Size-dependent
photoluminescence Stokes shifts (ΔE
s) universally exist in CsPbX3 (X
= Cl–, Br–, or I–) perovskite nanocrystals (NCs). ΔE
s values, which range from ∼15 to 100 meV for NCs with average
edge lengths (l) from approximately 13 to 3 nm, are
halide-dependent such that ΔE
s(CsPbI3) > ΔE
s(CsPbBr3) ≳ ΔE
s(CsPbCl3). Observed size-dependent Stokes shifts are not artifacts of ensemble
size distributions as demonstrated through measurements of single
CsPbBr3 NC Stokes shifts (⟨ΔE
s⟩ = 42 ± 5 meV), which are in near quantitative
agreement with associated ensemble (l = 6.8 ±
0.8 nm) ΔE
s values (ΔE
s ≈ 50 meV). Transient differential absorption
measurements additionally illustrate no significant spectral dynamics
on the picosecond time scale that would contribute to ΔE
s. This excludes polaron formation as being
responsible for ΔE
s. Altogether,
the results point to an origin for ΔE
s, intrinsic to the size-dependent electronic properties of individual
perovskite NCs.
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.
Using
a combination of density-gradient and analytical ultracentrifugation,
we studied the photophysical profile of CsPbBr3 nanocrystal
(NC) suspensions by separating them into size-resolved fractions.
Ultracentrifugation drastically alters the ligand profile of the NCs,
which necessitates postprocessing to restore colloidal stability and
enhance quantum yield (QY). Rejuvenated fractions show a 50% increase
in QY compared to no treatment and a 30% increase with respect to
the parent. Our results demonstrate how the NC environment can be
manipulated to improve photophysical performance, even after there
has been a measurable decline in the response. Size separation reveals
blue-emitting fractions, a narrowing of photoluminescence spectra
in comparison to the parent, and a crossover from single- to stretched-exponential
relaxation dynamics with decreasing NC size. As a function of edge
length, L, our results confirm that the photoluminescence
peak energy scales a L
–2, in agreement
with the simplest picture of quantum confinement.
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