High-efficiency red phosphorescent organic light-emitting diodes based on metal-microcavity structure

Multilayer electroluminescent (EL) diodes with red emission were fabricated using Bis(2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C3′) iridium (acetyl-acetonate) [(btp)2Ir(acac)] as dopant. Double metal-microcavity structure with a semitransparent Ag anode was introduced into the EL diodes. The cavity device structure was ITO/Ag/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine [NPB]/4,4′-N,N′-dicarbazole-biphenyl [CBP]: 5 wt% (btp)2Ir(acac)/2,2′,2″-(1,3,5-benzenetriyl)tris[1-phenyl-1H-benzimidazole] [TPBi]/LiF/Al. The sharp bright red emission of cavity device was observed. Compared with noncavity device the full width at half maximum (FWHM) of top normal emission spectrum was narrowed from 38 nm to 23 nm, and the maximum brightness of the cavity device was increased from 3500 cd/m2 to 5800 cd/m2, i.e., was improved by a factor of 1.7. The blue-shift of emission spectrum with increasing detection angle in traditional microcavity device was no longer observed, which attributed to a very steep rising of the EL-emission from (btp)2Ir(acac). Besides, it was importantly found that at higher current density EL efficiency was obviously higher for the cavity device than for noncavity device, i.e., at J = 200 mA/cm2 the former and later were 1.8 cd/A and 1.0 cd/A, respectively. And the increasing mechanism of the EL efficiency was ascribed to the shortened lifetime of triplet excited state (T1) and the weakened hole injection of Ag anode.