Annealing Temperature Dependence of Phonon Modes and Crystallinity on Perovskites Studied by Terahertz Spectroscopy

• Main Authors: Chen, Ting-Yang, 陳廷揚
• Other Authors: Ahn, Hyeyoung
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/6c9pyv

碩士 === 國立交通大學 === 光電工程研究所 === 107 === Hybrid halide perovskites have become one of the most promising solar cell materials, due to their great advantages of simple fabrication, lower cost, strong absorption, and good mobility. There have been tremendous efforts to improve the conversion efficiency of perovskite-based photovoltaic devices and now it reaches over 22.5 %. Recently, two-dimensional (2D) perovskites have attracted great attention in light emitting device community. Some studies have pointed out that 2D perovskites have higher stability under atmospheric environment. The thermal stability is another very important property for the out-door applications. In this work, we employed the broadband time-domain terahertz spectroscopy to study the low frequency phonon vibration mode of the alkylammonium lead iodides (XAPbI3); ethylammonium (EAPbI3), propylammonium ((PA)2PbI4), and butylammonium lead iodide ((BA)2PbI4). We found that the phonon mode corresponding to the bending vibration of I-Pb-I disappears for three layered perovskites and those at 1.5 and 2 THz due to the bonding length variety of Pb-I blueshift as the length of alkylammonium chain increases. In order to understand the thermal stability of layered perovskites, we measured the phonon modes under different thermal annealing temperatures. For three-dimensional perovskite, CH3NH3PbI3 (MAPbI3), the onset of blueshift of phonon mode is observed when the temperature reaches at about 65 ℃, where tetragonal-to-cubic phase transition occurs. When covered with monolayer graphene or transition metal dichalcogenides, the blueshift of mode frequency was not observed, implying the improvement thermal stability. For layered perovskites, the phonon modes at 1.5 and 2 THz is nearly unchanged as the temperature increases upto 100 ℃, indicating high thermal stability of 2D perovskites. The x-ray diffraction measurements show that the thermal annealing of 2D perovskites improves the crystallinity to ordered phase.