Morphological dependance of charge transport in nanostructured ZnO-based dye sensitized solar cells (original) (raw)
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The Journal of Physical Chemistry C, 2007
Nanocrystalline particles of ZnO and TiO 2 of approximately equal size (∼15 nm) were used to prepare mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed very similar results for ZnO and TiO 2 . Apparent activation energies for electron transport in nanostructured ZnO of e0.1 eV were calculated from the temperature dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO 2 cells, which was attributed to a lower electron injection efficiency from excited dye molecules and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was about 0.23 V lower than the opencircuit potential at the same light intensity. Results may be explained using a multiple trapping model, but as electrons are usually only shallowly trapped in ZnO, an alternative view is presented. If there is significant doping of the ZnO, resulting band bending in the nanocrystals will form energy barriers for electron transport and recombination that can explain the observed properties.
The Journal of Physical Chemistry A, 2009
Dye-sensitized solar cells based on ordered arrays of polycrystalline ZnO nanotubes, 64 µm in length, are shown to exhibit efficient electron collection over the entire photoanode array length. Electrochemical impedance spectroscopy, open-circuit photovoltage decay analysis, and incident-photon-to-current efficiency spectra are used to quantify charge transport and lifetimes. Despite the relatively thick photoanode, the charge extraction time is found to be faster than observed in traditional TiO 2 nanoparticle photoanodes. If the extraction dynamics are interpreted as diffusive, effective electron diffusion coefficients of up to 0.4 cm 2 s -1 are obtained, making these pseudo-1D photoanodes the fastest reported for an operating DSC to date. Rapid electron collection is of practical significance because it should enable alternative redox shuttles, which display relatively fast electron-interception dynamics, to be employed without significant loss of photocurrent.
Electrochimica Acta, 2014
Though one-dimensional (1-D) ZnOnanrods are promising transport electron transport material in the photoanode of Dye Sensitized Solar Cells (DSSC), 1-D ZnOnanorod based DSSCs exhibit poor energy conversion efficiencies. In this study, DSSCs were fabricated with 1-D ZnOnanorods having different aspect ratio and the dependence of solar cell performance on aspect ratio of ZnOnanorods was investigated. Photoanodes fabricated with different 1-D ZnOnanorods having aspect ratios of 4.4, 5.4, 5.8, 6.8 and 7.6 showed increasing solar cell performance with the increase of aspect ratio where 2.1 and 4.7% light conversion efficiencies were observed respectively for the lowest and highest aspect ratio of 1-D ZnO nanostructures. We study the electrical and operational differences between DSSC made with ZnO nanostructures with different aspect ratio. Electrochemical impedance spectroscopy (EIS) is used to quantify the aspect ratio depended electron transport properties, charge recombination, lifetime and charge diffusion lengths of excited electrons in 1D ZnOnanorods and electron transport properties are correlated to the observed cell performance. In addition, effect of aspect ratio of ZnOnanorodson dye loading amount and light scattering properties were also investigated.
Journal of Alloys and Compounds, 2011
ZnO film with a novel bilayer structure, which consists of ZnO nanowire (ZnO NW) arrays as underlayer and polydisperse ZnO nanocrystallite aggregates (ZnO NCAs) as overlayer, is fabricated and studied as dye-sensitized solar-cell (DSSC) photoanode. Results indicate that such a configuration of the ZnO nanocrystallite aggregates on the ZnO nanowire arrays (ZnO-(NCAs/NWs)) can significantly improve the efficiency of the DSSC due to its fast electron transport, relatively high surface area and enhanced lightscattering capability. The short-circuit current density (J sc) and the energy-conversion efficiency (Á) of the DSSC based on the ZnO-(NCAs/NWs) photoanode are estimated and the values are 9.19 mA cm −2 and 3.02%, respectively, which are much better than those of the cells formed only by the ZnO NWs (J sc = 4.02 mA cm −2 , Á = 1.04%) or the ZnO NCAs (J sc = 7.14 mA cm −2 , Á = 2.56%) photoanode. Moreover, the electron transport properties of the DSSC based on the ZnO-(NCAs/NWs) photoanode are also discussed.
Journal of Applied Physics, 2009
In order to explain the higher short-circuit current ͑J sc ͒ with comparable open-circuit voltage ͑V oc ͒ from dye-sensitized solar cells ͑DSCs͒ based on gallium-modified ZnO ͑ZnO:Ga͒ porous electrodes, the diffusion coefficient ͑D͒ and electron lifetime ͑͒ in DSCs with and without Ga-modified ZnO were studied by stepped light-induced transient measurements of photocurrent and voltage. In comparison to DSCs based on ZnO electrodes, the ZnO:Ga-based solar cells provided lower D and higher values. The results were interpreted according to the transport-limited recombination model, where the Ga modification induced a higher density of intraband charge traps. At matched electron densities, a decrease in V oc from DSCs based on ZnO:Ga was observed, suggesting a positive shift of the ZnO:Ga conduction band edge. The higher J sc can be explained by the positive shift of the ZnO:Ga conduction band edge in addition to the increased roughness factor of the electrode due to the Ga modification.
