Electronic model for self-assembled hybrid organic/perovskite semiconductors: Reverse band edge electronic states ordering and spin-orbit coupling

Abstract

Based on density functional theory, the electronic and optical properties of hybrid organic/perovskite crystals are thoroughly investigated. We consider the mono-crystalline 4F-PEPI as material model and demonstrate the optical process is governed by three active Bloch states at the Γ point of the reduced Brillouin zone with a reverse ordering compared to tetrahedrally bonded semiconductors. Giant spin-orbit coupling effects and optical activities are subsequently inferred from symmetry analysis.

Comment on “Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3” by Y. Wang et al., Phys. Chem. Chem. Phys., 2014, 16, 1424–1429

“…First, the band gap is strongly reduced when taking into account SOC. [3][4][5][6] This lowering is by far much larger than the small difference quoted by the authors between the PBE and the so-called "optB86b + vdWDF" exchange-correlation functionals. Next, as expected, the band gap calculated with SOC is strongly underestimated in the DFT ground-state computations.…”

“…Lastly, Bader (AIM) charges summarized in Table 3 of Yun Wang et al's paper are particularly striking, especially for the methylammonium cation, formally (CH 3 )NH 3 + . In fact, as stated in the manuscript "the NH 3 group is almost charge neutral; and most of the positive charge is the contribution from the CH 3 group".…”

“…In fact, earlier DFT calculations performed both for 2D 3 and 3D 4 lead-based hybrid and allinorganic perovskites 4 evidenced the dramatic effect of SOC on their optical properties, showing that their conduction bands undergo a giant spin-orbit split off. Moreover, in ref.…”

“…Indeed, for related compounds such as CH 3 NH 3 PbBr 3 or CH 3 NH 3 SnI 3 , DFT band gaps (without SOC) do not agree with experimental optical band gaps. [3][4][5][6]10 The numerical agreement between experiment and DFT (without SOC) stems from large error cancellations. First, the band gap is strongly reduced when taking into account SOC.…”

Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductors

“…The valence band consists mostly of Iodine p-like states and the conduction band is a mixture of hybridized Lead s and p-like states 1,34,36 in a similar form to III-V semiconductors. The value of the excitonic reduced mass is therefore…”