Fast electron transport in metal organic vapor deposition grown dye-sensitized ZnO nanorod solar cells (original) (raw)
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Morphological dependance of charge transport in nanostructured ZnO-based dye sensitized solar cells
2011 Saudi International Electronics, Communications and Photonics Conference (SIECPC), 2011
sg † Authors contributed equally to this work Zinc oxide (ZnO) photoanodes of two different morphologies nanorods and nanosheets are used to fabricate dye sensitised solar cells in order to study the influence of morphology on device performance. Characteristics such as dye loading capacity and light scattering ability of the photoanodes depend on the morphology to a large extent. This leads to change in the electron transport properties, which in turn affects power conversion efficiency. Here we report the difference in the charge transport properties of these morphologies evaluated using the time resolved photocurrent measurements. The photocurrent transients for ZnO nano-rod film exhibited dual peak behaviour, whereas transients for ZnO nano-sheet film exhibited only one peak as the time delayed second peak corresponding to slow electron diffusion was totally suppressed. The transient photocurrent decay also indicates that the charge transport rate is much faster in the ZnO nano-sheets, which allows the efficient charge collection over much larger thickness compared to other ZnO nano-rod film. The power conversion efficiency (PCE) of the devices was found to be 1.6% and 1.5 % for the ZnO-nano-rod and nano-sheet based dye sensitized solar cells respectively. A comparative study is performed and the photovoltaic parameters obtained in each case are correlated with the photocurrent transients enabling better understanding of the impact of morphology.
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.
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 Physical Chemistry B, 2004
Nanoporous ZnO/eosinY films prepared by electrochemical self-assembly have already shown promising characteristics for use in dye-sensitized solar cells, such as ease of preparation (no need for high-temperature sintering) and high dye loading. In this study, electron transport and back reaction in these films have been investigated by intensity modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). In contrast to sintered colloidal ZnO films, electrodeposited ZnO/eosinY films exhibit electron transit times (τ D ) that are much shorter than electron lifetimes (τ n ), leading to very efficient electron collection. The shorter transit times in the electrodeposited layers are due in part to the fact that the films are very thin, but in addition the electron diffusion coefficients are higher than in sintered colloidal ZnO films. Although the unusually high dye concentration in the electrochemically self-assembled film allows efficient light harvesting, it was found that not all dye molecules inject electrons. The low injection efficiency is probably due to the formation of dye aggregates.
ACS Nano, 2011
High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO 2 as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We present here a comparative study of electron mobility and injection dynamics in thin films of TiO 2 , ZnO, and SnO 2 nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO 2 nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO 2 , the process is predominantly concluded within a few picoseconds. These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent generated by the solar cell. We conclude that these injection dynamics are not substantially influenced by bulk energy level offsets but rather by the local environment of the dyeÀnanoparticle interface that is governed by dye binding modes and densities of states available for injection, both of which may vary from site to site. In addition, we have extracted the electron mobility in the three nanoporous metal-oxide films at early time after excitation from terahertz conductivity measurements and compared these with the time-averaged, long-range mobility determined for devices based on identical films. Comparison with established values for single-crystal Hall mobilities of the three materials shows that, while electron mobility values for nanoporous TiO 2 films are approaching theoretical maximum values, both early time, short distance and interparticle electron mobility in nanoporous ZnO or SnO 2 films offer considerable scope for improvement.
The Journal of Physical Chemistry C, 2012
Electron transport and recombination are the essential processes that determine the charge collection efficiency in dye-sensitized solar cells (DSSC). While nearly 100% of charges are collected in well-built ordinary DSSCs, this value can be sharply reduced by the use of redox couples other than iodide/triiodide due to fast electron recombination. To compensate, structures capable of fast electron transport are needed. Nanorod arrays that have this attribute tend to suffer from low surface area, resulting in low dye loading and reduced light harvesting. We have therefore developed a novel nanocomposite structure consisting of zinc oxide (ZnO) nanorods coated with titanium dioxide (TiO 2 ) nanoparticles using an electrostatic layer-by-layer (LbL) deposition technique. The titanium dioxide nanoparticle coating can add an order of magnitude of surface area and is compatible with known highperformance dyes. This composite nanostructure has been designed to take advantage of the improved electron transport along the nanorods and surface area provided by the nanoparticles, yielding good charge collection and light harvesting. Transient measurements indicate that the composite film can transport electrons at least 100 times faster than a nanoparticulate TiO 2 film. In tests using ferrocene/ferrocenium as a model alternative redox couple with fast recombination, current−voltage measurements indicate that the ZnO−TiO 2 hybrid films generate much higher currents than conventional TiO 2 nanoparticulate films. However, not all charges successfully transfer from TiO 2 to ZnO due to an energy barrier between the materials.
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.
Nanostructured ZnO electrodes for dye-sensitized solar cell applications
Journal of Photochemistry and Photobiology A: Chemistry, 2002
Dye-sensitized photoelectrochemical solar cells constitute a promising candidate in the search for cost-effective and environment-friendly solar cells. The most extensively studied, and to date the most efficient systems are based on titanium dioxide. In this paper, the possibilities to use nanostructured ZnO electrodes in photoelectrochemical solar cells are investigated. Various experimental techniques (e.g. infrared, photoelectron, femtosecond and nanosecond laser spectroscopies, laser flash induced photocurrent transient measurements, twoand three-electrode photoelectrochemical measurements) show that the thermodynamics, kinetics and charge transport properties are comparable for ZnO and TiO 2. The preparation techniques of ZnO provide more possibilities of varying the particle size and shape compared to TiO 2. However, the dye-sensitization process is more complex in case of ZnO and care needs to be taken to achieve an optimal performance of the solar cell.
Solid-state dye-sensitized solar cells based on ZnO nanocrystals
Nanotechnology, 2010
We report on the development of solution-processed ZnO-based dye-sensitized solar cells. We fabricate mesoporous ZnO electrodes from sol-gel processed nanoparticles, which are subsequently sensitized with conventional ruthenium complexes and infiltrated with the solid-state hole transporter medium 2, 2', 7, 7'-tetrakis-(N, N-di-p-methoxyphenylamine)-9, 9'-spirobifluorene (spiro-OMeTAD). Starting from ZnO nanorods synthesized from solution, we investigate the porous ZnO film morphology using various precursor