p-Type molecular doping of organic materials with high ionization energies (IEs) of above 5.50 eV is still a challenge, limiting the use of doping in high-performance organic light-emitting diodes (OLEDs). Here, we investigate the molecular dopant hexacyano-trimethylene-cyclopropane (CN6-CP) with a high electron affinity of 5.87 eV as p-dopant in OLEDs. We show that CN6-CP can be used not only as a dopant in the traditional hole transport material 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC, IE = 5.50 eV) but also effectively dopes the host material tris(4-carbazoyl-9-ylphenyl)amine (TCTA, IE = 5.85 eV), reaching a conductivity of 1.86 × 10 −4 S/cm at a molar ratio of 0.25. Using CN6-CP-doped TAPC as hole injection and transport layer, we achieve a low driving voltage of 2.92 V at the practical brightness of 1000 cd/m 2 and 3.18 V at a current density of 10 mA/cm 2 for a green phosphorescent OLED based on bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium-(III) (Ir(ppy) 2 (acac)), together with a maximum external quantum efficiency of 18% and a luminous efficacy of 78 lm/W. The device also exhibits a very low efficiency roll-off at high luminance. Further, by directly adopting CN6-CP-doped TCTA as the injection/transport layer, the driving voltage drops to 2.78 V at 1000 cd/m 2 and 2.93 V at 10 mA/cm 2 . Moreover, conductivity and absorption measurements suggest that CN6-CP could also dope CBP with an IE as high as 6.05 eV. The results show that CN6-CP is an excellent p-type dopant for efficient OLEDs and possesses great potential for future application in organic optoelectronic devices.
We investigate free-running dual-comb pulses with overlapping spectra and well-controlled repetition rate difference in a single birefringent fiber cavity. Multiple linear and nonlinear soliton formation mechanisms in an all-fiber laser with partial polarization maintaining fiber are experimentally observed and validated for switchable and tunable dual-comb pulse emissions. Linear polarization mode dispersion is first exploited to emit polarization-multiplexed pulses with the upper limit of repetition rate difference at kHz level. By further tailoring linear birefringence, birefringence filter effect and nonlinear polarization evolution are well leveraged to emit hybrid mode-locked pulses with the lower limit at 10-Hz level. The lower limit of ∼12 Hz and nearly two order-of-magnitude tunable range of repetition rate difference are highlighted. Moreover, overlapping spectra and the passive mutual coherence between pulses in the free-running state are clarified, indicating the potential of the simplification of amplification system and single-cavity dual-comb source.
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