應用二氧化鈦奈米管及硒化鎳/石墨烯於染料敏化太陽電池之研究

Abstract

This thesis aims to prepare the TiO2 as the photoanodes and highly efficient catalytic materials of the counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). There are three parts in this thesis: applying TiO2 nanotubes (TNTs) to the photoanodes, preparing the Pt-free catalysts for CEs in DSSCs (Chapter 5), and finally coupling the above prepared TiO2 electrodes with the Pt-free CEs (Chapter 5). In Chapter 3, one–dimensional TNT arrays (TNT-A) prepared by electrochemically anodizing Ti foils have been suggested to apply as the photoanode of DSSCs for many years to improve the charge transfer and reduce the charge recombination. In this study, to avoid back illumination, the detached TNT-A obtained through a two–step anodization process is transferred onto an fluorine-doped tin oxide (SnO2:F) (FTO) glass. The influence of the configuration of the detached TNT-A is studied by attaching single–end and double–ends opened TNT-A on FTO glasses with different sides facing up. By considering the dye loading amount and light utilization, the best light–to–electricity conversion efficiency of 8.46% was obtained for the DSSC with the photoanode fabricated by using a double–ends opened TNT-A with the larger–diameter-end facing up. Since high temperature sintering of a semiconductor film is not possible for a flexible DSSC, due to low thermal endurance of its plastic substrates. A new technique is developed in Chapter 4 to fabricate a flexible DSSC. TNT were obtained by anodization of Ti foil, detached, sintered, ground, and mixed with a paste of P25 (commercial TiO2 powders) and TTIP (tetraisopropoxide) in ethanol; the paste was coated on an ITO glass by a doctor-blade technique, and the thus obtained film was sintered at a low temperature of 150 ◦C to obtain the photoanode for a DSSC. Sintering temperature of the TNT was optimized with respect to photovoltaic performance of the DSSC. Different weight percentages of the sintered TNT were added to the above mentioned paste to prepare photoanodes, and the photovoltaic performance of the corresponding DSSCs was also studied. Electrochemical impedance spectra (EIS) analysis has revealed that the incorporation of the TNT in the paste has actually accelerated the electron transfer at the interface of the semiconductor film electrolyte. While the DSSC with glass substrates shows an η of 6.80% under one sun condition, the DSSC with semi-flexible photoanode and the fully flexible DSSC show η of 4.62% and 4.35%, respectively. The films are characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) patterns. The photovoltaic parameters are substantiated by EIS, UV-Vis absorption spectra, and incident photon-to-current conversion efficiency (IPCE) spectra. In Chapter 5, Ni3Se4/graphene composites were synthesized in an oil bath at a low temperature (90 oC) with the precursors of Ni and Se in the presence of graphene. By tuning the concentrations of the precursors, the size and distribution of the Ni3Se4 nanoparticles (NPs) on the graphene sheet could be controlled, which were crucial factors for approaching high electrocatalytic ability of the I3- reduction. Ni3Se4 NPs, with the size of 50 to 90 nm in diameter, were uniformly grown on the surface of graphene for 1.0 mM precursors. Due to the incorporation of graphene, which not only acted as a matrix to control the size and distribution of the Ni3Se4 NPs, but also acted as the linker to enhance the electrical connection between the NPs. Ni3Se4/graphene showed high electrocatalytic ability and low charge transfer resistance. As a result, a composite film of Ni3Se4 /graphene as the CE, exhibited an efficiency of 8.49%, which was higher than those of NixSey NPs (6.29%), graphene (2.61%) and Pt (8.24%) CEs. KEYWORDS: Anodized TiO2 nanotubes, Dye–sensitized solar cell, Flexible dye–sensitized solar cell,Free-standingTiO2 nanotubes, Ni3Se4/graphene, Sintering temperature