays an essential role in determining the dye loading and electron transportation, and hence the photon to electron conversion efficiency. To realize highly efficient DSSC, a nanostructured photoanode should possess several favorable intrinsic characteristics, such as large surface area to permit high dye loading, direct electron transport pathways for long electron diffusion lengths, and compatible energy levels to achieve high electron injection efficiency and high voltage. It is therefore highly desirable to develop a photoanode that meets the above requirements.

There have been many efforts to develop efficient photoanode materials such as TiO₂, ZnO, SnO₂, Nb₂O₅, and other composites. So far, TiO₂ has been widely investigated as a photoanode material for DSSCs. With a band gap similar to that of TiO₂, ZnO is another alternative photoanode material, which has attracted much attention due to its high mobility of about 115–155 cm²·V^-1·s^-1, which is much higher than that of TiO₂ (10^-5 cm²·V^-1·s^-1) [⁵,⁶]. Among the various morphologies, one-dimensional (1D) nanostructures such as nanorods, nanotubes, and nanowires, which can offer more superior electron transport pathways, have been attracting increasing attention. However, the power conversion efficiencies of DSSCs based on ZnO 1D nanostructures are still at relatively low levels, because of the insufficient internal surface area.

To overcome this shortcoming, it is necessary to design a novel core/shell photoanode by applying metal oxide shells such as SiO₂ [⁷], ZrO₂ [⁸], Nb₂O₅ [⁹–¹²], and Al₂O₃ [¹³] to the core nanostructures. The metal oxide shells synthesized on core nanostructures could increase the specific area of the electrode for dye loading, thereby enhancing the photocurrent density. Moreover, the layer formed by coating with these materials act as an energy barrier that decreases the electron recombination losses, shifts the conduction band downward, which increases the electron injection, and enhances the injection efficiency.

Nb₂O₅ is a promising metal oxide because it supports good N719 dye loading due to its basic character [¹⁴–¹⁷], and its conduction band level is more negative than that of TiO₂ [¹⁸,¹⁹]. Here, we applied a Nb₂O₅ coating on ZnO nanorod to obtain ZnO/Nb₂O₅ core/shell nanorod arrays, which were used as the photoanodes in DSSCs. The results showed that improved efficiencies of 1.609% and 1.995% were obtained for DSSCs based on ZnO/Nb₂O₅ core/shell nanorod synthesized in-low concentration NbCl₅ solution and in high-concentration NbCl₅ solution, respectively, compared with the efficiency of 0.856% for the DSSC based on bare ZnO nanorod photoanode.

Experimental

Synthesis of ZnO nanorod arrays

The ZnO seed layer on fluorine-doped tin oxide (FTO) glasses was prepared using a previously published procedure [²⁰]. Then, 0.455 g Zn(NO₃)₂·6H₂O and 0.555 g NH₄F were dissolved into 50 mL deionized water. After that, NH₃·H₂O was dropped into the above solution while stirring. In this process, the clear solution turned turbid when NH₃·H₂O was first dropped into the solution, and then gradually it turned clear with continued stirring. Subsequently, the prepared ZnO seed layer on FTO glass (with the FTO layer facing downwards) was placed into a Teflon-lined autoclave, which contained the above solution, and was then sealed. Finally, the Teflon-lined autoclave was heated at 70°C for 8 h to obtain ZnO nanorod arrays.

Synthesis of ZnO/Nb₂O₅ core/shell nanorod arrays

Figure 1 shows the process of growing ZnO/Nb₂O₅ core/shell nanorod arrays on FTO glass. First, 0.135 and 0.270 g NbCl₅ were dissolved into 50 mL anhydrous ethanol. After that, the obtained ZnO nanorod arrays were placed into the two Teflon-lined autoclaves, each containing one of the above two NbCl₅ solutions. Then, the Teflon-lined autoclaves were heated at 180°C for 24 h to obtain ZnO/Nb₂O₅ core/shell nanorod arrays with ZnO/Nb₂O₅(1) for 0.135 g NbCl₅ and ZnO/Nb₂O₅(2) for 0.270 g NbCl_5. Finally, the ZnO/Nb₂O₅ core/shell nanorod arrays were sintered at 500°C in air for 1 h.

