Non-doped red organic light-emitting diodes

A convenient and improved procedure has been developed for preparing the red fluorophore N-methyl-bis(4-(N-(1-naphthyl)-N-phenylamino)phenyl)maleimide (NPAMLMe) through the efficiently synthesized bis(4-bromophenyl)fumaronitrile, the necessary precursor in preparing NPAMLMe. This allows NPAMLMe to be an easily accessible material compared with other known red, organic light-emitting diode (OLED) materials. We also report an unusual approach in fabricating red OLEDs, which does not adopt a conventional red dopant but rather NPAMLMe as the host red emitter. The performance of the non-doped red devices has been studied in depth for the first time. Devices with varied layer thickness were fabricated for examining the compatibility of NPAMLMe with commonly known materials, electron-transporting tris(8-hydroxyquinolinolato)aluminium (Alq3) and hole-transporting 4,4′-bis(4-(N-(1-naphthyl)-N-phenylamino)phenyl)biphenyl (NPB). In the presence of a hole-blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), the devices emit pure red electroluminescence (EL), and it is essentially voltage-independent. Red EL with a brightness near 4600 cd m−2 and an external quantum efficiency as high as 1.6% has been achieved. The performance of such non-doped, red OLEDs is comparable with or better than contemporary, dopant-based, red OLEDs, and the simple fabrication is the advantage of the approach.