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Excited State Dynamics of a CsPbBr3 Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin-Orbit Coupling Suppresses Carrier Cooling.
Journal of the American Chemical Society 2019 Februrary 5
Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and light emitting devices due to their electronic properties which can be tuned/optimized via halide composition, surface passivation, doping, and confinement. Compared to bulk materials, certain excited-state properties in NCs can be adjusted due to electronic confinement effects such as suppressed hot carrier cooling and enhanced radiative recombination. Here we use spinor Kohn-Sham orbitals (SKSOs) with spin-orbit coupling (SOC) interaction as a basis to compute excited-state dissipative dynamics simulations on a fully-passivated CsPbBr3 NC atomistic model. Redfield theory in density matrix formalism is used to describe electron-phonon interactions which drive hot carrier cooling and non-radiative recombination (k_(non-rad)). Radiative recombination (k_rad) is calculated through oscillator strengths using SKSO basis. From comparing ratio of k_rad and k_rad+k_(non-rad) we computed a theoretical photoluminescence quantum yield (PLQY) of 53%. Computed rates of hot carrier cooling (k_cooling) compare favorably with what has been reported in the literature with rates on the order of 10-1 1/ps. Interestingly, we observe that hot electron cooling slows down near the band edge. This we attribute this to large SOC in the conduction band combined with strong confinement creating sub-gaps above the band edge. This slow carrier cooling near the band edge could have potential for extracting hot carriers before complete thermalization in photovoltaics (PVs). Implications of this work suggest that strong/intermediate confined APbX3 NCs are better suited towards applications in PVs, due to slower carrier cooling near conduction band edge, while intermediate/weak confined NCs are more appropriate for light emitting applications, such as LEDs.
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