Fe–tannic acid complex dye as photo sensitizer for different morphological ZnO based DSSCs

https://doi.org/10.1016/j.saa.2016.03.031Get rights and content

Highlights

  • Different ZnO nanostructures were synthesized by microwave hydrothermal methods.

  • The tannic acid and Fe–tannic acid complex dye were used for the first time in DSSCs.

  • ZnO plate has higher photovoltaic efficiency than rods and nanoparticles in DSSC.

  • Fe–tannic acid complex dye display broad spectrum responses.

  • Fe–tannic acid complex dye exhibited better photovoltaic performance than tannic acid.

Abstract

In this paper we have synthesized different morphological ZnO nanostructures via microwave hydrothermal methods at low temperature within a short time. We described different morphologies of ZnO at different Zn(NO3)2/KOH mole ratio. The ZnO nanostructures were characterized via X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and UV–vis spectrophotometry. All ZnO structures have hexagonal wurtzite type structures. The FESEM images showed various morphologies of ZnO such as plate, rod and nanoparticles. Dye sensitized solar cells have been assembled by these different morphological structures photo electrode and tannic acid or Fe–tannic acid complex dye as sensitizer. We have achieved at maximum efficiencies of photovoltaic cells prepared with ZnO plate in all dye systems. The conversion efficiencies of dye sensitized solar cells are 0.37% and 1.00% with tannic acid and Fe–tannic acid complex dye, respectively.

Introduction

Solar energy is the clean, green and plentiful renewable energy sources [1]. In 1991, O'Reagen and Gratzel developed dye sensitized solar cells (DSSCs) which were interested in many researcher's due to their low cost and environmental friendly applications [2], [3], [4]. The DSSCs comprise of a photo anode (metal oxide semiconductor), a dye sensitizer (organometallic or organic dye), an electrolyte (redox couple) and a counter electrode (catalyst) [5]. In DSSC, a photon excitation of the dye sensitized and dye molecule injected an excited electron into conductivity band of photo anode [5], [6]. Dye molecules which lost electrons are then oxidized. The injected electron travels through the counter electrode and concurrently extracted to load electrical energy. The injected electron transferred to electrolyte and electron bereft dye to replace the electron [6].

Zinc oxide (ZnO) is a wide band gap (3.37 eV) semiconductor having a variety of application in sensors, photodetectors, and DSSCs [7], [8]. Depending on the preparation methods for ZnO, different morphological properties can be obtained. Various morphologies of ZnO nanostructures have been used in DSSC, such as nanoparticles, nanoporous films, nanowires, nanosheets, tetrapods, nanorods, and nanospheres [7], [8], [9], [10], [11], [12], [13]. The power conversion efficiency of ZnO-based DSSCs are lower than that of TiO2-based DSSCs. The conversion efficiency of ZnO-based DSSCs was explained by dissolution of ZnO, dye-Zn2 + aggregates formation and lower injection efficiency, etc. However, ZnO has higher electron mobility than TiO2. Consequently, there is a significant enhancement in the electron transport in DSSCs using ZnO with different morphologies in photo anodes [7].

In DSSCs, the dyes play a key role in sunlight absorption and conversion of sunlight to electric energy. The highest efficiency of DSSCs formed by Ru-complex dyes and TiO2 reached 11–12%. However, Ru based dyes have several disadvantages such as costly produced noble metal contains in their structures including toxic materials and difficulty in synthesis [2]. Thousands of dyes have been synthesized and used in DSSCs until nowadays, and they can be divided into four categories. The first type sensitizers are the metal complexes dyes. The second type sensitizers are porphyrin and phthalocyanine. These dyes have some disadvantages such as poor solubility and difficulty in attachment on semiconductor's surface. The third type sensitizers are organic dyes. They are included in coumarin, indoline, carbazole, tetrahydroquinoline, triarylamine, triphenylamine, and perylene dyes. These dyes have some advantages such as low cost, easy synthesis and less environmental pollution problems due to presence of non-toxic metals in their structures, although they are poor chemical and light stability [14]. The fourth type sensitizers are natural dyes. The natural dyes are promising alternative to organic dyes for DSSC application [15]. Natural dyes include several pigments such as cyanin, carotenoids, tannins, chlophyll, anthocyanins, and betanins [2], [5], [14]. They are environmental friendly, cheap and have easy preparation methods. However, they have several disadvantages such as lower conversion efficiency and chemical stability.

