有機-鹵素化鉛鈣鈦礦太陽能電池薄膜之結晶行為解析與調控

Resolving and Modulating the Crystallization Behavior of the Organolead Perovskite Thin Film Solar Cells

賈皓中(Hao-Chung Chia)

指導教授 ： 鄭有舜

國立清華大學/工學院/化學工程學系/碩士(2017年)

摘要

本研究使用時間解析掠角X光散射 (In-situ GIXS)輔以光電子能譜 (XPS)及X光繞射 (XRD)研究以PbCl2+MAI為前驅物所形成的CH3NH3PbI3-xClx鈣鈦礦薄膜在高溫退火過程中的結晶行為。前驅物在退火過程初期先形成中間相(CH3NH3)2Pb(Cl2I2)·CH3NH3I此結構中之八面體堆疊的層狀結構與2-D鈣鈦礦結構一致，而能在加熱過程後期中透過氯離子的脫附而誘導與夾層的碘離子組合成3-D鈣鈦礦晶體(MAPbI3)結構。鈣鈦礦晶體成長的過程中可再細分成兩個階段，第一階段的來源為非晶前驅物，但此階段生成的鈣鈦礦薄結晶數量少；而第二階段來源則為前述—由中間相轉化結晶，其所產生的結晶繼承中間相的方向性及結晶大小。而受不同退火溫度下不同結晶動力之影響，兩種階段的比例也會改變:較高溫退火 (120 °C)時，第一階段產生的隨機取向晶粒比例會增加；而在較低溫 (105) °C則幾乎未有第一階段方向無序晶體出現，最後薄膜晶體特性則由第二階段所產生偏好(100)面朝上的立方晶體主導。這兩個階段結晶動力學參數皆經由Avrami分析取得鈣鈦礦晶體結晶的Avarami反應級數 (n)及反應常數 (k)。第一階段成長n值約為1，而第二階段約為3.5。而透過隨溫度變化取得的反應常數，所獲取相對應的結晶生成活化能在第一階段較高為180 kJ/mol，而第二階段活化能為66 kJ/mol與中間相的生成活化能124 kJ/mol相近，支持由中間相熔融轉化的結晶機制。而本文第二部分則在鈣鈦礦前驅物中加入表面覆有碘離職子配體的硫化鉛奈米晶體(PbS/I^-)，作為中間相的結晶催化核。而能有效的加速中間相的形成與相轉變，導致大幅增加鈣鈦礦結晶的速率、加強的方向性及結晶度，亦增加薄膜表面覆蓋度，而使元件光電效率也從14%提升至17%。

關鍵字

有機-無機鈣鈦礦 ； 結構演進 ； 中間相 ； X光散射

並列摘要

In this work, we have used combined in-situ grazing-incidence X-ray scattering, X-ray diffraction, and X-ray photoemission spectroscopy, to resolve the intermediate structure and crystallization kinetics of CH3NH3PbI3-xClx perovskite. After spin-cast from a DMF solution, the precursor PbCl2 and MAI of 3:1 molar ratio would preferentially form an intermediate phase (CH3NH3)2Pb(Cl2I2)·CH3NH3I in the early stage of high temperature (ca. 110 。C) annealing. The resolved intermediate structure (L1) comprises layers of 2-D ordered lead-halide octahedra intercalated with MAI layers, and could direct 3-D perovskite formation during prolong annealing, in a two-stage formation process. At 110 。C, formation of highly oriented L1 phase dominates over randomly oriented perovskite formation in the first stage; in the second stage, oriented perovskite crystals are formed mainly from the L1-templated conversion. Kinetic competition and conversion between the L1 phase and perovskite formation in the two-stage process are elucidated by Avrami analysis and the corresponding activation energy Ea extracted. The Avrami exponent n = 1 is obtained for the direct formation of perovskite in the first stage with Ea1 =180 kJ/mol, whereas n = 3.5 and Ea2 = 66 kJ/mol are obtained for the perovskite crystals formed from L1-templated conversion. The L1 phase is of a close Ea value of 124 kJ/mol, supporting the proposed L1-to-perovskite conversion.
In the second part of the study, we have used inorganic nanocrystals of PbS surrounded with MAI as embedded crystallization nuclei to enhance nucleation-dominated formation of the intermediate phase. With PbS nanocrystals mixed into the perovskite precursor solution, the hence spin-cast precursor film is found to have substantially enhanced perovskite crystallization rate owing to fast formation and conversion of the L1 phase at 110 。C annealing. Correspondingly observed are improved crystallinity, orientation along vertical direction, and surface coverage of perovskite. These lead to an enchantment in PCE to 17% from the 14% for a pristine case, proving the concept of using cryptographically aligned nanocrystals as seeded crystallization nuclei.

並列關鍵字

Organic-inorganic perovskite ； structural evolution ； intermediate phase ； grazing incidence X-ray scattering

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