Novel liquid crystalline organic semiconducting oligomers incorporating N-heterocyclic carbazole moieties for fluorescent OLEDs

Guang Hu, Stuart P. Kitney, Stephen M. Kelly, William Harrison, Mary O'Neill
2017 Liquid crystals (Print)  
A novel class of nematic liquid crystalline organic semiconducting oligomers incorporating Nheterocyclic carbazole moieties has been synthesised using simple and highly efficient reaction pathways. The electroluminescent colour of these novel oligomers can be varied in a controlled manner by molecular design. The values of the ionization potential and the electron affinity of these electroluminescent oligomers can also be matched by structural design to the HOMO energy level of the
more » ... king layer and the LUMO energy level of electron-transporting layer in the OLEDs to create low charge-injection barriers for electrons and holes, respectively leading to electroluminescence with an efficacy up to 4.1 cd A -1 . OLED Introduction Organic light emitting diodes (OLEDs) with high information content, active matrix addressing and full colour are a type of novel flat-panel display, which does not require backlighting and othermostly light-absorbing -components, such as the diffusers, colour filters and polarisers, used in LCDs [1] [2] [3] [4] [5] . Consequently, OLEDs are more efficient in terms of light emission due to the absence of so many light-absorbing or scattering layers and because no backlight is required. Particularly, multi-stack OLEDs with additional carrier transporting or blocking layers are introduced between the hole-/electron-transporting layer and the anode of tri-layer OLEDs, for example, to produce a lower barrier for hole injection into the other layers of the device. Many variants of multi-stack OLEDs have been developed with improved overall performance, e.g., in terms of brightness, power conversion efficiency, threshold voltage and operating voltage, etc. [6] [7] [8] [9] OLEDs require organic charge-transporting and electroluminescent materials. One of the advantages of using organic materials for electroluminescence is that they can be designed specifically with the desired functions, such as high hole or electron mobility and efficient electroluminescence. Furthermore, the electron affinity (EA, LUMO energy level) and ionisation potentials (IP, HOMO energy level) of charge-transport materials can be tuned to better match the work functions of electrodes [10] [11] [12] . These requirements for the material properties can be achieved by the design and synthesis of a variety of organic molecules. Specifically, compounds with a high concentration of highly conjugated aromatic rings, such as 1,4-disubstituted phenylene or 2,5disubstituted thiophene rings, in the molecular core, can form conjugated electron clouds with a high degree of delocalization, which renders aromatic organic compounds semiconducting under certain conditions [13] [14] [15] . Currently, the materials for OLEDs can be usually classified as lowmolecular-mass materials (LMMMs) [16] [17] [18] [19] [20] , polymers [21] [22] [23] [24] [25] [26] [27] [28] [29] and oligomers [30] [31] [32] [33] [34] [35] [36] . Liquid crystalline organic semiconductors [37] [38] [39] [40] [41] [42] [43] have found use in plastic electronic devices due to the tendency of such organic semiconductors in the fluid, self-assembling nematic phase -with an intrinsically low viscosity -to form uniform, thin layers or films, with a low concentration of charge traps, on device substrate surfaces. Deposition from solution from common organic solvents can be achieved using, for example, doctor blade, drop casting or spin coating techniques. The presence of a glassy nematic state above room temperature for oligomers means that there is no need to photochemically crosslink oligomeric liquid crystalline organic semiconductors in order to stabilize the fluid film on the device substrate. The presence of the hetero-atoms in aromatic heterocyclic moieties in some highly conjugated organic semiconductors contributes to the improved optoelectronic properties of such materials. For example, the carbazole moiety has long been recognized as an excellent construction of
doi:10.1080/02678292.2017.1306633 fatcat:hg2u2wvbandmdk5lkeqtmlw7i4