Untangling free carrier and exciton dynamics in layered hybrid perovskites using ultrafast optical and terahertz spectroscopy

Abstract

Layered hybrid perovskites (LPKs) are promising as alternatives or additives to 3D metal halide perovskites for optoelectronic applications including photovoltaic cells, LEDs and lasers due to their increased stability. However, high exciton binding energies in these materials mean that excitons are the majority species under the operating conditions of many devices. Although the efficiency of devices that incorporate LPKs has been increasing, much is still unknown about the interplay of excitons and free charge-carriers in these materials, which is vital information for understanding how optoelectronic properties dictate device efficiency. In this work, we employ optical pump/THz probe spectroscopy (OPTP) and visible transient absorption spectroscopy (TAS) to analyse the optoelectronic properties and charge-carrier dynamics of phenylethylammonium lead iodide (PEA)2PbI4. By combining these techniques, we are able to disentangle the contributions from excitons and free charge-carriers. We observe fast cooling of free charge-carriers and exciton formation on a timescale of ~400 fs followed by slower bimolecular recombination of residual free charge-carriers with a rate constant k2 ~109 cm3s-1. Excitons recombine via two monomolecular processes with lifetimes t1 ~11 ps and t2 ~83 ps. Furthermore, we detect signatures of exciton-phonon coupling in the transient absorption kinetic traces. These findings provide a new insight into the interplay between free charge carrier and excitons well as a possible mechanism to further understand the charge-carrier dynamics in LPKs.

Anisotropic Electron–Phonon Interactions in 2D Lead-Halide Perovskites

“…We separately compute spectra for the high-temperature (HT) and low-temperature (LT) phases (T c ≈ 274 K) of n = 1 BAPI based on resolved crystal structures from ref. [19] 21 , (see SI Figure S10). Surprisingly, the calculated spectrum of the LT phase matches the room temperature, i.e.…”

“…Due to a large impedance mismatch between the inorganic and organic layers in 2D perovskites, acoustic phonon propagation, is two times slower compared to 3D perovskites . Butylammonium lead iodide (BAPI) perovskites have been studied extensively, − but especially when synthesized through a synthesis protocol involving spin coating, , it is hard to directly obtain large-area crystals which are phase pure in terms of the thickness of the inorganic structure n . Room-temperature Raman microspectroscopy measurements in literature have revealed in-plane anisotropy of the Raman-active modes, which were shown to be strongly heterogeneously broadened upon lowering the temperature …”