ISP/Applied Sciences Laboratory, Washington State University, Spokane, Washington 99210, USA.
Phys Chem Chem Phys. 2019 Oct 16;21(40):22293-22301. doi: 10.1039/c9cp03080h.
Self-trapped excitons (STEs) in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) emit broadband white light, suggesting the great potential of 2D HOIPs in low-cost lighting and display applications. A prerequisite for understanding STEs' properties is a correct identification of the underlying interaction that leads to the STEs. Here, we show that the long-range polar coupling between electrons and optical phonons is quenched in 2D HOIPs' tightly bound excitons and cannot effect STEs. Rather, the STEs are induced by a short-range optical deformation potential (ODP) arising from the phonon-modulated Pb-X quantum-well thickness. Interaction between transition dipoles in adjacent PbX6 (X = Br or I) octahedra gives rise to highly anisotropic intra- and inter-layer exciton bandwidths. In flat (001) 2D HOIPs, both the ODP and the exciton bandwidths are susceptible to out-of-plane PbX6 tilting but not to the in-plane one, and their interplay can quantitatively account for the observed temperature and structure dependences of luminescence associated with STEs. In corrugated (011) 2D HOIPs, the exciton bandwidth is further reduced and the resultant STEs have a stronger lattice distortion and broader luminescence spectrum. Our results reveal the mechanism of STE formation and suggest ways of tuning STEs and associated broadband luminescence in 2D HOIPs.
二维（2D）杂化有机-无机钙钛矿（HOIPs）中的自陷激子（STE）发射宽带白光，这表明 2D HOIPs 在低成本照明和显示应用中有巨大的潜力。理解 STE 性质的前提是正确识别导致 STE 的基础相互作用。在这里，我们表明电子和光学声子之间的远程极性耦合在 2D HOIPs 的紧密束缚激子中被猝灭，并且不能影响 STE。相反，STE 是由源自声子调制的 Pb-X 量子阱厚度的短程光学变形势（ODP）诱导的。相邻 PbX6（X = Br 或 I）八面体中的跃迁偶极子之间的相互作用导致各向异性的内和层间激子带宽。在平面（001）2D HOIPs 中，ODP 和激子带宽都容易受到垂直于平面的 PbX6 倾斜的影响，但不受平行于平面的 PbX6 倾斜的影响，它们的相互作用可以定量解释与 STE 相关的观察到的发光的温度和结构依赖性。在波纹（011）2D HOIPs 中，激子带宽进一步减小，由此产生的 STE 具有更强的晶格畸变和更宽的发光光谱。我们的结果揭示了 STE 形成的机制，并提出了在 2D HOIPs 中调整 STE 和相关宽带发光的方法。
