Highly-efficient green phosphorescent organic light-emitting diodes with hybrid device geometry

We report on the performance of green phosphorescent organic light-emitting diodes (OLEDs) based on the well-known device structure of a hole-transport layer, an emissive layer with host 4,4'-di(carbazol-9-yl)-biphenyl [CBP] and the green phosphor emitter fac tris(2-phenylpyridinato-N,C-2,) iridium [Ir(ppy)(3)], a hole-blocking layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline [BCP] and and tris-(8-hydroxyquinolinato-N,O) aluminum [Alq(3)] as an electron-transport layer. Using spin-coated hole-injection/transport layers with increasing ionization potentials and decreasing hole mobilities, external quantum efficiencies of up to 18.1% at 100 cd/m(2) were measured in such devices. Furthermore, by removing the electron-transport layer of Alq(3) and increasing the thickness of BCP, devices with efficiencies of 21.2% and 72 cd/A at 100 cd/m(2) were obtained. Achieving such high efficiencies with a simplified hybrid structure in which one layer is processed from solution and only two other organic layers are deposited from the vapor phase is desirable for the fabrication of low-cost OLEDs.