Epitaxial inorganic metal-halide perovskite films with controlled surface terminations

Metal-halide perovskite (MHP) thin films for next-generation solar cells are typically fabricated by wet-chemical synthesis in which surface properties such as the orientation of surface facets and their termination cannot be controlled. MHP device efficiencies depend critically on those surface properties. We demonstrate the epitaxial growth of several nanometer thick purely (001)-orientated films of ${\mathrm{CsPbBr}}_3$ and ${\mathrm{CsSnBr}}_3$ MHPs on Au(001) by molecular beam epitaxy. The epitaxial films are in a cubic phase aligned with the substrate. Their surfaces are singly terminated and can be modified for the first time. While the CsBr and ${\mathrm{PbBr}}_2$ surface terminations differ in terms of their atomic structure and defect content, the films are intrinsic semiconductors irrespective of the termination. The work function of the ${\mathrm{PbBr}}_2$-terminated surface is increased by $0.7\mathrm{eV}$, which has drastic implications for the level alignment at MHP interfaces.

Figure 1

Atomic and electronic structure of epitaxial CsBr-terminated ${\mathrm{CsPbBr}}_3$(001) on Au(001). (a) Schematic illustration of the crystal lattice alignment. (b) LEED pattern. (c) Large-scale STM image $(300\mathrm{nm}\times 300\mathrm{nm})$ at $295\mathrm{K}$. The inset shows a line scan along the red line indicated in the main panel. Panels (d) and (e) present a corresponding LEED pattern and an STM image $(45\mathrm{nm}\times 45\mathrm{nm})$ measured at $90\mathrm{K}$ and $80\mathrm{K}$, respectively. (f) Atomically resolved ($5\mathrm{nm}\times 2.5\mathrm{nm}$) STM images at $80\mathrm{K}$. The blue squares in panels (d) and (f) mark the ($1\times 1$) unit cell of ${\mathrm{CsPbBr}}_3$, whereas the light and dark green rectangles in panel (f) indicate ($2\times 2$) and $(2\sqrt{2}\times \sqrt{2})\mathrm{R}{45}^{\circ }$ supercells, respectively. STM parameters (tunneling voltage/current): (c) $3.5\mathrm{V}$/$0.22\mathrm{nA}$, (e) $3.3\mathrm{V}$/$0.03\mathrm{nA}$, and (f) $2.5\mathrm{V}$/$0.02\mathrm{nA}$ (top), $2.3\mathrm{V}$/$0.02\mathrm{nA}$ (bottom). (g) UPS results from the vacuum edge at large and the valence bands at low binding energies $E_{\mathrm{B}}$, respectively. All investigated films had thicknesses between 8 and $14\mathrm{ML}$.

Figure 2

Atomic and electronic surface structure of ${\mathrm{PbBr}}_2$-terminated ${\mathrm{CsPbBr}}_3$(001) on Au(001). (a) Schematic illustration of the crystal lattice alignment. [(b), (c)] LEED patterns of $8\mathrm{ML}{\mathrm{CsPbBr}}_3$(001) at $90\mathrm{K}$ and $295\mathrm{K}$, respectively. [(d)–(f)] Corresponding large-scale $(150\times 150\mathrm{nm})$ and atomically resolved ($15\times 15\mathrm{nm}$ and $12.5\times 6.25\mathrm{nm}$) STM images. STM parameters (tunneling voltage/current): (d) $1.7\mathrm{V}$/$0.02\mathrm{nA}$, (e) $1.6\mathrm{V}$/$0.02\mathrm{nA}$, and (f) $-1.3\mathrm{V}$/$0.02\mathrm{nA}$ (top), $-1.5\mathrm{V}$/$0.02\mathrm{nA}$ (bottom). (g) UPS results of $14\mathrm{ML}{\mathrm{CsPbBr}}_3$(001) from the vacuum edge at large and the valence bands at low binding energies $E_{\mathrm{B}}$, respectively. The presented surfaces were prepared by adding $0.3–1\mathrm{ML}{\mathrm{PbBr}}_2$ onto a well-ordered MHP surface until the LEED pattern revealed a pure ($2\times 2$) reconstruction.