Lead-free double perovskites have attracted noteworthy attention due to their compositional flexibility and electronic diversity. In this study, we hydrothermally grow a new class of Cs₂Ag_xNa_1–xFeCl₆ (0 ≤ x ≤ 1) perovskite single crystals with high thermal stability. The substitution of B-site cation allows to regulate the crystallographic and band structure, which gives rise to enlarged band absorbance close to the near-infrared region (∼800 nm) via composition engineering. Ultrafast transient absorption spectroscopy (TAS) certifies that the decay time of excited-state absorption is 5.02 and 2450 ps in the case of Cs₂NaFeCl₆ and Cs₂AgFeCl₆, respectively. The corresponding charge carrier diffusion length accordingly enhances from 3.7 to 311 nm by means of increasing Ag dopant concentration. Structurally, the primitive cell shrinks due to the partial replacement of [NaCl₆]⁵⁻ octahedra by [AgCl₆]⁵⁻ octahedra. It is proved theoretically as well as experimentally that the introduction of Ag species can effectively enhance the electron mobility (from 1.06 to 15.3 cm² V^–1 s^–1) by ∼15 times through realizing stronger orbital coupling of the conductive ions, which enables such a novel double perovskite to be a potential candidate for the optoelectronic and photovoltaic applications.

中文翻译：

---

新兴Cs ₂ Ag _x Na _{1– x} FeCl ₆钙钛矿单晶的工程电子结构和载流子动力学

无铅双钙钛矿由于其成分的灵活性和电子多样性而引起了人们的关注。在这项研究中，我们水热生长一类新的Cs的₂的Ag _X娜_{1- X}的FeCl ₆（0≤ X ≤1）钙钛矿型单晶具有高的热稳定性。B-位阳离子的取代可以调节晶体学和能带结构，通过组成工程可以提高接近近红外区域（约800 nm）的能带吸收率。超快速瞬态吸收光谱（TAS）证明，在Cs ₂ NaFeCl ₆和Cs的情况下，激发态吸收的衰减时间为5.02和2450 ps分别为₂ AgFeCl ₆。相应地，通过增加Ag掺杂剂浓度，相应的载流子扩散长度从3.7nm增加到311nm。在结构上，原始细胞会因[NaCl ₆ ] ^5-八面体被[AgCl ₆ ] ^5-八面体部分取代而收缩。从理论上和实验上都证明，引入Ag物种可以有效地提高电子迁移率（从1.06到15.3 cm ² V ^–1 s ^–1通过实现更强的导电离子轨道耦合，可实现约15倍的光学耦合，这使得这种新型的双钙钛矿成为光电和光伏应用的潜在候选者。