Yuh-Yih Chien

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...

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.

Efficient bipolar carrier transport in oligofluorenes

Fluorene-based materials, such as polyfluorenes or oligofluorenes, are conventionally well known as efficient blue emitting materials. Intriguingly, it is recently unveiled that ter(9,9-diarylfluorene)s also exhibit efficient unusual nondispersive bipolar carrier-transport characteristics and high carrier mobilities of >10-3 cm2/Vs for both holes and electrons in the amorphous state. Making use of convenient substitution on C9 of fluorenes and corresponding variety in molecular structures and physical properties, in this work we systematically investigate influences of molecular structures, such as dialkyl-substitution vs. diaryl-substitution and oligomer length etc., on charge transport of oligofluorenes. Furthermore, utilizing liquid-crystalline properties of oligofluorenes with appropriate substitutions, comparative studies of carrier transport of oligofluornes in both vacuum-deposited amorphous states and aligned liquid-crystal glass states are also performed. Insights on intermolecular charge transport derived from the experimental observations and theoretical examinations are then discussed accordingly.