Optical Properties and Photostability Improvement of CH₃NH₃PbI₃ Treated by Iodide of Long H₃N(CH₂)₁₀COOH Bifunctional Cation in “2D/3D” and “Monolayer” Passivation Modes

Surface passivation by various organic molecules is a widely used approach to compensate surface defects and to improve a stability of hybrid halide perovskites. For commonly used cationic passivators, the formation of 2D phases and related heterostructures is considered as an essential part of the passivation process. However, there is an intriguing fundamental question: is it possible to achieve effective and stable passivation by the thinnest possible layer? In this article, we applied an iodide salt of a new bulky bifunctional 11-carboxy-decylammonium cation (further AUDA+) as a passivator which can not only passivate both VI• and VMA′ defects but can also expectedly form on the 3D perovskite surface a dense and stable “monolayer” assembled due to the strong hydrogen bonds between the terminal carboxylic groups and, hereby, suppressing the passivator migration in the bulk of the perovskite. Applying a wide set of methods such as steady-state and time-resolved photoluminescence spectroscopy, X-ray diffraction, and scanning electron microscopy, we revealed that the passivation regime could be controlled by adjustment of two treatment parametersthe concentration of AUDA+I– and the post-annealing temperature resulting in either the formation of 2D/3D heterostructures or surface defect passivation by the “monolayer” without the formation of additional phases. The “monolayer” regime was found to provide a greater improvement of optical properties: photoluminescence intensity and average charge carrier lifetime increase by 10 and 2.5 times, respectively, and demonstrate significantly better properties after long-time light soaking. These results indicate that a fine tuning of passivation conditions provides a significant increase in photostability even without formation of a 2D capping layer, thus revealing new possibilities to enhance perovskite solar cells’ lifetime.表面钝化（surface passivation）是补偿杂化卤化物钙钛矿（hybrid halide perovskites）表面缺陷、提升其稳定性的常用手段。对于常用的阳离子钝化剂（cationic passivators）而言，二维（2D）相及其相关异质结构（heterostructures）的形成被认为是钝化过程的核心环节。然而，一个值得深究的基础问题随之而来：能否通过最薄的钝化层实现高效且稳定的钝化效果？ 本文中，我们使用一种新型大体积双功能11-羧基癸基铵阳离子（11-carboxy-decylammonium cation）的碘盐（后文简称AUDA+）作为钝化剂：该钝化剂不仅可钝化VI•与VMA′两类缺陷，还可通过末端羧基间的强氢键（hydrogen bonds）作用在三维钙钛矿（3D perovskite）表面组装形成致密稳定的"单分子层（monolayer）"，从而抑制钝化剂向钙钛矿本体迁移。 我们采用稳态光致发光光谱（steady-state photoluminescence spectroscopy）、时间分辨光致发光光谱（time-resolved photoluminescence spectroscopy）、X射线衍射（X-ray diffraction）以及扫描电子显微镜（scanning electron microscopy）等多种表征手段，研究发现可通过调控两项处理参数——AUDA+I–的浓度与后退火温度（post-annealing temperature）——来调控钝化模式：要么形成2D/3D异质结构，要么通过"单分子层"实现表面缺陷钝化而不产生额外相。 研究表明，"单分子层"钝化模式可更显著地优化材料光学性能：光致发光强度（photoluminescence intensity）与载流子平均寿命分别提升10倍与2.5倍，且经过长时间光浸泡（light soaking）后仍表现出更优异的性能。 上述结果表明，即便不形成二维覆盖层，精准调控钝化条件也可显著提升材料的光稳定性（photostability），从而为延长钙钛矿太阳能电池（perovskite solar cells）的服役寿命提供了新的可行路径。