Nanowire-based Dye-sensitized Solar Cells Research Articles

ZnO@TiO2 Core/Shell Nanowire Arrays with Different Thickness of TiO2 Shell for Dye-Sensitized Solar Cells

The ZnO@TiO2 core/shell nanowire arrays with different thicknesses of the TiO2 shell were synthesized, through depositing TiO2 on the ZnO nanowire arrays using the pulsed laser deposition process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that these core/shell nanowires were homogeneously coated with TiO2 nanoparticles with high crystallinity, appearing to be a rather rough surface compared to pure ZnO nanowires. The efficiency of ZnO@TiO2 core/shell structure-based dye-sensitized solar cells (DSSCs) was improved compared with pure ZnO nanowires. This is mainly attributed to the enlarged internal surface area of the core/shell structures, which increases dye adsorption on the anode to improve the light harvest. In addition, the energy barrier which formed at the interface between ZnO and TiO2 promoted the charge separation and suppressed the carrier recombination. Furthermore, the efficiency of DSSCs was further improved by increasing the thickness of the TiO2 shell. This work shows an efficient method to achieve high power conversion efficiency in core/shell nanowire-based DSSCs.

Balancing surface area with electron recombination in nanowire-based dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) represent a promising technology in the renewable energy portfolio. In this study, tin-doped-indium oxide (ITO) nanowires are used to prepare DSSC photoanodes that overcome charge transport limitations associated with conventional nanoparticle-based photoanodes. Vertically-aligned ITO nanowires are grown by a thermal evaporation method and a porous TiO2 shell layer is uniformly coated on the nanowire surfaces to ensure high dye-loading. It is found that cell efficiency increased to 4.85% from 2.81% when a dense HfO2 blocking layer is inserted between ITO nanowire surfaces and the porous TiO2 shell. While more photoactive surface area associated with longer nanowires improves device performance, nanowires longer than 20μm show reduced efficiency from an increase in electron recombination. Therefore, 20-μm-long nanowires show the best efficiency, which is attributed to the balance of surface area and electron recombination in the photoanode. Although decreasing TiO2 annealing temperatures from 600 to 400°C increases electron recombination, the smaller TiO2 nanoparticles at 400°C lead to much higher dye-loading and, ultimately, the highest device efficiency of 5.59%.

Improving Performance via Blocking Layers in Dye-Sensitized Solar Cells Based on Nanowire Photoanodes.

Electron recombination in dye-sensitized solar cells (DSSCs) results in significant electron loss and performance degradation. However, the reduction of electron recombination via blocking layers in nanowire-based DSSCs has rarely been investigated. In this study, HfO2 or TiO2 blocking layers are deposited on nanowire surfaces via atomic layer deposition (ALD) to reduce electron recombination in nanowire-based DSSCs. The control cell consisting of ITO nanowires coated with a porous shell of TiO2 by TiCl4 treatment yields an efficiency of 2.82%. The efficiency increases dramatically to 5.38% upon the insertion of a 1.3 nm TiO2 compact layer between the nanowire surface and porous TiO2 shell. This efficiency enhancement implies that porous sol-gel coatings on nanowires (e.g., via TiCl4 treatment) result in significant electron recombination in nanowire-based DSSCs, while compact coatings formed by ALD are more advantageous because of their ability to act as a blocking layer. By comparing nanowire-based DSSCs with their nanoparticle-based counterparts, we find that the nanowire-based DSSCs suffer more severe electron recombination from ITO due to the much higher surface area exposed to the electrolyte. While the insertion of a high band gap compact layer of HfO2 between the interface of the conductive nanowire and TiO2 shell improves performance, a comparison of the cell performance between TiO2 and HfO2 compact layers indicates that charge collection is suppressed by the difference in energy states. Consequently, the use of high band gap materials at the interface of conductive nanowires and TiO2 is not recommended.

Highly efficient one-dimensional ZnO nanowire-based dye-sensitized solar cell using a metal-free, D-π-A-type, carbazole derivative with more than 5% power conversion.

