You-Ming Chen

A new benzimidazole/carbazole hybrid bipolar material for highly efficient deep-blue electrofluorescence, yellow–green electrophosphorescence, and two-color-based white OLEDs

The bipolar molecule CPhBzIm exhibits an excellent solid state photoluminescence quantum yield (ΦPL = 69%), triplet energy (ET = 2.48 eV), and bipolar charge transport ability (μh ≈ μe ≈ 10−6–10−5 cm2 V−1 s−1). We have used it to fabricate a non-doped deep-blue organic light emitting diode (OLED) exhibiting promising performance [ηext = 3%; CIE = (0.16, 0.05)] and to serve as host material for a yellow–green phosphorescent OLED [ηext = 19.2%; CIE = (0.42, 0.56)]. Exploiting these dual roles, we used CPhBzIm in a simple singly doped, two-color-based white OLED (ηext = 7%; CIE = 0.31, 0.33).

Anisotropic optical properties and molecular orientation in vacuum-deposited ter(9,9-diarylfluorene)s thin films using spectroscopic ellipsometry

This article reports on the investigation of anisotropic optical properties of vacuum-deposited thin films of high-efficiency blue-emitting ter(9,9-diarylfluorene)s using variable-angle spectroscopic ellipsometry. Under deposition conditions typical for thin-film organic devices, both real and imaginary parts of refractive indices of vacuum-deposited ter(9,9-diarylfluorene) films exhibit rather significant uniaxial anisotropy with the optical axis along the surface normal. In particular, for the absorption associated with the π–π* transition of the terfluorene backbone, they show substantially larger in-plane extinction coefficients than the out-of-plane extinction coefficients. It is thus inferred that the vacuum-deposited ter(9,9-diarylfluorene) molecules tend to align their molecular axes and π–π* transition dipole moments along the substrate surface as observed previously in spin-coated films of alkyl-substituted polyfluorenes or oligofluorenes, even though the present ter(9,9-diarylfluorene)s have ri...

Carbazole–benzimidazole hybrid bipolar host materials for highly efficient green and blue phosphorescent OLEDs

In this study, we synthesized a series of bipolar hosts (CbzCBI, mCPCBI, CbzNBI, and mCPNBI) containing hole-transporting carbazole and electron-transporting benzimidazole moieties and then examined the morphological, thermal, and photophysical properties and carrier mobilities of these bipolar host materials. Altering the linking topology (C- or N-connectivity of the benzimidazole) changed the effective conjugation length and led to different excited-state solvent relaxation behavior. The N-connected compounds (CbzNBI, mCPNBI) possessed higher triplet energies (ET) than those of their C-connected analogues (CbzCBI, mCPCBI) by 0.23 eV. The higher values of ET of CbzNBI and mCPNBI endowed them with the ability to confine triplet excitons on the blue-emitting guest. A blue PhOLED device incorporating mCPNBI achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 16.3%, 35.7 cd A−1, and 23.3 lm W−1, respectively; confirming the suitability of using N-connected bipolar hosts for the blue phosphor. The donor/acceptor interactions of the C-connected analogue resulted in a lower triplet energy, making it a suitable bipolar host for green phosphors. A green-phosphorescent device incorporating CbzCBI as the host doped with (PBi)2Ir(acac) achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 20.1%, 70.4 cd A−1, and 63.2 lm W−1, respectively.

Indolo[3,2-b]carbazole/benzimidazole hybrid bipolar host materials for highly efficient red, yellow, and green phosphorescent organic light emitting diodes

By incorporating electron-accepting benzimidazole and electron-donating indolo[3,2-b]carbazole into one molecule, two novel donor–acceptor bipolar host materials, TICCBI and TICNBI, have been synthesized. The photophysical and electrochemical properties of the hybrids can be tuned through the different linkages (C- or N-connectivity) between the electronic donor and acceptor components. The promising physical properties of these two new compounds made them suitable for use as hosts doped with various Ir or Os-based phosphors for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). PhOLEDs using TICCBI and TICNBI as hosts incorporated with Ir-based emitters such as green (PPy)2Ir(acac), yellow (Bt)2Ir(acac), and two new red emitters (35dmPh-6Fiq)2Ir(acac) (i3) and (4tBuPh-6Fiq)2Ir(acac) (i6) accomplished high external quantum efficiencies ranging from 14 to 16.2%. Nevertheless, the red PhOLED device incorporating TICNBI doped with the red emitter osmium(II) bis[3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate]dimethylphenylphosphine [Os(bpftz)2(PPhMe2)2] achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 22%, 28 cd A−1, and 22.1 lm W−1, respectively, with CIE coordinates of (0.65,0.35). The external quantum efficiency remained high (20%) as the brightness reached to 1000 cd m−2, suggesting balanced charge fluxes within the emitting layer, rendering devices with limited efficiency roll-off.

Spiroconjugation-enhanced intermolecular charge transport

We report charge-transport and related physical properties of terfluorenes with various C9 substitutions. Results show that by introducing slight spiroconjugation between a core chromophore and conjugated substitutions through spiro-linking, the bulky substitution not only serves as a spatial hindrance to enhance morphological stability and emission efficiency in thin films, but also bridges and enhances intermolecular charge transport under certain situations. Interestingly, such characteristics are acquired without altering major electronic properties of core chromophores, rendering it a flexible molecular scheme for tuning characteristics of functional molecules to meet various demands of different applications.

Exchange bias in Co/Fe/FexMn1−x/Cu(100) ultrathin films

Stable and well-grown face-centered-cubic Fe films were prepared on buffer layers with varying lattice constants by depositing FexMn1−x alloy film on Cu(100) single crystal. No ferromagnetic ordering was observed at the stage of 30 ML Fe on the FexMn1−x/Cu(100) systems in the temperature range from 100 to 350 K. Furthermore, capping of Co on Fe/FexMn1−x/Cu(100) was employed as the probe of antiferromagnetic ordering by study of exchange bias coupling in these films. The exchange bias of the hysteresis loops can be observed after field cooling of the films. Further analyses by varying the measurement temperature and Fe coverage of the films were also carried out to clarify the origin of the exchange bias coupling observed. The exchange bias field found here is attributed to the interlayer coupling between the Co and Fe–Mn films through the spacing layer Fe.