A nanocomposite p-type semiconductor film for possible application in solar cells: Photo-electrochemical studies
Introduction
Fuel shortage and energy crisis are two challenging issues of this century [1]. Photon-to-electron conversion is a promising scientific route for solving these energy-related problems [2]. In this route, using sunlight and a proper semiconductor-based device, the energy of photons is directly converted into electricity or solar fuels [3], [4], [5]. In these photonic devices, by irradiation of photons, the electrons are pumped from the valence band (VB) to conduction band (CB) of the semiconductor material; see Fig. 1. The result of this photo-excitation process is the generation of excess mobile electrons and holes in CB and VB, respectively. From chemical viewpoint, these photo-generated electrons and holes can be employed as potential reducing and oxidizing agents for redox reactions of electroactive species present at the semiconductor | electrolyte interface (Fig. 1).
In the fabrication of photonic devices, depending on the type of semiconductor material being applied, its role (reductive or oxidative) becomes determined; e.g. TiO2 is an n-type semiconductor having mobile electrons in its CB and this compound is conventionally utilized in the fabrication of photoanodes of liquid junction solar cells (DSSC or QDSC). By contrast, p-type semiconductor materials such as Cu2ZnSnS4 (CZTS) and CuInS2 have mobile holes in their VB and they are mostly applied in the fabrication of thin-film photovoltaics as well as photocathodes [6], [7].
As implicitly mentioned above, for liquid junction semiconductor photoelectrode systems (also known as photo-electrochemical cells/reactors), three major applications are recognizable: (1) electricity generation through transport of photogenerated electrons (or holes) into external circuit of a solar cell, (2) fuel production by transferring the photogenerated electrons to their recipients (e.g. H+) at solution part of the interface, and (3) degradation of environmental pollutants (e.g. carcinogenic dyes) via advanced oxidation process upon the electrode surface, using the photogenerated holes/electrons. All these applications are technologically important and the investigations in the area of facile fabrication of semiconductor (simple or composite) energy materials will be therefore crucial for forthcoming photon-based green technologies.
In the present paper, using an interesting/facile electrochemical route, some non-precious metallic cations such as Cu2+, Zn2+ and Sn4+ are co-deposited on a fluorine doped tin oxide (FTO) glass and a p-type composite oxide photoelectrode is fabricated. Because of their non-toxicity and abundance in the earth crust, the mentioned elements have attracted considerable attention particularly in the field of thin-film solar cells [8], [9]. Song et al. [6] and Peter [10] recently reviewed some electrochemical [11], [12], [13], magnetron sputtering [14], spray-pyrolysis [15], [16], pulsed laser deposition [17], and sol–gel spin coating [18] techniques for the preparation of CZTS thin films. Wang et al. [19] reported that the cost of photoelectrode being fabricated by vacuum-based methods can be decreased through solution-based (non-vacuum) approaches. Kahraman et al. [20] developed a four-step route via successive ionic layer deposition and reaction (SILAR) of the precursor species on soda lime glass. A simpler approach based on electrochemical deposition of stacked Cu–Sn–Zn layers, was also reported by Lin et al. [13], and the quaternary kesterite CZTS film was synthesized through sulfurization process of the stacked metallic layers in a reactive sulfur-containing atmosphere.
In all above mentioned methods, the objective is the synthesis of single-phase stoichiometric CZTS thin film [6]. In our best knowledge, however, there is no report on facile fabrication of composite semiconductor photoelectrodes using these earth-abundant elements (Cu, Zn, and Sn). Concerning CZTS family of semiconductors, moreover, there is no published work on production of new kind of solar energy materials via isoelectronic replacement of sulfur with oxygen atoms [6], [10]. Beside these points, the fabrication of oxide semiconductor films is of great interest from stability viewpoint, due to strong covalent bond existing between oxygen and other constituting (metallic) atoms.
In this work, the photo-electrochemical response of the composite electrode as photocathode of liquid junction solar cells will be also examined in a sulfide/polysulfide redox solution, a medium being commonly utilized as the electrolyte of quantum dot solar cells [21], [22]. Since Ag usually enhances the activity of photocatalytic systems [5] –through the phenomenon known as Surface Plasmon Resonance (SPR) which increases the photon absorption and produces some hot (high energy) electrons [23], [24], the silver effect on photoelectrochemical response of the composite photoelectrode will be also taken into consideration in the final part of this work.
Section snippets
Photoelectrode fabrication
The semiconductor oxide film containing copper (II), zinc (II) and tin (IV) cations, was electrochemically deposited on a 1.5×1.5 cm fluorine-doped tin oxide (FTO) glass (SOLARONIX, the electrical resistance: 10 Ω/sq). Therefore, an aqueous solution containing 0.01 M Cu2+ (CuSO4·5H2O; Fluka), 0.005 M Zn2+ (ZnSO4·7H2O; Fluka), 0.01 M Sn4+ (SnCl4; Alfa Aesar) and 2.35 g/lit trisodium citrate (Na3C6H5O7·2H2O; Fluka) was first prepared. By applying a negative potential, chosen −1.05 VSCE [25];in this
Composite film and XRD evidence
XRD pattern of the composite film is plotted in Fig. 2. Comparing with patterns of SnO2 (01-077-0452), ZnSnO3 (00-028-1486), CuO (01-072-0629), Cu–Sn alloy (Cu81Sn22, 00-031-0486) and ZnO (98-011-5666), the figure indicates that the composite material synthesized here is composed of tin, zinc tin and zinc oxide compounds, and a small amount of CuO and Cu–Sn alloy is also co-deposited during the electrodeposition process.
Optical response and band gap determination
The optical response of the composite material was investigated and the
Conclusions
Facile fabrication of low-cost semiconductor thin-film electrodes and synthesis of new earth-abundant solar energy materials, are the demand of current photon-based technologies, including solar cells and photo-chemical or photo-electrochemical reactors. Concerning this objective, through a simple as well as versatile electrochemical route, a composite Cu/Zn/Sn oxide semiconductor photoelectrode was for the first time fabricated. The approach was on the base of simultaneous electro-deposition
Acknowledgments
We gratefully acknowledge the anonymous referees for their constructive comments to enhance the quality of this paper.
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