Hydrothermally grown one-dimensional ZnO nanowire (1D ZnO NW) and a newly synthesized metal-free, D-π-A type, carbazole dye (SK1) sensitizer-based photovoltaic device with a power conversion efficiency (PCE) of more than 5% have been demonstrated by employing the cobalt tris(2,2'-bipyridyl) redox shuttle. A short-circuit current density (Jsc) of ∼12.0 mA/cm(2), an open-circuit voltage (Voc) of ∼719 mV, and a fill factor (FF) of ∼65% have been afforded by the 1D ZnO NW-based dye-sensitized solar cell (DSSC) incorporating [Co(bpy)3](3+/2+) complex as the one-electron redox mediator. In contrast, the identical DSSC with traditional I3(-)/I(-) electrolyte has shown a Jsc ≈ 12.2 mA/cm(2), a Voc ≈ 629 mV, and a FF ≈ 62%, yielding a PCE of ∼4.7%. The persuasive role of the inherent superior electron transport property of 1D ZnO NWs in enhancing the device efficiency is evidenced from the impoverished performance of the DSSCs with photoanodes fabricated using ZnO nanoparticles (NPs). The DSSCs having ZnO NP-based photoanodes have achieved the PCEs of ∼3.6% and ∼3.2% using cobalt- and iodine-based redox electrolytes, respectively. The electronic interactions between the SK1 sensitizer and ZnO (NWs and NPs) to induce the photogenerated charge transfer from SK1 to the conduction band (CB) of ZnO are evidenced from the significant quenching of photoluminescence and exciton lifetime decay of SK1, when it is anchored onto the ZnO architectures. The energetics of the SK1 dye molecule are estimated by combining the spectroscopic and electrochemical techniques. The electronic distributions of SK1 dye molecule in its HOMO and LUMO energy levels are interpreted using density functional theory (DFT)-based calculations. The electron donor-π linker-acceptor (D-π-A) configuration of SK1 dye provides an intramolecular charge transfer within the molecule, prompting the electron migration from the carbazole donor to cyanoacrylic acceptor moiety via the oligo-phenylenevinylene linker group. The D-π-A-mediated electron movement witnesses the favorable photoexcited electron transfer from the LUMO of SK1 dye to the CB of ZnO through the carboxyl anchoring group.

Improved performance of three-dimensional Ni–TiO2 core–shell nanowire photoanodes in dye-sensitized solar cells

Abstract

Tuning the Energy Levels of ZnO/ZnS Core/Shell Nanowires To Design an efficient Nanowire-Based Dye-Sensitized Solar Cell

By using the self-consistent charge density functional tight binding method we studied the electronic structure of ZnO/ZnS core/shell nanowire as a function of both core radius and shell thickness. By studying the band energy alignment, band structure, density of states, and band edge wave functions we envisage the efficacy of this particular nano heterostructure in dye sensitized solar cell. The strong localization of valence band maximum and conduction band minimum in ZnS shell and ZnO core, respectively, irrespective of core radius and shell thickness clearly indicates the spatial charge separation in this system. This spatial charge separation decreases the charge recombination rate thereby increasing the chance of better photovoltaic performance. We also investigated the electronic structure of anthraquinone (AQ) acid dye molecule–ZnO/ZnS nanowire composite system. We demonstrated that whether the composite system will form type I or type II band alignment that very much depends on the thickness of t...

ZnO/ZnO Core–Shell Nanowire Array Electrodes: Blocking of Recombination and Impressive Enhancement of Photovoltage in Dye-Sensitized Solar Cells

A multistep wet-chemistry route was developed, by combining electrodeposition, colloidal synthesis and spin coating, to obtain arrays of ZnO nanowires (NWs) coated by a ZnO nanocrystalline layer (i.e., ZnO/ZnO core–shell NWs). They were integrated as anodes in dye-sensitized solar cells. With the use of an iodide-based electrolyte, photovoltages as impressive as 870 mV were obtained with an enhancement of more than 250 mV with respect to devices based on bare NWs. A comprehensive device characterization study by means of impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) reveals a significant blockage of recombination upon NW shell deposition. To study the generality of this multistep method, electrodes of core–shell nanostructures based on commercial ZnO nanoparticles were also prepared. A decrease of recombination rate is also detected, although it is much more moderate than the observed for nanowire-based electrodes. The present ZnO synthetic approach allows obtaining nanowire-based dye-sensitized solar cells which exhibit longer electron lifetimes than nanocrystalline analogues. This finding implies a significant improvement of photovoltage with respect to the state of the art ZnO-based dye-sensitized solar cells.

Cobalt(II/III) Redox Electrolyte in ZnO Nanowire-Based Dye-Sensitized Solar Cells

In this work, we explore the use of cobalt complex redox shuttles in dye sensitized solar cells (DSCs) based on ZnO nanowires (NWs). Arrays of vertically aligned ZnO NWs produced by a low-cost hydrothermal method are used to fabricate DSCs with [Co(bpy)3](2+/3+) as electrolyte. A direct comparison of the performance of [Co(bpy)3](2+/3+)-based ZnO DSC with I(-)/I3(-)-based ones demonstrates the higher suitability of the cobalt complex, both in terms of a larger open circuit voltage (VOC) and a higher photocurrent. The [Co(bpy)3](2+/3+) electrolyte results in VOC enhancements above 200 mV. This VOC increase is associated to the better match between the cobalt complex redox potential and the oxidation potential of the dye. The incident photon-to-current efficiency (IPCE) enhancement is attributed to a less competitive visible light absorption of the cobalt redox couple. Thus the present study opens new opportunities to improve energy conversion efficiency in ZnO-based DSCs.

