Defect Tolerance Mechanism Revealed! Influence of Polaron Occupied Surface Trap States on CsPbBr3 Nanocrystal Photoluminescence: Ab Initio Excited-State Dynamics,Journal of Chemical Theory and Computation

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Defect Tolerance Mechanism Revealed! Influence of Polaron Occupied Surface Trap States on CsPbBr3 Nanocrystal Photoluminescence: Ab Initio Excited-State Dynamics
Journal of Chemical Theory and Computation ( IF 5.7 ) Pub Date : 2021-10-19 , DOI: 10.1021/acs.jctc.1c00691
Aaron Forde ₁ , Dmitri Kilin ₂

Affiliation

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.

中文翻译：

缺陷容忍机制揭晓！极化子占据表面陷阱态对 CsPbBr3 纳米晶体光致发光的影响：从头算激发态动力学

卤化铅钙钛矿 (LHP) 纳米晶体 (NC) 显示出卓越的缺陷耐受性，这归因于其独特的电子结构，其中缺陷能级未引入基本带隙内，以及极化子在从缺陷中屏蔽电荷载流子中的作用。在这里，我们使用ab initio原子模拟来探索用于钝化 CsPbBr ₃ NC 表面的各种表面化学物质（A = Cs ⁺ , R'NH ₃⁺ ; X = Br ^– , RCOO ^–）之间的相互作用及其对地面的影响态 (GS) 和激发态 (ES) 光物理特性。我们研究了原始的完全钝化表面和反映化学反应的 A-X 空位缺陷 A⁺ + X ^– → AX 在表面上，导致配体解吸。对于每个表面配置，在 GS 和最低 ES (L-ES) 电子配置中进行计算，近似光激发后的极化子形成。对于A-X表面空位模型，我们发现，局域电子陷阱表面（ST）状态出现原始小号~100-400兆电子伏以下_Ë的L-ES配置带由于极化子核重组。令人惊讶的是，这些陷阱态贡献了相对明亮的 S _h→ ST 光谱特征。为了测试这些表面陷阱状态在动态（热）情况下是否保持明亮，我们实施了激发态分子动力学模拟。发现表面缺陷模型显示出增强的非辐射复合率，这将光致发光量子产率 (PLQY) 从原始表面的 95% 降低到 75%。这伴随着 PL 强度的数量级降低和跃迁能的红移。这项研究提供了更多关于 LHP NC 的缺陷耐受性的证据，以及有助于有效光致发光的表面陷阱态的证据。对相对明亮的表面陷阱状态的观察可以提供对光物理现象的洞察，例如与尺寸相关的拉伸指数光致发光衰减和斯托克斯位移。

更新日期：2021-11-09

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