Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.

In this work, the structural, electronic and optical properties of novel atomically thin systems based on germanene and antimonene nanocomposites have been investigated by means of density functional theory. We find that the germanene and antimonene monolayers are bound to each other via orbital hybridization with enhanced binding strength. Most importantly, the band gap opening can be achieved. Our results demonstrate that the AAII pattern has a direct band gap characteristic with a moderate value of up to 391 meV, while the other three patterns have indirect band gaps tunable from 37 to 171 meV. In particular, changing the direction and strength of the external electric field (E-field) can effectively tune the energy gap of the germanene/antimonene bilayer over a wide range even with a semiconductor–metal transition. The work function of the heterobilayer in the AAII pattern which possesses a direct band gap can be tinkered up from −3.21 to 12.33 eV by applying different E-field intensities. In addition, the germanene/antimonene bilayer exhibits more pronounced optical conductivity. The tunable bandgaps and work function together with a superior visible light response capability make the germanene/antimonene bilayer a viable candidate for optoelectronic applications.

Bnd gap opening and enhanced optical performance are observed in the graphene/stanene heterobilayer.