Expanding the Cage of 2D Bromide Perovskites by Large A‑Site Cations

Two-dimensional
(2D) halide perovskites have outstanding optoelectronic
properties, and they feature a variety of organic cation spacers and
cage A-site cations that can be incorporated into their structures.
It has recently been reported that the Goldschmidt tolerance factor
can be relaxed and expanded in iodide 2D perovskites. Bromide 2D perovskites,
whose multilayer structures and optical properties are much less studied,
provide a great platform for studying structure–property relationships
for 2D perovskites with large A-site cations. Herein, we report the
synthesis and structure of three new 2D bromide perovskites(BA)2(MHy)2Pb3Br10 (BA, butylammonium;
MHy, methylhydrazinium), (BA)2(EA)2Pb3Br10 (EA is ethylammonium), and (BA)2(DMA)­Pb2Br7 (DMA is dimethylammonium). We compared them
with other 2D perovskites with different A-site cations but with the
same spacer and layer thickness. Single-crystal structures show that
the Pb–Br bonds are elongated to accommodate the large A-site
cations. Additionally, the octahedra in (BA)2(MHy)2Pb3Br10 and (BA)2(EA)2Pb3Br10 are highly distorted, and their
different stacking patterns of the inner and outer layers lead to
the formation of the n = 3 phases. Density functional
theory calculations show that 2D perovskites with larger A-site cations
(e.g., DMA, MHy, and EA) have smaller band dispersions and larger
effective masses than those with Cs+ and MA. (BA)2(MHy)2Pb3Br10 also exhibits one
of the largest Rashba splittings in the literature. Structures with
large cage cations also exhibit high band gaps within the same n number and short photoluminescence (PL) lifetimes. Temperature-
and power-dependent PL measurements reveal that the broad shoulder
in the PL peak originates from the trap states.

二维（2D）卤化物钙钛矿（halide perovskites）具有优异的光电特性，其结构中可引入多种有机阳离子间隔基与笼型A位阳离子。近期研究表明，碘化物基2D钙钛矿中的戈尔德施密特容忍因子（Goldschmidt tolerance factor）可被放宽与拓展。溴化物基2D钙钛矿的多层结构与光学特性研究相对较少，但其为探究含大尺寸A位阳离子的2D钙钛矿的构效关系提供了极佳的研究平台。本文报道了三种新型2D溴化物钙钛矿的合成与晶体结构：(BA)₂(MHy)₂Pb₃Br₁₀（BA为丁基铵（butylammonium），MHy为甲基肼阳离子（methylhydrazinium））、(BA)₂(EA)₂Pb₃Br₁₀（EA为乙基铵（ethylammonium））以及(BA)₂(DMA)Pb₂Br₇（DMA为二甲基铵（dimethylammonium））。我们将这三种材料与其他具有相同间隔基与层厚、但A位阳离子不同的2D钙钛矿进行了对比。单晶结构分析表明，为适配大尺寸A位阳离子，Pb-Br键发生了伸长。此外，(BA)₂(MHy)₂Pb₃Br₁₀与(BA)₂(EA)₂Pb₃Br₁₀中的八面体发生了显著畸变，其内外层不同的堆积模式形成了n=3相。密度泛函理论（density functional theory）计算结果显示，相较于含Cs⁺与MA（甲胺阳离子，methylammonium）的2D钙钛矿，含大尺寸A位阳离子（如DMA、MHy与EA）的钙钛矿具有更小的能带色散与更大的有效质量。其中，(BA)₂(MHy)₂Pb₃Br₁₀的拉什巴分裂（Rashba splittings）强度位列已有文献报道的最高值之一。含大尺寸笼型阳离子的结构在相同n值下同样具有较宽的带隙，且光致发光（PL）寿命较短。温度与功率依赖的光致发光测试表明，PL峰中的宽肩峰源于陷阱态（trap states）。