Sponge-like Porous ZnO Photoanodes for Highly Efficient dye-sensitized Solar Cells (original) (raw)
Related papers
Hierarchically Assembled ZnO Nanocrystallites for High-Efficiency Dye-Sensitized Solar Cells
Angewandte Chemie International Edition, 2011
Photoelectrochemical cells are promising devices for cheap, environmentally compatible, and large-scale solar energy conversion as an alternative to conventional solid-state semiconductor solar cells. Among excitonic cells, dyesensitized cells (DSCs) exhibit the highest performance in terms of energy conversion efficiency and long term stability, despite the fact that the efficiency remains below 13 % because of the intrinsic limitation in charge transport. The structure of the photoelectrodes is crucial in determining the functional properties of the photoelectrochemical system. In particular, the photoanode consists of a mesoporous wideband-gap oxide semiconductor film with a high specific surface (typically a thousand times larger than the bulk counterpart). To date, the highest photoconversion efficiency (PCE) has been achieved with film consisting of 20 nm TiO 2 nanocrystallites sensitized by different dye molecules (11.1 % for N719 dye, over 10 % for "black dye", and 11.4 % for C101, ). In addition to TiO 2 , a series of other ntype metal oxide semiconductors can in principle be used in DSCs, such as ZnO, SnO 2 , and In 2 O 3 . Much attention has been recently devoted to ZnO owing to its higher electron mobility and similar electronic band structure with respect to TiO 2 . Various strategies have been addressed to enhance PCE in ZnO-based DSCs, which are mainly based on tailoring the geometrical and structural features of ZnO. A possible solution to reduce electron recombination could be the use of one-dimensional nanostructures that are able to provide a direct pathway for the rapid collection of photogenerated electrons. However, only low PCE has been achieved to date, mainly because of the reduced internal surface area of the nanostructures. Hybrid structures have also been tested to improve light collection, such as combination of nanoparticles and nanowires (maximum PCE = 4.2 %), or hierarchical nanowires (maximum PCE = 2.63 %). Another strategy to enhance PCE is application of hierarchical photoanodes composed of large aggregates of nanocrystallites, which can act as light scattering centers while maintaining a high specific surface area. The synthetic procedure of photoanode preparation is crucial to improve PCE: an optimized photoanode composed of just ZnO nanoparticles without any geometrical feature for light confinement or enhanced electron transport resulted in the highest value of PCE (6.58 %) for a ZnO-based DSC. Herein we present the fabrication and characterization of hierarchically structured ZnO-based photoanodes in DSCs to enhance the PCE. Our approach addresses specifically the following points: 1) High optical density of the sensitized layer, allowing complete light absorption in the spectral range of the dye; 2) high light scattering of the absorbing layer, enhancing the time spent by light inside the sensitized film and improving light absorption; and 3) inhibition of back electron transfer between the conducting layer at the anode and the electrolyte. The films are prepared by the simple, cheap, and large-area-scalable spray pyrolysis method. The films are composed of polydispersed ZnO aggregates consisting of nanosized crystallites while submicrometer-sized aggregates act as efficient light scattering centers and nanoparticles provide the mesoporous structure and the large specific surface area needed for high dye loading. Additionally, a ZnO compact layer is intentionally formed between the conducting substrate and the layer composed of polydispersed aggregates. Such a layer acts as an efficient blocking layer for electron back reaction between the conducting glass at the anode and the electrolyte, improving the functional properties of the cells. This is the main innovation with respect to the work of Cao and co-workers, leading to unprecedented PCE up to 7.5 %, which is larger than ZnO nanoparticles (6.58 %), hierarchically structured ZnO without a blocking layer (5.4 %), and hierarchically arranged ZnO nanowires (2.63 %). As a further benefit, our method is extremely fast (no more than 1.5 h for the complete processing of a photoanode, while typically 8 h or 10 to 14 h are required), enabling its technological implementation.
Solution-derived ZnO nanostructures for photoanodes of dye-sensitized solar cells
Energy & Environmental Science, 2011
Solution-phase derived ZnO nanostructures have triggered considerable interest and become the mainstream route to obtain low-cost and large-scale electrode materials for dye-sensitized solar cells (DSSCs). The article reviews recent progress in liquid-phase synthesis methods for preparing ZnO nanostructures as the photoanodes of the DSSCs. A few classic paradigms and new advancements in the ZnO nanostructures made by our group are demonstrated. The effects of ZnO nanostructured films with different morphologies, prepared by solution-phase approaches, on the performance of DSSCs are discussed. Finally, various liquid-phase methods of ZnO nanostructure synthesis are summarized and compared to allow further exploration of the ways to improve the photoelectric conversion efficiency of DSSCs.
Nanoporous ZnO Photoelectrode for Dye-Sensitized Solar Cell
Journal of Nanomaterials, 2012
Nanoporous and macroporous structures were prepared by using self-assembled monolayer (SAM) onto ZnO thin films in order to investigate the efficiency of dye-sensitized solar cells (DSSCs) produced using these films. Using SAM on ZnO thin films, it is obtained successfully assembled large-area, highly ordered porous ZnO thin films. Varying nanoporous radius is observed between 20 and 50 nm sizes, while it is 500–800 nm for macroporous radius. The solar conversion efficiency of 2.75% and IPCE of 29% was obtained using ZnO nanoporous/N719 dye/I−/I3-electrolyte, while macroporous ZnO given solar conversion efficiency of 2.22% and IPCE of 18%.
