Promoting Hot Carrier Extraction in Zn-Ag-In-Te Nanocrystals By Irradiation of High-Energy Light
Authors: Kameyama Tatsuya , Sugiura Kota , Kuwabata Susumu , Okuhata Tomoki , Tamai Naoto , Torimoto Tsukasa 
Published: December 2020

Colloidal semiconductor nanocrystals (NCs), so-called quantum dots, offer attractive advantages such as low cost, large-scale solution processing, and tunable physicochemical properties depending on their size. Due to these advantages, it is expected to be used for highly efficient and low-cost energy conversion systems such as solar cells and photocatalysts. Since the relaxation rates of carriers in highly excited states are remarkably reduced by the discrete energy levels of semiconductor NCs, unique photoresponses of semiconductor NCs are induced by the absorption of photons with energy higher than the energy gap (Eg) of NCs. It has been reported that by extending the relaxation time of hot carriers, it is 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 NCs, 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. Recently, we prepared ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1-x)Te2: ZAITe) NCs with a rod shape, the Eg of which was tunable in near-IR region from 1.20 to 1.60 eV with an increase in Zn content.1 These NCs are expected to act as near-IR photosensitizers and enable the use of NC-generated hot carriers. However, their photoelectrochemical properties and hot carrier behavior have not been investigated to date. In this study, we clarified the behavior of hot carrier in ZAITe NCs by measurering the photoelectrochemical properties of ZAITe NC-immobilized photoelectrodes.2 Furthermore, we prepared ZAITe NC-Methylviologen (MV2+) complexes to evaluate the dynamics of hot electron transfer from ZAITe NCs to MV2+ under the excitation of lights with various photon energies3.
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 photoelectrochemical behavior of ZAITe NCs immobilized on ITO electrode with thickness of less than one monoparticle, resembling a p-type semiconductor photoresponse, was remarkably different from that of thicker 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 dependence of photocurrent-potential curves on the energy of irradiaited photons for thin ZAITe NC films were investivated. The magnitude of the cathodic photocurrent was increased with a negative shift of the applied pototential, regardless of the energy of excitation photons. 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. In contrast, 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. we conclude that excitation with photons of > ca. 2.5 eV enabled direct injection of hot holes from ZAITe NCs into ITO electrodes, resulting in enhancement of the photocurrent generetion efficiency of ZAITe NCs.
By using transient absorption spectroscopy, the migration of hot carriers in NCs can be directly observed, enabling better clarification of the photochemical properties of ZAITe NCs. Therefore, we also investivated the hot electron transfer from ZAITe NCs to MV2+ by masureing transient absorption spectra of the complexes. The details of this result will be discussed in the presentation.

Reference

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Kameyama, et al., ACS Adv., 2020, 10, 16361.