甲基銨碘化鉛單晶鈣鈦礦材料的光學物性探討

Abstract

本論文合成出單晶CH3NH3PbI3鈣鈦礦材料並對其進行變溫XRD及光激螢光實驗，探討其光學及物理性質。 在變溫XRD實驗中，我們觀察到從四方相轉變成立方相之相變溫度發生在320K，與文獻量測到的327.4K相差不遠，然而從四方相轉變成正交相之相變溫度發生在90K，明顯與文獻量測之相變溫度在162.2K有很大的差異，我們推測此一差異主要來自於以不同方式合成MAPbI3單晶所造成。 在變溫光激螢光實驗中，在360K至11K的溫度範圍下，我們僅觀察到一支光激螢光訊號，在相變溫度附近並沒有量測到其他螢光訊號。在290K至11K的溫度下，此螢光訊號的位置沒有發生改變；而在300K到360K的溫度下，螢光訊號的位置發生藍移的現象，此實驗結果與理論計算之推測相互衝突，此外，光激螢光訊號與吸收峰邊界的位置有所差距，我們推測此光激螢光訊號可能不是來自於能帶躍遷。 我們進行一連串實驗來驗證，但結果證明推測並不正確，光激螢光訊號與吸收峰邊界的差距並非是晶體表面的缺陷態及有機陽離子偶極矩排列所造成。最後我們引用band filling效應來進行推測，原因可能來自於光激發後載子累積在能帶邊界，造成能隙值增加，使得光激螢光訊號與吸收峰邊界的位置呈現偏差所致。

Organo-lead trihalide hybrid perovskites (MAPbX3 ; MA= CH3NH3+, X=Cl-, Br- or I-) have emerged as promising new materials for solar cells. Although efficiencies of perovskite solar cells have achieved exceeding 20%, there are many questions remained to be answered, such as the crystal chemistry of these materials, the mechanism of exciton separation, and the nature of charge transfer. Therefore, research in perovskite intrinsic properties are very important. In particular, solution-prepared MAPbX3 single crystals show long carrier diffusion lengths and low trap–state densities compared to photovoltaic quality silicon. Large MAPbX3 single crystals have been successfully synthesized via different methods over the past few years. In this work, we study the structral and photo-physical characteristics of CH3NH3PbI3 perovskite single crystals with a size of nearly 1 cm3 prepared by using inverse temperature crystallization method. Temperature dependent powder X-ray diffraction reveals phase transition from tetragonal to cubic phase at 320 K and from tetragonal to orthorhombic phase at 90 K, respectively. Measurements of temperature dependent photoluminescence from 360 K to 11 K and cathodoluminescence clearly indicated that the observed luminescence peaks from the MAPbI3 single crystals are not resulted from the surface states, defects or the excitonic recombination. We speculate that the observed PL peak above the absorption bandedge is originated from the band filling effect due to carriers accumulate near the bandgap.