Design, Prearation And Properties Of Electrode Materials Based On One-Dimensional Units For Dye-Sensitized Solar Cells

Developing green and renewable energy resources has become an urgent priority for the world with the accelerated depletion of the earth’s fossil fuels. Solar energy which is pollution free and abundant radiating all regions of the globe has been regarded as one of the perfect energy resources. It is urgent to develop cheap and efficient devices for converting light into electricity. Dye-sensitized solar cells (DSSCs), which own the merits of low cost, simple manufacture procedure and high theoretical energy conversion efficiencies have been attracting global attentions. One typical DSSC contains five components:a mechanical support coated with transparent conductive oxides, the semiconductor porous film composed of oxide, a sensitizer adsorbed onto the surface of the semiconductor, an electrolyte containing a redox mediator and a counter electrode capable of regenerating the redox mediator like platinum. Electrode materials are the key components in DSSCs. In this dissertation, we designed a seires of photoanode materials as anatase TiO2nanorods, hierarchical TiO2hollow spheres and in-situ growth of ZnO nanorods within the spheres, nanorod TiO2spheres and investigated their photovoltaic performance, from the point of large amount of dye adsorption, fast electron transport, strong light scattering; we also explored a novel three-dimensional TiN network as a Pt and FTO-free counter electrode; furthermore, all flexible DSSCs based on treated Ti foil and PET-ITO were fabricated, and the photovoltaic performance was systematically studied.(1) One-dimensional anatase TiO2nanorods were synthesized from titanium sulfate. Amorphous TiO2precipitate was obtained after the hydrolysis of titanium sulfate, then it was peptized using hydrogen peroxide and yielded the transparent and yellow peroxotitanium acid (PTA). PTA was treated under different hydrothermal conditions by adjusting the temperature and duration of the treatment. For the hydrothermal treatment at120℃for4h and24h, spindle-like TiO2nanorods of average diameter about7～8nm and length about20～50nm were formed. For the hydrothermal treatment at150℃for4h, the morphology of the nanorods was analogous to that of the nanorods obtained at120℃, however, chain-shaped TiO2nanorods were obtained when the duration was prolonged to24h. These high aspect ratio nanorods were grown from the{001} facet of the truncated octahedral units by oriented attachment and the driving force for the unit assembly process was originated from surface energy reduction. X-ray diffraction (XRD) patterns showed all the TiO2nanorods were pure anatase phase, and the nitrogen adsorption-desorption isotherms showed all the nanorods had specific surface areas greater than110m2g-1, much larger than those nanorods reported by the literature. Nanorods obtained at120℃and150℃for24h were chose to prepare DSSC photoanodes, and commercial P25TiO2was also used to prepare a DSSC for comparison.UV-vis spectra showed that photoanodes based on the synthesized nanorods adsorbed larger amount of dye molecules than the P25one. When the photoanodes were assembled as DSSCs, all the photovoltaic parameters of DSSCs based on the synthesized nanorods were much better than the P25one. The chain-shaped nanorods exhibited an energy conversion efficiency (η) of7.28%, showing a58%increment of efficiency compared with the P25DSSC, due to the faster electron transport rate, and longer electron lifetime as confirmed by the electrochemical impedance spectroscopy.(2) Hierarchical TiO2hollow spheres were synthesized by the2-step process consisting of thermal hydrolysis and subsequent solvothermal reaction. Firstly, spherical TiO2aggregates were obtained via adjusting the dielectric constant of the solution in the process of titanium sulfate hydrolysis. Secondly, spherical aggregates were subjected to solvothermal treatment in order to get better crystallinity. It was found that the hollow structure was formed in the process of solvothermal treatment. The inner nanoparticles with a larger surface energy than the outer ones will gradually dissolved due to the "Ostwald ripen". XRD pattern showed the hollow spheres were in anatase phase, while TEM observation revealed that the hollow spheres were consisted of tiny TiO2nancrystals with a size of about10nm. Nitrogen adsorption-desorption isotherm showed that the as-prepared hollow spheres had a high specific surface area of108.0m2g-1Photoanode made from the hollow spheres showed a strong light harvesting ability with the absorption value of0.6in the visible light region of700～800nm whereas the photoanode made from the nanocrystals showed a small value of0.1. DSSCs based on the TiO2hollow spheres showed an efficiency of5.47%due to the dual-funcion of large amount of dye adsorption and strong light scattering, and an improved efficiency of7.16%was obtained when the spheres were used as the a scattering layer on top of the nanocrystalline layer. Furthermore, one-dimensional ZnO nanorods were constructed in-situ growth within the hollow stuctures, and the DSSC showed an improved efficiency of5.25%due to the fast electron transport way.