Enhanced electron injection and stability in organic light-emitting devices using an ion beam assisted cathode

Abstract : In this article, we report a highly efficient bilayer OLED with a maximum luminance up to 62 000 cd/m2 and a threshold voltage of 3.8 V. The device structure is indium-tin-oxide (ITO)/N,N0-diphenyl-N,N0-bis(3- methylphenyl)-1,10-diphenyl-4,40-diamine (TPD)/tris-(8-hydroxyquinoline) aluminum (Alq3)/LiF/Al. Ion beam assisted deposition (IBAD) process is used to deposit the aluminum cathode on the LiF layer. The IBAD process improves the OLED performance both by increasing the maximum luminance by a factor of 3 and by reducing the threshold voltage for light-emission. The IBAD process also enhances the microstructure and morphology of the Al layer and leads to denser and more homogeneous layers. The resulting highly-packed microstructure acts as a barrier to moisture and oxygen and inhibits their penetration into the Alq3 layer, leading to the OLED lifetime and stability increasing. In order to optimize the IBAD process parameters, prior to the OLED deposition, we have characterized aluminum films deposited on glass substrates by using atomic force microscopy and X-ray diffraction.