Spectral Signatures of Positive and Negative Polarons in Lead-Halide Perovskite Nanocrystals

Abstract

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.

Mn2+‐Doped Metal Halide Perovskites: Structure, Photoluminescence, and Application

“…[ 83,129 ] Subsequently, Mn 2+ ‐doped perovskite NCs have attracted extensive attention. [ 130–133 ] Under the excitation at UV light, undoped CsPbCl 3 NCs only exhibit narrow band‐edge emission with the relatively low PLQY of <5%. After Mn 2+ doping, CsPbCl 3 :Mn 2+ NCs present bright Mn 2+ emission at ≈600 nm originating from the energy transfer from FEs to Mn 2+ ions, the overall PLQY dramatically improves up to 30−60% ( Figure a).…”

First-Principles Study on the Electronic Properties of PDPP-Based Conjugated Polymer via Density Functional Theory

“…80 Negative and positive injections here could correspond to chemical reduction and chemical oxidation in experiments, respectively, and also relate to the formation of polaron quasiparticles. 81,82 In a similar manner, we explore the influence of a positive charge-carrier injection, mimicking p-type doping, on the energy gap of the current semiconducting CP. Figure 3d shows that injecting holes into the system tends to considerably decrease the band gap to 0.29 eV.…”

Simulating the Coupled Structural–Electronic Dynamics of Photoexcited Lead Iodide Perovskites

“…33 Kilin and coworkers investigated the electronic dynamics in LHP nanoclusters from the viewpoint of quantum confinement, 34 spin− orbit coupling, 35,36 and polaron excited states. 37 Wang and coworkers elucidated the effect of interstitial iodine on hot carrier cooling. 38 In real situations, the electronic and structural degrees of freedom should be coupled with each other.…”

“…focused on the dependence of the hot carrier cooling rate on the A-site cation species . Kilin and co-workers investigated the electronic dynamics in LHP nanoclusters from the viewpoint of quantum confinement, spin–orbit coupling, , and polaron excited states . Wang and co-workers elucidated the effect of interstitial iodine on hot carrier cooling …”