Elsevier

Organic Electronics

Volume 86, November 2020, 105915
Organic Electronics

Enhancing the electron transfer process of TiO2-based DSSC using DC magnetron sputtered ZnO as an efficient alternative for blocking layer

https://doi.org/10.1016/j.orgel.2020.105915Get rights and content

Highlights

  • DC magnetron sputtering was utilized to provide the proper ZnO blocking layer on FTO substrates.

  • ZnO sputtered thin film was utilized as an efficient alternative for TiCl4 pre-treatment blocking layer.

  • The thickness of ZnO compact layer is a critical parameter in electron trapping states or blocking layer efficiency of photoanode.

Abstract

Developing dye sensitized solar cell (DSSC) technology by exploiting different alternative semiconductors has attracted research attentions. Among all types of semiconductors, ZnO nanostructures due to their unique electrical properties and the facile preparation of various morphologies, have considered as the promising materials for application in DSSCs. In the present study, DC magnetron sputtering method was utilized to prepare a ZnO thin film as an efficient alternative for TiCl4 pre-treatment to suppress the charge recombination process occurring at a conventional TiO2-based DSSC. Different thicknesses of ZnO seed layers on fluorine tin oxide conductive glass substrates (FTO) were prepared via various sputtering deposition times. Field emission-scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD) analyses were utilized to study the surface uniformity and crystallinity of the ZnO nanostructures. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were employed to investigate the photoanode interface properties via determination of the electron lifetime and density of electron in conduction band. The results demonstrated that the thickness of ZnO compact layer which either acts as electron trapping states or blocking layer has the important role in cell performance. Finally, by the optimum thickness of ZnO thin film, the highest efficiency was achieved at 5.1%.

Introduction

Recently, dye-sensitized solar cells (DSSCs) have been regarded as one of the most promising alternative renewable energy sources [1,2]. Due to their unique characteristics, such as simple fabrication process, low production cost, efficient performance under low light intensity conditions, will be considered for specially application in roofing, windows, flexible solar panels, and indoor uses [3]. The photoanodes are the main part of DSSCs which are made by semiconductor oxides. TiO2 nanostructures are as the first candidate semiconductors which are commonly used in DSSCs. Mainly, they can provide high surface area for efficient dye adsorption and diffusion pathway for charge transfer processes [4,5]. Despite their unique advantages, the whole recombination reactions which are occurring through FTO/electrolyte and TiO2/electrolyte interfaces with both oxidized electrolyte and dye molecules species decline a conventional TiO2–based DSSC efficiency [6]. In fact, the difference between the maximum theoretical and the experimental efficiencies of TiO2 mesoporous appears to be partly related to the charge recombinations [7]. So, the interface modification of the TiO2-based DSSCs to suppress the charge recombination reactions is an important topic in DSSCs research [8,9]. In general, TiCl4 pre-treatment is carried out to fabricate the blocking layer on FTO substrate to suppress recombination reactions in TiO2-based DSSCs. The tedious and hazards conditions for TiCl4 pre-treatment, attracted attentions to find the alternative process. ZnO appears to be the most promising alternative for TiCl4 pre-treatment with a very similar band gap to those of TiO2 nanostructures. Also, the attractive electrical properties of ZnO nanostructures, the high electron transfer properties of ZnO nanostructures (200 cm2 V−1s−1) which is much larger than that of TiO2 nanostructures (30 cm2 V−1s−1), and their facile synthesis with various morphologies make them as an appropriate candidate for application in DSCCs [10,11]. Despite the ZnO advantages, the overall efficiency of ZnO-based DSSCs is lower than that of TiO2 cells. The main reason of the much less efficiency of ZnO-based DSSCs could be due to the instability of ZnO nanostructures in commercial acidic ruthenium based dyes like N719 dye and the insufficient rate constant of electron injection from excited dye molecules into ZnO nanostructures which is much slower as compared to TiO2 nanostructures (more than 100 times) [12,13]. Briefly, by considering the all advantages points previously mentioned above, ZnO/TiO2 structure has been attended as the proper modified photoanode in DSSCs technology. Many studies have reported the electrode modification via ZnO nanostructures to decrease recombination reaction at the interface between the FTO and TiO2 nanostructures. The whole recombination reactions occurred in a ZnO/TiO2 photoanode through an FTO/electrolyte interface is indicated schematically in Fig. 1.

Several methods such as hydrothermal [14,15], spin coating [16,17], spray pyrolysis [18], sol-gel [19], dip coating [20], chemical bath deposition [21] and electrodepositing methods [22] were utilized by different researcher groups to prepare ZnO/TiO2 photoanode. Despite the simplicity and cost effective manner of these methods, the tedious, time-consuming and not efficient effect of their processes led to development of new fabrication methods. Recently, high technology modern deposition methods such as atomic layer and pulse laser deposition [23,24], magnetron sputtering and chemical vapor deposition methods [25,26] were developed to fabricate the ZnO nanolayer. The precise control of the thickness deposition makes them applicable, especially in the academic field.

In our group, fabrication of ZnO thin films via sputtering technique has been considered as an important issue [27]. In this study, a ZnO thin film consisting of nano-networks were deposited onto (FTO) substrates by DC sputter deposition at different sputtering deposition times (tdep) from 15 to 600 sec. X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM) experiments were carried out to investigate the structure and particle sizes of the prepared ZnO thin film. Furthermore, the electrochemical impedance spectroscopy (EIS), as an informative technique and cyclic voltammetry (CV) method, were employed to investigate the photoelectrode interface properties [28,29]. The thickness effect of the ZnO thin films on their function as either electron trap states or barrier layers was investigated via determination of the effective electron lifetime (τeff), the density of electron in conduction band (ns), charge transfer (Rct) and charge transport resistances (Rt) values and discussed.

Section snippets

Materials and apparatus

All chemicals were of analytical grade and were used without further purification. TiCl4 solution (Aldrich, 98%), and P-25 TiO2 powder containing anatase crystalline phase, were purchased from standard sources. N719 dye and FTO (fluorinated tin oxide) glass sheets with 15 Ω cm−2 resistances were obtained from Solaronix S.A. Co. The electrolyte solution in acetonitrile (Iodolyte ELT-ACN-I Sharif Solar, Iran) was purchased from Sharif Solar Co. Furthermore, DC magnetron sputtering system

Results and discussion

In this study, preparation of the ZnO thin film as an efficient alternative for TiCl4 pre-treatment in fabrication of the conventional TiO2-based DSSCs was studied. DC magnetron sputtering as an advanced coating technology with higher accuracy was considered to provide the proper ZnO blocking layer on FTO substrates. The basic principles of DC magnetron sputtering have been reported elsewhere [[35], [36], [37]]. In reactive sputtering, the deposited thin film is formed by chemical reaction

Conclusion

In summary, this research presented an advanced coating technique with higher accuracy to fabricate an excellent uniform ZnO thin film to improve the performance of TiO2 NPs-based DSSCs. The prepared ZnO blocking layer on the FTO could provide an alternative of TiCl4 pre-treatment to eliminate the tedious and hazards conditions of the TiCl4 pre-treatment process. Since, the engineering of the fabricated ZnO layers is a critical point to act as barrier layer, various tdep were studied to obtain

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

S. A. Mozaffari acknowledges the support rendered by the Iranian Research Organization for Science and Technology (IROST), and Iran Nanotechnology Initiative Council (INIC), Sharif solar company and Nanostructured Coating Co. for this research.

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