Interface Properties In Organic Light-emitting Diodes

Organic light-emitting diodes (OLEDs) have attracted considerable attention due to their diverse promising applications in large-area flat panel displays. However, there are still many problems to be solved, and the charge transport mechanism is not clear and its interface properties need to be further investigated.In this dissertation, by using traditional measurement methods and AC impedance spectroscopy technique, we have systematically studied the theory of charge carriers injection and transport, and the photoelectric characteristics of the organic heterojunction interface in OLEDs.Firstly, the charge carriers injection and transport mechanism of single-layer device ITO/Alq3(90nm)/Al was investigated by using the traditionally measurement methods. Our experimental results indicated that the charge carriers conduction mechanism of ITO/Alq3(90nm)/Al is trap-charge limited current (TCLC) with exponential distribution. At lower voltages, the currents increase linearly with applied voltages, which implies that the organic/metal interface are ohmic.Secondly, by using impedance spectroscopy technique, the conduction mechanism of the single-layer device ITO/Alq3(90nm)/Al was further studied, a half-cycle in its AC impedance spectroscopy was observed, indicating that no schottky barriers were formed at the ITO/organic interface and organic/Al interface. According to the impedance spectroscopy of these devices, it can be interpreted in terms of an equivalent circuit model of a contact resistance Rs in series with a parallel combination of a capacitance CP and a resistance RP. According to our experimental data, the values of RP, CP and RS can be figured out. Further, we have studied the impedance-voltage (Z-V), capacitance-voltage (C-V) and capacitance-frequency (Z-ω) of the single device, and its conduction mechanism was confirmed to be trap-charge limited current with exponential distribution and the dielectric relaxation time of the organic material decreased dramatically with increasing applied voltages.Lastly, by using traditional measurement methods, we have investigated the electroluminescence (EL) and photoluminescence (PL) of the interface in organic heterostructure diodes. By comparing the EL, PL and absorption spectra of pure TPD film and pure PBD film, bilayer structure film TPD/PBD and incorporated film...