Janke Svenja M, Qarai Mohammad B, Blum Volker, Spano Frank C
Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA.
Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
J Chem Phys. 2020 Apr 14;152(14):144702. doi: 10.1063/1.5139044.
For the prototypical two-dimensional hybrid organic-inorganic perovskites (2D HOIPs) (AE4T)PbX (X = Cl, Br, and I), we demonstrate that the Frenkel-Holstein Hamiltonian (FHH) can be applied to describe the absorption spectrum arising from the organic component. We first model the spectra using only the four nearest neighbor couplings between translationally inequivalent molecules in the organic herringbone lattice as fitting parameters in the FHH. We next use linear-response time-dependent density functional theory (LR-TDDFT) to calculate molecular transition densities, from which extended excitonic couplings are evaluated based on the atomic positions within the 2D HOIPs. We find that both approaches reproduce the experimentally observed spectra, including changes in their shape and peak positions. The spectral changes are correlated with a decrease in excitonic coupling from X = Cl to X = I. Importantly, the LR-TDDFT-based approach with extended excitonic couplings not only gives better agreement with the experimental absorption line shape than the approach using a restricted set of fitted parameters but also allows us to relate the changes in excitonic coupling to the underlying geometry. We accordingly find that the decrease in excitonic coupling from X = Cl to Br to I is due to an increase in molecular separation, which in turn can be related to the increasing Pb-X bond length from Cl to I. Our research opens up a potential pathway to predicting optoelectronic properties of new 2D HOIPs from ab initio calculations and to gain insight into structural relations from 2D HOIP absorption spectra.
对于典型的二维有机-无机杂化钙钛矿（2D HOIPs）(AE4T)PbX（X = Cl、Br和I），我们证明了弗伦克尔-霍尔斯坦哈密顿量（FHH）可用于描述由有机成分产生的吸收光谱。我们首先仅使用有机人字晶格中平移不等价分子之间的四个最近邻耦合作为FHH中的拟合参数来对光谱进行建模。接下来，我们使用线性响应含时密度泛函理论（LR-TDDFT）来计算分子跃迁密度，并基于2D HOIPs中的原子位置评估扩展的激子耦合。我们发现这两种方法都能重现实验观察到的光谱，包括其形状和峰值位置的变化。光谱变化与从X = Cl到X = I时激子耦合的降低相关。重要的是，基于LR-TDDFT且具有扩展激子耦合的方法不仅比使用一组受限拟合参数的方法与实验吸收线形状的吻合度更好，而且还使我们能够将激子耦合的变化与底层几何结构联系起来。因此，我们发现从X = Cl到Br再到I激子耦合的降低是由于分子间距增加，这又与从Cl到I时Pb-X键长的增加有关。我们的研究开辟了一条潜在途径，可从第一性原理计算预测新型2D HOIPs的光电性质，并从2D HOIPs吸收光谱深入了解结构关系。