Fabrication of DSSCs based on ZnO/Nb₂O₅ core/shell nanorod arrays

The ZnO/Nb₂O₅ core/shell nanorod arrays were dipped in a dye solution containing 0.5 mM⁾ N719 (Dyesol) dye for 24 h. Then, the ZnO/Nb₂O₅ electrode was scrapped to obtain an active area of 25 mm². The counter electrode was prepared by coating a 0.6 mM H₂PtCl₆·6H₂O solution in anhydrous ethanol onto the FTO substrate. After that, the ZnO/Nb₂O₅ electrode was assembled with the counter electrode by clamping a 25 mm thick polymeric film (Surlyn, DuPont). Then, an electrolyte solution was injected into the gap between the ZnO/Nb₂O₅ electrode and the counter electrode. The electrolyte solution contained 0.05 M LiI (Sigma-Aldrich), 0.03 M I₂ (Aldrich), and 0.5 M 4-tert-butylpyridine (Aldrich) in a solution containing acetonitrile (Aldrich). Finally, the injecting hole was sealed with an adhesive tape to obtain the completed device.

Characterization

A scanning electron microscope (SEM, Quanta200, FEI, Netherlands) and a transmission electron microscope (TEM, JEOL TEM-2010) were used to measure the structure and morphology of the nanomaterials. The current–voltage characterization was performed using a Keithley 2400 source meter under simulated AM 1.5 sunlight illumination (100 mW·cm^-2) provided by an Oriel solar simulator (Model 9119X, Newport Co.). The illuminated active area of the photovoltaic measurements was 0.16 cm². The electrochemical impedance spectra (EIS) of the devices were tested at −0.6 V for the range from 1 MHz to 1 Hz, with an advanced electrochemical system (PAR2273) under dark conditions.

Results and discussion

Structural and morphological characterization of ZnO/Nb₂O₅ core/shell nanorod

Figures 2(a) and 2(b) show the top-view and cross-view SEM images of the ZnO nanorod arrays, respectively. Figure 2(a) reveals that the top of the nanorods is uniform. From the tilted-view SEM image, we can clearly see that the high-density ZnO nanorods grew vertically on the FTO substrate. Their length is about 5 mm. After the coating of the Nb₂O₅ shell, the core-shell structure was investigated by SEM. Figure 2(c) shows the top-view SEM image of the ZnO/ Nb₂O₅ core-shell nanorod film. It clearly reveals that the diameter of the core-shell nanorod increased and its surface became rough. The corresponding tilted-view SEM image shows that the core-shell structure has a similar shape as that of the ZnO nanorod shown in Fig. 2(d). To investigate the coating of Nb₂O₅, the samples were synthesized in a low-concentration NbCl₅ solution (0.135 g NbCl₅) and a higher-concentration NbCl₅ solution (0.270 g NbCl₅). The results are shown in Fig. 3. By comparison, we could find that upon increasing the concentration of NbCl₅, the ZnO nanorods became rougher on the side surface and were fully covered with Nb₂O₅ nanoparticles on the top. This shows that a high concentration of NbCl₅ could facilitate significantly more growth of Nb₂O₅ on the top of the nanorod than on its side.

In the TEM image (Fig. 4(a)), obvious differences in terms of the contrast between the center and fringe parts of each individual rod-like structure are observed, which indicates that the rod-like structures are core-shell structures with core diameters of 150–200 nm and shell thicknesses of 30–40 nm. The corresponding sellected area electron diffraction (SAED) pattern obtained from the circled area in Fig. 4(a) is shown in Fig. 4(b). We can see single crystal diffraction spots in Fig. 4(b), corresponding to ZnO (103) and (203). In addition, Fig. 4(b) also shows an amorphous diffraction pattern, which is indicated by the dotted line. We measured the radius of the amorphous diffraction ring, and the spacing between the crystal planes ranged from 0.39 to 0.28 nm, which was consistent with the strong peak position of JCPDS (320711) Nb₂O₅. Therefore, we inferred that the amorphous diffraction ring was from the amorphous Nb₂O₅ wrapped on the surface of ZnO.

