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) _2 PbI _4 . 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 k _2 ∼ 10 ^9 cm ^3 s ^−1 . Excitons recombine via two monomolecular processes with lifetimes t _1 ∼ 11 ps and t _2 ∼ 83 ps. Furthermore, we detect signatures of exciton–phonon coupling in the transient absorption kinetic traces. These findings provide new insight into the interplay between free charge-carriers and excitons as well as a possible mechanism to further understand the charge-carrier dynamics in LPKs.

Keywords

• excitons
• 2D perovskites
• Ruddlesden-Popper
• terahertz spectroscopy
• layered hybrid perovskites
• transient absorption spectroscopy