In situ photocurrent spectra measurements during growth of three-dye-stacked structures by the liquid-phase molecular layer deposition

Abstract

Three-dye-stacked structures were grown on ZnO surfaces by the liquid-phase molecular layer deposition (LP-MLD) using electron-accepting p-type dyes of rose bengal (RB) and eosine (EO), and an electron-donating n-type dye of crystal violet (CV) as source molecules. It was found that the dye adsorption strength is strong for combinations of p-n, and weak for combinations of p-p and n-n, which satisfies requirements for the source molecules in LP-MLD when dyes are provided with a sequence of p-n-p-n- on n-type ZnO. Surface potential and photoluminescence measurements revealed that monomolecular-step growth is achieved in LP-MLD for growth of the three-dye-stacked structure of [ZnO/RB/CV/EO]. To evaluate the sensitization effects of multi-dye-stacked structures, photocurrent spectra induced in the dye-sensitized ZnO layers were measured in situ in each growth step of the dye molecule adsorption. In step 1, p-type RB was adsorbed onto an n-type ZnO surface to induce a photocurrent spectrum in a range of 400–580 nm. In step 2, n-type CV was provided to connect it to p-type RB. Then, photocurrents attributed to CV were superposed to the photocurrent spectrum attributed to RB, raising photocurrents in the longer wavelength region. In step 3, p-type EO was provided to grow the three-dye-stacked structure of [ZnO/RB/CV/EO]. This structure enhanced the photocurrents and, at the same time, widened the photocurrent spectrum extending from 390 nm to 610 nm, which is regarded as a superposition of RB, CV, and EO spectra, suggesting that the multi-dye-stacked structure acts as a light-harvesting antenna to enhance the sensitization effect on ZnO.