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 was recently revealed 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 substitutions vs. diaryl substitutions 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 oligofluorenes 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.