鈣鈦礦在太赫茲光譜的聲子模態研究

Abstract

近年來，鈣鈦礦展現很多優點，例如：簡單的製程、較低的成本、較強的吸收和好的載子遷移率。以鈣鈦礦維基底的元件在這幾年快速的發展，其轉換效率已經可以媲美最有效的無機材料了。隨著元件的快速發展，為了能更有效的提升元件性能，對於材料物理特性的理解與掌握就更為重要了。在這篇論文，我們使用太赫茲時域分析系統來研究分析鈣鈦礦在低頻的振動模式。鈣鈦礦MAPbI3是由共頂的金屬鹵化物八面體所組成的，在室溫下是呈現四方晶相。在室溫下長在係基板上的MAPbI3，我們觀察到在1 THz和2 THz有模態分裂的現象，這現象通常發生在低溫斜方晶相的MAPbI3。而我們認為這是由於鈣鈦礦與基板之間的晶格不匹配引發應變，造成無機八面體的扭曲。當MAPbI3的陽離子MA替換成碳鏈較長的乙基碘化銨(EA)，鈣鈦礦的結構會從三維結構轉為二維的層狀結構。EAPbI3在太赫茲光譜上隨著陽離子尺寸不同有兩個不同的變化：在1 THz的模態消失，以及在1.4 THz附近的模態有紅移。這可能是因為在結構轉換時，削弱了Pb-I-Pb的彎曲振動。而在1.4 THz的紅移可以理解為Pb-I之間的鍵長增加。

Hybrid halide perovskites have recently exhibited their great advantages of simple fabrication, lower cost, strong absorption, and good mobility. Development of perovskite-based photovoltaic devices have been accelerated in last few years and reaches the power conversion efficiency comparable with the most efficient inorganic materials. Synchronizing with the fast development of devices, a detailed understanding of the fundamental device physics is essential for the better control of performance of perovskite-based devices. In this work, we have characterized the low-frequency vibrational properties of hybrid halide perovskites using broadband time-domain terahertz spectroscopy. Methylammonium lead iodide (MAPbI3) perovskites consist of layers of corner-sharing metal halide octahedral and has tetragonal crystal structure at room temperature. For room-temperature MAPbI3 grown on Si substrates, we observed the splitting of modes near 1 and 2 THz, which is typically observed for orthorhombic MAPbI3 at low temperature below 160 K. We believe this mode-splitting is due to strain induced by the symmetry and lattice mismatch against the substrates, leading to the distortion of inorganic cages. The room-temperature mode-splitting was not observed for MAPbI3 grown on Si. When the cation component is replaced from MA to ethylammonium (EA) iodide, the longer cation chain changes the perovskite structure from corner-sharing to face-sharing in only one dimension. And three-dimensional structure changes to the two-dimensional layered structures of alkylammonium lead iodide perovskites. Terahertz spectroscopy on EAPbI3 shows two characteristics related with the dimensional transform; disappearance of phonon modes at 1 THz and a large redshift of the mode at 1.4 THz. This may be associated with the structure change from 3D to 2D, weakening the bending vibration of Pb-I-Pb links. The redshift of Pb-I stretching mode at 1.4 THz can be understood as the lengthening of Pb-I link as the size of the cation becomes bigger.