Page - 165 - in Photovoltaic Materials and Electronic Devices

However, theutilizationofFTOmaynotbethebestmethodfor improvingthe cell performance. One problem is that the small difference in the work function between ZnO and FTO does not supply sufficient driving force for the charge injection from the ZnO nanowires to FTO, which hints that new TCO materials should be used in ZnO-based DSSCs. Lee et al. use the ZnO:Al (AZO) film to replace the FTO layer as the TCO layer [4]. Their structure was accomplished by a three-step process, TCO, seed layer, and nanostructure, but this method was slight complicated. To simplify the procedures, we used a two-step process in this study, and present a detailed discussion. These characteristics were observed using X-ray diffraction (XRD), UV-visible spectrophotometry, field-emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), incident photon-to-electronconversionefficiency(IPCE),andsolarsimulation. 2. Experimental Figure 1 illustrates the schematic structures of DSSCs with ZnO nanorods of various lengths, which are shown in Figure 1. First, radio-frequency sputtering was used to deposit a ZnO:Al (AZO) seed layer (approximately 300 nm) on Corning-glass substrates with a sheet resistance of 18Ω/sq, and the defined area of the seed layer was 1 cm2. The Pt (H2PtCl6 solid content: <6%, viscosity: ~50cps, eversolar Pt-100) film was also deposited on the AZO/Corning-glass substrates by spin-coating. These substrates were used for growing ZnO NRs. The ZnO NRs were deposited using zinc nitrate (Zn(NO3)26H2O, Aldrich) and hexamethylenetetrasece (C6H12N4, HMT, Aldrich). Both mixtures were melted in deionized water to a concentration of 0.02 M and stored at 90 ˝C for 9 h. These solutions were replaced every 9 h, and the corresponding ZnO NRs were denoted by 18- and 27-h NRs. The hydrothermal chemical reactions for the ZnO NRsareexpressedas follows: C6H12N4` 6H2OÑ6HCHO` 4NH3 (1) NH3`H2OÑNH+4`OH´ (2) Zn2+`2OH´ÑZnpOHq2 (3) ZnpOHq2 Ñheat ZnO` H2O (4) After the reaction was complete, the resulting ZnO NRs were rinsed with deionized water to remove residual ZnO particles and impurities. A D-719 dye, cis-bis(isothiocyanato)bis(2,21-bipyridyl-4,41-dicarboxylato) ruthenium(II)bis-tetrabutylammonium, (Everlight Chemical Industrial Corp., Taipei, Taiwan) was dissolved in acetonitrile for preparing a 0.5 mM dye solution. DyesensitizationwaspropagatedbysoakingtheZnOphotoelectrodes intheD-719 165