Bismuth oxysulfide film electrodes with giant incident photon-to-current conversion efficiency: the dynamics of properties with deposition time

Abstract

It was demonstrated in our previous work that the photoelectrochemical (PEC) reduction processes occur with a giant incident photon-to-current conversion efficiency (IPCE ≫ 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.

Bismuth Oxysulfide Photoelectrodes with Giant Incident Photon‐to‐Current Conversion Efficiency: Chemical Stability in Aqueous Solutions

“…In such conditions, the BOS film gradually dissolves, and its thickness decreases. As it was demonstrated earlier, the IPCE value depends on a number of factors including film thickness . The change of the film thickness during the cathodic polarization curve record could be responsible for the appearance of the maximum at IPCE– E dependence (Figure b).…”

“…The chemically deposited BOS films are characterized by a very low lifetime of non‐equilibrium charge carriers. These values are in the range of a few tens of picoseconds depending on the film thickness, and the charge carriers diffusion coefficient is below 1 cm 2 s −1 . It leads to a negligible contribution of diffusion in a charge transfer across the films evidencing that this process is determined mainly by drift.…”

“…According to X‐ray photoelectron spectroscopy, the BOS films contain bismuth and sulfur atoms in the oxidation states +3 and −2, respectively. The giant IPCE effect is observed for the BOS films with a wide Bi/S atomic ratio: from 1.1 to 2.6 …”

“…Nowadays, there are a number of known bismuth oxysulfide compounds: Bi 2 O 2 S, Bi 2 OS 2 , Bi 3 O 2 S 3 ,, Bi 4 O 4 S 3 ,, Bi 9 O 7.5 S 6 , Bi 10 O 6 S 9 . These materials have a layered structure and are characterized by the pronounced anisotropy of electronic properties . Bi 3 O 2 S 3 and Bi 4 O 4 S 3 are of great interest for superconductive materials design ,.…”

“…It leads to a negligible contribution of diffusion in a charge transfer across the films evidencing that this process is determined mainly by drift. The main condition for the giant IPCE effect observation is the photogeneration of charge carriers over the entire film thickness . Therefore, the increase of the BOS film thickness at a fixed wavelength of monochromatic light, as well as the shortening of wavelength at a fixed thickness result in a reduction of giant IPCE …”