Effect of Blocking Layer to Boost Photoconversion Efficiency in ZnO Dye-Sensitized Solar Cells (original) (raw)
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Effect of a compact ZnO interlayer on the performance of ZnO-based dye-sensitized solar cells
Solar Energy Materials and Solar Cells
ZnO is a promising material for application in dye-sensitized solar cells, related to its attractive electrical properties and the facile preparation of nanomaterials, with excellent control over the structure and morphology. In this work ZnO-based dye-sensitized solar cells were prepared, and the effect of the presence of a compact ZnO interlayer between the transparent conducting oxide (TCO) electrode and the nanostructured, mesoporous ZnO film on the performance of the solar cell is reported. The compact interlayer was deposited using planar rf magnetron sputtering, and the ZnO nanomaterial was prepared by forced hydrolysis from zinc acetate in ethanol solution. The presence of the compact interlayer has a positive effect on the overall characteristics of the solar cell and decreases the recombination rate from the TCO substrate, resulting in a higher open circuit voltage under low light conditions. The best efficiency of non-optimized solar cells at 1 sun was 4.0%.
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.
Electrochemical design of ZnO hierarchical structures for dye-sensitized solar cells
Solar Energy Materials and Solar Cells, 2012
ZnO single-crystalline nanowires (NWs) are highly relevant 1D-nanostructures for efficient charge collection in dye sensitized solar cells (DSSCs). In a first part of the paper, ZnO-NWs have been grown by electrochemistry on various fluorine-doped transparent oxide (FTO) coated glass substrates. The effects of substrate treatments and of the presence of an underlayer of TiO 2 or ZnO on the cell performance are reported. In particular, we show that the presence of a TiO 2 underlayer gives rise to a dense array of ZnO-NWs with a high aspect ratio (4 30) and to the cells with the best performances. However, the roughness of the NW arrayed films were not sufficient for the efficient harvesting of sunlight and low fill factors were found. In a second step, hierarchical structures have been designed by growing electrochemically a secondary nanoporous epitaxial ZnO phase on the wire surface. We have found that the cell performances were significantly improved in that case due to a much larger specific surface area and to a much better fill factor. We also show that the cell efficiencies of hierarchical structures did not follow the performances of the starting ZnO-NWs. The volume occupied by the second phase, its thickness and its connection to the wires are important parameters for cell optimizing.
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.
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.
Dependence of ZnO-based dye-sensitized solar cell characteristics on the layer deposition method
Bulletin of Materials Science, 2015
The selection of a proper method for the semiconductor layer deposition is an important requirement towards a high efficiency for dye-sensitized solar cells (DSSCs). We compared three techniques for deposition of the semiconductor thin layer in ZnO-based DSSCs, in order to determine the dependence between the deposition method, the ZnO film properties and finally the DSSCs characteristics. For this purpose, we varied the method used for deposition of the semiconductor film and we replaced ZnO with Al-doped ZnO. The nanostructured films morphology was analysed by transmission electron microscopy, high-resolution transmission electron microscopy and selected area electron diffraction. The optical properties were examined by UV-visible spectroscopy and the bandgap energies were calculated using the Tauc equation. The higher fill factor value was registered for DSSCs based on the ZnO film obtained by electrochemical method, but the higher efficiency was registered for doctorblading method.
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.
ZnO Nanostructures for Dye-Sensitized Solar Cells
Advanced Materials, 2009
This Review focuses on recent developments in the use of ZnO nanostructures for dye-sensitized solar cell (DSC) applications. It is shown that carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSC photoelectrode as they offer larger surface areas than bulk film material, direct electron pathways, or effective light-scattering centers, and, when combined with TiO 2 , produce a core-shell structure that reduces the combination rate. The limitations of ZnO-based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO 2 , motivating further improvement in the power-conversion efficiency of DSCs.
In this research, ZnO photoelectrode and Platinum counterelectrode in dye-sensitized solar cells (DSSCs) were modified by sparking technique and investigated photoconversion properties. The ZnO photoelectrode was adjusted by using double-layered technique. Here, the DSSC structures were FTO/double-layered ZnO/N719/electrolyte/Pt counterelectrode. The efficiency characteristics for DSSCs were measured under illumination of simulated sunlight with the radiant power of 100 mW/cm 2 . It was found that the best results of DSSCs were observed with power conversion efficiency of 2.53% at 250 sparking cycles for ZnO nanopowder over-layered which was significantly higher than 1.83% of the reference cell. The efficiency enhancement can be explained by the arising of short-circuit photocurrent due to increasing of light scattering and dye adsorption for double-layered photoelectrode and the increasing of the active surface area of Platinum nanoparticles in counterelectrode.
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....