Construction of 3-dimensional ZnO-nanoflower structures for high quantum and photocurrent efficiency in dye sensitized solar cell
Graphical abstract
Introduction
Dye-sensitized solar cells (DSSCs) have attracted much attention in the last decade due to low cost and relatively efficient photovoltaic energy conversion [1], [2], [3]. The highest efficiency, to the best of our knowledge, reported on DSSCs was produced by Gratzel et al. and it was obtained 11% on nanoporous TiO2 by using ruthenium complex dye, containing I−/I−3 redox couple electrolytes and platinum counter electrode [2], [3], [4]. Although the conversion efficiency for TiO2 are much higher than that achieved with ZnO based cells (0.3–6%), ZnO nano-semiconductor materials are considered as an alternative to TiO2 due to its ease of crystallization and anisotropic growth [5]. ZnO nanostructures are one of the most promising semiconductor materials for optoelectronic and photonic applications due to its wide, direct band gap (3.37 eV), a large exciton binding energy (60 meV) and a huge magneto-optic effect [6], [7]. Many different ZnO nanostructures have been synthesized under specific experimental conditions and investigated for DSSC applications due to the large specific surface area offered by these nanostructures [8]. The large specific surface area is one of the most important requirements for the photo-electrode films in DSSCs mainly because of the need to maximize dye molecule adsorption on semiconductor materials [9]. Therefore, it was reported that the surface structure, high surface area and the porosity are all important factors for obtain high solar conversion efficiency in DSSCs [10], [11].
Especially, three-dimensional nanoflower and one-dimensional nanowire have attracted great interest for DSSC applications due to their unique optical, electronic, mechanical properties [12], [13], [14], [15]. However, the uses of ZnO nanoflowers consisting of outstretched branches have been also reported for high efficiency in DSSC [16]. This is based on the consideration that the one-dimensional structures may not capture the photons completely due to the existence of intervals inherent in the morphology [16]. Therefore, nanoflower structures have nanoscale branches that stretch to fill these intervals and, provide both a larger surface area and a direct pathway for electron transport along the channels from the branched petals [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].
In this study, ZnO nanoflowers were growth on FTO substrate by hydrothermal method. X-ray diffraction (XRD) studies of the ZnO nanoflowers showed a high crystallinity and preferred orientation along the (0 0 1) plane of the wurtzite structures. Scanning electron microscopy (SEM) images showed that the nanoflowers can be obtained with average diameters of 10–50 nm and in the range of 1–5 μm in size. Raman spectra of the ZnO nanoflowers showed that the peaks at around 582 cm−1 to be related to the E1 mode of the ZnO nanoflowers. The room temperature photoluminescence (PL) measurements confirm that the nanoflower samples have near band edge emissions and broad deep level emission. The UV–Visible absorbance spectrums of the ZnO nanoflower show a strong absorption edge between 360 and 400 nm and high transmittance of higher than 92%. As a result, wide band gaps ZnO nanoflowers were successfully used as photoanode in DSSCs and it was compared with one-dimensional nanowire in terms of solar conversion efficiency.
Section snippets
Preparation of ZnO nanoflowers
ZnO nanoflowers were grown on FTO (SnO2:F), substrate by hydrothermal method using ammonia as the base source. The growth was carried out at 175 °C for 24 h with Zn (NO3)2 as a source of Zn2+ ions. First of all, FTO substrates were cleaned carefully by dipping for 2 min each in Propan-2-ol, de-ionized water, methanol and de-ionized water, in sequential manner. After the cleaning process, 10 mM Zn(NO3)2·6H2O (aq) solution was prepared and pH of the solution was adjusted to ∼10 using ammonia
Results and discussion
SEM images with different magnification (Fig. 1a–c) show that ZnO nanocrystals with flower-like shapes were successfully obtained uniformly throughout on FTO substrate using an ammonia solution with a pH of 10.0. The diameters of the ZnO nanoflowers were observed to be (from the SEM images) varying between 50 and 1000 nm. When increase in the pH of the solution (pH 11) were obtained ZnO nanowire on substrate as shown before study [11]. The morphology of ZnO obtained from aqueous solution is
Conclusion
The growth of ZnO nanoflower was performed on FTO substrates by hydrothermal method. Structural characterizations showed that the morphology of ZnO nanostructures changed as a function of pH value, from nanoflower (pH 10.0) to nanowire (pH 11.0). The highest solar-to-electric energy conversion efficiency of 5.119% and IPCE of 60% was obtained by using the ZnO nanoflowers/N719 dye/I−/I−3 electrolyte. The nanoflowers structures leads to a significant increase of sample conductivity. It was found
Acknowledgment
The authors gratefully acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) for the grant 2214- PhD research scholarship program.
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