Size- and Composition-Dependent Photoelectrochemical Properties of Cu-Zn-Sn-S Multinary Semiconductor Nanoparticles

Colloidal semiconductor nanoparticles have attracted great attention as a precursor (nanocrystal ink) to fabricate thin film solar cells or efficient quantum dot-based solar cells at low cost. So far various kinds of compound semiconductor nanoparticles have been prepared by a solution-based strategy. Among them, Cu₂ZnSnS₄(CZTS) seems to be one of most promising materials for practical photovoltaic application because of their low toxicity and high absorption coefficient. Since semiconductor nanoparticles exhibit size-dependent unique physicochemical properties, the determination of electronic energy structure is of considerable importance for CZTS nanoparticles to estimate the energy band diagram formed in the devices. We previously reported that CZTS nanoparticles of 5-6 nm in diameter had the potentials of the valence band edge (E(VB)) and the conduction band edge (E(CB)) at +0.3 and −1.2 V vs. Ag/AgCl, respectively. However, there has been little investigation of the size dependence of energy levels. In this study, we synthesized CZTS nanoparticles with average diameters ranging from 2.8 to 5.2 nm via a solution-based approach and then experimentally clarified the size dependence of their electronic energy structure [1].

CZTS nanoparticles were prepared by thermal decomposition of corresponding metal diethyldithiocarbamate complexes in 1-dodecanethiol (DDT) with addition of a small amount of oleylamine (OLA) (0.13-2.0 mmol) at 120-280 ^oC for 30 min under an N₂atmosphere. Total volume of DDT and OLA was set to 3.0 cm3. To control particle size, the amount of OLA added and/or the reaction temperature were systematically varied.

TEM measurement revealed that spherical CZTS particles were formed and their average diameter was enlarged with an increase in the amount of OLA added or with elevation of the reaction temperature. Figure 1a shows the absorption spectra of CZTS nanoparticles with various diameters. The onset wavelength was blue-shifted with a decrease in particle size due to the quantum size effect. The Eg of CZTS particles was determined from Tauc plots. CZTS particles with diameters larger than ca. 5 nm had Eg similar to that of the bulk value (1.45 eV), while Eg of particles with diameters smaller than ca. 5 nm increased rapidly with decrease in particle size. This behavior agreed with that theoretically expected: the quantum size effect is remarkably observed for particles with diameters smaller than twice the Bohr radius (a_B= 2.5-3.3 nm for CZTS crystal). The chemical composition of the resulting particles was almost constant regardless of particle size, so the change in Eg originated from the quantum size effect but not from change in the chemical composition of CZTS nanoparticles.

Thus-obtained CZTS nanoparticles were layer-by-layer-deposited on an ITO electrode using 1,2-ethanedithiol as cross-linking agent. The photoelectrochemical properties of the thus-obtained nanoparticulate film electrodes were measured in an aqueous solution containing Eu²⁺as an electron scavenger.

A cathodic photocurrent was observed with light irradiation (λ > 350 nm) in each electrode, indicating that the immobilized CZTS nanoparticles behaved as a p-type semiconductor regardless of particle size. Action spectra of the cathodic photocurrent agreed well with the absorption spectra of the corresponding nanoparticles. Thus, it can be concluded that the photocurrent was produced by photoexcitation of CZTS particles on the ITO electrodes. In many instances of p-type semiconductors, the onset potentials of the cathodic photocurrent could be regarded as E(VB) and then the potential of E(CB) were calculated by subtracting Eg from E(VB). Figure 1b shows the potentials of E(VB) and E(CB) of CZTS nanoparticles as a function of average diameter. The potential of E(VB) shifted positively from +0.20 to +0.38 V vs. Ag/AgCl and E(CB) shifted negatively from −1.47 to −1.81 V vs. Ag/AgCl with decrease in the size of CZTS particles from 4.9 to 2.8 nm, indicating that the quantum confinement of the charge carriers changed the electronic energy structure of CZTS nanoparticles, the degree being remarkable with a decrease in particle size.

In conclusion, we clarified the electronic energy structure of CZTS nanoparticles depending on their size. This requires precise size control of the nanoparticles for their application to photovoltaic devices. The findings in the present study will be important for investigating and designing the energy band diagram in quantum dot-based solar cells fabricated with CZTS nanoparticles. 

Reference [1] H. Nishi, et al., Phys. Chem. Chem. Phys., in press, (2013). DOI:10.1039/C3CP53946F.