White organic light-emitting diodes based on tandem structures

White organic light-emitting diodes made of two electroluminescent (EL) units connected by a charge generation layer were fabricated. Thus, with a tandem structure of indium tin oxide/N,N^′-di(naphthalene-1-yl)-N,N^′-diphenyl-benzidine (NPB)/9,10-bis-(β-naphthyl)-anthrene (ADN)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/tris(8-hydroxyquinoline) aluminum (Alq₃)/BCP:Li/V₂O₅/NPB/Alq₃:4-(dicyanomethylene)-2-t-butyle-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran (DCJTB)/Alq₃/LiF/Al, a stable white light with Commission Internationale De L’Eclairage chromaticity coordinates from (0.35, 0.32) at 18 V to (0.36, 0.36) at 50 V was generated. It was clearly seen that the EL spectra consist of red band at 600 nm due to DCJTB, green band at 505 nm due to Alq₃, and blue band at 435 nm due to ADN, and the current efficiency and brightness equal basically to the sum of the two EL units. As a result, the tandem devices showed white light emission with a maximum brightness of 10 200 cd/m² at a bias of 40 V and a maximum current efficiency of 10.7 cd/A at a current density of 3.5 mA/cm².