RESUMEN
It was demonstrated in our previous work that the photoelectrochemical (PEC) reduction processes occur with a giant incident photon-to-current conversion efficiency (IPCE â« 100%) at bismuth oxysulfide (BOS) semiconductor films in aqueous solutions containing acceptors of photoelectrons ([Fe(CN)6]3-). The anomalously high IPCE was related to the photoconductivity of the semiconductor. In this work, we analyze the dynamics of the chemical and phase composition of BOS films with variation of their deposition time, as well as the dependence of photocurrent on the film thickness and wavelength of the incident light. We demonstrate that in the case of illumination with a short-wavelength light (λ = 465 nm), the photocurrent is reduced down to a complete disappearance with an increase in the film thickness in the range of 0.3-1.3 µm, and for a fixed thickness of the bismuth oxysulfide film, the photocurrent decreases with the reduction of the wavelength indicating that photogeneration of the charge carriers over the entire thickness of the film is necessary for the giant IPCE effect. Using the light induced transient grating (LITG) method, the lifetime of the charge carriers (τ) was determined in the range of 25-80 ps depending on the film thickness, whereas the diffusion coefficient (D) does not exceed 1 cm2 s-1 meaning that the charge transport across the films is determined only by drift.
RESUMEN
Nanostructured layered bismuth oxysulfide films synthesized by chemical bath deposition reveal a giant incident photon-to-current conversion efficiency (IPCE). This study shows that surprisingly for the cathodic photocurrent in the photoreduction process, the IPCE reaches ≈2500% in aqueous solutions containing [Fe(CN)6 ]3- . The giant IPCE is observed starting from a certain minimal oxidizer concentration (c > 10-3 m for [Fe(CN)6 ]3- ) and decreases nonlinearly with an increase of illumination intensity. Giant IPCE is determined by the decrease in resistivity of the bismuth oxysulfide film under illumination with photoconductivity gain, which provides the possibility of charge carriers from an external circuit to participate in the photoreduction process. Giant IPCE is observed not only in [Fe(CN)6 ]3- solutions, but also in electrolytes containing other photoelectron acceptors: Fe3+ , I3- , quinone, H2 O2 . In all, solution-processed layered bismuth oxysulfide films offer large-area coverage, nontoxicity, low cost, and compatibility with a wide range of substrates. Abnormally high photoelectrochemical activity, as well as a band gap energy value favorable for efficient conversion of solar light (1.38 eV, direct optical transitions), proves the potential of bismuth oxysulfide photoelectrodes for a new generation of high-performance photoconverters.