Archiviertes Physik-Kolloquium:

17. Nov. 2016, 16:00 Uhr, Gebäude NW1, Raum H3

Towards the Epitaxial Growth of Lead Halide Perovskites

Dr. Clemens Simbrunner, Institute of Semiconductor and Solid State Physics Johannes Kepler University Linz, Austria

Organic-inorganic perovskites have emerged as a promising semiconductor material class for (opto)electronic device development, including photon detectors operating in all spectral regions between the near infrared and Gamma rays, field effect transistors, light emitting diodes, lasers, and photo-voltaic devices1. These devices benefit from properties such as high exciton diffusion lengths and electric field induced electron mobility, a high tolerance of the crystal structure to defects, and an insensitivity to grain boundaries. In addition, the devices also benefit from the simple processing of perovskites to thin films, based on solution processing, or vapor deposition or even on combined efforts. As a key towards highest performing devices, not only the exact chemical composition of the organic-inorganic perovskites is considered to be important, but also the film morphology and the obtained polymorphic phase. Solar cells with the highest photo-voltaic performance (power conversion efficiencies greater than 20 %) are described for instance to be based on formamidinium lead iodide films with a (111)-preferential crystallographic orientation2. Thus, controlling the orientation of perovskite materials is of uppermost importance for the optimization of device properties. The materials are commonly synthesized in two step processes, which yields weak control of the orientation and leads to polymorphous films. This motivated my scientific work towards epitaxial synthesis during my substitute Professorship in Bremen. Within this talk I would like to give an overview on these results. In the first part the epitaxial growth of lead halides on muscovite mica will be discussed. PbI2 represents a direct semiconductor (Eg=2.55 eV) which is built up in a sheet like structure in trigonal crystal structure3. The sequence of the layers is repeated in units of I-Pb-I held together by van der Waals forces4 which also recommend PbI2 as 2D-material5. Beside its importance as precursor for the fabrication of organic-inorganic lead halide perovskites, PbI2 crystals have been studied as a very promising material with large applicability as room temperature γ- and X-ray detectors6. First growth studies on muscovite mica (001) underline that PbI2 thin films can be grown by van der Waals epitaxy, yielding a well defined epitaxial relationship. Whereas thin layers are characterized by atomically smooth and closed layers, increasing layer thickness leads to the formation of pyramids thus following Stranski–Krastanov like growth. The size and density of these pyramids can be nicely controlled by tuning the substrate temperature. In the second part results on the epitaxial growth of methylammonium lead halide (MAPbI3 and MAPbBr3) thin films on muscovite mica will be presented. Experiments underline that muscovite mica and MAPbI3 as well as MAPbBr3 represent outstanding and promising material combinations for the fabrication of highly oriented perovskite films. Again, a well defined epitaxial relationship can be demonstrated, yielding quasi single crystalline thin films. By comparing the obtained results with the epitaxially grown PbI2 layers we will finally discuss the influence of two-step (conversion of PbI2 by exposure to methylammonium iodide [MAI]) versus one-step (coevaporation of PbI2 and MAI) fabrication techniques on the obtained anisotropy which certainly plays an important role also for wet-chemical processing of organic-inorganic lead halide perovskite layers.