Atomic structure of Methylammonium-PbI3 in its room-temperature tetragonal phase.

Dynamics of Perovskite Photovoltaics

Between 2012 and 2015 a rat race for the highest efficiency perovskite solar cell has occupied many materials scientists worldwide. It emerged out of the initial report that organo-metal halide perovskites can function as a photovoltaic dielectric. In only three years an astonishing increase from 10% in 2012 to more than 20% efficiency in 2015 has been achieved.

This material is made up of elements abundantly found in nature, has a simple production procedure and can therefore be used to produce cheap solar cells. However, the main issue preventing perovskites solar cells from going to market is their lack of stability. Where silicon solar cells have life times of about 20 years, a perovskite solar cells typically breaks down after several days. The bad material strength, however, is at the same time accompanied with the presence of polar phonons and ’freely’ rotatable methylammonium (MA) molecules in the material. The vibrations of the ionic lattice and the intrinsic dipole moment of the MA molecule can screen slowly oscillating electric fields.

In a foregoing study we have started to understand the role of these polar phonons in photo excited state of the perovskite. Currently, we study the finite temperature structure of hybrid perovskites close to the phase transition temperature. Since the orientation of the MA molecules cannot be uniquely obtained from experiment, large scale molecular dynamics can be of help. With these calculations we would like to find an answers to the superseding question:

Do the methylammonium molecules in the record breaking MAPbI3 perovskite make an essential contribution to its solar cell success, and if so, how?