The electron transport in dye-sensitized solar cells with a MOCVD (metal organic vapor deposition)-grown ZnO nanorod array (ZnO-N) or a mesoporous film prepared from ZnO colloids (ZnO-C) as the working electrode was compared. The electrodes were of similar thickness (2 µm) and sensitized with zinc(II) mesotetrakis(3-carboxyphenyl)porphyrin, while the electrolyte was I -/I 3 -in 3-methoxypropionitrile. Electron transport in the ZnO-C cells was comparable with that found for colloidal TiO 2 films (transport time ∼ 10 ms) and was light intensity dependent. Electron transport in solar cells with ZnO-N electrodes was about 2 orders of magnitude faster (∼30 µs). Thus, the morphology of the working ZnO electrode plays a key role for the electron transport properties.Highly ordered nanostructures of ZnO, 1 TiO 2 , 2 and other wide band gap semiconductors are promising alternatives to the mesoporous nanoparticle thin films that are currently used for dye-sensitized solar cells (DSSCs) and other devices. DSSCs prepared from colloidal solutions are characterized by complex interfaces. 3 The possibility to employ a well-defined semiconductor layer morphology in DSSCs is attractive, as it could allow control of the surface functionalization and also improve the fundamental understanding of the interfacial electronic processes that are the basis for solar energy conversion. These are key premises for improving the performance of solar cells.Over the past few years, ZnO has emerged as the material of choice for the preparation of nanostructured electrodes, especially those based on crystalline, columnar structures generally called nanorods or nanowires. 4 In particular, the directionality of transport of the injected electron through the vertical nanorods is expected to be high. ZnO nanorods have been prepared using various methods, including vapor-liquid-solid processes, chemical vapor deposition, and chemical synthesis through aqueous solutions. 1,5,6 Among them, metal organic vapor deposition (MOCVD) is an excellent growth technique as it leads to well-ordered arrays of ZnO nanorods that are vertically aligned, single-crystalline, and with a low density of defects. 7 Films of MOCVD-grown ZnO nanorods are abbreviated in this paper as ZnO-N to distinguish them from ZnO nanorods grown by other methods. The functionalization of ZnO-N films has been recently published. 8 The few DSSCs prepared from ZnO nanorods (grown from aqueous solutions) reported to date have exhibited efficiencies similar to those observed for cells prepared from colloidal nanoparticles. 4 While the electron mobility of a nonsensitized ZnO nanorod layer and of single nanorods has been measured using the field-effect transistor technique (1-5 cm 2 V -1 s -1 ), 4a,5,9 experimental data on the electron transport in a working solar cell with ZnO nanorods sensitized with a dye and in the presence of electrolyte is not available. Determination of the electron transport properties of ZnO nanorod materials in a working solar cell will improve our understanding...

Two bifunctional linkers, a rigid-rod p-ethynyl-isophthalic acid capped with a Ru(II)-polypyridyl complex and 3-mercaptopropionic acid, were covalently bound to ZnO nanotip films grown by metal-organic chemical vapor deposition (MOCVD) technology. This highly vertically aligned, crystalline form of ZnO had not been functionalized before. The binding was studied by Fourier transform (FT) IR and UV spectroscopies and probed, in the case of the Ru complex, by static and dynamic fluorescence quenching. The molecules did bind through the carboxylic acid groups, and the FT-IR attenuated total reflectance spectra are indicative of a bidentate carboxylate binding mode. Other molecules (heptanoic acid, isophthalic acid, and trimethoxy(2-phenylethyl)silane) were also bound to the ZnO nanotips. A comparison was made with epitaxial ZnO films grown by MOCVD and ZnO mesoporous films prepared from colloidal solutions to investigate the effect of the ZnO morphology. The ZnO nanotips were excellent binding substrates, particularly for the rigid-rod linker. Since ZnO films are etched at low pH (< 4), novel nanotip films made of ternary MgxZn1-xO, which is formed by alloying ZnO with MgO and is more resistant to acids, were developed. The MgxZn1-xO nanotip films were employed to use linkers with acidic groups and to study the effect of pH pretreatment of the surface on the binding.

Well-aligned zinc oxide (ZnO) nanotip arrays were grown on fluorinated tin oxide coated glass using metal organic chemical vapor deposition. Dye (N719) sensitized photoelectrochemical cells comprising of ZnO nanotip arrays were fabricated and characterized. It is found that the power conversion efficiency of the cells increases with the length of the ZnO nanotips. The cells with 3.2μm ZnO nanotip array exhibited an incident photon-to-current conversion efficiency of 21.1% (at 550nm) and a power conversion efficiency of 0.55% under 1 sun irradiance. Light harvesting in ZnO nanotips also contributes to the photocurrent in the UV range.

Dye sensitized solar cell (DSSC) is a promising and low-cost photovoltaic device. In the DSSC, a monolayer of dye is anchored on the surface of a semiconductor oxide.Photoexcitation takes place inside the dye, and photogenerated charges are then separated at the dye/oxide interface. Nanostructured oxide films are particularly attractive for DSSCs as they provide large surface area for dye anchoring. The crystalline quality of oxide films has significant impact on the charge transport. It is important to reduce the charge traps in the oxide to speed up the charge transport. In the DSSC, the transparent conducting oxide serves as an optical window, which determines the amount of light entering the device, and as the electrode, which extracts photocurrent. In the cell design, the selection of semiconductor oxide and corresponding transparent electrode is critical to achieve efficient light harvesting, charge separation and extraction.ii In this dissertation, a novel photoelectrode, consisting of well-aligned ZnO nanotips and a Ga-doped ZnO (GZO) TCO film, is developed for DSSC applications. The n-type semiconductor ZnO nanotips provide large surface area for dye anchoring in conjunction with direct conduction pathways for charge transport, while the GZO film acts as the transparent electrode. ZnO nanotips and GZO films are grown by metalorganic chemical vapor deposition (MOCVD). GZO films (~400 nm) with sheet resistance of ~ 25 Ω/sq and transmittance over ~ 85% are achieved. ZnO nanotips have single crystalline quality and show free exciton emissions at room temperature. Photoelectrochemical cells are fabricated using liquid redox electrolyte and semi-solid gel electrolyte, respectively. UV light harvesting, which is directly generated by ZnO photoelectrode, is observed. The DSSC using liquid electrolyte exhibits a quantum efficiency at ~530nm of 65% and power conversion efficiency of 0.77%. By replacing the liquid electrolyte with gel electrolyte in the DSSC with the same structure, the open circuit voltage is increased from 610 mV to 726 mV and the overall power efficiency is increased to 0.89%. The aging testing shows that the DSSC using gel electrolyte has better stability than its liquid electrolyte counterpart.iii

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