Electroluminescent devices based on cross-linked polymer blends

We report the electrical and optical properties of two-component blends of electron and hole transporting materials in single and bilayer structures for organic light emitting diode (OLED) applications. The materials considered were a blue-emitting bipolar transporting polyfluorene, poly(9,9-di-n-hexylfluorene) (DHF), and a hole-transporting material, poly-[4-n-hexyltriphenylamine] (HTPA). We compare the steady state OLED performance, transport, and optical properties of devices and describe morphology studies of the polymer films based on cross-linkable (x) blends with the analogous non-cross-linkable blends. The cross-linkable blends exhibit highest efficiency at low concentrations of the hole transporting material. At these concentrations the single layer OLEDs reach efficiencies greater than 0.1%, and are higher than for single layer x-DHF or the binary non-cross-linkable blend by more than an order of magnitude. Bilayer structures with homogeneous x-HTPA as hole transport layer show efficiencies between 0.08% and 0.96%, depending on the blend concentration in the emitting layer and on the top contact. We interpret these results in terms of the relative degree of phase segregation in the cross-linked networks versus the non-cross-linkable blends. (C) 2003 American Institute of Physics.