Dye-sensitized solar cells based on ZnO nanoneedle/TiO 2 nanoparticle composite photoelectrodes with controllable weight
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To possess the merits of both building blocks, i.e., the rapid interfacial electron transport of ZnO nanoneedles (NNs) and the high surface area of TiO2 nanoparticles (NPs), the ZnO NN and TiO2 NP composite photoelectrodes were prepared with controllable weight ratio. The dye-sensitized solar cell (DSSC) prototypes were fabricated based on this composite photoelectrodes, and the photoelectrical properties have been systematically studied. The results indicate that the composite cells achieve higher power conversion efficiency compared to pure TiO2 NP cells by rational tuning the weight ratio of ZnO NNs and TiO2 NPs. The DSSC with 1 wt% ZnO NNs yields the highest g of 5.16%. It is elucidated by the interfacial electron transfer of DSSC with different weight of ZnO NNs using the electrochemical impedance spectra. And it is found that the DSSC with 1 wt% ZnO NNs displays the fastest interfacial electron transfer. I. INTRODUCTION 1–4
Since the breakthrough work was done by Grätzel in 1991, regenerative dye-sensitized solar cells (DSSCs) have attracted intensive attention, because they possess excellent photoelectrical conversion efficiency (g). The DSSC’s working principle is based on the electron injection from the excited state of the sensitizer into the conduction band of the semiconductor photoelectrode, a suitable redox shuttle is needed to reduce the oxidized dye, and diffusion of the oxidized form of the shuttle to the counter electrode completes the circuit.5–8 It has been established that there are a number of factors determining the overall conversion efficiency. Among them, the structural and physical properties of the nanocrystalline semiconductor oxide are predominant.9 Anatase TiO2 nanoparticle (NP) film is an excellent starting photoelectrode for DSSCs due to its easy fabrication, minimal cost, and controllable over light scattering.10,11 However, the primary weakness of the small electron diffusion coefficient limits the random walk of electrons through the NP film to the millisecond timescale.12 Recently, one-dimensional (1D) semiconductor photoelectrodes have been becoming research focus because of their merit of accelerated electron transport.13–15 Yang et al.16 demonstrated that 1D nanowire (NW) photoelectrodes had faster electron transfer than TiO2 or ZnO NP films in operating cells (several hundred times). Several works based on 1D ZnO photoelectrodes have been developed.17–19 However, compared to NP DSSCs, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.350 2982
J. Mater. Res., Vol. 27, No. 23, Dec 14, 2012
the lower g has been obtained because the lower surface area of 1D photoelectrode materials results in the low dye adsorption. 20 Therefore, the 1D nanomaterial/NP composite can be an ideal structure for the fabrication of DSSC photoelectrode. The NPs can provide high surface area for the dye adsorption, while the 1D nanomaterials can enhance the photogenerated electron transport. So far, there have been several investigations on this system, includ
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