(3) Sub-micrometer sized rutile TiO2nanorod microspheres in diameters of500-700nm and with a specific surface area of63.7m2g-1were synthesized from a salt-assisted hydrothermal method. Morphological evolution as a function of reaction time was carried out to gain insight into the formation process of the nanorod microspheres, and a mechanism of "nucleation-assembly-dissolution-recrystallization" was proposed. The presence of NaCl played a crucial role in yielding the radial microsphere structure from the aspects of adjusting the ionic strength of the solution and promoting anisotropic growth. The nanorod microspheres could provide dual-functions of adsorbing dye molecules and strong light-harvesting efficiency when they were fabricated as a scattering overlayer in DSSCs. Furthermore, the inherent nanorods would provide excellent electron percolation pathways for charge transfer as confirmed by electrochemical impedance spectroscopy. Consequentially, DSSC with the scattering overlayer exhibited a39%increment of cell efficiency (7.32%) compared with the DSSC without one (5.28%), and the efficiency was also a little higher than the DSSC with the same thickness composed of only nanocrystallites (7.14%).(4) TiO2microspheres assembled by single crystalline rutile TiO2nanorods were synthesized by the one-pot solvothermal treatment at180℃based on the aqueous-organic mixture solution containing n-hexane, distilled water, titanium n-butoxide and hydrochloric acid. The spheres were with a radiative structure from the center, and their diameters were controlled to range from1to5μm by adjusting the volume of the reactant water. Nitrogen adsorption-desorption isotherms showed that all the as-prepared microspheres had relatively high specific surface areas of about40m2g-1. The1μm sized TiO2nanorod microspheres were fabricated as a scattering overlayer in DSSCs, leading a short-circuit current density (Jsc) of17.4mA cm-2, an open-circuit voltage (Voc) of736mV, and a fill factor (FF) of64.2%, leading to an energy conversion efficiency (η) of8.22%. IPCE analysis showed that the nanorod microspheres were much more efficient than the commercial rutile scattering centers.(5) Three-dimensional TiO2network was constructed on the surface of Ti foil after the acid cleaning and the subsequent hydrogen peroxide treatment. XRD patterns showed that the network was in pure anatase phase. The surface roughness was much improved and the effective area increased from1μm2to1.615μm2。 After coating a layer of nanocrystalline TiO2, it was fabricated as an all flexible DSSC with the aid of an ITO-PET counter electrode. J-V curve showed that the efficiency was improved from3.74%to4.98%when the three-dimensional TiO2network was introduced. An increment of33.1%can be attributed to the following three aspects, a lower contact resistance between the Ti foil and the oxide layer, the better adhesion, and the network was also effective on inhibiting recombination and beneficial to electron transport. Further, large area DSSCs were fabricated and their bent stability was investigated. DSSC based on the treated Ti foil showed a comparable efficiency after alternate bending for20times, whereas the efficiency of the DSSC based on the as-received Ti foil decreased a lot, implying the three-dimensional TiO2network were effective to enhance the bent stability.(6) A novel TiN counter electrode was develped to replace the the traditional Pt counter electrode. TiN was obtained by nitridizing the hydrogen peroxide treated Ti foil in the NH3atmosphere at800℃. EDS and HRTEM confirmed the existence of cubic TiN. Cyclic voltammetry analysis showed that the current density of anode peak for the TiN network was2.2mA cm-2which was larger than the Pt with the value of1.7mA cm-2, however, the TiN network showed poor performance on the reduction of triiodide to iodide, and the problem was addressed when some Pt nanoparticles were dispersed on the TiN network. Finally, DSSC using the TiN network/Pt counter electrode made from the Ti foil showed an efficiency of5.10%which was comparable with the conventional FTO-based Pt counter electrode.