Abstract
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.