Teng-Chih Chao

Employing ambipolar oligofluorene as the charge-generation layer in time-of-flight mobility measurements of organic thin films

Strong absorption of oligofluorenes at wavelengths of a few commonly used nanosecond pulsed lasers, their bipolar transport characteristics, and high mobilities for both holes and electrons make them useful as the general and effective charge-generation material for the time-of-flight mobility measurement of organic materials. In this letter, we demonstrate the use of the terfluorene as the charge-generation material for measuring hole and electron mobilities of various organic materials. Such a scheme has the advantages of simplifying the instrumentation and reducing material consumption in the measurements.

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.

Enhancement of bipolar carrier transport in oligofluorene films through alignment in the liquid-crystalline phase

We investigate the formation of aligned liquid-crystal glasses of an oligofluorene and perform comparative studies of carrier-transport properties of the oligofluorene in both amorphous films and glassy liquid-crystal films. With mesophase-mediated molecular alignment, the bipolar carrier-transport capability of oligofluorene solid films is enhanced by more than an order of magnitude. High bipolar carrier mobilities, up to 2.0×10−2cm2∕Vs for holes and up to 2.3×10−2cm2∕Vs for electrons, are observed in the aligned glassy liquid-crystal films.

Excited-State Backbone Twisting of Polyfluorene As Detected from Photothermal After-Effects

By means of time-resolved photoluminescence and photothermal techniques, after-effects from excited-state dynamics, energy migration, and conformational rearrangement of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) and its homologues has been examined and interpreted with rotational potential maps from quantum mechanical calculations. Steady-state photoluminescence spectral changes and time-resolved photoluminescence measurements of oligofluorenes and PFO diluted in toluene suggest excited state ring torsion occurring within 30 ps of photoexitation. With all effects from internal conversion/intersystem crossing processes properly accounted for, we show that the conformational changes associated with this twisting motion can be quantitatively probed by means of photothermal methods. Results suggest mean torsion between neighboring fluorene units by ca. 40 degrees upon excitation, in agreement with the shift of rotational potential minimum from +/-40 degrees (and +/-140 degrees) in the ground state to +/-20 degrees (and +/-160 degrees) in the first excited singlet state according to results of quantum mechanical calculations.

Anisotropic ambipolar carrier transport and high bipolar mobilities up to 0.1cm2V−1s−1 in aligned liquid-crystal glass films of oligofluorene

By taking advantage of the rich morphological characteristics of oligofluorenes and the mesophase-mediated molecular assembly/alignment, we studied carrier-transport properties of solid-state oligofluorenes in different morphologies and orientations. Compared to the amorphous phase, the uniaxially aligned liquid-crystal glass of oligofluorenes could give up to 100-fold enhancement in bipolar carrier-transport mobilities, bringing both hole and electron mobilities to nearly 0.1cm2∕Vs. Furthermore, significant anisotropic bipolar transport, i.e., preferential bipolar carrier transport along the oligomer chain direction is observed.

Tuning stimulated emission of organic thin films by molecular reorientation

In this letter, employing the spectroscopic ellipsometry, we study the phenomena of molecular reorientation in amorphous thin films of a ter(9,9-diarylfluorene) compound T3 upon annealing around or above its glass transition temperature. Molecular reorientation results in changes in the in-plane optical constants, which in turn could alter the waveguiding characteristics of a thin-film slab waveguide. Employing such properties, we demonstrate continuous tuning of the wavelength of amplified spontaneous emission of a T3 slab waveguide within one sample. Such an approach has the particular advantage of permitting facile tuning of stimulated emission after device fabrication.

Novel oxygen sensor based on terfluorene thin-film and its enhanced sensitivity by stimulated emission

Ter(9,9-diarylfluorene) (TDF) neat film featuring oxygen sensing with advantages of fast response, reversibility and high efficiency is reported. The fast O2 fluorescence quenching process (4.0 × 1010 M−1 s−1) is unique and is mainly due to intact amorphous morphology of the TDF film, providing ample porous sites in nature so that O2 can travel therein freely. This, together with a large S1–T1 energy gap of TDF, leads to efficient O2 sensitization, i.e. the O2 induced S1–T1 intersystem crossing. The sensitivity can be further enhanced up to ∼10-fold and ∼20-fold in amplified spontaneous emission (ASE) and lasing action, respectively. The work thus demonstrated a new class of organic materials suited for high-speed, high-sensitivity oxygen sensing.

Influences of substitution on electronic structures of oligofluorenes

The influences of substitutions on the electronic structures of oligofluorenes were systematically investigated. Electronic states of oligofluorenes with various types of C9 substitutions were studied by both ultraviolet photoemission spectroscopy and theoretical calculation. It is found that the substituents could have significant interaction with the backbone and substantially modify the electronic structures of oligofluorenes, depending on the types of substitutions. Consequently, the electronic properties that would affect the operations of electronic devices also considerably depend on the substitutions of the oligofluorenes.

Highly efficient organic light emitting diodes based on solution/evaporation hybrid process

The solution/evaporation hybrid processed organic light emitting devices composed of newly developed host material CzDBS, green PG-02 dopant, and orange PO-01-TB and PO-06 dopants are studied. Orange, green and two-peak white light emitting devices were fabricated by using spin coating of HIL, HTL and emission layers followed by thermal evaporation of ETL and cathode. The orange devices achieved efficacies of 26.9 cd/A and 14.6 lm/W at the brightness of 1538 nits. The green devices, on the other hand, achieved efficacies of 31.3 cd/A and 19.7 lm/W at the brightness of 1590 nits. The two-peak white devices, composed of blue FIrpic and orange PO-01-TB, achieved efficacies of 36.9 cd/A and 18.4 lm/W at the brightness of 1150 nits.

Recent progress on solution processable small molecules for organic light-emitting diodes in ITRI

A series of novel thieno[3,2-c]pyridine-based RGBY phosphorescent Ir complexes have been synthesized and characterized. Solution-processable double-layered OLEDs fabricated with theses materials as the light-emitting layer exhibit high efficiencies. The monochromatic large-area of 40 x 83 mm2 Yellow and Green OLEDs have also been demonstrated.