Bi 13 S 18 X 2 -Based Solar Cells ( X = Cl, Br, I): Photoelectric Behavior and Photovoltaic Performance

Bismuth chalcohalides of ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_{2}$ (X = $\mathrm{Cl},\mathrm{Br},\mathrm{I}$) are solvothermally synthesized and systematically characterized as a light absorber for photovoltaics. Rietveld refinements reveal that ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_{2}$ compounds adopt the same hexagonal structure with space group P3 and a chemical composition of $({\mathrm{Bi}}^{3+}{)}_{12}({\mathrm{Bi}}_{2}^{4+}{)}_{0.5}({\mathrm{S}}^{2\ensuremath{-}}{)}_{18}({X}^{\ensuremath{-}}{)}_{2}$, where a triangular tunnel is constructed from [${\mathrm{Bi}}_{4}{\mathrm{S}}_{6}$]-ribbon spokes in the c-axial direction, ${X}^{\ensuremath{-}}$ is located in the tunnel sites, and Bi(II) dimers of (${\mathrm{Bi}}_{2}^{4+}$) are located at the hexagonal center. The photoelectric behavior is investigated using ultraviolet-to-visible-to-near-infrared (UV-vis-NIR) absorption spectroscopy, ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, Mott-Schottky plots, and conductivity measurements. The ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_{2}$ compounds are indirect n-type semiconductors with narrow band gaps of 0.76, 0.80, and 0.81 eV for ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Cl}}_{2}$, ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Br}}_{2}$, and ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{I}}_{2}$, respectively, and exhibit strong light absorbance over a wide wavelength range from UV to NIR. A facile physical vapor deposition approach is developed for the fabrication of a homogeneous ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_{2}$ thin film to study the photovoltaic performances. Solar cells with the architecture fluorine-doped tin oxide/${\mathrm{Ti}\mathrm{O}}_{2}$/${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{X}_{2}$/(${\mathrm{I}}_{3}^{\ensuremath{-}}/{\mathrm{I}}^{\ensuremath{-}}$ redox couple)/$\mathrm{Pt}$ demonstrate power conversion efficiencies of 0.91, 1.12, and 0.75% for ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Cl}}_{2}$, ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Br}}_{2}$, and ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{I}}_{2}$, respectively. The highest cell performance of ${\mathrm{Bi}}_{13}{\mathrm{S}}_{18}{\mathrm{Br}}_{2}$ can be attributed to its higher light-absorption ability.