Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India.
ACS Appl Mater Interfaces. 2023 Apr 12;15(14):18473-18481. doi: 10.1021/acsami.3c00369. Epub 2023 Mar 28.
Surface-tailored lead-free halide double-perovskite (CsAgBiX) thin films are utilized for ion migration studies. A thin surface layer of BiOBr/Cl is grown via intentional annealing of the halide films in ambient conditions. Herein, we physically stacked the two films, viz., CsAgBiBr and CsAgBiCl, to thermally activate the halide ion migration at different temperatures (room temperature (RT)-150 °C). While annealing, the films' color changes from orange to pale yellow and transparent brown to yellow as a result of the migration of Br ions from CsAgBiBr to CsAgBiCl and Cl ions from CsAgBiCl to CsAgBiBr, respectively. Annealing helps in homogenizing the halide ions throughout the films, consequently leading to a mixed phase, i.e., CsAgBiClBr/CsAgBiBrCl ( = 0 to 6) formation. The movement of ions is understood by absorption studies performed at regular time intervals. These investigations reveal a redshift (from 366 to 386 nm) and a blueshift (from 435 to 386 nm) in absorption spectra, indicating the migration of Br and Cl toward CsAgBiCl and CsAgBiBr, respectively. The films characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveal the presence of a peak at 2θ = 10.90° and binding energy of 158.1 eV, respectively, corresponding to the formation of Bi-O bonds at the film surface. Also, XRD studies show a lower 2θ shift of the diffraction peak in the case of CsAgBiCl films and a higher 2θ shift in the case of CsAgBiB films, which further confirms the migration of Cl and Br from one film to the other. XPS investigations confirm the compositional change with a gradual increment in the concentration of Br/Cl with an increase in heating time for CsAgBiCl/CsAgBiBr films. All these studies confirm thermal diffusion of halide ions in double-perovskite films. Further, from the exponential decay of the absorption spectra, the rate constant for halide (Br) ion diffusion is calculated, which shows an increment from 1.7 × 10 s at RT to 12.1 × 10 s at 150 °C. The temperature-dependent rate constant follows Arrhenius behavior and renders an activation energy of 0.42 eV (0.35 eV) for bromide (chloride) ion mobility. A larger estimated value as compared to the reported values for CsAgBiBr wafers (∼0.20 eV) reveals a slow mobility of halide ions in thin films of CsAgBiBr/Cl. The formation of a BiOBr passivation layer at the surface of CsAgBiBr thin film might be one of the plausible causes of the slow anion diffusion in the present work. Slow ion migration is an indication that the films are stable and of high-quality.
用于离子迁移研究的表面修饰的无铅卤化物双钙钛矿（CsAgBiX）薄膜。通过在环境条件下故意退火卤化物薄膜，生长出薄薄的 BiOBr/Cl 表面层。在此，我们物理地堆叠了两种薄膜，即 CsAgBiBr 和 CsAgBiCl，以便在不同温度（室温（RT）-150°C）下热激活卤化物离子迁移。退火过程中，薄膜的颜色从橙色变为浅黄色，从透明棕色变为黄色，这是由于 Br 离子从 CsAgBiBr 迁移到 CsAgBiCl 和 Cl 离子从 CsAgBiCl 迁移到 CsAgBiBr 的结果。退火有助于在整个薄膜中均匀化卤化物离子，从而导致混合相，即 CsAgBiClBr/CsAgBiBrCl（=0 至 6）的形成。通过在固定时间间隔进行吸收研究来理解离子的运动。这些研究表明，吸收光谱中出现红移（从 366nm 到 386nm）和蓝移（从 435nm 到 386nm），分别表明 Br 和 Cl 向 CsAgBiCl 和 CsAgBiBr 的迁移。使用 X 射线衍射（XRD）和 X 射线光电子能谱（XPS）表征的薄膜分别在 2θ=10.90°和结合能 158.1eV 处显示出峰值，这分别对应于薄膜表面形成 Bi-O 键。此外，XRD 研究表明，CsAgBiCl 薄膜的衍射峰的 2θ 偏移较小，而 CsAgBiB 薄膜的 2θ 偏移较大，这进一步证实了 Cl 和 Br 从一个薄膜到另一个薄膜的迁移。XPS 研究证实了组成的变化，随着 CsAgBiCl/CsAgBiBr 薄膜加热时间的增加，Br/Cl 的浓度逐渐增加。所有这些研究都证实了卤化物离子在双钙钛矿薄膜中的热扩散。此外，从吸收光谱的指数衰减中，计算出卤化物（Br）离子扩散的速率常数，该常数在室温下从 1.7×10 s 增加到 150°C 时的 12.1×10 s。温度相关的速率常数遵循阿仑尼乌斯行为，并为溴化物（氯化物）离子迁移提供了 0.42eV（0.35eV）的活化能。与 CsAgBiBr 晶片的报道值（约 0.20eV）相比，估计值较大，表明 CsAgBiBr/Cl 薄膜中卤化物离子的迁移率较慢。CsAgBiBr 薄膜表面形成的 BiOBr 钝化层可能是本工作中阴离子扩散缓慢的一个可能原因。离子迁移缓慢表明薄膜稳定且质量高。
