Pre-defense of promotion work: REQUIREMENTS FOR ENERGY LEVEL DETERMINATION OF ORGANIC MATERIALS USING PHOTOELECTRON YIELD SPECTROSCOPY AND SCANNING KELVIN PROBE by Raitis Gržibovskis
Raitis Gržibovskis (ISSP UL) speaks at the scientific workshop of ISSP UL Doctoral School “Functional Materials and Nanotechnologies” on December 13, 13:00 at ISSP UL 2nd floor hall.
The efficiency of devices made of organic materials is determined not only by the efficiency of the material itself for each specific application, but also by its compatibility with other materials used in the device. Very often, devices made of organic materials consist of several layers: electron or hole injection layers, electron or hole blocking layers, active media, which may consist of a mixture of two or more substances, electrodes. The compatibility of different materials is determined by the values of the energy levels (ionization energy of the molecule, affinity of the electrons), which in turn determine the efficiency of charge carrier transfer between the individual layers. Therefore, convenient and relatively simple methods are needed to determine the energy levels of materials, as well as their changes near the boundary surfaces.
Most commonly, energy levels are determined by ultraviolet photoemission spectroscopy (UPS), but this method is complicated and expensive because of the need for ultra-high vacuum. In the case of a UPS, only the surface of the material is investigated, since the scanning depth is up to 2 nm. As an alternative method to determine the ionization energy of a material, photoelectron yield spectroscopy (PEYS) method can be used.
In this work, PEYS is used to measure pure material thin-film ionization energy, electrode-organic compound boundary surface and its influence on energy level values studies, and for organic-organic material boundary surface studies. Organic compound - organic compound has been studied both in classical planar structures and in a very small volume studied with mixed substances.
An insight into surface potential measurements with the Kelvin probe and problems related to the interpretation of the results is given.
Thesis to be defended:
- In the case of very thin as well as inhomogeneous films in FEIS measurements, the signal is obtained from both the test substance and the electrode. These signals must be separated to determine the ionization energy of each material. Measuring the thickness of the film with a profilometer may not give true information about the quality of the film. In the case of nearby islands or crystallites, the average height of these islands is obtained.
- Using FEIS method, it is possible to observe shifts in energy levels organic matter-organic matter in the vicinity of the boundary surface. In order to monitor the effects on the values of the energy levels, the maximum number of molecules located near this boundary surface is required. This can be accomplished by forming volumetric heterojunction samples from a volatile solvent.
- The principle of superposition of FEIS signals may limit simultaneous studies of the energy levels of both materials in volumetric heterojunction systems.
- In Kelvin probe measurements, the value of the surface potential depends not only on the metal output work, but also on the film thickness. The relationship between changes in surface potential and changes in metal output work decreases with increasing film thickness and conductivity of the studied material.