(Invited) Hot Hole Transfer from Zn-Ag-in-Te Nanocrystals Photo-Excited with High-Energy Photons
Authors: Kameyama Tatsuya , Sugiura Kota , Kuwabata Susumu , Okuhata Tomoki , Tamai Naoto , Torimoto Tsukasa 
Published: May 2020

Semiconductor nanocrystals (NCs), so-called quantum dots, exhibit unique size-dependent physicochemical properties different from bulk materials and have attracted intense attention for the application to solar light energy conversion systems. Since the relaxation rates of carriers in highly excited states are remarkably reduced by the discrete energy levels of semiconductor NCs due to the phonon bottleneck effect, unique photoresponses of semiconductor NCs are induced by the absorption of photons with energy higher than the energy gap (Eg) of NCs. For example, by extending the relaxation time of hot carriers, it was possible to transfer excess kinetic energy before releasing it as heat. Thus, next-generation solar cells of outstandingly high efficiency beyond the Shockley-Queisser limit can be developed if photogenerated carriers are harvested from a semiconductor NC without energy loss of incident photons.
I-III-VI2 semiconductor-based nanoparticles, such as CuInS2 or AgInS2 and their solid solution with ZnS, have been intensively studied as a visible-light-driven photocatalyst because of their excellent optical properties (e.g. direct band gaps in the visible-light region, and large absorption coefficients). Recently, we prepared ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1-x)Te2: ZAITe) NCs with a rod shape, the Eg of which was enlarged from 1.20 to 1.60 eV with an increase in Zn content.1 ZAITe NCs exhibited a narrow band-edge PL peak with high QYs, and the PL peak wavelength was tunable in the near-IR wavelength region depending on their energy gap (Eg). These NCs are expected to act as a photosensitizer for near-IR light, but their photoelectrochemical properties have not been investigated. In this study, we clarified the electronic energy structure of ZAITe NCs for utilization as a photosensitizer and then investigated the photoelectrochemical properties of ZAITe NC-immobilized photoelectrodes.2 Since photons in the visible light region have much larger energies than the Egs of ZAITe NCs, it is important to examine the influence of photogenerated carriers with excess energies on the photoelectrochemical properties of ZAITe NCs with the goal of application to photovoltaic devices.
ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. TEM observations revealed that rod-shaped NCs with width of ca. 4 nm and length of ca. 11 nm were formed regardless of chemical composition of particles, that is, the x value. We revealed the electronic energy structures of ZAITe NCs with the compositions of x= 1.0, 0.75, and 0.5, the Eg values of which were 1.20, 1.27, and 1.46 eV, respectively. The band-edge potentials of conduction band (ECB) and valence band (EVB) were negatively and positively shifted with an increase in the Eg of NCs, the degree being more remarkable for the change in ECB.
The photocurrent-potential curves of ZAITe NCs-immobilized on ITO electrode with thickness of less than one monoparticle layer resembled a p-type semiconductor photoresponse, but the behavior was remarkably different from those of ZAITe NC multilayer films: The onset of cathodic photocurrent was shifted more positively than that of EVB of ZAITe NCs and the photocurrent generation efficiency was significantly enlarged, when photons of energy higher than ca. 2.5 eV were irradiated. The photocurrent onset potential was also dependent on the energy of irradiaited photons for thin ZAITe NC films. Irradiation of photons with lower energy than 2.43 eV gave the onset potential at around 0 V vs. Ag/AgCl, being comparable to the EVB of ZAITe NCs with x = 0.75, -0.05 V vs. Ag/AgCl. On the other hand, the increases of photon energy to 2.95 and 3.40 eV gave more positive onset potentials of 0.1 and 0.3 V vs. Ag/AgCl, respectively. Thus we conclude that the excitation with photons of > ca. 2.5 eV enabled the direct injection of hot holes from ZAITe NCs into ITO electrodes, resulting in enhancement of the photocurrent generetion efficiency of ZAITe NCs.

Reference

1. T. Kameyama, et al., J. Mater. Chem. C, 2018, 6, 2034.
2. T. Kameyama, et al., ChemNanoMat, 2019, 5, 1028.