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Highly ordered anodized Nb2O5 nanochannels for dye-sensitized solar cells

https://doi.org/10.1016/j.elecom.2013.12.011Get rights and content

Highlights

  • For the first time, highly ordered Nb2O5 nanochannels were used in DSSCs.

  • DSSCs with 10 μm films had the best performance compared with other film thicknesses.

  • Thicker films decreased the electron lifetime and the charge transfer resistance.

  • More embedded impurities in thicker films hindered the performance of the DSSCs.

Abstract

Highly ordered anodized Nb2O5 nanochannels are synthesized and utilized as the photoanodes for dye-sensitized solar cells (DSSCs). We characterize these DSSCs to determine the optimum photoanode thickness for the best cell performance. The samples with thicknesses from 5 to 25 μm are obtained in glycerol based electrolyte at 180 °C and their photoconversion properties are investigated utilizing various techniques including photocurrent–voltage characteristic, photovoltage decay and electrochemical impedance spectroscopy. Overall, the DSSC incorporating a 10 μm thick Nb2O5 photoanode shows the highest efficiency of 4.48%. We analyze the factors that limit the efficiency of DSSCs.

Introduction

Potentially, Nb2O5 is a suitable material for developing dye-sensitized solar cells (DSSCs) due to its relatively high conduction band edge, electron injection efficiency and chemical stability [1], [2], [3], [4]. So far, the best photoconversion efficiencies of Nb2O5 films have exceeded ~ 6% using hydrothermally grown nanorods at a thickness of ~ 11 μm [3]. However, the preparation of such films is time consuming (over 24 h) and involved toxic materials, which limits their applicability. As an alternative synthesis method of nanostructured Nb2O5, anodization has been recognized as a fast approach for forming highly ordered nanoporous Nb2O5 at relatively mild synthesis conditions. To achieve high conversion efficiencies, well-ordered, large surface-to-volume ratio, low embedded structural impurities and highly crystalline photoanodes are required. We have previously introduced an efficient anodization method to obtain well-ordered, highly porous and low embedded impurity Nb2O5 nanovein-networks for DSSCs [1], [2]. Despite the presence of such desirable characteristics, these DSSCs only showed a maximum conversion efficiency of 4.1% due to the lack of film integrity, which could not result in thicknesses exceeding 5 μm. An anodization method that can produce a higher level of order should solve this problem.

Recently, highly ordered Nb2O5 nanochannels with thicknesses of up to 25 μm has been demonstrated by anodizing Nb foil in a glycerol based electrolyte at elevated temperatures [5]. This kind of nanostructure can be potentially employed as a photoanode for DSSCs in which high dye absorption is expected due to a very large surface area. Therefore, in this work, we fabricated DSSCs based on highly ordered anodized Nb2O5 nanochannelled films with the thicknesses ranged from 5 to 25 μm. The DSSCs and their associated films were comprehensively characterized.

Section snippets

Material and methods

Niobium foil with dimensions of 1.2 cm × 1.5 cm (99.9% purity, Sigma Aldrich) was anodized at 15 V for 3 to 15 min to produce 5 to 25 μm Nb2O5 nanochannels. The anodization was conducted using the electrolyte containing 10 wt.% K2HPO4 (98% purity, Sigma Aldrich) in anhydrous glycerol (99% purity, Sigma Aldrich), at 180 °C as suggested previously [5]. The as-anodized samples were annealed in air at 450 °C for 30 min (ramp up/down of 2 °C/min). The Nb2O5 samples were then characterized using scanning

Results and discussions

The morphologies of the annealed Nb2O5 nanochannelled films were characterized using SEM. Fig. 1b represents a ∼ 10 μm thick nanochannelled film, which is formed after 7 min of anodization at 15 V. The top image of the oxide film shows a highly porous structure (inset in Fig. 1b). Furthermore, there are secondary sets of porous layers underneath the initial oxide layer, which consist of more organized nano-sized pores with average diameters ranging from 30 to 50 nm. The side walls are around 10 to 15

Conclusion

The performances of DSSCs based on different thicknesses of anodized Nb2O5 nanochannelled films were investigated. The results revealed that the DSSCs with 10 μm thick Nb2O5 films exhibited the highest efficiency of 4.48%. Further characterizations using photovoltage decay and EIS showed that increasing the photoanode thickness resulted in shorter electron lifetime and faster electron recombination. This is associated to the increase of the path length for the injected electron and free charge

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  • Sol-gel processed niobium oxide thin-film for a scaffold layer in perovskite solar cells

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    As the other candidate, niobium oxide (Nb2O5) is expected to exhibit greater blocking effect than TiO2 [33]. So far, Nb2O5 has been applied mainly in dye-sensitized solar cells, where the short-circuit photocurrent (Jsc) and open-circuit voltage (Voc) were improved by effective blocking of the recombination [33–42]. More recently, Nb2O5 film has started to be applied as a blocking layer in a PSC [31,43,44].

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