Molecularly doped polymer light emitting diodes utilizing phosphorescent Pt(II) and Ir(III) dopants

The use of molecular phosphorescent dyes in polymer-based organic light emitting diodes (OLED) of different architectures was investigated by incorporating several phosphorescent dopants into poly(N-vinylcarbazole) (PVK)-based single layer and single heterostructure light emitting diodes (LEDs). In particular, cis-bis[2-(2-thienyl)pyridine-N,C-3] platinum(II) (Pt(thpy)(2)) and platinum(II) 2,8,12,17-tetraethyl-3,7,13,18-tetramethyl porphyrin (PtOX), and an Ir(III) compound, fac-tris[2-(4',5'-difluorophenyOpyridine-C'(2),N] iridium(III) (FIrppy) were used. The maximum external quantum efficiency of phosphorescent devices exceeds 0.6% for the two Pt dopants and reaches approximate to 1.8% for FIrppy. An overall increase in LED efficiency compared to similar devices based on fluorescence is attributed to the fact that phosphorescent dopants allow both singlet and triplet excitons to be involved in emission. In addition to finding an energetically suitable dopant, such parameters as dopant concentration and organic layer thickness influence the performance of the LEDs. Introduction of an electron injecting layer of tris(8-hydroxyquinoline) aluminum(III) causes an increase of quantum efficiency of up to 1.8-2.8%. The second order quenching process present in these OLEDs, which is prevalent at high current densities, is most likely not due to T T annihilation of excitons trapped at dopant sites in these OLEDs. T T annihilation in the PVK matrix or trapped charge-triplet annihilation are more likely explanations for the decrease. (C) 2001 Elsevier Science B.V. All rights reserved.