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/$N,N^{\prime }$ -di(naphthalene-1-yl)-$N,N^{\prime }$ -diphenyl-benzidine (NPB)/9,10-bis-($\beta$ -naphthyl)-anthrene (ADN)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/tris(8-hydroxyquinoline) aluminum $\left(\mathrm{Al}{\mathrm{q}}_3\right)∕\mathrm{BCP}:\mathrm{Li}∕{\mathrm{V}}_2{\mathrm{O}}_5∕\mathrm{NPB}∕\mathrm{Al}{\mathrm{q}}_3:4$ -(dicyanomethylene)-2-$t$ -butyle-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)4H-pyran $\left(\mathrm{DCJTB}\right)∕\mathrm{Al}{\mathrm{q}}_3∕\mathrm{Li}\mathrm{F}∕\mathrm{Al}$ , a stable white light with Commission Internationale De L’Eclairage chromaticity coordinates from (0.35, 0.32) at $18\mathrm{V}$ to (0.36, 0.36) at $50\mathrm{V}$ was generated. It was clearly seen that the EL spectra consist of red band at $600\mathrm{nm}$ due to DCJTB, green band at $505\mathrm{nm}$ due to $\mathrm{Al}{\mathrm{q}}_3$ , and blue band at $435\mathrm{nm}$ 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 $10200\mathrm{cd}∕{\mathrm{m}}^2$ at a bias of $40\mathrm{V}$ and a maximum current efficiency of $10.7\mathrm{cd}∕\mathrm{A}$ at a current density of $3.5\mathrm{mA}∕{\mathrm{cm}}^2$ .