The Electrical and Mechanical Study of the Flexible Organic Light Emitting Diodes

Abstract

This work begins from depositing the same organic light emitting diode (OLED) structure on the glass substrate and the Polyethylene terephthalate (PET) substrate. The result suggests that the transparency of the substrates, the microcavity effect arising from different anode thicknesses and the surface roughness of the anode all come into play to lower the device efficiency of the PET device. The Young’s moduli of the Tris-(8-hydroxyquinoline) aluminum (Alq3) and N,N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPB) are measured using the nanoindentation technique. Although the substrate effect cannot be completely filtered out due to the thin film thickness and the softness of base (PET), with a Young’s modulus of around 40-100 GPa, the small molecule layer can still be considered to be applied in the flexible devices. The stress in the indium-tin-oxide (ITO) of the multi layer OLED device when being bent is then calculated using this elastic parameter. A buffer layer inserted between the ITO and PET substrate with an optimized Young’s modulus and film thickness is suggested so that, according to theoretical calculations, the stress in the ITO layer can be reduced by up to 60%. Finally the flexible OLED devices were fabricated and their electroluminescence (EL) was measured when the OLED layers were applied with a tensile strain and compressive strain. It is observed that the compressive strain caused less damage than the tensile strain during the bending. It is also observed that a 4 µm thick buffer layer does protect the OLED device in terms of EL brightness while bending with the radius of curvature R = 6 mm. Without the buffer layer, the brightness of the device drops by 40% compares to merely 10% with this buffer layer.
It is finally concluded that with the proper mechanical design of the device structure, the small molecule based OLED devices using the ITO as anode can still be considered as one of the candidates of the flexible display or lighting devices.