(Invited) Inorganic Solid-State Solar Cells Using Plasmon-Induced Charge Separation
Authors: Oshikiri Tomoya , Shi Xu , Ueno Kosei , Misawa Hiroaki 
Published: February 2020

[Introduction]
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.

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