Defended Dissertations in 2023


Dissertation title: Charge carrier generation and extraction in non-fullerene organic solar cells
Fields of science: Physics N 002
Scientific supervisor: prof., habil dr. Vidmantas Gulbinas
Defence of the dissertation: January 5, 2023

SUMMARY: In the search for alternative power generation sources, organic photovoltaics seem to be one of the most attractive options for certain niche applications. Recently, their power conversion efficiency has skyrocketed thanks to the introduction of a new generation of acceptor molecules. Nevertheless, the physical understanding of the underlying mechanisms remains poorly understood. This dissertation is devoted to the study of all important dynamic processes, such as the generation and extraction of free charge carriers in non-fullerene organic solar cells. Various transient measurement techniques and their improvements have been employed to study each step in the operation of organic solar cells over an exceptionally wide temporal range, from femto- to microseconds. In this work, the effects of spatial trapping of charge carriers in bulk heterojunction organic solar cells, as well as the dynamics of charge carrier extraction dynamics at close to real working conditions in organic solar cells, and the specifics of charge carrier generation in novel organic solar cells based on non-fullerene acceptors have been investigated. This has led to a better understanding of the physical processes in organic photovoltaic devices and contributed to their further improvement.

Dissertation title: Investigation of detection properties of planar microwave diodes based on A3B5 semiconductor compounds in millimeter–wavelength range
Fields of science: Electrical and electronic engineering T001
Scientific supervisor: prof., dr. Algirdas Sužiedėlis
Defence of the dissertation: February 10, 2023

SUMMARY:  Successful development of microwave technologies requires electromagnetic detectors capable of sensing high frequency signals at low levels of microwave power. Bulk barrier planar microwave diodes operating based on the major carrier phenomena are promising in high frequency electromagnetic radiation sensing applications. The dissertation aimed to develop and investigate new original planar microwave diodes with a lower spread of their electrical parameters and capable of detecting an electromagnetic signal in the millimeter wavelength range. The first chapter reviews the physical properties of microwave diode based detectors with quasi linear and non linear current voltage characteristics and microwave diodes with a two dimensional electron gas channel. Then, the application technologies for microwave detectors and methods for their investigation using appropriate probing systems are discussed. The second chapter covers the aspects of the development of planar semiconductor microwave diodes based on GaAs, AlGaAs, and AlGaAs/GaAs compounds. Next, the methodology for investigation methods of electrical parameters and detection properties of the microwave diodes is presented. The third chapter presents the results of experimental investigations of the electrical parameters and detection properties of planar dual microwave diodes based on a semi insulating or low resistivity GaAs substrate, including current voltage characteristics, detected voltage on power characteristics and dependence of voltage sensitivity on frequency in the millimeter wavelength range. The fourth chapter presents theoretical estimations and experimental investigations of the electrical and detection properties of bow tie type microwave diodes with partial gate above a two dimensional electron gas channel based on a selectively doped GaAs/AlGaAs heterostructure. The dissertation presents new developed planar microwave diodes, advanced techniques for investigating their properties, and ways for enhancing their detection properties. Five scientific papers were published on the topic of the dissertation: three papers in scientific journals included in the list of Clarivate Analytics Web of Science database with an impact factor, and two papers in conference proceedings included in the Clarivate Analytics Web of Science and Scopus databases. A European patent based on the results of the dissertation has been granted, and twelve reports, including the results of the dissertation, were presented at national and international scientific conferences.


Dissertation title: Development and application of advanced methods based on the application of a polyethylene naphthalate scintillation detector for recording and identification of ionizing radiation
Fields of science: Physics N 002
Scientific supervisor: dr. Artūras Plukis
Defence of the dissertation: May 5, 2023

SUMMARY: In this work we investigated new and advanced scintillation detectors capable of separating neutrons from the accompanying gamma particle radiation. Separation of particles is carried out by recording the light pulses (scintillations) generated by the detector and analyzing their shape.  Conclusions: 1. PEN is a suitable material for measuring the energy of various types of ionizing radiation particles in a wide energy range from 50 keV to 5.5 MeV. 2. The PEN material is suitable for use as a neutron flux detector, which can separate neutrons from a mixed gamma and neutron flux, the separation quality parameter FOM > 1 is obtained when the energy left in the ionizing radiation particle detector exceeds 200 keVee.  3. Thin PEN and Makrofol detectors with a FeB converter layer are suitable for recording the products of reactions caused by thermal neutrons in the converter layer.  4. The use of computer neural network models improves the separation of ionizing radiation particles when the amount of energy left by interacting ionizing radiation particles is low (<0.2 MeV), and the separation parameter FOM is less than 1.

