Organic light emitting diode (OLED) is a new, promising technology in the field of lighting and display applications due to the advantages offered by its organic electroactive derivatives over inorganic materials. OLEDs have prompted a great deal of investigations within academia as well as in industry because of their potential applications. The electroactive layers of OLEDs can be fabricated from low molecular weight derivatives by vapor deposition or from polymers by spin coating from their solution. Among the low-molar-mass compounds under investigation in this field, carbazole-based materials have been studied at length for their useful chemical and electronic characteristics. The carbazole is an electron-rich heterocyclic compound, whose structure can be easily modified by rather simple reactions in order to obtain 2,7(3,6)-diaryl(arylamino)-substituted carbazoles. The substituted derivatives are widely used for the formation of OLEDs due to their good charge carrier injection and transfer characteristics, electroluminescence, thermally activated delayed fluorescence, improved thermal and morphological stability as well as their thin film forming characteristics. On the other hand, relatively high triplet energies of some substituted carbazole-based compounds make them useful components as host materials even for wide bandgap triplet emitters. The present review focuses on 2,7(3,6)-diaryl(arylamino)-substituted carbazoles, which were described in the last decade and were applied as charge-transporting layers, fluorescent and phosphorescent emitters as well as host materials for OLED devices.

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.

Low color temperature candlelight organic light-emitting diodes (LEDs) are human and environmentally friendly because of the absence of blue emission that might suppress at night the secretion of melatonin and damage retina upon long exposure. Herein, we demonstrated a lighting device incorporating a phenoxazine-based host material, 3,3-bis(phenoxazin-10-ylmethyl)oxetane (BPMO), with the use of orange-red and yellow phosphorescent dyes to mimic candlelight. The resultant BPMO-based simple structured candlelight organic LED device permitted a maximum exposure limit of 57,700 s, much longer than did a candle (2750 s) or an incandescent bulb (1100 s) at 100 lx. The resulting device showed a color temperature of 1690 K, which is significantly much lower than that of oil lamps (1800 K), candles (1900 K), or incandescent bulbs (2500 K). The device showed a melatonin suppression sensitivity of 1.33%, upon exposure for 1.5 h at night, which is 66% and 88% less than the candle and incandescent bulb, respectively. Its maximum power efficacy is 23.1 lm/W, current efficacy 22.4 cd/A, and external quantum efficiency 10.2%, all much higher than the CBP-based devices. These results encourage a scalable synthesis of novel host materials to design and manufacture high-efficiency candlelight organic LEDs.