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.

Organic light-emitting diodes (OLEDs) have played a vital role in showing tremendous technological advancements for a better lifestyle, due to their display and lighting technologies in smartphones, tablets, television, and automotive industries. Undoubtedly, OLED is a mainstream technology and, inspired by its advancements, we have designed and synthesized the bicarbazole-benzophenone-based twisted donor–acceptor–donor (D-A-D) derivatives, namely DB13, DB24, DB34, and DB43, as bi-functional materials. These materials possess high decomposition temperatures (>360 °C) and glass transition temperatures (~125 °C), a high photoluminescence quantum yield (>60%), wide bandgap (>3.2 eV), and short decay time. Owing to their properties, the materials were utilized as blue emitters as well as host materials for deep-blue and green OLEDs, respectively. In terms of the blue OLEDs, the emitter DB13-based device outperformed others by showing a maximum EQE of 4.0%, which is close to the theoretical limit of fluorescent materials for a deep-blue emission (CIE_y = 0.09). The same material also displayed a maximum power efficacy of 45 lm/W as a host material doped with a phosphorescent emitter Ir(ppy)₃. Furthermore, the materials were also utilized as hosts with a TADF green emitter (4CzIPN) and the device based on DB34 displayed a maximum EQE of 11%, which may be attributed to the high quantum yield (69%) of the host DB34. Therefore, the bi-functional materials that are easily synthesized, economical, and possess excellent characteristics are expected to be useful in various cost-effective and high-performance OLED applications, especially in displays.

Over the past few decades, organic light-emitting diodes (OLEDs) find applications in smartphones, televisions, and the automotive sector. However, this technology is still not perfect, and its application for lighting purposes has been slow. For further development of the OLEDs, we designed twisted donor-acceptor-type electroactive bipolar derivatives using benzophenone and bicarbazole as building blocks. Derivatives were synthesized through the reaction of 4-fluorobenzophenone with various mono-alkylated 3,3′-bicarbazoles. We have provided a comprehensive structural characterization of these compounds. The new materials are amorphous and exhibit suitable glass transition temperatures ranging from 57 to 102 °C. They also demonstrate high thermal stability, with decomposition temperatures reaching 400 °C. The developed compounds exhibit elevated photoluminescence quantum yields (PLQY) of up to 75.5% and favourable HOMO-LUMO levels, along with suitable triplet-singlet state energy values. Due to their good solubility and suitable film-forming properties, all the compounds were evaluated as blue TADF emitters dispersed in commercial 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP) host material and used for the formation of emissive layer of organic light-emitting diodes (OLEDs) in concentration-dependent experiments. Out of these experiments, the OLED with 15 wt% of the emitting derivative 4-(9′-{2-ethylhexyl}-[3,3′]-bicarbazol-9-yl)benzophenone exhibited superior performance. It attained a maximum brightness of 3581 cd/m², a current efficacy of 5.7 cd/A, a power efficacy of 4.1 lm/W, and an external quantum efficacy of 2.7%.