Dissertation title: Charge carrier transport in hybrid Perovskites
Fields of science: Physics N 002
Scientific supervisor: dr. Marius Franckevičius
Defence of the dissertation: May 31, 2023

SUMMARY: The accumulation of charge carriers in the perovskite layer under forward voltage causes the  electroluminescence overshoot effect, while the ion redistribution induced by the electric field minimizes the accumulation of charge carriers and the overshoot effect in perovskite solar cells. An unfavorable space charge distribution leads to a roll-off of the external quantum efficiency in hybrid perovskite light-emitting diodes, which can be avoided by a balanced injection current. Electric field-induced redistribution of ions of a single type in the perovskite layer causes complex multiphase dynamics of electroluminescence and current in MAPbI3 perovskite light emitting diodes. Electric field-induced luminescence quenching in MAPbI3 perovskites is determined by the direct effect of the electric field on the charge carrier distributions and the influence on the spatial redistribution of ions in the perovskite layer. There are potential barriers to lateral carrier movement in the polycrystalline hybrid perovskite films that affect carrier mobility, diffusion and recombination rates. 

Dissertation title: Research into variability of permanent magnet synchronous motor parameters by magnetic field modelling
Fields of science: Electrical and electronic engineering T 001
Scientific supervisor: prof. dr. Algirdas Baškys, 2017-2018, prof. dr. Voitech Stankevič 
Defence of the dissertation: June 15, 2023

SUMMARY: The dissertation deals with permanent magnet synchronous motors (PMSM) classical control theory and also by applying magnetic field theory approach. The purpose of the thesis is to create the mathematical model of the PMSM based on analytical equations of magnetic field accounting torque ripple, inductance distributions and back electromotive force, to develop the measurement system for direct measurement of magnetic flux density and to propose PMSM parameter estimation method. In order to solve the stated problem and reach the goal following main tasks of the thesis are formulated: create PMSM model based on the analytical equations of magnetic flux density; develop vector control method of PMSM and investigate it experimentally; develop PMSM parameter estimation method; develop and implement a magnetic flux density measurement system; perform experimental measurement of magnetic flux density of PMSM using the developed system. The dissertation consists of the introduction, three chapters, general conclusions, references, and the list of scientific publications of the author on the topic of the dissertation. The introduction discusses the research problem, the relevance of the dissertation, the object of the research, formulates the aim and tasks, de- scribes the research methodology, the scientific novelty of the dissertation, de- fended statements and the practical value of the research findings. At the end of the introduction, the dissertation and the structure of the dissertation are provided. The first chapter reviews the PMSM design, rotor and stator types, magnetic field analytical solutions for predicting magnetic flux in the PMSM and magnetic field measurement systems. The second chapter presents an analysis of the following topics of PMSM’s magnetic field distribution: the field created only by the rotor (open-circuit) and the field created only by the stator (armature reaction). Next, the effect of the stator slot on the magnetic field distribution is analysed. Finally, the enhanced PMSM model concept is stated based on magnetic field analysis. The third chapter presents the magnetic flux density measurement systems and compares the experimental results with the simulation results obtained using the proposed enhanced PMSM model. The main results of the dissertation were published in seven scientific publications: four in the Clarivate Analytics Web of Science database with an impact factor and three in other databases. The results were presented at eight scientific conferences. 

Dissertation title: Application of dosimeters and calibrated ionization chambers in optimization of exposure in nuclear medicine procedures
Fields of science: Physics N 002
Scientific supervisor: dr. Arūnas Gudelis
Defence of the dissertation: September 7, 2023

SUMMARY: In the thesis, the risk probability of tumour complications among Lithuanian health care specialists working in nuclear medicine was investigated and evaluated. The time interval was chosen for over 25 working years. The non-definition of activity meters of radiopharmaceutical preparations used by Lithuanian nuclear medicine departments was also carried out. Special attention is paid to the evaluation of exposure to workers and patients. During the work, the task was to investigate the evaluation of the whole body Hp(10) exposure received by nuclear medicine workers when working with automatic injectors and to perform a detailed analysis of the Hp(0,07) exposure of the hands when working with 99mTc and 18F radionuclides. A detailed analysis of the prescribed administered activity to patients was evaluated and the local diagnostic reference levels used in nuclear medicine were proposed and updated, considering the relevant assigned activities..