Direct Evidence of Chlorine-Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO2

摘要

Mixed halide hybrid perovskites CH3NH3PbI3-x,Cl-x (MAPIC1) show higher charge carrier diffusion lengths, improved solar cell characteristics, and enhanced stability as compared to their single halide counterparts CH3NH3PbI3 (MAPI). This is surprising in view of the fact that the actually observed Cl content is very low (x < 0.1) as the incorporation of large amounts of Cl in the MAPI lattice is crystallographically not possible. Understanding the role of chlorine in these systems is therefore of the utmost importance and has been the subject of several experimental and theoretical studies. The main conclusions are that Cl addition leads to larger grain sizes and preferential crystalline orientation; however, so far a more quantitative description of these aspects is lacking. Synchrotron X-ray diffraction microscopy is a very promising technique in this respect, as it allows for the direct imaging of crystalline domains oriented in a selected direction with a resolution of some hundreds of nanometers. When imaging perovskite thin films deposited on an FTO-covered glass substrate containing a dense polycrystalline TiO2 layer at the perovskite (220) reflection, diffraction micrographs show an increase in surface coverage of oriented grains from 2.5% (MAPI) to 5.5% (MAPIC1; x = 0.07). This is a clear evidence of the increased preferential orientation in the mixed perovskite. At the same time the integrated intensity increases by a factor of 4, which can be related to a larger perovskite grain size. When a single crystalline TiO2(001) substrate is used for the perovskite film deposition, both effects are dramatically enhanced in the case of MAPIC1, leading to a surface coverage of 80%, a 55-fold increase in integrated diffracted intensity with respect to MAPI, and a mean crystallite size of 2.5 mu m. In contrast, MAPI does not show any differences when grown on TiO2(001), which demonstrates that chlorine locates preferentially at the TiO2 perovskite interface and induces the growth of larger crystallites showing preferential orientation along the (110) direction.