近年來鈣鈦礦材料由於其高能量轉換效率引起許多研究學者關注，而methylammonium lead iodide (CH3NH3PbI3, MAPbI3) 被認為是極具潛力的鈣鈦礦材料，然而化學穩定性是該材料應用於太陽能電池上必須面臨的一大挑戰。有許多文獻研究了熱分解反應的機制且提供了兩種可能的熱分解反應路徑，兩種路徑之間的差異就在於碘離子反應的對象，碘離子和CH3NH3+中N上的H反應生成HI及CH3NH2，而跟C反應則會得到CH3I以及NH3。本研究利用密度泛函理論 (Density Functional Theory, DFT) 計算MAPbI3不同熱分解反應過程中構型及位能的變化，並且建立了兩套計算模型模擬實際熱分解過程，分別為氣體模型以及晶格模型。在氣體模型中我們發現反應較容易生成HI及CH3NH2，因為反應的初始結構透過Boltzmann distribution的計算發現在氣態狀態下碘離子是無法直接與C反應，所以我們推斷在氣體模型中主要的熱分解產物是HI及CH3NH2；接著透過晶格的模型計算我們發現晶格的環境可以同時提供兩種反應位向，而且兩種反應的活化能是相似的，意味著在晶格模型中MAPbI3兩種熱分解反應都會發生。我們的研究提供了純粹的晶格環境來去探討分解的機制。本研究目的是了解MAPbI3熱分解機制並希望能夠提供相關研究學者一些參考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 is one of the challenge problems retarding for its wide applications. The thermal degradation mechanism of MAPbI3 perovskite remains unclear. In this study, we employ firstprinciple density functional theory to investigate the thermal degradation mechanisms of MAPbI3 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 substituent reaction (SN2) using the iodine as the nucleophile to attack the carbon atom of CH3NH3+ to yield the products of iodomethane (CH3I) and ammonia (NH3). Our calculations based on crystal structure of MAPbI3 perovskite provides an kinetics data under an “environment-free” conditions allowing us to assess the “intrinsic” (in)stability of MAPbI3 perovskite under heat stress. Our calculations show the SN2 reaction is a two-step one, where the CH3-I-Pb-X intermediate formed in crystal after the release of NH3 gas. In addition, both pathways have comparable activation energy indicating that extrinsic factors could lead to different degradation pathways.
