The design and synthesis of efficient charge transporting and TADF materials for OLEDs is a real challenge, and serious considerations are required.

Solution-processable bipolar materials acting as hosts, soluble in common organic solvents, are key materials for thermally activated delayed fluorescence (TADF) emitters. Bipolar hosts are importantly anticipated to encourage solution-processable high-efficiency TADF organic light-emitting diodes (LEDs). Herein, we have successfully designed and synthesized solution-processable bipolar carbazole-benzophenone derivatives as host materials, i.e. 4,4′-di(3-phenylcarbazol-9-yl)benzophenone (BPBCzO) and 4,4′-di[3-[4-(carbazol-9-yl)phenyl] carbazol-9-y] benzophenone (BCzBCzO). These newly synthesized hosts exhibited high-triplet energies, balanced charge-transporting properties, suitable molecular orbital energy levels, good thermal stability, and good solubility, which are required to realize green TADF based organic LEDs. Initially, we fabricated two different types of TADF organic LEDs using these hosts and commercially available guest 4CzIPN as a green dopant and compared their device characteristics. A green TADF based organic LED employing BPBCzO host displayed excellent performance with a maximum external quantum efficiency (EQE_max), current efficacy (CE_max), and power efficacy (PE_max) as high as 23.2%, 70.7 cd A⁻¹, and 55.6 l m W⁻¹, respectively. In particular, over 90% of EQE was reserved (EQE of 21.3%) at the practical luminance of 1000 cd m⁻², which is advantageous for display technology. At last, green OLED was also fabricated with a cross-linkable hole transport material, i.e. 3,6-bis(4-vinylphenyl)-9-ethylcarbazole (VPEC) and realized PE_max of 63.6 l m W⁻¹ with EQE of 25.3%, which could be very effective for lighting and display devices. These excellent outcomes demonstrate the big potential of the carbazole-benzophenone derivatives as host materials for next-generation solution-processable display and lighting technologies.

Herein, we discuss an imidazole-based molecular framework, which can successfully transform triplet excitons present in high triplet levels into singlet states. We explain the working mechanisms of different methods for collecting triplet excitons, including hot excitons or HLCT states. After the development of an hot exciton material by Ma and Yang, many studies have demonstrated that the organic conjugated molecules having imidazole core have possibilities to show high efficiencies via hot exciton pathways. Finally, we provide a detailed investigation of recently published hot exciton luminogens based on imidazole molecular frameworks. This review provides an overview of the molecular structures, frontier molecular orbital information, and glass transition temperature of developed luminogens as well as the efficiency of organic light-emitting diodes (OLED) devices.

Since their initial introduction at Eastman Kodak in 1987, OLEDs have continuously advanced along a longer route. Because of their intriguing qualities, OLEDs are currently highly regarded in display and lighting industries. OLED device technology has experienced constant expansion in the past as one of the study fields that sparks and propels advancements in the academic community and industry. Currently, OLED devices have achieved remarkable progress with three types of emitting materials, i.e. fluorescence, phosphorescence, and TADF. The appropriate design and synthesis of some effective green dopants in academia and industries are incorporated in this review. In addition, this review concentrates on the recent advancement of green emitting materials in terms of the molecular design approach and device efficiency, presenting a brief outline of highly competent well-known green phosphorescent TADF emitters and MR-TADF emitters. Recent green phosphorescent TADF emitters have been chosen to discuss their device efficiency with respect to their molecular design evolution. Moreover, some green MR-TADF emitters are also mentioned because of their crucial requirement in upcoming display technology.