An Efficient Pyridine- and Oxadiazole-Containing Hole-Blocking Material for Organic Light-Emitting Diodes: Synthesis, Crystal Structure, and Device Performance

In this paper, we focus on the synthesis and structure of the new bis(1,3,4-oxadiazole) system 2,5-bis[2-(4- tert -butylphenyl)-1,3,4-oxadiazol-5-yl]pyridine (PDPyDP). We have fabricated light-emitting diodes (LEDs) using poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH−PPV) as the emissive material, with and without a thermally evaporated electron-injection/hole-blocking layer of either PDPyDP or its vinylene analogue (E)-1,2-bis[2-(4- tert -butylphenyl)-1,3,4-oxadiazol-5-yl]ethene (PDVDP) or its phenylene analogue 1,4-bis[2-(4- tert -butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (PDPDP). PDPDP is the para isomer of OXD-7, which is a widely used molecular electron-transporting material. Electroluminescence spectra indicate that light is emitted only from the MEH−PPV layer. Using aluminum as the cathode, the bilayer LED with PDPyDP is considerably more efficient than the corresponding single-layer device or devices with PDVDP or PDPDP as the electron-injection layer.