Ultrafast Exciton Dynamics of CH3NH3PbBr3 Perovskite Nanoclusters.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr3 Perovskite Quantum Dots.

Exciton-phonon interactions elucidate structure-function relationships that aid in the control of color purity and carrier diffusion, which is necessary for the performance-driven design of solid-state optical emitters. Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) reveal that thermally activated exciton-phonon interactions originate from structural distortions related to vibrations in cubic CsPbBr3 perovskite quantum dots (PQDs) at room temperature. Exciton-phonon interactions cause performance-degrading PL line width broadening and slower electron-hole recombination. Structural distortions in cubic PQDs at room temperature exist as the bending and stretching of the PbBr6 octahedra subunit. The PbBr6 octahedral distortions cause symmetry breaking, resulting in thermally activated longitudinal optical (LO) phonon coupling to the photoexcited electron-hole pair that manifests as inhomogeneous PL line width broadening. At cryogenic temperatures, the line width broadening is minimized due to a decrease in phonon-assisted recombination through shallow traps. A fundamental understanding of these intrinsic exciton-phonon interactions gives insight into the polymorphic nature of the cubic phase and the origins of performance degradation in PQD optical emitters.