Fate of Optical Excitons in FAPbI3 Nanocube Superlattices

Abstract

Understanding the nature of the photoexcitation and ultrafast charge dynamics pathways in organic halide perovskite nanocubes and their aggregation into superlattices is key for potential applications as tunable light emitters, photon-harvesting materials, and light-amplification systems. In this work, we apply two-dimensional coherent electronic spectroscopy (2DES) to track in real time the formation of near-infrared optical excitons and their ultrafast relaxation in CH(NH₂)₂PbI₃ nanocube superlattices. Our results unveil that the coherent ultrafast dynamics is limited by the combination of the inherent short exciton decay time (≃40 fs) and the dephasing due to the coupling with selective optical phonon modes at higher temperatures. On the picosecond time scale, we observe the progressive formation of long-lived localized trap states. The analysis of the temperature dependence of the excitonic intrinsic line width, as extracted by the antidiagonal components of the 2D spectra, unveils a dramatic change of the excitonic coherence time across the cubic to tetragonal structural transition. Our results offer a new way to control and enhance the ultrafast coherent dynamics of photocarrier generation in hybrid halide perovskite synthetic solids.

Figure 1

(a) Cartoon of FAPI nanocubes, hosting quantum confined excitons, and FAPI superlattices. (b) Absorption spectrum (green area) of the FAPI superlattices sample measured in this work. The green solid lines represent a free carriers edge and two peaks, as obtained from Elliott analysis of the room temperature absorbance (Supporting Information, Section S1-D). (c) Scheme of 2DES experiment. (d) Sketch of a typical 2D spectrum (vertical axis: excitation, horizontal axis: detection) where, as opposed to 1D experiments (top panel), homogeneous and inhomogeneous line widths and correlations between spectral features can be resolved.

Figure 2

2D spectra of the FAPI superlattice measured at 200 K with 175 μJ/cm² excitation fluence for three different t₂ delays: (a) t₂ = 0 fs, (b) 200 fs, and (c) 3 ps. The top panels show the sample absorption spectrum (red) and the laser pulse spectrum (blue).

Figure 3

(a) 2D spectrum of FAPI superlattices measured at 30 K and t₂ = 0 fs, with 175 μJ/cm² excitation fluence. (b) Antidiagonal profile of the 2D spectrum in (a) taken along the dotted line in (a). The black dots are the experimental data points, the red line is the profile fit, and the green and blue filled areas are peaks A and B associated with, respectively, exciton and biexciton resonances. (c) and (d) display a 2D spectrum and an antidiagonal profile analogous to (a) and (b), but collected at a higher sample temperature (290 K). (e) Fluence dependence of the integrated amplitude I_i of peaks A and B, extracted from fitting the 2D spectra antidiagonal profiles at 200 K and t₂ = 0 fs. The black dashed lines represent the power-law fit to the data. (f) Dynamics of the exciton (peak A) and biexciton (peak B) peak amplitudes I_i at 200 K for 175 μJ/cm² excitation fluence. (g) Sketch of the relaxation channels for excitons, which can decay at a γ_x rate or bind into biexcitons at a rate βN_x, N_x being the instantaneous number of excitons. (h) Representation of the energy levels scheme (left) and fifth-order rephasing double-sided Feynman diagrams (right) for the biexciton formation process.

Figure 4

(a) Temperature dependence of the exciton (green circles, left axis) and biexciton (blue squares, right axis) homogeneous line widths extracted from simultaneous fitting of diagonal and antidiagonal profiles at short time delay t₂. The black solid line represents the line width broadening originating from interaction with an optical phonon mode at E_OP = 17 meV. The purple shaded area represents the temperature range where FAPI nanocubes have cubic lattice structure and where the deviation from the expected temperature-dependent trend is observed. (b) Top panel: FAPI crystal structure in the high-temperature cubic phase with the organic cation subject to orientational disorder. Bottom panel: low-temperature tetragonal phase of the FAPI perovskite. The black squares highlight the unit cell. The green lines and blue arrows indicate the I–Pb–I bending mode that softens in the proximity of the second-order tetragonal-to-cubic phase transition.

Figure 5

(a) t₂ dynamics of the 2DES signal measured at 200 K and integrated over the two regions of interest depicted in the top left inset, which select the diagonal (orange) and off-diagonal (purple) structures. (b) Sketch of the energy level structure and population dynamics of trap states originating from defects or self-trapped excitons. (c) Temperature dependence of the off-diagonal spectral feature at t₂ = 0 fs as compared to the diagonal excitonic resonance. The yellow markers report the ratio between the signal intensities obtained from the integration of the 2D spectra over the areas indicated in the inset of Figure 5a. The blue dashed line indicates the divergence of a second-order phase transition order parameter scaling as (1 – T/T_c)^−γ; here γ = 1 and T_c = 240 K.