Investigation of excitons fission and annihilation processes in Rubrene based devices by utilizing magneto-electroluminescence curves

Abstract

That the energy of triplet exciton in Rubrene is about half of its singlet leads to energy resonance. This resonance not only allows two triplets to annihilate into a singlet, but also makes a singlet probably fission into two triplets in different molecules. On the other hand, the π-π conjugation of two Rubrene molecules could be formed during molecules stacking, and this spatial relationship will affect the charge transport property enormously. In this article, we use organic magnetic-field effect as a convenient approach to explore the influence of the energy resonant excited states in the Rubrene molecules and the π-π conjugation between the different molecules on the luminescence property of Rubrene. Firstly, we fabricate organic light emitting diodes based on pure Rubrene and modulate the thickness of Rubrene. Experimental measurements of these devices at room temperature exhibit that the thickness can affect the devices' magneto-electroluminescence (MEL) curves substantially. Values of high-field MEL increase with the thickness of Rubrene and gradually saturate after reaching 30 nm. This can be attributed to the fact that the ratio of π-π conjugation in Rubrene molecules to the stacking will grow with increasing thickness, and then saturate at a proper thickness. Subsequently, we modulate the concentration of Rubrene by doping Buthocuproine (BCP) in the active layer. Experimental results at room temperature show that the values of high-field MEL decrease as the concentration of Rubrene decreases. These 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 exhibit a high-field decay at low temperatures since the endothermic fission process in the Rubrene molecules becomes weaker as the temperature decreases, and the longer triplet lifetime at lower temperatures also enhances the process of triplet annihilation. Besides, the extensively existent intersystem crossing between singlet and triplet polaron pairs may affect these devices as well. Finally, the MEL curves of 20% Rubrene device at room temperature changing with various currents are successfully fitted through the combination of two exponential functions and a Lorentzian function. By means of the fitting, we confirm 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 reveal that the triplet-triplet exciton annihilation rate increases more obviously than the singlet exciton fission rate with increasing current. Compared with the rates of the two bimolecular interactions given before, the change of the intersystem crossing rate could be neglected because of its small magnitude. This work is helpful to expand the understanding of the internal mechanism of organic optoelectronic devices.