High Efficiency Dye-Sensitized Solar Cells Based on Three-Dimensional Multilayered ZnO Nanowire Arrays with “Caterpillar-like” Structure

A 3D ZnO nanowire-based dye-sensitized solar cell (DSSC) with unique "caterpillar-like" structure was designed. Because of the significant improvement of the total ZnO nanowire surface area, the amount of light absorption was substantially increased. This increase in the light harvesting efficiency enables us to achieve an overall power conversion efficiency as high as 5.20%, which is the highest reported value to date for ZnO nanowire-based DSSCs. A branched-multilayered design of ZnO nanowire arrays grown from ZnO nanofiber seed layers proves to be very successful in fabricating 3D ZnO nanowire arrays. Practically, electrospun ZnO nanowires were used as the seeds in multilayer growth of ZnO nanowire arrays with a unique "caterpillar-like" structure. This unique structure significantly enhances the surface area of the ZnO nanowire arrays, leading to higher short-circuit currents. Additionally, this design resulted in closer spacing between the nanowires and more direct conduction pathways for electron transfer. Thus, the open-circuit voltage was so significantly improved as a direct result of the reduction in electron recombination.

Effects of ZnO nanowire synthesis parameters on the photovoltaic performance of dye-sensitized solar cells

Determination of the effects of ZnO nanowires on the efficiency of ZnO nanowire-based dye-sensitized solar cells (DSSCs) is important. In this study, we determined the effects of different OH- precursors, concentrations, the ratio of zinc nitrate to hexamethylene tetramine (HMT), and the hydrothermal synthesis temperature on the physical, crystal, and optical properties of ZnO nanowires and investigated the performance of the resulting DSSCs. We observed that ZnO nanowires synthesized using an equimolar ratio of HMT to zinc nitrate yielded a DSSC with high incident photon-to-current efficiency (IPCE), cell efficiency, short circuit current density (Jsc), and fill factor (FF), and low ZnO–dye–electrolyte interface resistance due to an increased amount of dye and a decreased density of defects. Furthermore, ZnO nanowires made using optimal concentrations and ratios of zinc nitrate to HMT had a high surface area and low defect density. All the photovoltaic performance parameters of DSSCs assessed such as IPCE, cell efficiency, Jsc, open circuit potential (Voc), and FF increased with synthesis temperature, which was related to a decrease in the resistance at the ZnO–dye–electrolyte interface. We attributed these results to an increased amount of dye facilitated by a large nanowire surface area and fast electron transfer because of the improved crystalline structure of the ZnO nanowires and their low defect density. By optimizing the ZnO nanowires, we increased DSSC efficiency to 0.26% using ZnO nanowires synthesized with 25mM of both zinc nitrate and HMT at 90°C, while only a 0.02% increase in efficiency was obtained when NH4OH was used as OH− precursor.

Successive ionic layer adsorption and reaction of ZnSe shells for ZnO nanowire-based dye-sensitized solar cells

Thin ZnSe layers were deposited on ZnO nanowires by a novel successive ionic layer adsorption and reaction technique in order to solve recombination problems in ZnO nanowire-based dye-sensitized solar cells (DSSCs). Cell efficiency increased from 0.1 to 1.3–1.4% with the deposition of a 9- to13-nm-thick ZnSe shell on ZnO nanowires due to a large increase in JSC. The dramatic increase in JSC and cell efficiency is due to the facilitation of electron transfer related to ambipolar diffusion by the formation of a type II band alignment and the suppression of recombination in the presence of the ZnSe shell.

An Interfacial and Bulk Charge Transport Model for Dye-Sensitized Solar Cells Based on Photoanodes Consisting of Core–Shell Nanowire Arrays

Dye-sensitized solar cells (DSSCs) based on ordered photoanode morphologies, such as nanotubes and nanowires, are widely gaining attention because these geometries are believed to enhance interfacial charge transfer and bulk charge transport. Unfortunately, experimental results have yet to show substantial improvement to conversion efficiency over nanoparticle-based DSSCs. A model is developed to characterize the performance of an idealized photoanode based on an ordered array of transparent conductive nanowires coated with an anatase titania shell. The role of the interfacial electric field in nanowire-based DSSCs is explored computationally by turning electron migration ON or OFF. The results show that back-reaction rates are most strongly influenced by the electric field. These electron loss mechanisms can be reduced by several orders of magnitude, leading to improvements in short-circuit current, open-circuit voltage, and fill factor.

