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.