Metallic nanostructures which show localized surface plasmon resonances have received considerable attention as a light harvesting optical antenna for light-energy conversion systems such as solar cells as well as artificial photosynthesis [1-5]. Recently, we have successfully fabricated a plasmonic photoelectric conversion device using a gold nanostructured titanium dioxide photoelectrode with a nickel oxide (NiO) thin film as a hole transport layer [6]. However, the conversion efficiency was still low. To overcome this problem, we investigated the modulation of the diffusion potential of the junction to control the cell voltage and rectification characteristics. Yajima et al. reported that an interface dipole in an atomic scale strongly affects the rectification characteristics [7]. In this study, we fabricated plasmonic solar cells composed of n-strontium titanate (SrTiO₃), gold nanoparticles (Au-NPs) and p-NiO. Also, we inserted a unit cell (uc) of lanthanum aluminate (LaAlO₃) as an interface dipole between SrTiO₃ and NiO to modulate the diffusion potential.

[Experimental]

We prepared the plasmonic solar cells as the following procedure. At first, a 0.05wt% niobium-doped SrTiO₃ single crystal (100) was treated with HF and NH₄F to expose the TiO₂ surface of SrTiO₃ (t-SrTiO₃). Au-NPs were fabricated on the t-SrTiO₃ by the deposition of the gold thin film by a pulsed laser deposition (PLD) and subsequent annealing of the substrate at 800°C. After that, the following different cells were prepared. (i) NiO was deposited on Au-NPs/t-SrTiO₃ with a thickness of 100 nm by PLD (NiO/Au-NPs/t-SrTiO₃); (ii) 100 nm thick NiO was deposited after the deposition of 1uc of LaAlO₃ (NiO/LaAlO₃/Au-NPs/t-SrTiO₃); (iii) 100 nm thick NiO was deposited after the deposition of 1uc of SrO and 1uc of LaAlO₃ (NiO/LaAlO₃/SrO/Au-NPs/t-SrTiO₃). A gold thin film with a thickness of 4 nm was deposited by the helicon sputtering as an electrode. Photoelectric properties of the plasmonic solar cell were measured by the solar simulator (AM 1.5G).

[Results and Discussion]

First of all, we evaluated current-voltage characteristics of cells under the irradiation of AM 1.5G. The open circuit voltages were dramatically changed by the insert of the LaAlO₃ layer and showed large values in the order of (ii)>(i)>(iii). Additionally, the cell (ii) showed most clear rectification characteristics. Furthermore, the cell (ii) showed the largest solar energy conversion efficiency. These results indicate that the diffusion potential was increased in the case of cell (ii) and decreased in the case of (iii). It is considered that the difference is derived from the deposition order of LaAlO₃ layer. It was reported that LaAlO₃ was deposited in the order of LaO⁺ and AlO₂^- on the TiO₂ surface of SrTiO₃ [7]. In contrast, LaAlO₃ was deposited in the order of AlO₂^- and LaO⁺ on the SrO surface of SrTiO₃. As a result, the opposite dipole direction had an opposite effect on the diffusion potential.

We also measured action spectra of incident photon-to-current efficiency to evaluate the contribution of the plasmon to photoelectric conversion. As a result, all cells showed a visible light responsivity in the region of plasmon resonance, indicating plasmon-induced charge separation works well even in the presence of the interface dipole.

References

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3. Ueno, K., Misawa, H. NPG Asia Mater. 2013, 5, e61.

4. Zhong, Y., Ueno, K., Mori, Y., Shi, X., Oshikiri, T., Murakoshi, K., Inoue, H., Misawa, H. Angew. Chem. Int. Ed. 2014, 53, 10350−10354.

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