Direct Observation of Polaron Vibronic Progression in 2D Layered Perovskites

Hybrid perovskites have emerged as a new class of materials with unique electronic properties, which are inherited from both their organic and inorganic constituents. The inorganic framework provides a basis for the semiconducting band structure, whilst the organic molecules stabilize the lattice, indirectly controlling the optical properties. These soft and ionic crystals possess many new and unexpected properties bridging the worlds of classic and organic semiconductors.<br/><br/>A direct consequence of the soft ionic lattice is the significant coupling of charge carriers to the ions of the lattice (i.e. the electron-phonon interaction). The microscopic description of the electron-phonon coupling is nontrivial, essentially due to the fact that it is mediated by a large anharmonicity and dynamic disorder, which requires the introduction of polarons – a quasiparticle representing charge carriers coupled to the lattice vibrations. Polarons are often regarded as a charge carrier “dressed” in a phonon cloud and are characteristic excitations for organic and ionic semiconductors. Polarons are widely invoked to understand the low mobilities of charge carriers, the long carrier lifetimes and diffusion lengths in metal halide perovskites.<br/><br/>Despite tremendous progress in understanding of the fundamental properties of 2D layered halide perovskite, unresolved questions of paramount importance remain to be elucidated. The most important is the origin of the complex optical response of these materials, which cannot be understood by considering only the band structure of these materials. Starting with the pioneering work of Noboru Miura, already 30 years ago, the peculiar line shape of both emission and absorption, composed of equidistant signals separated by tens of meV, is still under debate. Although multiple explanations were suggested in the intervening years, to date the understanding of linear optical response of 2D layered perovskites remains vague requiring further clarification, not only for fundamental understanding of these materials, but also for successful deployment in future opto-electronic devices.<br/><br/>In this work we provide, for the first time, a conclusive proof that the complex structure observed in the optical response of 2D layered perovskites is due to the formation of polarons. Using resonant Raman scattering to investigate various 2D layered perovskites, we observe a spectacular difference of the Raman response with respect to current literature data. Under resonant excitation conditions, the Raman scattering spectrum is dominated by a feature in the high-frequency region &gt;200 cm^-1 (&gt;25 meV), in striking contrast to typical non-resonant Raman spectra, where the scattering is dominated by the expected low-frequency optical modes (&lt;50 cm^-1). This newly observed Raman signal is exactly at the energy corresponding to the spacing observed in absorption/emission (i.e. 280 cm^-1 (35 meV) for (PEA)₂PbI₄) and possess all the characteristics of polarons, allowing for the first time to comprehensively understand the emission and absorption spectra of 2D perovskites. This is the first direct correlation between absorption spectra and Raman studies in these materials.<br/><br/>An analysis of the polaronic response allows us to determine important physical parameters, such as the Huang-Rhys factor, and polaron binding energy. For all investigated samples, we find a consistently large Huang-Rhys factor S&gt;6, indicating a charge carrier – lattice coupling approaching the strong coupling regime. Importantly, the parameters determined in this work are first of their kind for 2D layered perovskites, and could serve as a benchmark for future band structure theory of 2D layered perovskites, including carrier-lattice coupling.