Substrate controls photovoltage, photocurrent and carrier separation in nanostructured Bi2S3 films

Abstract

Bi₂S₃ is a narrow bandgap (1.2 eV) semiconductor of interest for the construction of solar cells and photoelectrodes. While many researchers have reported on the use of Bi₂S₃ as a sensitizer for photoelectrodes, the photoelectrochemical (PEC) properties of pure Bi₂S₃ films are less documented. Here, phase pure Bi₂S₃ films (Bi/S elemental ratio of 0.66) were grown on FTO, ITO, Au, Mo substrates by electrochemical deposition of bismuth, followed by sulfurization in a single-zone tube furnace. The nanostructured Bi₂S₃ films are 5 ± 1 μm thick and crystallize in an orthorhombic Stibnite type structure. They have a bandgap of ∼1.24 eV based on UV-Vis diffuse reflectance and are n-type based on a negative surface photovoltage (SPV) signal. X-ray photoelectron spectroscopy (XPS) places the Fermi level at 0.95–0.91 eV above the valence band edge. According to X-ray diffraction the substrates have no influence on the crystallographic properties of the Bi₂S₃ films. However, the PEC properties of Bi₂S₃ films in 0.5 M Na₂S(aq) are sensitively controlled by the Bi₂S₃/substrate interface. Under 100 mW cm⁻² simulated solar (AM 1.5) illumination and 1.23 V_RHE applied bias, Bi₂S₃ photoelectrodes produce anodic photocurrents of 9.3 mA cm⁻², 6.1, 3.6 and 1.8 mA cm⁻² for FTO, Mo, Au and ITO substrates, respectively, and photovoltages of 0.9–0.25 V. For FTO, the photocurrent reported here is among the best reported for phase pure Bi₂S₃ photoanodes. The lower performance of the other substrates is due to a Schottky barrier at the Bi₂S₃/substrate interface, which retards electron transfer. These findings explain why FTO is a preferred substrate for Bi₂S₃ photoanodes and they highlight the importance of matching substrate workfunction to the Bi₂S₃ Fermi level for efficient majority carrier extraction. Lastly, the work demonstrates the use of electrochemical deposition combined with single-zone furnace-based sulfurization as a pathway to form high-quality Bi₂S₃ films for solar energy conversion.