Understanding Hybrid Perovskites from MD Simulations: From MAPI to FAPI
Marcelo Carignano a b
a Qatar Environment and Energy Research Insitute, PO BOX 5825, DOHA, 0, Qatar
b Hamad bin Khalifa University
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, Marcelo Carignano, presentation 092
Publication date: 7th November 2016

First principles molecular dynamics simulations offer the possibility to probe the internal ionic degrees of freedom of materials with a level of detail hardly reachable with experimental techniques. In this work we considered methylammonium lead iodide (MAPI) and formamidinium lead iodide (FAPI) as prototypical examples of hybrid perovskites for photovoltaic applications and investigate the rotational degrees of freedom of the organic cations and other static and dynamic properties. We also studied systems containing a mix of the two cations in different proportions. The simulations were performed on a variety of supercells with different number of atoms, obtained by replicating the stoichiometric building unit. Interestingly, there is a strong odd-even effect in the behavior of the system with respect to the number of replicas of the building unit in the simulation supercell. In our studies the simulated systems contains up to 1500 atoms. Careful analysis of the results reveals that a large simulation supercell is indeed needed to capture the complexity of these materials. For the MAPI case, only one rotational direction is relevant. For the FAPI case, the molecular architecture imposes the use of at two directions. The rotational behavior of the organic cations is analyzed in terms of an expansion in cubic harmonics. In all cases it is observed that the perovskites have a rotational heterogeneous dynamics as the system cools down, implying the existence of rotational glassy states at lower temperatures. Further analysis of the temperature evolution of the systems based on group theory will be discussed.

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