Tannins are water-soluble polyphenolic compounds which are important classes of secondary metabolites and have molecular weight ranged to 500–20,000 Da [16]. Since ancient time tannins are able to tan animal skins from leather. Tannins are a type of polyphenols extracted from different parts of trees such as in barks, fruits, leaves and woods [16], [17]. Tannins are water-soluble compounds and their modified forms are used as adsorbent for metals from wastewater [16], [17], [18]. Tannins are divided in four major groups: gallotannins, ellagitannins, complex, and condensed tannins [16]. Tannic acid is a specific commercial form of gallotannin which are esters of glucose and gallic acid.

Tannic acid has a unique form of polyphenol and used in different subjected area. We have used tannic acid and Fe–tannic acid for DSSCs application because there is not any application of tannic acid as a dye for solar cell in the literature. The Fe tannic acid complex is a stable dye and it can conform to dye sensitized solar cell application. Ever since, O'Reagen and Gratzel [3] invented the first dye sensitized solar cell in 1991 numerous research effort have focused on DSSCs based different natural dye, organic dye etc. due to the environmental friendliness and low cost product. The researchers have effort for improvement the cell efficiency. Many researchers have improved separately different type of dyes, but these dyes are insufficient in providing the expected efficiencies and hence researchers have been looking for a new type dyes and/or alternatives. We thought that the tannic acid and especially Fe–tannic acid complex which has higher efficiencies can become potential alternative natural dye for DSSCs.

In the present work, the photo-sensitizing properties of tannic acid and Fe–tannic acid complex dyes on different morphological ZnO nanostructure based photoanodes have been studied for DSSC application. Fe–tannic acid complex dye was synthesized in using tannic acid and FeCl3. The light absorption characteristics of the dyes were investigated using UV–Vis spectrophotometer. To the best of our knowledge, the tannic acid and Fe–tannic acid complex dyes have never been used as sensitizer in DSSC and the present work provides a comprehensive view on the use of these dyes in the DSSC. The ZnO nanostructures were synthesized by microwave assisted hydrothermal method, and the crystalline structure and morphologies of the prepared ZnO nanostructures were investigated by XRD and SEM, respectively. The photoanode was prepared by coating various ZnO nanostructures layer on FTO coated glass substrate. The photovoltaic parameters of DSSCs fabricated of these natural dyes were measured under simulated solar light and the results were discussed and compared.

Section snippets

Materials

In the current study, the used materials are mentioned as follows: Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich), potassium hydroxide (KOH, Merck), tannic acid (Merck), iron (III) chloridehexahydrate(FeCl3·6H2O, Sigma Aldrich), di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719 dye, Sigma Aldrich), fluorinated tin oxide glass (FTO glass, Sigma Aldrich 7 Ω/sq.), ethyl cellulose (Sigma Aldrich), ethanol (Merck), dihydrogen

ZnO nanostructure characterization

The X-ray diffraction patterns of microwave-hydrothermal synthesized ZnO nanostructures are given in Fig. 1. As shown in Fig. 1, all the diffraction peaks of ZnO samples can be indexed in hexagonal wurtzite phase (JCPDS 36-1451) with the calculated lattice parameters for all samples a and c of 0.325 and 0.521 nm, respectively. No peaks of impurities and other phases such as Zn(OH)2 are detected and it demonstrates that all of the samples have high purity. The sharpness of the peaks indicates

Conclusion

Three different morphological ZnO nanostructures were synthesized via microwave hydrothermal methods. The ZnO nanostructures were characterized by XRD, FE-SEM, and UV–vis (diffused reflectance). The tannic acid and the Fe–tannic acid complex dye were prepared and characterized via FT-IR, UV–vis and CV to determine the physical and electrochemical properties. Then the DSSC was fabricated using these dye and the ZnO nanostructures. A solar cell efficiency for these systems of the ZnO-1 (plate)

Acknowledgements

M.Ö. acknowledges the partial financial support from the Turkish Academy of Sciences (TUBA).

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