Figure 1.

Exciton recombination and spin relaxation dynamics in FA_0.9Cs_0.1PbI_2.8Br_0.2. a) Microscopy image of FA_0.9Cs_0.1PbI_2.8Br_0.2 crystal. b) Spectra of photoluminescence (blue) excited at 1.96 eV and photoluminescence excitation (green), as well as of TRKR amplitude at zero time delay (red). c) Differential reflectivity dynamics ΔR/R (dots) and fitted with bi‐exponential function (line). d) Dynamics of KR amplitude (dots) with mono‐exponential fit (line) at B _|| = 100 mT. The dynamics in (c,d) are measured with the laser photon energy of 1.513 eV. e) Sketch of direct bandgap at the R point of the Brillouin zone. Valence band is formed by 6s‐type lead $|{\mathrm{s}}_{\text{Pb}}\rangle$ orbitals strongly hybridized with iodine 5p‐type $|{\mathrm{p}}_{\mathrm{I}}\rangle$ orbitals, conduction band is formed by 6p‐type lead orbitals with minor admixture of 5s iodine orbitals. Nomenclature follows ref. [41] and S _e,h here denotes the component of the angular momentum of the Bloch function.