Importance of Spin–Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications

TLDR

Three‑dimensional hybrid perovskites CH₃NH₃PbX₃ are emerging as key materials for dye‑sensitized and hybrid semiconductor photovoltaic cells, with two‑dimensional analogues offering additional flexibility, and their room‑temperature optical transitions arise from a spin‑orbit split‑off band in the cubic lattice. Density‑functional calculations show that a giant spin‑orbit coupling dominates the band gap of CH₃NH₃PbX₃, leaving optical absorption states largely insensitive to lattice distortions, and that the CH₃NH₃PbX₃/TiO₂ interface provides an effective electron‑driving layer, thereby establishing a foundational understanding comparable to conventional semiconductors.

Abstract

Three-dimensional (3D) hybrid perovskites CH3NH3PbX3 (X = Br, I) have recently been suggested as new key materials for dye-sensitized solar cells (DSSC) leading to a new class of hybrid semiconductor photovoltaic cells (HSPC). Thanks to density functional theory calculations, we show that the band gap of these compounds is dominated by a giant spin–orbit coupling (SOC) in the conduction-band (CB). At room temperature, direct and isotropic optical transitions are associated to a spin–orbit split-off band related to the triply degenerated CB of the cubic lattice without SOC. Due to the strong SOC, the electronic states involved in the optical absorption are only slightly perturbed by local distortions of the lattice. In addition, band offset calculations confirm that CH3NH3PbX3/TiO2 is a reference material for driving electrons toward the electrode in HSPC. Two-dimensional (2D) hybrids are also suggested to reach further flexibility for light conversion efficiency. Our study affords the basic concepts to reach the level of knowledge already attained for optoelectronic properties of conventional semiconductors.