Characterization of photovoltaic performances for DSSCs

Figure 5 shows the optical absorption spectra of the ZnO and ZnO/Nb₂O₅ nanorod films. For the bare ZnO nanorod film, the onset of the band gap transition is at ~420 nm. The presence of Nb₂O₅ shell increases another adsorption edge at ~370 nm relative to Nb₂O₅. Compared with the ZnO nanorod arrays, the coating of the Nb₂O₅ moved the onset of absorption to a lower wavelength, and the light absorption intensity was enhanced. The larger band gap of Nb₂O₅ semiconductor is related to the more negative conduction band potential and the larger open-circuit photovoltage of the Nb₂O₅ cell. This result is consistent with the following photovoltaic performances.

The photocurrent density-voltage curves of DSSCs based on the photoanodes of bare ZnO nanorods, ZnO/Nb₂O₅(1) core/shell nanorod (0.135 g NbCl₅), and ZnO/Nb₂O₅(2) core/shell nanorod (0.270 g NbCl₅) 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 (J_sc), open-circuit voltage (V_oc), 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/Nb₂O₅(1) core/shell nanorod exhibited improved performances with J_sc, V_oc, FF, and PCE of 5.52 mA·cm^-2, 569 mV, 0.512, and 1.609%, respectively. The increased J_sc is mainly attributed to the increased surface area of ZnO/Nb₂O₅ core/shell nanorod, which was caused by the small Nb₂O₅ nanoparticles wrapped on the ZnO nanorod. The increased V_oc is attributed to the fact that the Fermi level of ZnO/Nb₂O₅ core/shell nanorod is higher than that of bare ZnO nanorod, since Nb₂O₅ has a higher conduction band than ZnO. This increases the V_oc value because the maximum V_oc of DSSC is mainly decided by the difference between the Fermi level of the photoanode and the redox potential of I^-/I₃^-. In addition to the increased J_sc and V_oc, the device based on ZnO/Nb₂O₅(1) core/shell nanorod exhibited a higher FF of 0.512 than the device based on ZnO nanorod (0.357). This indicates that the Nb₂O₅ shell provided another charge-transporting channel other than the ZnO nanorod, resulting in decreased series resistance. As for the device based on ZnO/Nb₂O₅(2) core/shell nanorod, the J_sc, FF, and PCE were further increased to 5.95 mA·cm^-2, 0.592, and 1.995%, respectively. Compared with the ZnO/Nb₂O₅(1) core/shell nanorod, the ZnO/Nb₂O₅(2) core/shell nanorod had a thicker diameter and had additional Nb₂O₅ particles on top of the nanorod. Such a core/shell nanorod structure makes the ZnO/Nb₂O₅ 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 Zn²⁺-dye aggregates, the structure of the ZnO crystals was easily destroyed after loading the Ru-complex dyes. However, after being coated with Nb₂O₅, 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/ Nb₂O₅ interface. Thus, J_sc and V_oc of the ZnO/ Nb₂O₅ nanorod photoanode are much higher than that of the bare ZnO photoanode.

Conclusions

In summary, ZnO/Nb₂O₅ core/shell nanorods were successfully developed by solvothermal synthesis and applied as photoanodes for DSSCs. ZnO nanorod core was first synthesized by a hydrothermal process, and then Nb₂O₅ shell was directly synthesized on the ZnO nanorod core by solvothermal reaction in NbCl₅ solution. SEM and TEM images revealed that the ZnO nanorods were uniformly wrapped by the Nb₂O₅ shell layers with a thickness of 30–40 nm. Photovoltaic characterization showed that improved efficiencies of 1.609% and 1.995% were obtained for DSSCs based on ZnO/Nb₂O₅ core/shell nanorod synthesized in low-concentration NbCl₅ solution and high-concentration NbCl₅ solution, respectively, compared with the efficiency of 0.856% for the DSSC based on bare ZnO nanorod photoanode. Our work offers a facile strategy to develop functional composite nanomaterials for photovoltaic devices.

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