Role of Exciton Binding Energy on LO Phonon Broadening and Polaron Formation in (BA)2PbI4 Ruddlesden-Popper Films

The effect of carrier-carrier and carrier-phonon interactions is presented in n = 1 (2D) (BA)₂PbI₄ Ruddlesden-Popper thin films, and their effect is compared to that of conventional MAPbI₃. While temperature-dependent photoluminescence shows the well-studied structural phase transitions and evidence of longitudinal optical (LO) phonon scattering in MPbI₃, the 2D (BA)₂PbI₄ films produce subtler properties. At low temperatures, evidence of two competing intrinsic exciton transitions is observed (P1 and P3), in addition to several extrinsic transitions attributed to the impurities in the (BA)₂PbI₄ film. At higher temperatures, (BA)₂PbI₄ is dominated by the two intrinsic excitons that are attributed to varying degrees of localization caused by deformations of the PbI₄ framework mediated by structural relaxation. Although both exciton complexes are well separated from the continuum (490 meV or greater), their interaction with phonons is quite different. In the case of the more strongly bound complex (P3), the strong excitonic nature and short-range interaction are mediated by the emission of LO phonon replicas. In the case of the exciton with the smaller binding energy (P1), the more extended wavefunction manifests itself in strong carrier-phonon scattering and evidence of large, stable polarons.