Investigation Of Magnetic Filed Effects In Organic Light Emitting Devices Based On Rubrene

In recent years, organic semiconductor has received extensive attention for the character of flexibility and abundant types. It has shown great vitality and bright prospects in new energy, illumination and electric circuit devices. Amongst so many kind of materials, there are special kinetic properties with Rubrene due to its unique molecular structure. So Rubrene have been used in organic light emit diode(OLED), organic field transistor and organic photovoltaics(OPV) to improve their behaviors. To date, there is so many approaches to investigate organic semiconductor devices, but magnetic field effect have held a peculiar place for it can explore the inner interaction between different kinds of micro particles. Although there have been reported some papers investigating Rubrene with magnetic field effect, the great variety of structure and sophisticated micro mechanism still be ambiguity. So we can do some further work to take a deep insight about the micro dynamic mechanics of Rubrene based OLED.In this dissertation, we fabricated several structure of OLED based on Rubrene, and the use magneto electroluminescence(MEL), which measure the relative variation of electroluminescence with magnetic field, as an approach to analysis the interaction of the devices under different bias voltage and temperature. The MELs exposure the affection of structure and environment for the device. This will be helpful for the advancement of the design of organic semiconductor, and may give some useful advice to increase the photoelectric conversion efficiency.The main chapters were listed below:In the first chapter, the basic knowledge of organic semiconductor was introduced, including the fundamental structure and mechanism of OLED and OPV. The first step elucidated the process of charge injection, extraction and the interaction between different excited states. Then details about the energy of excited state of Rubrene were introduced, focused in the fundamental theory of Triplet-Triplet annihilation and Singlet Fission of it and the potential for the advancement of OLED and OPV. The formation of π-π conjugation between two Rubrene molecules during molecules stacking and the affection of this spatial relationship to the charge diffusion property were introduced. Then the influence of π-π conjugation on the revolution of micro particles in Rubrene was described at last.In the second chapter, the procedure of how to fabricate OLED and measure the physical property of these devices were related.In the third chapter, the OLEDs with various thickness of Rubrene layer were prepared. The structure of them is ITO(120nm) /copper phthalocyanine(15 nm)/ N,N′-bis(naphthalene-1-y)-N, N ′-bis(pheny1)benzidine(60 nm) / Rubrene(X nm)/ Bathocuproine(80-X nm)/ LiF(1 nm)/ Al(120 nm), X represents the thickness of Rubrene layer. The experimental measurements of these devices under room temperature exhibit that the thickness can affect the devices’ magneto-electroluminescence(MEL) curves substantially. The values of high-field MEL increase with the thickness of Rubrene and gradually saturate after 30 nm. This can be attributed to that the ratio of π-π conjugation in Rubrene molecules stacking will grow with the thickness increasing, and then saturates at a proper thickness. Subsequently, we modulated the concentration of Rubrene by doping BCP in the active layer. The experimental results under room temperature present that the values of high-field MEL decrease as the concentration of Rubrene decreasing. This results verify that the influence of π-π conjugation is not only on the MEL curves, but also on the singlet fission. Furthermore, all the MEL curves presented a high-field decay at low temperature due to that the endothermic fission process in Rubrene molecules becomes weaker as the temperature decreasing, and that the longer triplet lifetime at lower temperature also enhances the process of triplet annihilation. Besides, the extensively existent intersystem crossing between singlet and triplet polaron pairs affects these devices as well. Finally, the MEL curves of 20% Rubrene device in room temperature with various current were successfully fitted through the combination of two exponential functions and a Lorentzian function. By means of the fitting, we confirmed that the singlet exciton fission, the triplet-triplet exciton annihilation and the intersystem crossing between singlet and triplet polarons coexist in the devices. Therefore, the varieties of these MEL curves can be attributed to the competition of these processes. The fittings revealed that the triplet-triplet exciton annihilation rate increases more obviously than the singlet exciton fission rate with current increasing.In the fourth chapter, a series of bifunctional devices(OLEDs and photovoltaics) were fabricated using Rubrene and fullerene(C60) as the active layer, in which the electroluminescence threshold voltage(~1.1 V) was half the value of the bandgap of Rubrene. MEL of planner heterojunction diodes exhibited a small increase in response to a low magnetic field strength(<20 mT); however, a very large decay was observed at a high magnetic field strength(>20 mT). When a hole-transport layer with a low mobility was included in these devices, the MEL response reversed in shape, and simultaneously, the electroluminescence threshold voltage became larger than the bandgap voltage. When bulk heterojunction device was examined, the amplitude of MEL curves presented an anomalous voltage-dependence. The analysis of the MEL responses of these devices indicted that the electroluminescence of half-bandgap-voltage device is originated from bimolecular triplet-triplet annihilation in the Rubrene film, rather than from singlet excitons that formed via an interface auger recombination.