The construction of tandem dye-sensitized solar cells from chemically-derived nanoporous photoelectrodes
Graphical abstract
Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable interest as a low cost and renewable means of harnessing solar energy [1], [2]. For this type of device to be competitive to other solar cells, many attempts have been made in terms of enhancing the conversion efficiency [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Recently, the concept of ‘tandem-DSSC’ has started evolving where both working and counter electrodes in the DSSC are photoactive [13], [14], [15], [16]. The tandem-DSSC consists of an n-type photoanode linked to a p-type photocathode via the electrolyte, and this configuration can offer improved open-circuit voltage compared to the single-junction (half cell) DSSCs.
For the fabrication of photoelectrodes of DSSCs, doctor-blade method [17] and screen printing [18] of hydrothermally-synthesized TiO2 nanoparticles have been widely used. As another candidate for the photoelectrodes, sputtering deposition has the merits of high uniformity, large-area deposition, and enhanced reproducibility. Therefore, sputtering has been extensively utilized in industrial fields to obtain various coating layers and high-quality functional films, which are typically considered as advantages of dry process sputter deposition [19], [20], [21], [22], [23], [24], [25]. However, electrodes grown by sputtering cannot adsorb a large number of dye molecules, because of the lower specific surface area resulting from the compact nanostructures, compared to the nanoparticle-based films [23], [24]. In order to modify the inefficient surface area, we adopted a selective etching process in sputter-deposited films [26], [27].
In this study, both nanoporous photocathode (NiO) and photoanode (TiO2) were successfully fabricated by simple sputtering deposition and selective etching. Through combining these two electrodes with an intermediate electrolyte layer, we originally suggest sputter-deposited tandem-DSSCs. The photovoltaic properties of tandem-DSSCs were analyzed under various fluxes of the incident light, by comparing the cell parameters in both the experimental and ideal cases. Also, the recombination resistance was investigated with the modified one-diode model to understand the working principles of the tandem-DSSCs. Furthermore, impedance spectra of the tandem cell was correlated with the parameters from each of the n-type or p-type DSSC, to confirm the design of the fabricated tandem structures.
Section snippets
Experimental procedure
To fabricate nanoporous NiO film, Ni–Al alloy films were deposited on fluorine-doped tin oxide (FTO, Pilkington, Japan) substrates by rf magnetron sputtering using Ni and Al targets. Sputtering was performed in an Ar atmosphere with a working pressure of 10 mTorr at room temperature. The optimized sputtering power condition was found to be 40 W for Ni and 160 W for Al. To remove Al from Ni–Al alloy films, the as-deposited Ni–Al alloy films were first immersed in 0.1 M sodium hydroxide (NaOH,
Results and discussion
After the Ni–Al-alloy films being immersed in NaOH, the Ni–Al phase disappeared (diffraction in Fig. S1), indicating a complete removal of Al, and subsequent heat treatment of the as-dealloyed sample led to the crystalline NiO phase. The film morphologies for the before/after dealloying were confirmed by FE-SEM (Fig. S2). The dealloyed-NiO film clearly shows porous characteristics compared to the compact Ni–Al alloy. The porous characteristics of the NiO films were also evaluated by small-angle
Conclusions
The nanoporous electrodes were rendered by a simple chemical dealloying, and they were combined to construct a p-n junction tandem cell. The analysis of recombination resistance from the ideal one-diode model for the flux-dependent J–V curves, together with modeling the impedance spectra provided clues on the principle of the tandem-cell operation. The optimization of tandem-DSSC is a breakthrough challenge to effectively combine two photoactive working and counter electrodes.
Acknowledgments
This research was supported by the National Research Foundation of Korea (NRF): 2013R1A1A2065793 and 2010-0029065.
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