We studied polymer light emitting diodes where the chain order of polymers were enhanced on a photoalignment layer. The threshold voltage is significantly reduced and the power efficiency is increased with increasing the chain order of electroluminescent (EL) polymers. Our EL device is composed of a glass substrate, an anode (indium tin oxide as a hole injecting electrode), a photoalignment polymer, an EL polymer, and a cathode (aluminum as an electron injecting electrode). A thin alignment layer of poly(methyl methacrylate) based photopolymers, spin coated on the anode, was treated with a linearly polarized ultraviolet (LPUV) light. The electroluminescent polymer layer of the poly(2-methoxy,5-(2-ethyl-hexoxy)-1,4-phenylenevinylene) (MEH-PPV), was partially aligned on the photoalignment layer. The photopolymer was found to control the molecular alignment of the EL polymer depending on the polarization state of the UV. The reduction in the operation voltage was attributed to the increase in the conjugation length of the partially ordered MEH-PPV and the reduction in the trap density.

This work was supported in part by Korea Research Foundation Grant (KRF-2004-005-D00165) through Hanyang University and Ministry of Science and Technology of Korea through the 21st Century Frontier Research and Development Program at the Information Display Center.