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?

In recent years the halide perovskites, a new highly promising material for light harvesting technology, caught the attention of many scientists around the world. The atomic crystal structure of this material is an intricate electrostatic puzzle that we wanted to solve. The structure can not be described within the classical picture of atoms connected by springs that jiggle around at room temperature. Its inorganic framework of lead and halide atoms enclose organic molecules in small cells. These molecules can 'rattle and flip' inside the cells, thereby breaking and forming weak bonds with the framework. In this project, we have shown that also custom made 'toy' model hamiltonians and classical force fields do not have the required first principles accuracy to solve this puzzle. Therefore, we have developed a new method to 'learn' a highly flexible force field based on just a couple of hundred very accurate calculations made with the Vienna Ab-initio Simulation Package (VASP). Based on machine-learning techniques, the computer automatically learns a model of the potential energy surface of the crystal. We refer to this model as a Machine-Learning Force Field (MLFF). With the MLFF we simulated the movement of the atoms of the most famous halide perovskite Methylamonnium-PbI3 in real-time. The method is so accurate that we could simulate the movements of the molecules while the crystal went through a so called phase transition. With purely VASP calculations this would not have been possible, because they would require years of compute time even on a modern supercomputer. The agreement between the simulated and experimentally measured lattice constants and phase transition temperatures is impressive and unprecedented for such a highly 'Dynamic Solid'. The trajectories of the molecules obtained with the MLFF method were used to obtain a deeper understanding of the physical mechanisms inside the 'puzzle'. The effect of the electrostatic dipolar interactions between the molecules on the total energy as described by 'toy' models was analysed. We showed that at room temperature and above these interactions have a negligible effect on the crystal structure. This agrees with our prediction of the crystal structure of the fully inorganic CsPbI3 perovskite. Even though such dipolar interactions are absent in this material, the change of the crystal structure is as function of temperature is similar. The MLLF method now opens up the possibility to simulate the atomic structure of many other 'Dynamic Solids' with near first-principles accuracy, incorporating anharmonic interactions and bond forming/breaking events. This is, for instance, important in developing new materials for batteries with moving ions and thermoelectrics with ultra-low thermoconductivities