An X-ray scattering study of the nanoscale structure of C70-pentacene heterostructures and metal-halide perovskite nanocrystallites

Roemer, Janina Melanie (2019): An X-ray scattering study of the nanoscale structure of C70-pentacene heterostructures and metal-halide perovskite nanocrystallites. Dissertation, LMU München: Fakultät für Physik

Abstract

The growing worldwide demand for electronic devices increases the importance of finding climate-friendly production pathways. This setting motivates the investigation of semiconductors that can be processed energy efficiently, especially those that show potential for photovoltaic applications. The two material classes of organic semiconductors and metal-halide perovskites match these conditions. Understanding the nanoscale structure of these materials and how it influences the optoelectronic properties can provide important information for an advancement of the fields. In this work several complementary X-ray scattering techniques were applied to examine various structural properties of two different material systems. Molecular beam deposited pentacene-C70 bilayers were investigated, as an example of organic ambipolar heterostructures. Here, the question of how crystal structure, disorder, and aggregate distribution can be adjusted by controlling the vacuum deposition parameters was addressed. The second material class investigated are metal halide perovskite nanoparticles, processed by spatial confinement in porous thin films. This method, developed and applied by members of the Johannes Kepler University Linz, can be used to control the perovskites emission wavelength directly within device-relevant structures. Here, information about the size and shape of perovskite crystallites as well as of the surrounding pores is important to allow for a well-directed tuning of the optoelectronic properties. X-ray reflectivity measurements of pentacene-C70 bilayers performed at the High Resolution Beamline P08 at Deutsches Elektronen-Synchrotron (DESY) provided suitable data for quantifying the out-of-plane lattice spacing as well as its regularity. Applying the paracrystal theory developed by Hosemann revealed that the paracrystalline distortion of the C70 layers can be adjusted in a wide range by choice of the sample temperature during C70 deposition. A minimum paracrystalline distortion of 2.3 % occurs in C70 deposited at 75 °C, while a paracrystalline distortion of 5.4 % was found for deposition at 25 °C. The same data revealed a 1.4 % increase of the C70 out-of-plane lattice spacing for a deposition temperature increase of 80 °C. The respective changes of the three-dimensional crystal structure were investigated by grazing-incidence wide-angle diffraction. The differences of the diffraction patterns could be explained by a continuous transition from a rhombohedral phase to face-centred cubic structure of the fullerene. Grazing-incidence small-angle scattering revealed two characteristic length scales in the morphology of C70 nanoaggregates on pentacene films, which are assigned to the distances of aggregates along and across pentacene step edges. The combined results provide a sound basis for future experiments investigating the effect of nanoscale structure on different (opto)electronic properties of the materials, such as charge transport or the efficiency of charge carrier separation at the heterojunction. Accessing the size and shape of perovskite nanocrystals confined in 1-40 µm thick porous films was possible by high-energy X-ray microbeam wide-angle diffraction measurements performed at beamline P07 at DESY. Aligning the several-millimetre long films parallel to an incoming ~100 keV X-ray beam that was focussed onto their transverse plane enhanced the scattering signal and allowed for separation of in- and out-of-plane information. In nanoporous silicon, almost isotropic crystallites as small as 1.8 nm were observed. They increase in size with increasing etching current density that was applied to prepare the porous film. In nanoporous alumina, strongly anisotropic crystallites were observed, with an in-plane size of 13 nm and twice that size along the surface normal. Complementing this information with an analysis of small-angle scattering data of unfilled porous films indicates that the crystallite size in nanoporous silicon is only about half the pore diameter, while the crystallites in nanoporous alumina appear to fill the entire cross section of regularly arranged tubular pores. These results provide insights into the process of crystallite formation within the pores, helping to understand the observed optoelectronic properties and how they are affected by the differences of the two porous materials. This dissertation helps to clarify how structure and morphology of the two investigated material systems can be tuned by the choice of processing parameters, thereby providing an experimental building block towards a better understanding of the correlation between structural and (opto)electronic properties.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Mai 2019
• 1. Bericht­erstatter:in: Nickel, Bert