Charge Carrier Relaxation in Colloidal FAPbI3 Nanostructures Using Global Analysis

Abstract

We study the hot charge carrier relaxation process in weakly confined hybrid lead iodide perovskite colloidal nanostructures, FAPbI3 (FA = formaminidium), using femtosecond transient absorption (TA). We compare the conventional analysis method based on the extraction of the carrier temperature (Tc) by fitting the high-energy tail of the band-edge bleach with a global analysis method modeling the continuous evolution of the spectral lineshape in time using a simple sequential kinetic model. This practical approach results in a more accurate way to determine the charge carrier relaxation dynamics. At high excitation fluence (density of charge carriers above 1018 cm−3), the cooling time increases up to almost 1 ps in thick nanoplates (NPs) and cubic nanocrystals (NCs), indicating the hot phonon bottleneck effect. Furthermore, Auger heating resulting from the multi-charge carrier recombination process slows down the relaxation even further to tens and hundreds of picoseconds. These two processes could only be well disentangled by analyzing simultaneously the spectral lineshape and amplitude evolution.

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-Induced Stark and Phonon Bottleneck Effects

“…35 This effect was thus attributed to the hot phonon bottleneck, as measured in weakly confined nanostructures of FAPbI3. 8,36 However, the intrinsic cooling time (i.e. at low excitation fluence) was found much faster, leading to an energy-loss rate of about 3.5 eV.ps -1 .…”

“…[1][2][3][4][5][6][7] Time-dependent carrier temperatures are extracted by fitting the high-energy tail of the band-edge bleaching in the transient spectra, which reflects the population of the continuous energy levels above the bandgap. 1,8 Cooling dynamics in halide perovskites typically range within several hundreds of femtoseconds for low excitation fluences but are strongly delayed to several orders of magnitude at high charge carrier density (starting at about 10 18 cm -3 ). [2][3][4]9 This hot phonon bottleneck effect, commonly observed in inorganic polar semiconductors, is linked to the reabsorption of long-lived longitudinal optical (LO) phonons, while their interaction with the hot charge carriers controls the first step of the relaxation.…”

“…We thus used a global target analysis 25 with a sequential model 8 to fit the full evolution of the TA spectra in time and extract the initial and final state associated spectra (EASi), as well with the characteristic rates (ki) of their temporal evolution. 19 The relative change in linewidth of the band-edge absorption, (ωON-ωOFF)/ωOFF = Δω/ω0, is about 33 % for the smallest excitation fluence, while the relative change in area, (AON-AOFF)/AOFF = ΔA/A0, is only about -0.3 %.…”

Naphthalenediimide/Formamidinium-Based Low-Dimensional Perovskites

“…Furthermore, the TA spectra of ⟨n⟩ = 2 compositions reveal an additional bleaching signal in the 625−750 nm range that is characteristic for the 3D perovskite phase (Figure 5f). 38 Having evidenced a rapid charge transfer between the organic and the inorganic components of the hybrid composite, time-resolved microwave conductivity (TRMC) measurements were used to probe changes in the conductivity of hybrid perovskites by using high-frequency microwaves after the excitation by either a high energy electron pulse or a laser (Section S7, SI). 7,9,14,39 The generated free charge carriers absorb part of the microwave power and this fraction of the absorbed microwave power (ΔG) is proportional to the change in conductivity of the material (Δσ), which, in turn, is defined as the product of charge mobility (μ) and quantum yield of free charge carrier formation (φ).…”

“…The spectral shape does not change over time and the generated species are long-lived, in accordance with the formation of radical anions. Furthermore, the TA spectra of ⟨ n ⟩ = 2 compositions reveal an additional bleaching signal in the 625–750 nm range that is characteristic for the 3D perovskite phase (Figure f) …”