Improved dye-sensitized solar cell with a ZnO nanotree photoanode by hydrothermal method
© Kuo et al.; licensee Springer. 2014
Received: 16 February 2014
Accepted: 22 March 2014
Published: 2 May 2014
This study investigated the influence of ZnO nanostructures on dye adsorption to increase the photovoltaic conversion efficiency of solar cells. ZnO nanostructures were grown in both tree-like and nanorod (NR) arrays on an AZO/FTO film structure by using a hydrothermal method. The results were observed in detail using X-ray diffraction, field-emission scanning electron microscopy (FE-SEM), UV-visible spectrophotometry, electrochemical impedance spectroscopy, and solar simulation. The selective growth of tree-like ZnO was found to exhibit higher dye adsorption loading and conversion efficiency than ZnO NRs. The multiple ‘branches’ of ‘tree-like nanostructures’ increases the surface area for higher light harvesting and dye loading while reducing charge recombination. These improvements result in a 15% enhancement in power conversion. The objective of this study is to facilitate the development of a ZnO-based dye-sensitized solar cell.
Dye-sensitized solar cells (DSSCs) have attracted much attention as the next-generation solar cell. DSSCs have been widely researched because of their low cost and high energy conversion efficiency. In a functioning DSSC, photoexcited electrons in the sensitizer are injected into the conduction band of a semiconductor. A charge mediator, i.e., a proper redox couple, must be added to the electrolyte to reduce the oxidized dye. The mediator must also be renewed in the counter electrode, making the photoelectron chemical cell regenerative . At present, the photoelectrochemical system of DSSC solar cells incorporates a porous-structured wide band gap oxide semiconductor film, typically composed of TiO2 or ZnO.
The single-cell efficiency of 12.3% has persisted for nearly two decades . This conversion efficiency has been limited by energy damage that occurs during charge transport processes. Specifically, electrons recombine with either oxidized dye molecules or electron-accepting species in the electrolyte [3–5]. This recombination problem is even worse in TiO2 nanocrystals because of the lack of a depletion layer on the TiO2 nanocrystallite surface, which becomes more serious as the photoelectrode film thickness increases .
In response to this issue, this study suggests ZnO-based DSSC technology as a replacement for TiO2 in solar cells. Like TiO2, ZnO is a wide band gap (approximately 3.3 eV at 298 K) semiconductor with a wurtzite crystal structure. Moreover, its electron mobility is higher than that of TiO2 for 2 to 3 orders of magnitude . Thus, ZnO is expected to show faster electron transport as well as a decrease in recombination loss.
However, reports show that the overall efficiency of TiO2 DSSCs is far higher than that of ZnO. The highest reported efficiency of 5.2% for ZnO DSSCs is surpassed by 6.3% efficiency for TiO2 thin passivation shell layers . The main problem is centered on the dye adsorption process in ZnO DSSCs. The high acidity of carboxylic acid binding groups in the dyes can lead to the dissolution of ZnO and precipitation of dye-Zn2+ complexes. This results in a poor overall electron injection efficiency of the dye [8–10].
There are multiple approaches for increasing the efficiency of ZnO DSSCs. The introduction of a surface passivation layer to a mesoporous ZnO framework is one possibility, but it may complicate dye adsorption issues. Alternatively, the internal surface area and morphology of the photoanode could be changed to replace the conventional particulate structures. However, the diffusion length and the surface area are incompatible with one another. Increasing the thickness of the photoanode allows more dye molecules to be anchored, but electron recombination becomes more likely because of the extended distance through which electrons diffuse to the TCO collector. Therefore, the structure of the charge-transporting layer should be optimized to achieve maximum efficiency while minimizing charge recombination.
The insufficient surface is as of 1D nanostructures limit the performance of DSSCs to a relatively low level. Accordingly, a photoanode with a highly branched network could yield greater photoconversion efficiency than 1D nanostructures because dye loading can be enriched without sacrificing electron transport properties . In addition, the highly branched tree-shaped structure possesses larger pores, creating a better transport route for electrolyte diffusion. Researchers have studied many 1D nanostructures, namely, nanowires [11–14], nanoflowers , nanotubes [11, 16], nanosheets [17, 18], nanobelts [11, 16], and nanotips . These nanostructures are expected to significantly ameliorate the electron diffusion length in photoelectrode films. By providing a direct conduction pathway for the fast collection of photogenerated electrons, they decrease the potentiality of charge recombination during interparticle percolation by replacing random polycrystalline TiO2 nanoparticle networks with ordered crystalline ZnO semiconductor nanowires (NWs). In the past studies, ZnO nanostructures were typically grown by chemical bath deposition (CBD) [20, 21]. This paper presents a discussion on the different surface characterizations of ZnO nanostructures using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), UV-visible spectrophotometry, electrochemical impedance spectroscopy (EIS), and solar simulation.
