Demonstration of the Order-Disorder Character of the CH3NH3PbI3 Polymorphic Transition and its Implications on the Lattice Stability
antonino la magna a, emanuele smecca a, giovanni mannino a, ioannis deretzis a, alessandra alberti a, salvatore sanzaro, youhei numata b, tsutomu miyasaka b
a CNR-IMM, zona industriale strada VIII n°5, Catania, 95121, Italy
b Graduate School of Engineering, Toin University of Yokohama, Kurogane-cho 1614, Aoba-ku, Yokohama, 225-8503
c Department of Mathematical and Computational Sciences, Physics and Earth Sciences, University of Messina, V. le F. Stagno d’Alcontres 31, Messina 98166, Italy
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
Oral, alessandra alberti, presentation 120
Publication date: 7th November 2016

Issues on stability and durability of hybrid perovskites are dramatically delaying the large dissemination of low cost/high yield related technologies for photovoltaic, light sensing and emitting purposes. While several combinations of cations and anions are under investigation to solve the issues, the advance of the technologies is currently forced to pass through the rationalization of the phenomena occurring into the hybrid lattice under conditions which mimic the material operation. In this framework, we study the structural modifications of the MAPbI3 lattice upon recursive thermal cycles from 30°C to 80°C in different annealing environments. Although it is known that the MAPbI3 lattice structure changes from tetragonal to cubic above 60°C, here we demonstrate the existence of a temperature-dependent order parameter which pilots a continuous lattice modification by approaching the transition temperature. This modification involves the entire lattice through the progressive change of the rotational state of the organic MA+ cations that gradually gain a z component out of the starting x-y bidirectional ordering, culminating in the symmetrization of the three crystallographic axes after full MA+ randomization (T~60°C). We reveal that this order-disorder transition opens a weakness-point in the structure that facilitates the access of external species into the cage. This has irrelevant effects in presence of N2 molecules, but dangerous impacts in air. Our findings extend to predict the transition temperature to be critical also with regards to the diffusion of atomic species from boundary materials and indeed invoke tailored solutions.

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