In this thesis, the dielectric behavior including dielectric constants and dielectric characteristics of poly-(p-phenyleneethynylene)s (PPEs) and their influence on organic electronic devices are investigated.
In the first part, a method to create gels that compose of PPE and an ionic liquid was invented and named π-ion gels. π-ion gels exhibit a high dielectric constant so that excellent performances are displayed when π-ion gels are applied to the light-emitting electrochemical cells (LECs). The turn-on time, the brightness, the current density of LECs based on π-ion gels are improved by 10 times (0.7~2 s), four times, and 10 times (~20 A/cm2) in comparison to LECs fabricated by a drop- casting method, respectively. Furthermore, π-ion gels are applied to a new type of transistors, π- ion gel transistors (PIGTs). PIGTs display the on/off ratio of ~105, the hole carrier mobility of (0.4 cm2/V s), and the response time of ~20 μs, respectively. Especially, the response time of 20 μs is the fastest among electrochemical based transistors.
In the second part, a novel concept for the control of dielectric properties was developed. dipolar units of o-difluorobenzene were incorporated in both poly-(p-phenyleneethynylene)s (PPEs) and liquid-crystalline oligo-(p-phenyleneethynylene)s (PEs). When o-difluorobenzene is introduced in PPEs, ferroelectric behavior is observed as the first example of ferroelectric conjugated polymer based on molecular rotations. Furthermore, a liquid-crystalline PE forms a dipole-aligned crystal via a dual control of the electric field and the temperature (called 2D control). In both cases, the compounds are applied to metal-insulator-metal (MIM) diodes, exhibiting anisotropic currents.
Overall, the approaches to control dielectric constants and dielectric characteristics are beneficial for organic electronic devices. The concept is novel and feasible to apply to other materials
