Metal-halide hybrid perovskites with 3D and 2D inorganic frameworks are currently of great interest for low-cost manufacture of solar cells and light-emitting devices, respectively. Obviously, the design of functional devices requires a thorough understanding of underlying physical mechanism that may be gained from concepts and tools of solid-state physics.

In this work, we show that the prerequisite for a realistic modelling of such hybrids are twofold: both spin-orbit coupling (SOC) and the proper space group should be taken into account. In fact, Pb-based perovskites are the most studied but, despite the relativistic effects expected for lead, the giant effect of SOC on the conduction band has only been evidenced recently. [1] It happens that DFT band gaps may fortuitously match experimental ones because SOC and many body effects are large and act in opposite directions. [1] This was recently confirmed by self-consistent GW-SOC calculations. [2] SOC is also mandatory to describe accurately metal substitution, interfaces, intensity and polarization of optical absorption, effective masses or to accurately implement empirical approaches. [1] Moreover, 3D and 2D hybrids undergo phase transitions and symmetry breaking. From DFT and k.p perturbation approaches, we show that tetragonal or orthorhombic strain lead to band gap shifts and, due to large SOC for lead-based materials, electronic states involved in optical absorption are little perturbed by local distortions of the lattice.

Next, spin-splitting subsequent to lack of centrosymmetry combined with SOC may lead to Rashba-Dresselhaus effects. This gives the opportunity to manipulate the spin with potential applications in spintronics. Rashba and Dresselhaus effects were originally defined in würtzite and zinc-blende semiconductors bulk-structures, respectively. We show that interplay of SOC and ferroelectric distortions for α (P4mm space group) and β (I4cm space group) phases of CH3NH3MI3, M=Pb,Sn , induces such spin-splittings. [1] This was recently confirmed by GW+SOC and tight-binding approaches. [2,3] We further evidence that similar Rashba-type spin-splitting occurs in 2D hybrid perovskites. This affords complementary routes for material engineering based on Rashba-Dresselhaus effects.

[1] J. Even et al., Phys. Rev. B. 2012, 86, 205301; J. Phys. Chem. Lett. 2013, 4, 2999; Phys. Status Solidi RRL 2014, 8, 31.

[2] P. Umari et al., Sci. Rep. 2014, 4, 4467; F. Brivio et al., Phys. Rev. B 2014, 89, 155204.

[3] A. Amat et al., Nano Lett. 2014, doi:10.1021/nl5012992; M. Kim et al., Proc. Natl. Acad. Sci. 2014, 111, 6900.