D-719 dye, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)bis-tetrabutylammonium (Everlight Chemical Industrial Corp., Taipei, Taiwan), was dissolved in acetonitrile for the preparation of the 0.5 mM dye solution. Dye sensitization was conducted by soaking the ZnO photoelectrodes in D-719 dye at room temperature for 2 h. A sandwich-type configuration was employed to measure the presentation of the DSSCs. An active area of 1 cm2 was assembled by using a Pt-coated FTO substrate as a counter electrode and the Pt/FTO was heated at 200°C for 30 min in air. The DSSC cell was sealed using the polymer resin to act as a spacer. The electrolyte was injected into the space between the electrodes from these two holes, and then these two holes were sealed completely by Surlyn (DuPont, Taipei, Taiwan).
Results and discussion
Electrochemical and photovoltaic parameters of DSSCs
Regarding branch-free rods, less accumulation on the electrode layer leads to poor electrolyte filling, improving the recombination pathway and raising the charge transport resistance. The surface charge density and trap level of the ZnO layer also play an important role in deciding the charge transport resistance by depleting the space charge layer. The drift transport that occurs when a 1D photoanode contacts electrolyte redox carriers is detrimental to efficacious charge transport. This requires further discussion [22, 12]. EIS measurement was used to obtain the Bode plots of the lifetimes displayed in Table 1. This table shows that the tree-like ZnO structure DSSCs exhibit a longer electron lifetime (τeff = 3.91 ms) than that of the NRs DSSCs (τeff = 3.28 ms). The longer lifetime implies lower recombination rate and increased electron-collection efficiency, and thus the parameter can be related to the improvement in cell efficiency.
where the Imax and I0 are the maximum current density and dark current density, respectively, in Equation 2. This equation predicts that the suppression of the dark current density (I0) results in a higher Voc, and the enhancement of Jsc is almost 12%. Accordingly, Figure 6b shows that the dark current density of DSSC with ZnO tree-like nanostructure was lower than that with ZnO nanorod. The dark current density supplies qualitative information on dye coverage on the photoelectrode surface . The lower dark current density in the tree-like ZnO nanostructure photoelectrode is caused by efficient dye coverage on the surface of the ZnO branches, as well as proper electrolyte penetration. These factors result in low recombination damages at ZnO/dye interfaces. Furthermore, the Voc increase in tree-like nanostructure DSSCs can be explained in two ways: (1) Higher dye loading fosters more charge injection from the dye sensitizer to the conduction band of ZnO. The result is an upward shift in the ZnO quasi-Fermi level, thus enhancing the potential difference between ZnO and the redox species. (2) Sufficient electrolyte pore filling in vertically branched structures leads to efficient hole scavenging at ZnO/dye interfaces, lowering the locus of recombination .
Although the power conversion efficiency of the present work is lower than the highest value reported in the literature , our principal concern is on whether the tree-like nanostructure can improve on the conversion efficiency of a DSSC composed of nanorods. This study determined that a tree-like ZnO nanostructure synthesized through effortless and gentle reaction conditions is highly efficient and economically viable as a photoelectrode for DSSCs. Further work will improve the cell configuration and conversion efficiency.
This study prepared tree-like ZnO structures and ZnO nanorods for use as photoanodes in DSSCs. DSSCs composed of tree-like ZnO nanostructures were found to show greater photovoltaic performance than DSSCs containing nanorods. Comparatively, tree-like ZnO structures exhibit a larger internal surface area for efficient dye loading and light harvesting, a greater available pore volume, reduced charge recombination, and improved interconnectivity for faster electron transport than ZnO nanorods. These improvements yield a 15% enhancement in power conversion.
This work was supported by the Green Technology Research Center of Chang Gung University and the National Science Council (NSC) of Taiwan under contract numbers NSC100-2815-C-155-013-E, NSC100-2112-M-182-004, and NSC101-2112-M-182-003-MY3.
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