Project Details
Projekt
Dielectric Effects in Hybrid Perovskites and Charge Selective Trap States
Subject Area
Synthesis and Properties of Functional Materials
Term
since 2019
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 424708448
This project is designed to continue the investigations on dielectric effects in metal halide perovskite solar cell absorber materials and the charge type - selective study of carrier traps.The softness of the crystal lattice in the halide perovskites and the concurrent large dielectric constant values have proven to be extremely relevant in explaining the long mean free paths of electronic charge carriers and the very low influence of defects on the related charge transport. The dielectric model introduced by us describing these aspects has been confirmed by over 100 follow-up publications. It combines the classical Fröhlich polaron with charge carrier screening provided by the rotation of small polar molecules. In the project we have been able to demonstrate, that the dielectric constant stays low, if this property is not present in the material, e.g. in the purely inorganic perovskite CsPbBr3. Consequently, charge transport is much hampered by ionic point defects in the crystals. The first funding period was concerned with establishing processing routes for single crystal growth and thin films of 3D and 2D halide perovskites beyond simple solvent crystallization and deposition. All processing routes have been successfully applied including the Bridgman technique. Beyond the routes promised in the proposal, solid state reactions have also proven successful. Refining crystal quality and defect concentrations as well as refinement of the Bridgeman method are ongoing.The metal insulator semiconductor thermally stimulated current technique has been successfully established for charge selective and aging dependent trap density of states characterization. These measurements were very successful for perovskite thin films and crystallites with thicknesses of up to 400 µm. For the second funding period, this will enable the quantification of grain boundary and interface related trap states, as well as their charge selective impact on perovskite aging for new and existing MHP materials.In the search for alternative absorbers using small molecules, we have identified new lead containing and lead free acetidinium compositions, so far unreported in literature. The goal of the second funding period is to build on the ground work we have performed so far. We will determine the majority charge carrier types in ten different compounds, apply the MIS-TSC technique with its versatility to develop detailed charge selective trap DOS pictures, determine the anisotropies in conductivity and dielectric response in 2D systems and determine which charge carrier dynamics are dominant in which frequency range. We are now able to manufacture all absorbers as single crystals and thin films. In the context of very low frequency response we will analyze the concurrent accumulation of point defects at grain boundaries locally. For the second funding period, the focus of our work shifts from material synthesis to characterization.
DFG Programme
Priority Programmes
Co-Investigator
Privatdozent Dr. Vladimir V. Shvartsman
