Strongly Polarized Emission and Sub-Band-Gap Excitation of Ultrasharp Defect Emitters in MAPbI3 Single Crystals

Photoluminescence excitation (PLE) and polarization spectroscopy are used to investigate the origin of the ultrasharp defect-center luminescence of MAPbI3 (MA = CH3NH3) single crystals at low temperature. We find that the excitation of the defect states through the free-exciton (FE) resonance is inefficient. Instead, the defect luminescence is strongly enhanced under sub-band-gap excitation, approximately 20 meV below the band gap. This channel enhances the sharp lines and the broad photoluminescence background nonselectively and is attributed to a bound-to-free (BF) transition. Polarization spectroscopy across multiple crystals reveals a degree of linear polarization as high as 40% for the ultrasharp defect lines, while a maximum of 16% is reached for the near-band-edge (NBE) emission. Distinct polarization axes are observed for the two emissions. This anisotropy suggests a preferential orientation of the defect dipoles, governed by the local symmetry of the emitting centers rather than by the crystal symmetry. The excitation and polarization results are consistent with a donor–acceptor pair (DAP) origin of the ultrasharp emission and open the way to a deeper understanding of the electronic properties of defects in halide perovskites (HPs).