Two types of self-trapped excitons in a quasi-one-dimensional crystal piperidinium tribromoplumbate

Photo-excited states of piperidinium tribromoplumbate (C5H10NH2PbBr3) single crystals were investigated by spectroscopic methods. The strong anisotropy was observed in polarized reflection spectra. A peak with large oscillator strength appears at 3.9 eV only for the light polarized parallel to the [PbBr3](n)(1-) chain, which is assigned as the lowest exciton transition. These polarized reflection spectra indicate that electrons and holes are strongly confined within the chain, suggesting that the system can be considered as natural multiple quantum wires. Two luminescence bands appear around 3.0eV (V emission) and 1.9 eV (R emission) under excitation into the intrinsic absorption region. The large Stokes-shifts of the luminescence bands indicate strong electron-phonon coupling in the excited states. Temperature dependence of luminescence intensities and decay lifetimes reveal that population transfer from the initial state of the V emission to that of the R emission is thermally activated via a potential barrier of about 35 meV. The excitation spectra and the decay kinetics of the luminescence bands strongly suggest that the initial states of luminescence bands are qualitatively different kinds of self-trapped excitons.