Exciton and Carrier Dynamics in Two-Dimensional Perovskites

Abstract

2D perovskites has become a major topic in the field of perovskite optoelectronics. Here, we aim to unravel the ultrafast dynamics governing the evolution of charge carriers and excitons. We use a combination of ultrabroadband time-resolved THz spectroscopy (TRTS) and fluorescence upconversion spectroscopy to observe and model this evolution. We find that sequential carrier cooling and exciton formation best explain the observed dynamics, where exciton-exciton interactions play an important role in the form of Auger heating and biexciton formation. In fact, we can show that the presence of a longer-lived population of carriers is due to these processes and not to a Mott transition. Therefore, in these materials, excitons still dominate at laser spectroscopy excitation densities. We use kinetic modeling to compare the phenethylammonium and butylammonium organic cations while investigating the stability of the resulting films. In addition, we demonstrate the capability of using ultrabroadband TRTS to study excitons in large binding energy semiconductors through spectral analysis at room temperature.

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

“…The tandem spectroscopy results demonstrate therefore that excitons are always a minority species and are immersed in a plasma of unbound charge carriers. Clearly excitons are still formed, since PL is emitted from excitons, consistently with recent reports, [38][39][40][41][42] but they are not the majority species.…”

“…[ 33–37 ] In spite of the large exciton binding energy, unbound charge carriers have been reported, so that it is not clear yet how much energy needs to be spent in solar cells to split bound excitons. [ 38–43 ]…”

“…[33][34][35][36][37] In spite of the large exciton binding energy, unbound charge carriers have been reported, so that it is not clear yet how much energy needs to be spent in solar cells to split bound excitons. [38][39][40][41][42][43] Rapid advances in perovskite photovoltaics have produced efficient solar cells, with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites. A major reason for the success of perovskite photovoltaics is the presence of free carriers as majority optical excitations in 3D materials at room temperature.…”

Charge Carrier Dynamics in Sn-Doped Two-Dimensional Lead Halide Perovskites Studied by Terahertz Spectroscopy

“…The decay lifetime of the early stage seems not to be sensitive to pump fluence, which is likely caused by the fact that the hot excitons make the major contribution of THz conductivity. The lifetime of the fast component is primarily determined by the process of exciton-phonon scattering, i.e., the cooling process (Burgos-Caminal et al, 2020). The Auger recombination due to excitonexciton scattering is much slower with insignificant impact on the early-stage dynamics.…”

Impact of Dimensionality on Optoelectronic Properties of Hybrid Perovskites

“…The (CH 3 NH 3 ) 2 Pb(SCN) 2 I 2 film shows a sharp absorption peak at 586 nm (2.1 eV) which is attributed to the 1 s exciton absorption. In low-dimensional systems including 2D (CH 3 NH 3 ) 2 Pb(SCN) 2 I 2 perovskites, there is both quantum and dielectric confinement due to the difference in permittivity between the ionic perovskite layer and the bulky organic cation [32]. These effects enhance the electron hole correlations which are manifested in absorption spectrum of these systems as a sharp excitonic peak (Figure 2).…”