Stability Improvement of Copper Nanoparticles for All-Solid-State Photovoltaic Cells Based on Plasmon Induced Charge Separation with p-Type Semiconductors
Authors: Nakano Yurika 
Published: December 2020

[Introduction] When the resonant light irradiates to a metal nanoparticle, enhanced electric fields on the surface of the particle are generated based on the collective oscillation of the free electrons. This phenomenon is called localized surface plasman resonance (LSPR). Furthermore, when the resonant light irradiates to a metal nanoparticle combined with an n-type semiconductor, the electron is transferred from the metal nanoparticle to the semiconductor. This phenomenon, which is called plasmon-induced charge separation (PICS), can be applied to photocatalysts and solar cells[1].
The most popular plasmonic metal nanoparticles consist of gold or silver (AuNPs and AgNPs), because they are highly stable for thermal and chemical treatments. Furthermore, their resonant wavelengths are generally in visible to near-infrared region. On the other hand, copper nanoparticles (CuNPs) are easily oxidized in the ambient air resulting in disappearance of the properties based on LSPR. Therefore, CuNPs have not been actively studied even their resonant wavelength exhibits in similar region to AuNPs.
Even more importantly, an n-type semiconductor, such as TiO2 and ZnO2, was mostly employed for combination with metal nanoparticles in PICS system so far. In the case of n-type PICS, reverse electron transfer might cause the low efficiency of PICS, because metal nanoparticles can function as an electron acceptor. Furthermore, positive charge generated in metal nanoparticles often results in their oxidative dissolving. To solve these problems, our group are focusing PICS between a metal nanoparticle and a p-type semiconductor, because p-type PICS was expected to occur charge separation opposite direction.
In this study, we developed a PICS system consisting of CuNPs, which are less expensive than AuNPs and AgNPs, and a p-type semiconductor. Furthermore, we improved the stabilities of CuNPs by introducing an Al2O3 nanomask[2] in order to realize simple all-solid-state photovoltaic cells based on p-type PICS without any electron or hole transport materials.

[Experimental] An Al2O3 nanomask was prepared by dip-coating method from a precursor solution containing a block copolymer, ethanol, tetrahydrofuran, NH3, and AlCl3·6H2O on an indium tin oxide (ITO)-coated glass substrate, followed by annealing at 500 oC for 1 h as reported before[2]. Then, CuNPs were electrodeposited through an Al2O3 nanomask on the ITO electrode. A NiO film was coated on the electrode by a spray pyrolysis method from an aqueous solution containing Ni(NO3)2 and LiNO3 at 350 °C, and then annealed at 500 °C for 2 h. As a counter electrode, gold film was sputtered on the NiO layer resulting in the all-solid-state ITO/Al2O3 nanomask/CuNP/NiO/Au cell. For a control experiment, an ITO/Al2O3 nanomask/NiO/Au cell (without CuNPs) was also prepared. The photoelectrochemical measurements were performed by irradiating to the cell with a solar simulator (100 mW · cm−
2) equipped with a UV (<400 nm) sharp cut filter.

[Results and Discussion]

It was reported that the thermal stabilities of AuNPs and AgNPs could be improved by introducing an Al2O3 nanomask, because the nanomask can prevent the nanoparticles from coalescing[2]. In a similar way, we tried to improve the thermal stability of CuNPs. The average size of CuNPs without an Al2O3 nanomask increased from ~83 to ~112 nm after annealing at 350 °C for 5 min (Fig. 1(a)). In contrast, the average sizes of CuNPs with the nanomask before and after annealing were almost same (from ~53 to ~56 nm) (Fig. 1(b)). Accordingly, we could improve the thermal stabilities of CuNPs. As a result, overcoating of the NiO film at 350 °C on the CuNPs became available. We confirmed the plasmonic absorption based on CuNPs was observed even after covered with the NiO film.
Then photoelectrochemical measurements of the ITO/Al2O3 nanomask/CuNP/NiO/Au cell was carried out. We observed photocurrents and photovoltages of the cell in the opposite direction to those of the all-solid state cells (ITO/Al2O3nanomask/AuNP or AgNP/TiO2/In) based on n-type PICS which were reported before[3]. The photovoltage of the prepared cell (with CuNPs) exhibited much more negative than that of the control cell (without CuNPs). These results suggested that the p-type PICS at the interface between CuNP/NiO can be observed.
In summary, we succeeded improvement of the thermal and chemical stabilities of CuNPs. Due to the high stable CuNPs, the all-solid-state ITO/Al2O3 nanomask/CuNP/NiO/Au cell could be fabricated. Photoelectrochemical measurements suggested that p-type PICS could proceed at the interface between CuNP/NiO. It will lead to designs of cupper-based plasmonic cells.
[1] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 2015, 127, 7632.
[2] Y. Takahashi, T. Tatsuma, Nanoscale
2010, 2, 1494.
[3] Y. Takahashi, T. Tatsuma, Appl. Phys. Lett.
2011, 99, 182110.

Figure 1