Ligand field states and defect levels synergism: A close look at the band alignment of 4T1‑Mn-CdS/Bi2S3-co-sensitized photoanodes

Abstract

Low-band gap chalcogenides such as Bi2S3 (<1.7 eV) have been widely used in optoelectronic devices such as quantum dot sensitized solar cells (QDSSCs), due to their high sunlight harvesting capability, absorbing low-energy photons close to the IR region. Nonetheless, Bi2S3 offers a poor band alignment with large-band gap semiconductors such as TiO2, achieving low photoconversion efficiencies. Accordingly, we studied how the presence of both Mn2+ 4T1 ligand field electronic states and structural defects as Cd–Cd energy levels produced during Mn-CdS synthesis influenced on the band structure and thereby, the charge carrier transport into co-sensitized boron, nitrogen and fluorine-co-doped TiO2 nanotubes (X–Mn–Y–CdS–Bi2S3). Carrier transfer pathways provided by both Cd–Cd defects and Mn2+ 4T1 states allowed to obtain a suitable 0.7–Mn–4–CdS–2–Bi2S3 based electrode, with a narrowed band gap of 2.16 eV and an appropriate II-type heterostructure. These features improved both the carrier separation and mobility from Mn-CdS/Bi2S3 interface to co-doped nanotubes. Additionally, the electron lifetime into the composite photoanode was 10 times higher compared with a Mn2+-absent 4–CdS–2–Bi2S3 material. Hence, the synergistic behavior between structural defects and ligand field electronic states explained here offers an insight for establishing adequate heterostructures to be useful in QDSSCs.