Nanoscale research letters, 2014
Mesoporous ZnO nanoparticles have been synthesized with tremendous increase in specific surface area of up to 578 m 2 /g which was 5.54 m 2 /g in previous reports (J. Phys. Chem. C 113:14676-14680, 2009). Different mesoporous ZnO nanoparticles with average pore sizes ranging from 7.22 to 13.43 nm and specific surface area ranging from 50.41 to 578 m 2 /g were prepared through the sol-gel method via a simple evaporation-induced self-assembly process. The hydrolysis rate of zinc acetate was varied using different concentrations of sodium hydroxide. Morphology, crystallinity, porosity, and J-V characteristics of the materials have been studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), BET nitrogen adsorption/desorption, and Keithley instruments.
Influence of morphology on the performance of ZnO-based dye-sensitized solar cells
ZnO nanomaterials with different morphologies, obtained by a sonochemical synthesis method at pH values of 5.5, 8, 10 and 12, have been used as starting materials for the fabrication of dye-sensitized solar cells. The morphology of the nanomaterials and the texture of the films deposited using screen printing depend on the synthesis pH, and the various exposed surface facets interact in a different manner with the dye and electrolyte solutions. The best cell performance was obtained with the morphology that resulted from the synthesis at pH 10, where the {100} and {110} crystal forms are predominant, and where dye coverage was largest. Interestingly, the BET total surface area was lowest for this nanomaterial illustrating the importance of morphology. The influence of the synthesis pH was also evident in the energetics and recombination kinetics of the solar cells. For the ZnO material synthesized at pH 5.5, the band edges appear to be shifted to more negative potentials, which could have resulted in a larger open circuit potential based on thermodynamic considerations. However, the electron life time for the pH 5.5 ZnO material is significantly smaller than for the other three synthesis pH values, indicating that the recombination kinetics are significantly faster for these cells as well, resulting in a smaller open circuit potential based on kinetics arguments. The balance between these two effects determines the experimentally observed open circuit potential. Overall, the results indicate that the dependence of the dye adsorption characteristics on ZnO nanomaterial morphology and film texture are the dominating factors that determine the solar cell performance.
Journal of Materials Science: Materials in Electronics, 2014
Novel ZnO core/shell nanostructures were constructed by depositing a porous ZnO layer directly on the surfaces of pre-fabricated ZnO nanowires through a facile chemical method. The morphology and structure of the obtained products have been investigated by fieldemission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction analysis. In these unique nanostructures, the porous overlayer exhibits a large surface area for sufficient dye loading to enhance light harvesting and the ZnO nanowire cores provide direct conduction pathways for the photogenerated electron transport to diminish the chance of electron recombination. The obtained ZnO nanostructures were used as photoanode material in dye-sensitized solar cell which showed an increase in performance of 141 % compared with an equivalent solar cell employing ZnO nanowire arrays as photoanode. This result was achieved mainly due to an increase in photogenerated current density directly resulting from improved light harvesting of the porous layer.
Self-assembled ultra small ZnO nanocrystals for dye-sensitized solar cell application
Journal of Solid State Chemistry, 2014
We demonstrate a facile chemical approach to produce self-assembled ultra-small mesoporous zinc oxide nanocrystals using sodium salicylate (SS) as a template under hydrothermal conditions. These ZnO nanomaterials have been successfully fabricated as a photoanode for the dye-sensitized solar cell (DSSC) in the presence of N719 dye and iodine-triiodide electrolyte. The structural features, crystallinity, purity, mesophase and morphology of the nanostructure ZnO are investigated by several characterization tools. N 2 sorption analysis revealed high surface areas (203 m 2 g À 1) and narrow pore size distributions (5.1-5.4 nm) for different samples. The mesoporous structure and strong photoluminescence facilitates the high dye loading at the mesoscopic void spaces and light harvesting in DSSC. By utilizing this ultra-small ZnO photoelectrode with film thickness of about 7 μm in the DSSC with an open-circuit voltage (V OC) of 0.74 V, short-circuit current density (J SC) of 3.83 mA cm À 2 and an overall power conversion efficiency of 1.12% has been achieved.
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.
JOM, 2019
ZnO nanostructures were synthesized by solvothermal (STT), solution combustion (SCT), and template synthesis (TS) techniques, showing the formation of rod-like, dot-like, and wire-like morphology. Pure-phase wurtzite structure was observed for STT and SCT samples, and mixed-phase wurtzite structure for the TS sample. Strong excitonic peaks appeared for STT and TS samples, whereas the excitonic peak tended to shift for the SCT sample. Dye-sensitized solar cell device structures using natural anthocyanin dye were fabricated and their I-V characteristics studied. The ZnO nanowire-based device showed the maximum open-circuit voltage (V oc) and short-circuit current density (I sc) in comparison with the rod-like and flake-like ZnO nanostructures. The photoconversion efficiency (PCE) was found to be 3.2%, 4.4%, and 5.4% for the rod-, dot-, and wire-like morphology, respectively. The enhancement in the PCE can be attributed to increased charge collection at the interface of the ZnO photoanode and electrolyte layer.
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.