Ultrafast Charge Carrier Relaxation in Inorganic Halide Perovskite Single Crystals Probed by Two-Dimensional Electronic Spectroscopy

Abstract

Halide perovskites are promising optoelectronic materials. Despite impressive device performance, especially in photovoltaics, the femtosecond dynamics of elementary optical excitations and their interactions are still debated. Here we combine ultrafast two-dimensional electronic spectroscopy (2DES) and semiconductor Bloch equations (SBEs) to probe the room-temperature dynamics of nonequilibrium excitations in CsPbBr3 crystals. Experimentally, we distinguish between excitonic and free-carrier transitions, extracting a ∼30 meV exciton binding energy, in agreement with our SBE calculations and with recent experimental studies. The 2DES dynamics indicate remarkably short, <30 fs carrier relaxation at a ∼3 meV/fs rate, much faster than previously anticipated for this material, but similar to that in direct band gap semiconductors such as GaAs. Dynamic screening of excitons by free carriers also develops on a similarly fast <30 fs time scale, emphasizing the role of carrier–carrier interactions for this material’s optical properties. Our results suggest that strong electron–phonon couplings lead to ultrafast relaxation of charge carriers, which, in turn may limit halide perovskites’ carrier mobilities.

Coherent Two-Dimensional and Broadband Electronic Spectroscopies

“…They have clearly distinguished the contribution associated with the exciton and free carriers and estimated an exciton binding energy of 30 meV. 403 The optoelectronic properties of perovskites have been explained by the large polaron formation model by Zhu and co-workers. 400,404 According to this model, the excited charge carriers are protected by the deformation of the inorganic lattice (PbX 3 − ).…”

Perovskite Bulk Crystals Grown through Antisolvent Droplet-Assisted Crystallization and Associated Wavelength-Dependent Photoluminescence Dynamics

“…All-inorganic perovskite crystals possess excellent optoelectronic properties including the tunable optical absorption and band gap, high carrier mobility, and high quantum yields, , allowing them suitable for a wide range of applications in solar cells, light-emitting diodes, and lasers and photodetectors. − In particular, the perovskite bulk crystals (BCs) offer distinct advantages over the nanocrystals (NCs), such as low trap density, long carrier diffusion length, and high stability. − Until now, all-inorganic perovskite CsPbBr 3 BCs have been prepared by several available methods, including the antisolvent vapor-phase-assisted crystallization, , inverse temperature crystallization, low-temperature-gradient crystallization, and Bridgman growth method. , Among them, the antisolvent crystallization allows relatively easy control over the nucleation and growth dynamics, which has proven to be a simple method to obtain high-quality perovskite BCs. Followed by such progress, the optical properties and carrier transport behaviors of the perovskite BCs have been extensively investigated in the past periods. − Recently, the polarons, that is, the coupling of excess electrons or holes with their ion lattice in the neighborhood, were proposed to reduce the mobility of carriers in perovskites, thereby prolonging the carrier relaxation time. − Thus, the in-depth investigations of the carrier–lattice interactions in different crystalline forms (e.g., BCs and NCs) are of fundamental importance in understanding the size-dependent control of such interactions under the operation of perovskite optoelectronic devices.…”

Nanoscale-Resolved Surface-to-Bulk Electron Transport in CsPbBr3 Perovskite

“…At charge carrier densities above ∼10 18 cm –3 , complex many-body phenomena can be observed in LHPs, − such as hot-phonon bottleneck − and band-filling effects, Auger processes become an important recombination channel for photocarriers, , an excitonic Mott transition occurs, and the polaronic transport picture breaks down . Photophysical studies of LHPs in this convoluted yet technologically relevant regime are theoretically and experimentally challenging. − …”