Author, institution: Nadzeya Kukhta, Kaunas University of Technology
Science area, field: Technology Sciences, Materials Engineering – 08T
The Doctoral Dissertation is available at the library of Kaunas University of Technology (K. Donelaičio St. 20, Kaunas)
Scientific Supervisor: Assoc Prof. Dr. Jolita OSTRAUSKAITĖ (Kaunas University of Technology, Technological sciences, Materials engineering 08T)
Dissertation Defence Board of Materials Engineering Science Field:
prof. habil. dr. Sigitas Tamulevičius (Kaunas University of Technology, Technological sciences, Materials engineering 08T) – chairman
prof. dr. Saulius Grigalevičius (Kaunas University of Technology, Technological sciences, Materials engineering 08T)
prof. habil. dr. Saulius Antanas Juršėnas (Vilnius University, Technological sciences, Materials engineering 08T),
dr. Roman LYTVYN (Kaunas University of Technology, Physical sciences, Chemistry – 03P),
prof. dr. Andrew MONKMAN (Durham University, United Kingdom, Physical sciences, Physics – 02P).
Annotation:
Recently, organic semiconductors have found wide application in the preparation of such devices like organic light emitting diods (OLEDs), organic field effect transistors (OFETs), and organic photovoltaic devices (OPVs). For the achievement of high device performance, such factors as chemical and thermal stability and durability, charge transport, light absorption, high photoluminescence quantum efficiency, appropriate energy levels and suitable morphological properties, should be taken into account in the development of organic semiconductors. Consequently, deep understanding of the electronic structure along with the resulting properties of the materials is necessary for the manufacturing of efficient devices and the design of new compounds.
Bipolar compounds employing fluorene, carbazole, triphenylamine donors and triazine, triphenylbenzene, fluorenone, benzonitrile and naphthalenetetracarboxilic acid dianhydride derivatives as acceptors, were thoroughly investigated in recent years. Easy functionalization and variation of conjugation strength leads thus to the great diversity of structures. Miscellanious conjunctions of the above mentioned fragments give rise to the bipolar molecules with a wide range of necessary characteristics.
The aim of this work was design, synthesis and thorough investigation of properties of the new bipolar compounds for the application in optoelectronics. Comparison of practically obtained characteristics with the theoretically predicted ones and estimation of the applicability of new compounds in optoelectronic device was the other goal of the study.
The following objectives were raised for the achievement of the aim of the thesis:
In current work the design, synthesis and investigation of properties by the joint theoretical and experimental approach of bipolar branched and electron-accepting brick-type compounds is presented.
Star-shaped derivatives of 2,4,6-triphenyl-1,3,5-triazine and 1,3,5-triphenylbenzene, linked to fluorenyl arms by various linkages, were found to emit light in blue region with high photoluminescence quantum efficiency (up to 0.7) and demonstrate high thermal stability (decomposition temperature reaching 434 oC) and high hole mobility. The highest hole mobility was displayed by the compounds utilizing ethenyl linking bridge.
Intense blue emission with photoluminescence quantum yields reaching 0.93 and triplet energies ranging from 2.44 to 2.68 eV, as well as moderately low ionization potential values (5.45-5.50 eV) were detected for the boomerang-shaped compounds with various central units and bicarbazolyl side arms. Bicarbazolyl group proved to be a stronger donor than single carbazole fragment, providing lower ionization potential and superior thermal and electrochemical stability.
Donor-acceptor multichromophore fluorenone-based compounds demonstrated superior values of decomposition (up to 500 oC) and glass transition temperature (up to 293 oC), ambipolar charge transport behavior. High photoluminescence quantum yields (up to 0.90) in non-polar media and aggregation induced emission enhancement were observed for these compounds as well. On the basis of experimental and theoretical results a mechanism of emission in the investigated materials was suggested.
The orientation of substituents was found to affect greatly the properties of the bipolar 1,3,5-triphenylbenzene-based compounds, utilizing carbazolyl donor and nitrile acceptor. Para-conjugation resulted in higher decomposition and glass transition temperatures, lower ionization potential, exclusively hole transport and appearance of delayed fluorescence; the compounds with meta-linkage displayed ambipolar charge transport and higher triplet energies. A derivative with exclusively para-conjugation displayed high performance in OLED as a blue fluorescent emitter, while a compound with meta-linkages proved to be a successful host matrix in combination with a para-substituted guest.
Two series of symmetrical and asymmetrical electron transport materials based on 1,4,5,8-naphthalenetetracarboxylic dianhydride demonstrated high thermal stability (decomposition temperature in the range 413–507 oC), proving that the materials are suitable for vacuum processed organic solar cells, as well as high conductivity (up to 4 · 10‑5 S cm-1) and electron mobility (up to 10-3 cm² Vs-1) and high electron affinity values (3.84-4.15 eV). The most promising asymmetric compound, tested in solar cell, showed a power conversion efficiency of 7.9 %.