Synthesis of a ZnS Shell on the ZnO Nanowire and Its Effect on the Nanowire-Based Dye-Sensitized Solar Cells

A thin ZnS shell was formed on ZnO nanowires using ZnSO4 and thiourea in an NH4OH (ZTA) precursor solution, and its effect on the performance of a dye-sensitized solar cell (DSSC) was investigated. As compared to hydrothermal and successive ionic layer adsorption and reaction methods, it was observed that the ZnS shell was effectively synthesized on the ZnO nanowires using the ZTA solution. ZnO nanowires with a ZnS shell had lower absorption intensities over the entire wavelength range compared with those of nanowires without the ZnS shell, which can cause dye to absorb more light in the DSSC operation. Because the intensity of visible emission decreased after the formation of a ZnS shell on the ZnO nanowires, it is proposed that surface defects that provide recombination sites will be reduced at the anode/electrolyte interface. The cell performance of the ZnO nanowire-based DSSC was greatly improved by the deposition of a thin ZnS shell onto the ZnO nanowires in the ZTA solution, which resulted from the ...

The effect of annealing on the photoconductivity of carbon nanofiber/TiO2 core-shell nanowires for use in dye-sensitized solar cells

Electrical transport properties and photoresponse of individual TiO2-coated carbon nanofibers were studied in an attempt to elucidate the limiting factors of core-shell nanowire-based dye-sensitized solar cells (DSSC). The role of the semiconductor shell microstructure was investigated by comparing as grown and thermally annealed samples. Steady state I-V and transient photoconductivity measurements suggest that improving the microstructure leads to reduced resistivity and contact resistance, a decrease in charge traps, improved surface stoichiometry for dye adsorption, and reduced absorption of visible light by the semiconductor, all of which may improve nanowire-based DSSC performance.

Al-Doped ZnO Thin Film: A New Transparent Conducting Layer for ZnO Nanowire-Based Dye-Sensitized Solar Cells

In this study, an aluminum-doped zinc oxide (AZO) layer was used as a transparent conducting oxide (TCO) layer in ZnO nanowire (NW)-based dye-sensitized solar cells (DSSCs). The well aligned, single crystalline ZnO NW arrays that were grown on the AZO films exhibited a better DSSC performance (an increased photocurrent density and fill factor) than those grown on the fluorine doped tin oxide (FTO) films. The I−V characteristics and electrochemical impedance spectroscopy measurements for the ZnO NW arrays on the AZO and FTO films clearly showed that the superior DSSC performance was caused by the facilitated charge injection from the ZnO NW to AZO, resulting from the formation of an ohmic contact. This study demonstrates that the AZO films are more favorable for highly efficient ZnO NW-based photoenergy conversion devices.

Hydrothermal Growth and Application of ZnO Nanowire Films with ZnO and TiO2Buffer Layers in Dye-Sensitized Solar Cells

This paper reports the effects of the seed layers prepared by spin-coating and dip-coating methods on the morphology and density of ZnO nanowire arrays, thus on the performance of ZnO nanowire-based dye-sensitized solar cells (DSSCs). The nanowire films with the thick ZnO buffer layer (~0.8–1 μm thick) can improve the open circuit voltage of the DSSCs through suppressing carrier recombination, however, and cause the decrease of dye loading absorbed on ZnO nanowires. In order to further investigate the effect of TiO2buffer layer on the performance of ZnO nanowire-based DSSCs, compared with the ZnO nanowire-based DSSCs without a compact TiO2buffer layer, the photovoltaic conversion efficiency and open circuit voltage of the ZnO DSSCs with the compact TiO2layer (~50 nm thick) were improved by 3.9–12.5 and 2.4–41.7%, respectively. This can be attributed to the introduction of the compact TiO2layer prepared by sputtering method, which effectively suppressed carrier recombination occurring across both the film–electrolyte interface and the substrate–electrolyte interface.

Nanowire-based dye-sensitized solar cells

We describe the design and performance of a ZnO nanowire-based dye-sensitized solar cell. ZnO nanowires with a branched structure were employed as the wide-band-gap semiconductor to construct dye-sensitized solar cells which exhibit energy conversion efficiencies of 0.5% with internal quantum efficiencies of 70%. The nanowires provide a direct conduction path for electrons between the point of photogeneration and the conducting substrate and may offer improved electron transport compared to films of sintered nanoparticles. The devices have light harvesting efficiencies under 10%, indicating that current densities and efficiencies can be improved by an order of magnitude by increasing the nanowire surface area.