Direct Observation of Intrinsic Twin Domains in Tetragonal CH3NH3PbI3
Mathias Uller Rothmann a, Wei Li b, Ye Zhu, Udo Bach, Leone Spiccia, Joanne Etheridge, Yi-Bing Cheng
a Monash University, Department of Materials Science and Engineering, 22 Alliance Lane, Monash University, VIC 3800, Australia
b Monash Centre for Electron Microscopy, 10 Innovation Walk, Monash University, VIC 3800, Australia
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Poster, Mathias Uller Rothmann, 116
Publication date: 7th November 2016

Organic­-inorganic hybrid perovskite solar cells have attracted a lot of interest recently due to their high efficiency (over 22 %) solution-based processing. While much research has been done into the general structure of the CH3NH3PbI3 (MAPbI3) perovskite material using X­-ray, neutron diffraction, and spectroscopy analyses, transmission electron microscopy (TEM) investigations have not been done extensively. This lack is possibly due to the highly damaging effect the electron beam has on the soft perovskite material, leading to many inconsistent findings being reported. In this work, low dose conditions have been specifically developed which allow for detailed study of the micro- and crystal structure of MAPbI3 using TEM. It was found that fragile crystallographic twinning mirrored across the {112} planes is intrinsic to MAPbI3 at room temperature, but disappears after a few minutes of very low electron dose rate (approximately 102 e/(nm2 s)) exposure. The twinning was found to disappear once the MAPbI3 was heated above its tetragonal to cubic phase transition temperature (57 °C), but reappeared when allowed to cool back down to room temperature. This indicates that the twinning is a result of strain alleviation in the crystal following a change in the unit cell volume during phase transition. Local high symmetry twin planes in other materials have been shown to act as charge transport highways, significantly facilitating movement of charges throughout the crystal. Since most reported MAPbI3 films undergo an annealing step above the phase transition temperature, the twins can be considered to be ubiquitous in MAPbI3-based solar cells and can be part of the explanation of the excellent charge transport properties of this material.

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