Analysis of Electron Transfer Properties of ZnO and TiO 2 Photoanodes for Dye-Sensitized Solar Cells
Mesoporous TiO 2 nanoparticle films are used as photoanodes for high-efficiency dye-sensitized solar cells (DSCs). In spite of excellent photovoltaic power conversion efficiencies (PCEs) displayed by titanium dioxide nanoparticle structures, the transport rate of electrons is known to be low due to low electron mobility. So the alternate oxides, including ZnO, that possesses high electron mobility are being investigated as potential candidates for photoanodes. However, the PCE with ZnO is still lower than with TiO 2 , and this is typically attributed to the low internal surface area. In this work, we attempt to make a one-to-one comparison of the photovoltaic performance and the electron transfer dynamics involved in DSCs, with ZnO and TiO 2 as photoanodes. Previously such comparative investigations were hampered due to the morphological differences (internal surface area, pore diameter, porosity) that exist between zinc oxide and titanium dioxide films. We circumvent this issue by depositing different thicknesses of these oxides, by atomic layer deposition (ALD), on an arbitrary mesoporous insulating template and subsequently using them as photoanodes. Our results reveal that at an optimal thickness ZnO exhibits photovoltaic performances similar to TiO 2 , but the internal electron transfer properties differ. The higher photogenerated electron transport rate contributed to the performances of ZnO, but in the case of TiO 2 , it is the low recombination rate, higher dye loading, and fast electron injection.
The Journal of …, 2006
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 3in 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.
Micromachines
This paper reports on the synthesis of ZnO nanowires (NWs), as well asthe compound nanostructures of nanoparticles (NPs) and nanowires (NWs+NPs) with different coating layers of NPs on the top of NWs and their integration in dye-sensitized solar cells (DSSCs). In compound nanostructures, NWs offer direct electrical pathways for fast electron transfer, and the NPs of ZnOdispread and fill the interstices between the NWs of ZnO, offering a huge surface area for enough dye anchoring and promoting light harvesting. A significant photocurrent density of 2.64 mA/cm2 and energy conversion efficiency of 1.43% was obtained with NWs-based DSSCs. The total solar-to-electric energy conversion efficiency of the NWs+a single layer of NPs was found to be 2.28%, with a short-circuit photocurrent density (JSC) of 3.02 mA/cm2, open-circuit voltage (VOC) of 0.74 V, and a fill factor (FF) of 0.76, which is 60% higher than that of NWs cells and over 165% higher than NWs+a triple layer of NPs-based DSSCs....
Photovoltaic Characteristics of ZnO Nanotube Dye-Sensitized Solar Cells and TiO 2 Nanostructure
The electrical transport in nanotube is extremely sensitive to local electrostatic environment due to their small size, large surface to volume ratio and high mobility. Among them, Oxide Zinc and Titanium are friendly for environment and promised materials. Dye sensitized solar cell (DSSC) is the only solar cell that can offer both the flexibility and transparency but its conversion efficiency is affected by physical and morphological parameters, like thickness, series resistance (Rs), ideality factor (n), saturation current (Is), shunt resistance (Rsh) and photocurrent (Iph) during elaboration as well as their normal use. This paper presents a simulation of photovoltaic characteristics of ZnO nanotube dye-sensitised solar cells and TiO2 nanostructure, by extracting the solar cell parameters which influence directly on solar cell output: conversion efficiency, fill factor, the short circuit photocurrent densities Isc and open-circuit voltage Voc. Furthermore, we review the relationship between geometry and output parameters.
Photochemical performance of ZnO nanostructures in dye sensitized solar cells
Solid State Sciences, 2015
In this work, the photoconversion efficiencies of ZnO having diverse microstructures and structural defects have been investigated. A conversion efficiency of 1.38% was achieved for the DSSCs fabricated with as prepared ZnO nanorods having minimum vacancy defects and a favourable one dimensional directional pathway for electron conduction. The DSSCs fabricated with ZnO nanoparticles exhibited relatively low conversion efficiency of 1.004% probably due to multiple trapping/detrapping phenomena within the grain boundaries and ZnO flowers though exhibited a high dye adsorption capability exhibited the lowest conversion efficiency of 0.59% due to a high concentration of structural defects. Based on the experimental evidences, we believe that the type of defects and their concentrations are more important than shape in controlling the overall performance of ZnO based DSSCs.