Controllable Synthesis Of One Dimensional Single-crystalline TiO₂Nanoarrays And Their Application In Solar Cells

Highly ordered single-crystalline one-dimensional（1D） TiO₂nanoarrays has been recognized as one ofthe most promising semiconductor nanostructures. The unique photoelectrical properties and geometricaladvantage of1D TiO₂nanoarrays render it excellent material for photocatalytic, photochromism andphotovoltaic, as well as lithium cell, sensor and micro&nano electron devices. Synthesizing freestandingordered1-D TiO₂nanostructures directly on transparent conductive oxides would be most desirablebecause of direct connection of the point of photogeneration with the collection electrode, which canimprove the collection efficiency of inject electron and enhance the performance. On the other hand, usingsuch structures can facilitate the fabrication of photoelectrical devices. However, unlike ZnO, crystalstructure and symmetry of TiO₂make the growth of oriented anisotropic single-crystalline TiO₂films verydifficult. In this dissertation, we present a straightforward method to prepare single crystal rutile TiO₂nanowire arrays directly onto FTO glass under simple hydrothermal conditions and discuss the applicationof the as-prepared TiO₂nanoarrays in different kinds of solar cell. The main results are listed below:1. A facile, low temperature hydrothermal method was developed to grow oriented,single-crystalline rutile TiO₂nanowire （rod） films on transparent conductive fluorine-doped tin oxide （FTO）substrates. The diameter and length of the nanoarrays could be varied by changing the growth parameters,such as growth time and growth cycles. The epitaxial relation between the FTO substrate and rutile TiO₂with a small lattice mismatch plays a key role in driving the nucleation and growth of the rutile TiO₂nanowires （rods） vertically oriented from FTO substrate with a preferred （001） orientation. The growthprocess and mechanism of the1D TiO₂nanoarrays were discussed detailedly and we also investigated thetransformation of nanowires （rods） to nanotubes in the conditions of acid etch. 2. The prepared ordered1D TiO₂nanostructures were used as photoanode to assembledye-sensitized solar cells （DSSCs）. The DSSC assembled with about2.0μm TiO₂nanowire array byhydrothermal growth of10h has a short-circuit current （Jsc） of5.8mA/cm²and an open-circuit voltage （Voc）of0.65V, which exhibits the best performance with an overall power conversion efficiency （η） of1.99%and a fill factor （FF） of0.53. By investigating the relationship between the efficiency of different length ofTiO₂nanowire photoanodes and the approach for nanowire arrays formation, we have found that not onlythe internal surface area, but also the photon loss in traversing the nanowire photoelectrodes and the contactat the FTO_nanowire interface have a great effect on the photovoltaic performance of nanowire-basedDSSCs.3. For assembly of quantum dots-sensitized solar cells （QDSSCs）,1D TiO₂nanostructure arrayspossess the superiority over other nanomaterials due to its more open structure which was preferable forboth sensitizer and electrolyte filling. CuInS₂quantum dots （QDs） were deposited onto TiO₂nanorod arraysfor different cycles by using successive ionic layer adsorption and reaction （SILAR） method. The effect ofSILAR cycles on the light absorption and photoelectrochemical properties of the sensitized photoelectrodeswas studied. With optimization of CuInS₂SILAR cycles and introduction of In2S3buffer layer, quantumdot-sensitized solar cells assembled with3μm thick TiO₂nanorod film exhibited a short-circuit currentdensity （Isc） of4.51mA cm2, an open-circuit voltage （Voc） of0.56V, a fill factor （FF） of0.41, and a powerconversion efficiency （η） of1.06%, which is higher than the CuInS₂QDs sensitized TiO₂nanocrystallinefilms as the photoanode. These results manifest the superior charge transport of single-crystalline TiO₂NRAs to disordered TiO₂nanoparticle films when used as the host material in QDSSCs.4. In colloidal quantum dot （CQD） solar cells, the ordered1D nanostructure array design wouldensure efficient separation and collection of all charge carriers while dramatically improving charge transport and simultaneously minimizing the surface area for recombination. The TiO₂nanostructure arrayswere used as the electron-accepting electrode to construct depleted bulk heterojunction （DBH） CQD solarcells by incorporating of CuInS₂QDs. The nanocomposite bulk heterojunction of p-type CuInS₂and n-typeTiO₂displayed preferable visible light absorption ability and photogenerated charge separation andtransport performance. The effect of different CuInS₂deposition cycles on the photovoltaic performance ofTiO₂/CuInS₂hetero-interface structure has also been studied.