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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Lead halide perovskite (LHP) nanocrystals (NCs) show exceptional defect tolerance which has been attributed to their unique electronic structure, where defect energy levels are not introduced inside the fundamental bandgap, and the role of polarons in screening charge carriers from defects. Here, we use ab initio atomistic simulations to explore the interplay between various surface chemistries (A = Cs+, R′NH3 +; X = Br–, RCOO–) used to passivate a CsPbBr3 NC surface and their impact on the ground-state (GS) and excited-state (ES) photophysical properties. We investigate pristine fully passivated surfaces and A–X vacancy defects that reflect chemical reactions A+ + X– → AX on the surface, which result in ligand desorption. For each surface configuration, calculations are performed in the GS and lowest ES (L-ES) electronic configurations, approximating polaron formation after photoexcitation. For models with A–X surface vacancies, we find that localized electron surface trap (ST) states emerge ∼100–400 meV below the pristine S e band in the L-ES configuration due to polaronic nuclear reorganization. Surprisingly, these trap states contribute relatively bright S h → ST spectral features. To test if these surface trap states remain bright in a dynamic (thermal) situation we implement excited-state molecular dynamics simulations. It is found that the surface defected model shows an enhanced nonradiative recombination rate which reduces the photoluminescence quantum yield (PLQY) from 95% for the pristine surface to 75%. This is accompanied by an order of magnitude reduction in PL intensity and a red shift of the transition energy. This study provides more evidence of the defect tolerance of LHP NCs along with evidence of surface trap states contributing to efficient photoluminescence. The observation of relatively bright surface trap states could provide insight into photophysical phenomena, such as size-dependent stretched-exponential photoluminescence decay and Stokes shifts.

Lead halide perovskites have gained attention as an active material in solid-state dye-sensitized photovoltaics due to their high absorption of visible light and long charge-transport lengths. In perovskite-based dye-sensitized photovoltaic architectures the perovskite material is typically paired with a hole-transport material, such as spiro-OMeTAD, which extracts a hole from the photoexcited perovskite to generate free charge carriers. In this study, we explored two competing charge-transfer pathways at the interface between lead halide perovskite and spiro-OMeTAD: “through-bond” and “through-space”. For the through-bond pathway we use a segment of spiro-OMeTAD that contains methoxy linker groups, which will be referred to as “dye with methoxy linker groups” (DML). For the through-space pathway we use a segment of spiro-OMeTAD with removed linker groups, triphenylamine, which will be referred to as “dye”. Four atomistic models were studied: (I) a periodic cesium lead iodide (CsPbI3) perovskite nanowire (NW) that is paired with the dye molecule, (II) a periodic CsPbI3 perovskite NW paired with the DML molecule where the linker groups form coordination bond to the surface of the nanowire, (III) a CsPbI3 perovskite thin film (TF) paired with the dye molecule, and (IV) a CsPbI3 perovskite TF paired with the DML molecule. Charge-transfer dynamics, providing rates of electron/hole relaxation and relaxation pathways, are calculated using reduced density matrix formalism using Redfield theory. It was found that the terminal surface of the perovskite (Pb–I vs Cs–I) has important implications for energetic alignment at the perovskite–dye interface due to band bending. Computed charge-transfer rates match well with upper and lower bounds of reported experimental results where “fast” picosecond rates correspond to through-bond pathway and “slow” nanosecond rates correspond to through-space.

Fully inorganic lead halide perovskite nanocrystals (NCs) are of interest for optoelectronic and light-emitting devices because of their photoluminescence (PL) emission properties, which can be tuned/ optimized by (I) surface passivation and (II) doping. (I) Surface passivation of the NC affects PL capabilities, as an underpassivated surface can introduce trap states, which reduces PL quantum yields. (II) Doping NCs and quantum dots with transition-metal ions provides stable optical transitions. Doping perovskite NCs with Mn 2+ ions provides highintensity 4 T 1 → 6 A 1 optical transitions in addition to the bright intrinsic NC emission. Here, we use noncollinear density functional theory (DFT) to investigate the roles of surface passivation and doping on the PL emission stability of perovskite NCs. Two models are investigated: (i) a pristine NC and (ii) a NC doped with the Mn 2+ ion. The noncollinear DFT includes spin−orbit coupling (SOC) between different spin states and produces spin adiabatic molecular orbitals. These orbitals are used to calculate the transition dipoles between electronic states, oscillator strengths, radiative transition rates, and emission spectra. It was found that the noncollinear spin basis with SOC slows down hole relaxation in the doped NC by 2 orders of magnitude compared to spin-polarized basis. This is attributed to "spin-flip" transition from the perovskite NC to the Mn 2+ dopant and low-probability nonradiative d-d transition.

APbX 3 (A = Cs, methylammonium {MA}; X = I, Br, Cl) lead halide perovskites are of interest for light-emitting applications due to the tunability of their bandgap across the visible and near-infrared spectrum (IR) coupled with efficient photoluminescence quantum yields (PLQYs). It is widely speculated that photoexcited electrons and holes spatially separate into large (Frolich) negative and positive polarons which are stabilized by the A cations. Polarons are expected to be optically active, with recent IR transient absorption experiments showing spectral features consistent with photoionization of the polaron into the continuum band states. For large polarons in the intermediate coupling regime, it would also be expected to observe spectral signatures of transitions within the polaronic potential well producing polaron excited-states. From the polaron excited-state we predict that large polarons should be capable of spontaneous emission (photoluminescence) in the mid-IR to far-IR regime based on the concept of inverse occupations within the polaron potential well. To test this hypothesis, we use density-functional theory (DFT) based calculations using a CsPbBr 3 nanocrystal atomistic model as a host material for either negative (electron) or positive (hole) polarons. We dynamically couple electronic and nuclear degrees of freedom by computing nonadiabatic couplings which allow us to explore nonradiative relaxation of excited polaronic states. Radiative relaxation of excited polaronic states is found from Einstein coefficients for spontaneous emission. Efficiency of polaron emission is determined from rates of nonradiative recombination (k NR ) and radiative recombination (k R ) as k R /(k R + k NR ). It is found that both the positive and negative polaron show bright absorption features and photoluminescence from the relaxed-excited state (RES) to the polaron ground states (PGS), but it is an inefficient process (PLQY ∼ 10 −4 −10 −7 ). Methodology considerations for improving the computed PLQY of polaron emission are discussed, such as Marcus rate corrections and coherence. This work provides computational support for observation of IR polaron absorption and a potential direction toward extending the emission capabilities of APbX 3 perovskites into the mid-IR to far-IR regime.