New derivatives of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine for dopant-free organic light emitting diodes with low turn-on voltages

N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) is a hole-transporter widely used in optoelectronic devices due to the low energetic disorder of its layers, high drift mobility of holes and suitable energy levels. In this work, new hole-transporting derivatives of NPB are developed with the aim of achieving lower ionization energies and the smooth transport of holes to the recombination sites of organic light-emitting diodes (OLEDs). Aiming at low turn-on voltages and enhanced efficiency and stability of the devices, electron-donating methoxy and dimethylamine groups are attached to the NPB core. The modification results in the manifestation of intramolecular charge transfer. While NPB is characterized by high crystallinity, the newly synthesized compounds are capable of the formation of molecular glasses. The compounds exhibit higher thermal stability (5 % weight loss at 388–411 °C) compared to that of NPB. The highest hole drift mobility of 1.85 × 10⁻³ cm²/V × s at the electric field of 2.5 × 10⁵ V/cm was detected for the compound with para-methoxy moieties. A series of doping-free OLEDs were fabricated to test the performance of the compounds as hole-transporting materials. It is demonstrated that turn-on voltages as low as 2.4 V can be achieved for the devices utilizing the layers of the synthesized hole-transporting compounds. Such low turn-on voltages are manifestation of the smooth barrierless charge transport necessary for high efficiency and operational stability of OLED. With another emitter chosen, namely CzPCN, the interface exciplexes between the hole-transporter and emitter were formed highlighting the same ability as for NPB.