Uniaxial Expansion of the 2D Ruddlesden–Popper Perovskite Family for Improved Environmental Stability

The
unique hybrid nature of 2D Ruddlesden–Popper (R–P)
perovskites has bestowed upon them not only tunability of their electronic
properties but also high-performance electronic devices with improved
environmental stability as compared to their 3D analogs. However,
there is limited information about their inherent heat, light, and
air stability and how different parameters such as the inorganic layer
number and length of organic spacer molecule affect stability. To
gain deeper understanding on the matter we have expanded the family
of 2D R–P perovskites, by utilizing pentylamine (PA)2(MA)n−1PbnI3n+1 (n = 1–5,
PA = CH3(CH2)4NH3+, C5) and hexylamine (HA)2(MA)n−1PbnI3n+1 (n = 1–4, HA = CH3(CH2)5NH3+, C6) as the organic
spacer molecules between the inorganic slabs, creating two new series
of layered materials, for up to n = 5 and 4 layers,
respectively. The resulting compounds were extensively characterized
through a combination of physical and spectroscopic methods, including
single crystal X-ray analysis. High resolution powder X-ray diffraction
studies using synchrotron radiation shed light for the first time
to the phase transitions of the higher layer 2D R–P perovskites.
The increase in the length of the organic spacer molecules did not
affect their optical properties; however, it has a pronounced effect
on the air, heat, and light stability of the fabricated thin films.
An extensive study of heat, light, and air stability with and without
encapsulation revealed that specific compounds can be air stable (relative
humidity (RH) = 20–80% ± 5%) for more than 450 days, while
heat and light stability in air can be exponentially increased by
encapsulating the corresponding films. Evaluation of the out-of-plane
mechanical properties of the corresponding materials showed that their
soft and flexible nature can be compared to current commercially available
polymer substrates (e.g., PMMA), rendering them suitable for fabricating
flexible and wearable electronic devices.

2D层状Ruddlesden–Popper（R–P）钙钛矿独特的杂化特性，不仅赋予其电子性质的可调控性，还使其相较于三维同类材料，能够制备出环境稳定性更优异的高性能电子器件。然而，目前关于其固有热、光、空气稳定性，以及无机层数目、有机间隔分子长度等不同参数如何影响稳定性的相关信息仍较为有限。为了对该问题获得更深入的理解，我们拓展了2D R–P钙钛矿的家族：以戊胺（pentylamine, PA）和己胺（hexylamine, HA）作为无机层间的有机间隔分子，分别构建了(PA)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁（n=1~5，PA=CH₃(CH₂)₄NH₃⁺，记为C5）与(HA)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁（n=1~4，HA=CH₃(CH₂)₅NH₃⁺，记为C6）两个全新的层状材料系列，最高分别对应n=5与n=4的层数。所得到的化合物通过物理与光谱学方法组合进行了全面表征，其中包括单晶X射线衍射分析。借助同步辐射光源的高分辨粉末X射线衍射研究，首次揭示了高层数2D R–P钙钛矿的相变行为。有机间隔分子长度的增加并未对其光学性质产生影响，但却对所制备薄膜的空气、热及光稳定性有着显著作用。针对有封装与无封装条件下的热、光、空气稳定性开展的系统性研究表明，部分特定化合物可在相对湿度（relative humidity, RH）为20%~80%±5%的环境中保持空气稳定性超过450天；而通过对相应薄膜进行封装，可使材料在空气中的热与光稳定性呈指数级提升。对该系列材料的面外力学性能进行评估后发现，其柔软且具备柔性的特性可与当前商用聚合物基底（如聚甲基丙烯酸甲酯，PMMA）相媲美，因此适合用于制备柔性可穿戴电子器件。