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

Abstract

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.

Influence of the Vibrational Modes from the Organic Moieties in 2D Lead Halides on Excitonic Recombination and Phase Transition

“…However, even with unrealistic parameters, the phonon contribution remained significant, with a phonon energy that is quite consistent: 55 meV, falling in between the two Raman and IR modes in 52 and 59 meV (Figure 2e). In physical terms, a few conditions must be considered: 1) The PLE and TRES spectra indicate that these two bands have the same excitonic origin, which suggests that defects do not influence in the emission, in accordance with other reports; [ 33,35,38 ] 2) the exciton binding energy and oscillator strength of these materials are high, which explain their predominantly excitonic optical behavior; [ 30,39 ] 3) the determined parameters canceling the ionized‐defect term are physically realistic, with acceptable errors; 4) these materials are known to present strong exciton–phonon coupling due to the “softness” of their crystal structure; [ 16 ] and 5) the contribution of acoustic phonons is dominant in temperatures below 200 K; [ 18,37 ] thus, in the conditions of our analysis, optical phonons are the main homogeneous broadening factor (i.e., Fröhlich scattering).…”

“…[ 37 ] This model was developed for MQW systems grown by physical methods (e.g., molecular beam epitaxy), but it has been used for 2D metal halides, providing valuable information. [ 24,25,38 ] The equation is as follows. where Γ 0 is the temperature‐independent linewidth of the emission related to the inhomogeneous broadening; Γ ac , Γ opt are the exciton–phonon coupling parameters for acoustic and optical phonons, respectively; E opt is the energy of the optical phonon that interacts with the exciton; Γ in is the broadening parameter related to ionized defects; and E in is the average binding energy of these defect‐emissive states.…”

“…For the fit in Figure 2d (red line), we set the parameters of the ionized defects to be zero and disregarded the acoustic parameter, considering only the optical phonon contribution and Γ 0 (equation in Figure 2d), as reported in other works. [ 18,21,38 ] This decision is mathematically and physically justifiable. In mathematical means, the fit with the full equation gives physically unrealistic values of the parameters and large errors (Figure S4, Supporting Information).…”

“…that reported the same 57 meV for the phonon coupled with the low‐temperature‐phase exciton in BAPI (FE 242 ; see Figure 3a). [ 38 ] Given the unusual high energy of this phonon, the authors suggested that it is due to a strong polaronic effect in the system.…”

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

“…[ 84,101,160,161 ] Further, it also appears that variations in exciton binding energies have an effect on the energies of the polaronic state formed in the case of Coulombically bound electron–hole pairs. [ 121,162 ] For example, a recent study has argued that two excitonic states are present in (BA) 2 PbI 4 (BA = butylammonium), and that the different binding energies of the two excitons lead to different couplings with the lattice, with a weak exciton binding energy leading to the formation of large polarons. [ 162 ]…”