Electrodeposited nanoporous ZnO films exhibiting enhanced performance in dye-sensitized solar cells
Introduction
Dye-sensitized solar cells (DSSCs) based on porous thin film of wide bandgap oxide semiconductor modified by dye molecules have received considerable attention as a cost-effective alternative to conventional solar cells [1], [2], [3]. One advantage of DSSCs is the flexibility in the choice of material, in which all included components can be varied and their properties may be fine-tuned. Various combinations of dyes and semiconductor materials (such as TiO2, SnO2 and ZnO) have been studied in an attempt to improve the cell performance [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Due to the novelty of the nanostructured semiconductor materials, it is essential to compare different materials for a fundamental understanding of nanostructured systems. Even for a certain semiconductor material, its band gap, band gap position, surface structure, particle size and shape, doping density, porosity and film thickness should be considered for design of high efficient solar cells.
ZnO is one of the potential semiconductor materials in DSSCs due to its stability against photocorrosion and photochemical properties similar to TiO2. Dye-sensitized single-crystal ZnO electrodes were studied by Gerischer and Tributsch as early as 1969 [5], [6], [7]. Later, DSSCs based on the nanostructured ZnO films were extensively investigated [8], [9], [10], [11], [12], [13], [14]. Recently, ZnO nanowire-based dye-sensitized solar cell was reported [15], [16]. However, the energy conversion efficiencies of ZnO film-based DSSCs are relatively low (<3%) except that Keis et al. [12], [13] obtained the energy conversion efficiency of 5% under low intensity illumination (10 mW/cm2). The reason for low efficiency is that ZnO grain size is too big and/or the efficient effective surface area is too low. It is well known that nanoporous film like TiO2 film reported by Grätzel and co-workers [2], [3] can lead to very high solar-to-electric energy conversion efficiency (above 10%). Therefore, the preparation of nanoporous ZnO film is a key to improve the performance of ZnO-based DSSCs. ZnO thin films have been prepared by a wide variety of techniques such as pulsed laser deposition [17], sputtering [18], and electrodeposition [19], [20], [21], [22], [23], [24], [25], [26]. In particular, the electrodeposition method has advantages over other processes because of its simplicity, low equipment cost and the possibility in making large area thin films. Yoshida et al. [24] reported the cathodic electrodeposition of ZnO/dye (eosin Y) hybrid thin films and their photoelectrochemical performance. The surface morphologies of the electrodeposited ZnO films can be varied by the addition of surfactants such as ethylenediamine (EDA) [25] and sodium laurylsulfate [26]. In this paper, we investigated the electrodeposition of ZnO film assisted by polyvinylpyrrolidone (PVP) surfactant and obtained porous ZnO films with grain sizes between 20 and 40 nm for the first time. The feasibility of employing such ZnO thin films as photoelectrode materials for high efficiency DSSCs was studied.
Section snippets
Electrodeposition of ZnO films
The electrodeposition procedure was carried out in the electrolyte containing 0.1 M Zn(NO3)2 (AR, Beijing Chemical Factory, China) and 0.1 M KNO3 (AR, Beijing Chemical Factory, China) solution with the addition of 0–8 g/L PVP (K30, AR, MW ≈ 40000, Shanghai Bio Life Science & Technology Co.). The bath temperature was controlled at 60 °C and the electrolyte was stirred continuously using a magnetic stirrer. The transparent conducting optically glass (TCO glass: fluorine doped SnO2 coated glass, 10–15
Results and discussion
The general scheme of cathodic electrodeposition of ZnO thin films from nitrate solution is found elsewhere [19], [22], [23]. The preparation of ZnO thin films is thought to proceed via the reduction of nitrate ions according to the Eq. (1). The electrochemically generated hydroxide ions then chemically react with Zn2+ ions in the solution to form Zn(OH)2 at the cathode. Subsequently, Zn(OH)2 is spontaneously dehydrated into ZnO (Eq. (2)).NO3− + H2O + 2e− → NO2− + 2OH−Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2O
The
Conclusion
We have obtained nanoporous ZnO films with grain size of 20–40 nm by cathodic electrodeposition from an aqueous zinc nitrate solution containing PVP surfactant under potentiostatic condition. These films are used in DSSCs devices and the thicker film lead to the better cell performance. The cell based on 8 μm thick nanoporous ZnO films on a 200 nm thick compact nanocrystalline ZnO film gives Voc = 0.605 V, Jsc = 6.98 mA/cm2, ff = 0.638, yielding a conversion efficiency of η = 5.08%. This conversion
Acknowledgements
This work was supported by the Project-sponsored by SRF for ROCS, SEM, China and Natural Science Foundation of Hubei Provience, China (2005ABA039).
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