Grain boundaries in ultrathin organic semiconductors

Walter, Lisa Sophie (2022): Grain boundaries in ultrathin organic semiconductors: toward controlling their emergence, electrical properties, and impact on device performance. Dissertation, LMU München: Fakultät für Physik

Abstract

Organic semiconductors stand out over their inorganic counterparts, since they can be processed in thinner films and at lower energies, onto nearly any surface, and with tunable optical and electrical properties for specific purposes. These unique properties of organic semiconductors allow to save material, energy, space and cost and make them ideal for applications in customer-specific end-products. Although organic semiconductors have been implemented in semiconductor devices such as organic solar-cells, organic light-emitting diodes, organic field-effect transistors or sensors for years, these devices still show inferior performances compared to inorganic devices, especially in terms of reduced mobility, efficiency, reproducibility and stability. It is widely accepted that grain boundaries in organic semiconductors are one of the main responsibles for these drawbacks, since they act as trapping, recombination and/or degradation sites. However, why and how grain boundaries emerge (structurally and energetically), and which properties of grain boundaries mainly influence the device performance is still under investigation. Since addressing these questions will help to control charge transport in organic semiconductors and improve device performance, this work presents a fundamental investigation of grain boundaries in monolayer-thin films of an organic small molecule. These films stand out due to high crystallinity and atomically smoothness across grain boundaries. This, as well as their thinness, allows to characterize single grains and grain boundaries at the location where charge transport takes place in organic field-effect transistors, namely at the semiconductor-insulator interface. By Kelvin probe force microscopy (KPFM) grain boundaries are found as a first result to act as energy barriers or valleys, and different thin-film application techniques are presented resulting in films in which either a specific type of grain boundary predominates, or in films where barriers and valleys coexist. While it is particularly advantageous for future experiments to be able to control the existence of different types of grain boundaries in organic materials, the films with both types prove the fundamental difference between energy barriers and valleys. KPFM measurements not only allow a qualitative differentiation of barriers and valleys, but also a quantitative description of „grain boundary heights“. Valley depths and barrier heights can both be decreased by increasing the charge-carrier density in the organic semiconductor-film. However, they only vanish at charge-carrier densities above the typical operating regime of organic solar-cells and organic light-emitting diodes, which underlines the relevance of investigating charge transport at grain boundaries. Consequently, time-resolved KPFM measurements are conducted to investigate the trapping and detrapping mechanisms at grain boundaries and other local impurities, as well as their influence on global device parameters. While valleys trap charge carriers in deep traps, barriers backscatter electrons, but also indicate an increased trap-state density at the organic-semiconductor interface, thereby leading to a stronger reduction of charge transport than valleys. Valleys, on the contrary, are found to mainly define the global device parameters such as the turn-on and threshold voltage or the qualitative behavior of hysteresis. This finding underlines the need to be able to control not only the grain-boundary density in organic semiconductors, but also their type and absolute height. However, since it is challenging to control the emergence and electric properties of grain boundaries in organic semiconductors by experimental methods, an alternative experiment is presented with the aim to manipulate charge transport across grain boundaries by illumination with far-infrared light. It is assumed that photons from this light source are absorbed, leading to the excitation of charge carriers out of valleys or across barriers and thus to a measurable photocurrent. This photocurrent can be measured energy-resolved by using a modified Fourier transform infrared spectrometer, which allows to detect and characterize grain boundaries even in bulk-like materials. Finally, charge transport in a novel metal-organic framework is investigated directionally, globally and locally, to put the role of grain boundaries in organic semiconductors into a context. It is found that in this special material grain boundaries do not play an as important role as the stacking direction of single planes of the metal-organic framework. To summarize, the findings of this work lead toward controlling the properties of grain boundaries in organic semiconductors and their role in organic semiconductor devices such as field-effect transistors, organic solar-cells or organic light-emitting diodes.

• 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: 14. Dezember 2022
• 1. Bericht­erstatter:in: Weitz, Thomas