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 $\left(A_m{B}_n{X_m}_{+3n}\right)$. 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.

Figure 1

(a) Optical absorption spectra, (b) Tauc plots, and (c) $\frac{d{\left(\alpha h\nu \right)}^2}{d\left(h\nu \right)}$ vs $h\nu$ plots of $A_m{B}_n{X}_{m+3n}$ thin films. Composition of the compounds are shown in legends.

Figure 2

X-ray diffraction (XRD) patterns of different $A_m{B}_n{X}_{m+3n}$ compounds. Intensity axis has been presented in a logarithmic scale for better visualization of weak peaks as well. Patterns arising out of unreacted AgI and $\mathrm{Bi}{\mathrm{I}}_3$ are marked with asterisk (*) and hash (#) symbols, respectively.

Figure 3

(a) Topography (upper panel) and contact potential images (lower panel), (b) distribution of contact potential difference (CPD), and (c) average surface work function ($W_{\mathrm{F}}$) and Fermi energy ($E_{\mathrm{F}}$) from Kelvin probe force microscopy (KPFM) measurements of different $A_m{B}_n{X}_{m+3n}$ thin films. Error bars in (c) represent full width at half maximum (FWHM) of the CPD histograms.

Figure 4

(a) $dI/dV$ vs tip voltage, (b) histogram of conduction band (CB) and valence band (VB) energies, and (c) absolute band edges with respect to vacuum $\left(E_{\mathrm{vac}}=0\mathrm{eV}\right)$ from combined Kelvin probe force microscopy (KPFM) and scanning tunneling spectroscopy (STS) studies of different $A_m{B}_n{X}_{m+3n}$ thin films as stated in the legends.