近幾年鈣鈦礦材料的高能量轉換效率引起許多研究者關注的吸光材料，而methylammonium lead iodide (CH3NH3PbI3, MAPbI3) 被認為是極具潛力的鈣鈦礦材料之ㄧ，然而其化學不穩定性是將該材料應用於太陽能電池上所必須面臨的一大挑戰。近來有許多針對熱穩定性研究的文獻提供了兩個可能的熱分解反應，兩個分解反應之間的差異就在於與碘離子反應的對象，碘離子和CH3NH3+中N上的H反應生成HI及CH3NH2，而跟C反應則得到的產物則為CH3I以及NH3。本研究利用密度泛函理論 (Density Functional Theory, DFT) 計算MAPbI3兩種熱分解反應過程中構型以及位能的變化。我們建立了兩套計算模型試圖模擬實際熱分解過程，分別為自由氣體模型以及晶格表面模型，在自由氣體模型中生成CH3I以及NH3所需的活化能較生成HI及CH3NH2低，然而由於初始結構透過Boltzmann distribution的計算發現在氣態狀態下碘離子是無法跟C反應的。透過晶格表面的模型計算得到的活化能結果與氣態模型類似，且碘離子與C或N上的H反應的分布是接近，依此結果推論出熱分解的主要產物應為CH3I以及NH3。本研究的結果希望能夠提供相關研究者一些線索去設計具有良好熱穩定性的鈣鈦礦太陽能電池。;The poor stability of organometallic halide perovskite, especially the CH3NH3PbI3 (MAPbI3), in high temperature environment (up to 85 ºC) is one of the challenge problems retarding for its wide applications. The thermal degradation mechanism of CH3NH3PbI3 perovskite remains unclear. In this study, we employ firstprinciple density functional theory to investigate the energetic and structural mechanisms of the thermal degradation of CH3NH3PbI3 perovskite. We focus on studying the two reaction pathways of iodine with the CH3NH3+: one reaction is the proton abstraction reaction from methylammonium to iodine and the products are hydrogen iodide (HI) and methylamine (CH3NH2). The other reaction is a SN2 substituent reaction using the iodine as the nucleophile to attack the carbon atom of CH3NH3+ to yield the products of iodomethane (CH3I) and ammonia (NH3). The crystal structures of CH3NH3PbI3 perovskite with PbI2-terminated and MAI-terminated surfaces are employed in this study. Our calculations suggest that the SN2 substituent reaction requires lower activation energy than the proton abstraction reaction. Namely, the main products of the thermal detraction of CH3NH3PbI3 perovskite are CH3I, NH3, and PbI2. Our study gives clues to rationally design of the thermally stable organic cations. To verify the overall mechanism, we will scan the 2D energy profiles to make sure that PbI2 generates in our future work.
