Heterogeneous Cation–Lattice Interaction and Dynamics in Triple-Cation Perovskites Revealed by Infrared Vibrational Nanoscopy

Abstract

Hybrid organic–inorganic perovskites exhibit extraordinary photovoltaic performance. This is believed to arise from almost liquid-like low-energy interactions among lattice ions and charge carriers. While spatial variations have recently been identified over multiple length scales in the optoelectronic response of perovskites, the relationship between the heterogeneity and the soft cation–lattice interactions has remained elusive. Here, we apply multivariate infrared vibrational nanoimaging to a formamidinium (FA)–methylammonium (MA)–cesium triple-cation perovskite by using the FA vibrational resonance as a sensitive probe of its local chemical environment. The derived correlation among nanoscale composition, cation–lattice coupling, and associated few-picosecond vibrational dynamics implies a heterogeneous reaction field and lattice contraction that we attribute to a spatially nonuniform distribution of cesium cations. The associated spatial variation in elasticity of the lattice leads to disorder in charge–phonon coupling and related polaron formationthe control of which is central to improving perovskite photovoltaics.

Ultrafast infrared nano-imaging of far-from-equilibrium carrier and vibrational dynamics

“…Such disordered polaron–cation coupling is likely associated with a non-uniformity in the dynamic lattice elasticity. This interpretation is supported by other recent experimental and theoretical investigations 59 , 60 , 62 , 63 , which have identified a heterogeneity in chemical composition and resulting local lattice disorder and strain in perovskite films. With HPP IR s -SNOM probing the excited-state vibrational absorption, we resolve spatial heterogeneities in polaron–cation coupling arising from lattice disorder, which directly impacts polaron formation, lifetime, transport and, as such, photovoltaic device performance.…”

“…As another unique aspect, lead halide perovskites exhibit heterogeneity over multiple length scales in their optoelectronic responses in, e.g., photoluminescence intensity, carrier lifetime, or open circuit voltage 11 . While several works have addressed the nanoscale heterogeneity in the lattice strain and elasticity in the electronic ground-state 59 , 60 , the direct relationship between the nonuniform optoelectronic response and the underlying polaronic heterogeneity has not yet been established. Ultrafast infrared vibrational spectroscopy has previously elucidated the coupling between a molecular cation and a photoinduced polaron in perovskites 35 , 61 (Fig.…”

Double Charge Transfer Dominates in Carrier Localization in Low Bandgap Sites of Heterogeneous Lead Halide Perovskites

“…Perovskite films used in solar cells or light emitting diodes (LEDs) are intrinsically heterogeneous. [ 1,2 ] They typically consist of grains, that are of different sizes, have varying surface and/or bulk defect numbers, and exhibit different interactions with other grains. The heterogeneity is particularly large when perovskite films contain nanoparticles or other low‐dimensional structures where spatial confinement changes the state energies.…”

“…[ 9 ] Larger binding energy of ≈40 meV was evaluated for MAPBr 3 perovskites. [ 10 ] Exciton binding energies in 2D perovskites are of the order of hundreds of meV, [ 2,11,12 ] consequently the majority of charge carriers in these structures are expected to be bound in excitons. Therefore, efficient charge carrier generation and their mobility still remain important issues limiting performance efficiency of solar cells utilizing 2D perovskites.…”

Exciton–Photonics: From Fundamental Science to Applications

“…Beyond optical response in the visible range, additional information from the material can be identified from the infra-red region using near-field technique. Recently, infrared vibrational spectroscopic studies have shown label-free chemical information and cation vibrational dynamics in liquids (258) and perovskites (259), respectively.…”