Energy landscape in silver-bismuth-iodide rudorffites: Combining scanning tunneling spectroscopy and Kelvin probe force microscopy

We report the energy landscape of an upcoming solar cell material series, namely, silver-bismuth-iodide rudorffites $({A}_m{B}_n{{X}_m}_+3n)$. We formed the compounds through compositional engineering and characterized them. We employed Kelvin probe force microscopy (KPFM) to derive the surface work function and thereby the Fermi energy; scanning tunneling spectroscopy (STS), on the other hand, provided conduction and valence band edges with respect to the Fermi energy of the rudorffites. By combining KPFM and STS studies, we obtained a composition-dependent tuning of the Fermi energy in rudorffite thin films, namely, ${\mathrm{Ag}}₃\mathrm{Bi}{\mathrm{I}}₆$, ${\mathrm{Ag}}₂\mathrm{Bi}{\mathrm{I}}₅$, $\mathrm{AgBi}{\mathrm{I}}₄$, $\mathrm{Ag}{\mathrm{Bi}}₂{\mathrm{I}}₇$, and $\mathrm{Ag}{\mathrm{Bi}}₃{\mathrm{I}}₁₀$ with a gradual change in electronic conductivity from $p$ to $n$ type. While such behavior has been correlated to point defects, which are possible to form in the different members of the rudorffite series, we more importantly show that a combination of KPFM and STS studies can provide the complete energy landscape of a semiconductor.