High electron mobility layers of triazines for improving driving voltages, power conversion efficiencies, and operational stability of organic light-emitting diodes

We have shown that triazine compounds of 2-biphenyl-4-yl-4,6-bis-(4^′-pyridin-2-yl-biphenyl-4-yl)-[1,3,5]triazine (DPT) and 2,4-bis-biphenyl-4-yl-6-(4^′-pyridin-2-yl-biphenyl-4-yl)-[1,3,5]triazine (MPT) work as excellent electron-transport layers (ETLs) of organic light-emitting diodes (OLEDs). By replacing a typical ELT of tris(8-hydroxyquinoline) aluminum (Alq₃) with the DPT ETL and the MPT ETL, driving voltages and power conversion efficiencies of OLEDs operated at a current density of 50 mA cm^-2 are improved from 7.7 ± 0.2 V and 1.41 ± 0.10 lm W^-1 (Alq₃) to 6.0 ± 0.1 V and 1.62 ± 0.09 lm W^-1 (DPT) and 5.2 ± 0.1 V and 1.88 ± 0.18 lm W^-1 (MPT), respectively. Half lifetimes of the OLEDs operated at the same current are also enhanced from 3200 h (Alq₃) to 4200 h (MPT) by using the MPT ETL. The reduction in driving voltage is found to arise from more efficient electron injection at interfaces of DPT/cathode (a barrier height of ≈0.61 eV) and MPT/cathode (a barrier height of ≈0.51 eV) than at an interface of Alq₃/cathode (a barrier height of ≈0.73 eV) as well as better electron mobilities of DPT (8.7 × 10^-5 cm² V^-1 s^-1 at an electric field of 5.0 × 10⁵ V cm^-1) and MPT (1.4 × 10^-4 cm² V^-1 s^-1 at the same field) than that of Alq₃ (4.1 × 10^-6 cm² V^-1 s^-1 at the same field [Naka et al., Synth. Met. 111-112 (2000) 331.]). A high-mobility electron-transport material of 4,7-diphenyl-1,10-phenanthroline (BPhen) is also used as an ETL for comparison of OLED characteristics.