Temperature- and Pressure-Dependent Raman and Photoluminescence Studies of Corrugated Imidazolium-Methylhydrazinium Lead Bromide

Alternating cations in interlayer space (ACI) perovskites are attractive optoelectronic materials. In this work, temperature- and pressure-dependent Raman and photoluminescence studies of (110)-derived ACI perovskite comprising imidazolium (IM+) and methylhydrazinium (MHy(+)) cations are reported. Temperature-dependent Raman spectra show a gradual increase in the lattice dynamics and weak changes at the structural phase transition near 350 K. Photoluminescence studies reveal that IMMHyPbBr(4) exhibits strong broadband emission related to self-trapped excitons and narrow emission attributed to free and localized excitons. Small value of the activation energy (107 +/- 10 meV) and large Stokes shift proves that the self-trapped excitons are localized on deep defects. Pressure-dependent Raman studies reveal that at the low pressure, compression is accommodated mainly by squeezing of the organic layers, which leads to tilts of the interlayer IM+ cations and increase of hydrogen-bond strength. The presence of a strongly first-order pressure-induced phase transition is observed between 1.25 and 1.53 GPa and analysis of the spectra indicates that this phase transition is associated with sudden freezing of molecular dynamics accompanied by tilting and distortion of the PbBr6 octahedra. This freezing and decrease of the organic layer thickness should affect the dielectric constant of the organic layers and thus modify the dielectric confinement and exciton binding energy. Raman spectroscopy provides, therefore, evidence that pressure-enhanced hydrogen bonding plays a major role in tuning structural and optoelectronic properties of ACI perovskites. Pressure-dependent photoluminescence confirms this assumption, showing bandgap narrowing on compression due to the combined effect of Pb-Br bond shortening and decrease of the dielectric confinement. Photoluminescence studies also show remarkable stability of the broadband emission on compression, making this material attractive for light-emitting applications in broad temperature and pressure ranges.