Dynamics in perovskite photovoltaics

A rat race for the highest efficiency perovskite solar cell has emerged out of the initial report that organo-metal halide perovskites can function as a photovoltaic dielectric. The last three years have seen an astonishing increase from ~10% in 2012 to more than 20% efficiency in 2015. 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 live 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. In an attempt to stabilize the perovskite under ambient conditions researchers have placed a single layer of the 2D-material hexagonal Boron-Nitride on top. This material is transparent for light, but does not allow humidity to pass trough. How this interface affects the molecular ordering is however unknown and will influence the solar cells efficiency. We propose to focus on the orientation of the MA molecules in the bulk material as well as at the interface with a 2D material and determine under which conditions they show a long range ordered behaviour. The orientation of the MA molecules cannot be uniquely obtained from experiment, however large scale molecular dynamics have shown to do just that. 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?”