Preparation and Characterization of Organic Electroluminescent Devices Using Fluorescent Polyimides as a Light-Emitting Layer

Abstract

The electroluminescence (EL) properties of the organic electroluminescent devices (OELDs) using three kinds of fluorescent polyimides, ODPA/DCHM (HFPI-1), 10FEDA/DCHM (HFPI2), and P2FDA/DCHM (HFPI-3), for light emitting layers were investigated. The HOMO energy levels of HFPI-1 and HFPI-2 were determined as 6.7 and 6.9 eV, respectively, using cyclic voltammetry. Single-layer EL devices having the ITO/HFPI/Al structure were fabricated using HFPI-1 and HFPI-2, but these devices did not exhibit any current and EL properties. This is due to the quite large energy gaps between ITO (work function: 4.7 eV) and the HOMO levels of polyimides (>2.0 eV). Further, multilayer devices were fabricated using carrier transport materials. For these devices, the carrier injection properties from both electrodes were expected to be improved. However, no current and EL properties were observed for all devices. These experimental facts indicate that fluorescent polyimides have little carrier transportability. The EL properties of the devices using TPD-dispersed fluorescent poly-imides as hole transport layers were also investigated. In case of using HFPI-3 whose excitation spectrum is overlapped with the emission of Alq3, the device emits a distinct EL at 520 nm under the operation at 15 V. Further, the emission was displaced to 560 nm under the operation at 30 V, which can be explained by the energy transfer to HFPI-3 or the displacement of the recombination zone of electrons and holes from Alq3 to HFPI-3+TPD layer. This device mechanism might be promising as a novel wavelength-tunable OELD.