Carrier recombination in materials with strong spin-orbit coupling*

Materials that have large spin-orbit coupling and no inversion symmetry can display interesting spin texture. The hybrid perovskites, which are receiving a lot of attention for photovoltaic applications, are prominent examples in which the spin texture exhibits unexpected patterns. Based on model calculations, it has been suggested that due to mismatched spins the radiative recombination is severely suppressed. We have performed first-principles calculations to compute the spin texture in the prototypical hybrid perovskite, CH₃NH₃PbI₃. We find that the spin texture is dynamically evolving in various patterns with the rotation of the organic molecule [1]. However, the spin-orbit coupling always leads to spin-allowed optical transitions and the momentum splitting affects the radiative recombination coefficient by less than a factor of two [2]. The spin-orbit coupling does, however, significantly enhance the Auger coefficient due to a coincidental resonance between the band gap and interband transitions to spin-orbit-split conduction bands [3]. These insights demonstrate the importance of accurate treatment of spin-orbit coupling in describing materials properties, and also offer new computational approaches to quantitatively calculate the spin texture and its impact on carrier recombination.
[1] X. Zhang, J.-X. Shen, and C. G. Van de Walle, J. Phys. Chem. Lett. 9, 2903 (2018).
[2] X. Zhang, J.-X. Shen, W. Wang, and C. G. Van de Walle, ACS Energy Lett. 3, 2329 (2018).
[3] J.-X. Shen, X. Zhang, S. Das, and E. Kioupakis, C. G. Van de Walle, Adv. Energy Mater. 10, 1801027 (2018).