Excited-State Dynamics of a CsPbBr₃ Nanocrystal Terminated with Binary Ligands: Sparse Density of States with Giant Spin–Orbit Coupling Suppresses Carrier Cooling

Abstract

Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for photovoltaic and lightemitting devices due to optoelectronic properties that 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 by 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 CsPbBr 3 NC atomistic model. Redfield theory in the density matrix formalism is used to describe electron−phonon interactions which drive hot carrier cooling and nonradiative recombination (k nonrad ). Radiative recombination (k rad ) is calculated through oscillator strengths using SKSO basis. From k rad and k rad + k nonrad , we compute a theoretical photoluminescence quantum yield (PLQY) of 53%. Computed rates of hot carrier cooling (k cooling ≈ 10 −1 1/ps) compare favorably with what has been reported in the literature. Interestingly, we observe that hot electron cooling slows down near the band edge, which we attribute to large SOC in the conduction band combined with strong confinement, which creates subgaps above the band edge. This slow carrier cooling could potentially impact hot carrier extraction before complete thermalization in photovoltaics (PVs). Implications of this work suggest that strong/intermediate confined APbX 3 NCs are better suited to applications in PVs due to slower carrier cooling near the conduction band edge, while intermediate/weak confined NCs are more appropriate for light-emitting applications, such as LEDs.

On the absence of a phonon bottleneck in strongly confined CsPbBr3 perovskite nanocrystals

“…By simultaneously fitting the kinetics probed at these two features, the X 2 to X 1 relaxation time constant is determined to be τ r = 410 ± 30 fs. This fast relaxation is similar to that previously reported for bulk-like CsPbBr 3 NCs,17–20 indicating that quantum confinement does not slow down hot exciton relaxation in these NCs and that there are relaxation mechanisms bypassing the phonon bottleneck which, however, are overlooked in the calculations in ref. 26.…”

“…In light of these previous studies, it is interesting to examine whether strong quantum confinement can enable a phonon bottleneck and hence slow down hot carrier relaxation in lead halide perovskites, particularly because the Auger-type, electron-to-hole energy transfer mechanism responsible for sub-ps hot electron relaxation in typical CdSe quantum dots (QDs)23–25 should be unavailable in these perovskite materials with similar electron and hole effective masses. This phonon bottleneck was theoretically predicted in a very recent study which calculated hot carrier cooling in CsPbBr 3 NCs using the Red-field theory 26. Experimentally, however, the effect of quantum confinement on hot carrier/exciton relaxation in perovskite NCs has never been systematically studied.…”

Halide Perovskites: A New Era of Solution‐Processed Electronics

“…The defect properties of QDs involve a complex interplay between lattice termination, ligands, solvents, and size dependent strain effects. [ 55,56 ]…”

Effect of ligand groups on photoexcited charge carrier dynamics at the perovskite/TiO2 interface

“…The exited electrons appear to have similar relaxation times for both models. (18) and n(z,t) from eqn (20). The green background color represents no change from the ground state charge density distribution, while the brighter yellow represents high electron density and darker blue represents high hole density compared to the ground state.…”

“…Lead halide perovskites have been a popular research topic over the last several years for their applications to both photovoltaics [1][2][3][4][5][6] and photoluminescence. [7][8][9][10] Lead halide perovskites demonstrate promising features that include bandgap tunability, 11,12 long range charge transfer, 13 extended lifetime of excited-states, [14][15][16] high quantum yield of photoluminescence, [17][18][19][20] and high defect tolerance. [21][22][23][24] Noticeable efficiency of perovskite devices 25,26 can be attributed to the large values of their dielectric constants, which effectively cancel electron-hole interactions and allow for easy spatial separation of photoinduced electrons and holes.…”