Longer Cations Increase Energetic Disorder in Excitonic 2D Hybrid Perovskites

Abstract

We synthesize and characterize derivatives of the two-dimensional hybrid perovskite (2DHP) phenethylammonium lead iodide ((PEA)2PbI4) in which the para H on the cation is replaced with F, Cl, CH3, or Br. These substitutions increase the length of the cation but leave the cross-sectional area unchanged, resulting in structurally similar PbI4 2– frameworks with increasing interlayer spacing. Longer cations result in broader, blue-shifted excitonic absorption spectra with reduced or eliminated structure, indicating greater energetic disorder. Photoluminescence spectra are largely invariant and insensitive to cation length, suggesting polaron formation stabilizes a structural and electronic minimum. Temperature-dependent line width analysis reveals excitons couple to a vibration on the organic framework that is weakly sensitive to these cation substitutions, and Raman spectra and electronic structure calculations support the presence of such a cationic mode. Despite carriers being confined to the inorganic framework, the length of the organic cation alters the optical and electronic properties of 2DHPs.

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

“…The PL spectra are nearly invariant in width but have an increased Stokes shift as the cation becomes larger, 6 indicating that excitons find energetic minima before recombining, consistent with disorder and/or polaron formation. 5,9,13,[17][18][19][20][21] As the temperature is decreased from 300 K to 10 K, the excitonic absorption and PL resonances for 2-F, 2-Cl, and 2-Br linearly red-shift ( Figure S5 reports by us and others for Pb-based hybrid perovskites 6,22,23 as well as lead chalcogenides. [24][25][26] α is much smaller than that we observed for (PEA)2PbI4 and its derivatives which have longer cations with unchanged cross-sectional area, where α ranged from -0.14 to -0.16 meV/K, 27 indicative of a weaker temperature dependence of the electronic properties of 2DHPs as the Pb-I-Pb bond angle decreases and the band gap increases.…”

“…This hypothesis is consistent with the smaller energetic spacings in 2-F and 2-Cl and the absence of structure in 2-Br compared to (PEA)2PbI4, because introducing additional off-axis mass would increase the moment of inertia and decrease the energy of a torsional mode. 5,6 To further investigate the vibrational modes of the cations, we carried out densityfunctional theory calculations for isolated PEA, 2-F, 2-Cl, and 2-Br cations and analyze the similarity of vibrational modes between the cations. In our previous work, we identified vibrational modes in lead halide 2DHPs containing 4-substituted PEA cations that showed nearperfect similarity to the 411 cm -1 (51 meV) mode in the PEA cation.…”

“…(PEA)2PbI4, 2-F, 2-Cl, and 2-Br are synthesized by slowly cooling a hot hydriodic acid solution containing perovskite precursors following a previously published procedure. 6 Briefly, unstabilized aqueous hydriodic acid (57% w/w, Sigma-Aldrich) is purified by performing successive liquid-liquid extractions using 10% v/v tributylphosphate (99+%, Acros Organics) in chloroform (Fisher Chemical) until clear in color immediately before use. 6 Powder diffraction patterns are taken on thin films using a Rigaku SmartLab X-ray diffractometer using Cu-kα radiation operating at 40 kV and 30 mA in θ-2θ geometry.…”

Revealing Excitonic Phonon Coupling in (PEA)2(MA)n−1PbnI3n+1 2D Layered Perovskites

“…9,13,17,35,[37][38][39][40][41] Despite the fact that the band-edge states of 2D perovskites are derived from metal and halide orbitals 14,[42][43][44] the absorption and emission properties are affected by the octahedral templating (steric effects), 39,42,[45][46][47] dielectric confinement 17,20 and the strength of the electron-phonon coupling. 32,35,48,49 The possibility to tune the electron-phonon coupling strength, unique for 2D perovskite semiconduc-tors, allows to tune their emissive properties from monochromatic to white-light 11,35,50 as well as controlling the carrier cooling rate 51 and transport properties 48,[52][53][54] crucial for any device. Although 2D perovskites have already been studied for several decades 18,55,56 many fundamental physical aspects underlying their usefulness in real devices remain to be explored.…”

Microscopic Picture of Electron–Phonon Interaction in Two-Dimensional Halide Perovskites

“… 25 Indeed, in the case of the bulk (PEA) 2 PbI 4 crystals the phonon sideband can be resolved up to around 100 K . 20 This observation may indicate that the electron–phonon interaction can be substantially modified in thin layers of 2D perovskites due to possible residual strain induced by the organic cation interacting with the encapsulating layer . 52 It is also worth to note that in encapsulated layers the LO phonon energy is slightly lower than in bulk samples 20 , 25 , 36 which also points to the non-negligible impact of encapsulation.…”