Sensitization of ZnO in Stacked Structures Containing Multiple Dyes Grown Using Liquid Phase Molecular Layer Deposition
Two-dye sensitization can be applied to dye-sensitized solar cells as shown in Fig. 1. In the n/p/n structure of n-type ZnO/p-type dye/n-type dye, electrons excited in the p-type dye are directly injected into ZnO, and electrons excited in the n-type dye are also injected into ZnO through the p-type dye. Consequently, the photocurrent spectrum arising from the p-type dye and that of the n-type dye can be superimposed to widen the photocurrent spectra.
P-type dyes such as fluorescein (FL), eosine (EO) and rose bengal (RB), which can accept electrons, and n-type dyes such as crystal violet (CV) and brilliant green (BG), which can donate electrons,2 have absorption peaks at 510, 540, 570, 600, 660 nm, respectively. By stacking these five dyes, the photocurrent spectra can be widened, covering a wavelength range from ~450 nm to 700 nm. However, to date, sensitization has been carried out by combining only two dyes, limiting the degree of the spectral widening.
In the present work, stacked structures containing three or four kinds of dyes were constructed for further spectral widening. The structures were fabricated using liquid-phase molecular layer deposition (LP-MLD)3-5 using the electrostatic force between molecules. The p-n attraction and the p-p and n-n repulsion enable self-limiting growth to construct the n/p/n structures with strong connections.
Fig. 2 shows an example of the LP-MLD process for growth of a four-dye stacked structure on ZnO. In step 1, p-type dyes of p1 and p2 are deposited on a ZnO surface to form a monomolecular layer of p1+p2. Once the surface is covered with p1 and p2, the dye molecule deposition is automatically terminated because p-type dye molecules cannot be connected to other p-type dye molecules. After removing p1 and p2, n1 and n2 are deposited in step 2 to connect them to p1 or p2, resulting in four types of stacked structures: p1-n1, p1-n2, p2-n1, and p2-n2.
Fig. 3(a) shows a schematic illustration of an energy level scheme for the sensitization of ZnO using the four-dye containing stacked structure. This structure resulted in a broad photocurrent spectrum of a superposition of photocurrents generated by the four kinds of dyes. The spectrum is shown in Fig. 3(b).
Photocurrents were measured by applying a 3 V potential across slit-type ITO electrodes with a slit gap of 60 μm on glass substrates. Approximately ~5-μm-thick ZnO powder layers were formed on the substrates.
Fig. 4(a) shows the photoluminescence (PL) spectrum of ZnO powder layers with the added dyes. The two-dye stacked structure of ZnO/RB/CV exhibits a broad PL spectrum compared with the single dye structure of ZnO/RB. The four-dye stacked structure of ZnO/RB+EO/CV+BG exhibits a broader spectrum than ZnO/RB/CV as expected.
Fig. 4(b) shows the photocurrent spectra. In ZnO/EO, photocurrents were generated between ~450 nm and ~550 nm. The ZnO/EO/CV+BG stack was prepared after depositing EO on ZnO in step 1, and then CV and BG in step 2. This stack had a broad photocurrent spectrum extending to the long wavelength region beyond 600 nm and covered almost the whole visible region from 450 nm to 700 nm. The photocurrents generated in the wavelength region from ~550 nm to ~700 nm were attributed to CV and BG which have strong absorption in the long wavelength region. This result suggests that the three-dye stacked structure provides effective sensitization.
In the case of the two-dye stacked structure containing four kinds of dyes of ZnO/RB+EO/CV+BG, a strong sensitization was observed in the long wavelength region extending beyond 700 nm. However, the photocurrent decreased in the short wavelength region. The reason for this has not been determined, and is being investigated at present by considering the energy levels determined using photoemission yield spectroscopy in air (PYSA).6
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3 T. Yoshimura, H. Watanabe, and C. Yoshino, J. Electrochem. Soc., 58, 51(2011).
4 T. Yoshimura, Japanese Patent, Tokukai Hei3-60487 (1991) [in Japanese].
5 T. Yoshimura, Japanese Patent, Tokukai 2009-60487 (2009) [in Japanese].