Novel thermally activated delayed fluorescence materials for optoelectronic applications

Organic light-emitting diode (OLED) technology is steadily gaining more and more popularity, and, as of 2019, is the most commercially successful application of organic electronics. Compared to light-emitting diodes (LEDs), OLEDs possess several advantages such as colour tunability, quick switching times, higher brightness, better contrast, larger fields of view, the possibility to use lighter substrates instead of commonly used glass, and, potentially, cheaper production costs. Improving the performance of OLED devices could open new opportunities for the development of the technology and might be crucial for it to strengthen its position in the market. Upon electrical excitation singlets and triplets are formed in 1:3 ratio, and one of the challenges is to design materials that can efficiently utilise triplet excited states to generate electroluminescence. In purely organic fluorescent compounds normally only singlets are emissive at room temperature, so in fluorescent devices only 25% of the excited states are used. Phosphorescent OLEDs are known for their high efficiency compared to their fluorescent predecessors, however the structure of these materials is limited to heavy metal-containing organic compounds. Thus, creation of purely organic light emitting materials is of a great interest for the field. One of the solutions is to use thermally activated delayed fluorescence (TADF) emitters where dark triplets can be converted into singlet excited states and thus 100% internal quantum efficiency could be achieved. The design and synthesis of new TADF emitters is the main scope of this thesis. Chapter 1 gives a brief overview of the history of lighting technology with an emphasis on solid-state lighting. The milestones in the development of organic semiconductors are also described. The main concepts of organic electronics and the working principles of OLED devices are briefly approached. Furthermore, the existing ways of enhancing the efficiency of luminescent organic materials in light-emitting devices is discussed. A substantial part of the chapter is dedicated to the description of the main principles used for the design of materials exhibiting thermally-activated delayed fluorescence (TADF), recent advances in their development and future perspectives. Chapter 2 describes the photophysical properties of new helicene-based TADF emitters, in particular their behaviour in the solid state. This includes the presentation of time resolved measurements of helical compounds in crystals and films. The performance of the materials in OLEDs was also investigated. In Chapter 3 the synthesis and physical properties of novel-star-shaped donor-acceptor structures based on a truxene-benzothiadiazole-truxene core are discussed. The application of these materials in fluorescent OLEDs and as colour-converters in hybrid LEDs is described. Chapter 4 demonstrates a new way of preparing materials exhibiting delayed fluorescence (DF) from non-DF starting materials. These compounds exhibit either TADF or triplet-triplet annihilation (TTA) depending on the relative position of donor and acceptor moieties in the molecule. They also show aggregation-induced emission and mechanochromic properties. The photophysical behaviour of these compounds was investigated mainly in the solid state. The experimental procedures for chapters 2–4 are presented in Chapter 5