Dopant Effect on the Charge Injection, Transport, and Device Efficiency of an Electrophosphorescent Polymeric Light-Emitting Device

Abstract

The dopant effect of heavy-metal complexes on the charge injection, transport, and device efficiency in electrophosphorescent polymeric light-emitting devices is studied. Two types of polyfluorenes are used as the host medium, and different types of Ir complexes are used as dopants. The current–voltage (I–V) characteristics are analyzed at different dopant concentrations. Four regions are indicated in the I–V characteristics, with each region being controlled by a different process depending on the device configuration and applied field. The effect of the dopant concentration on the potential-barrier height of the interface is estimated using the Fowler–Nordheim model. It is found that the Ir complex may alter the potential barrier between the highest occupied molecular orbital energy level of the host polymer and the adjacent layer's work function. It is shown that the dopant reduces the potential barrier for hole injection into the polymer, thus reducing the turn-on voltage. The results here are compared with those of other authors who have observed an increase in the turn-on voltage with increasing dopant concentration. The effect of Ir complexes on the electron transport and exciton formation is significant; the dopant increases the free-electron density and enhances electron mobility by reducing the characteristic energy of the electron trap, Et. At high applied fields, the electron-transport mechanism is controlled by space-charge-limited current with high-concentration exponentially distributed intrinsic deep-trap states of the electron. Et and the power factor, m, are determined as a function of dopant concentration. The effect of the Ir-complex dopant on the external quantum efficiency (EQE) and luminance intensity depends on whether the dopant acts to balance carrier (electron and hole) injection and transport, or to disturb the existing balance. For poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2/6) end-capped with bis(4-methyl phenyl)phenylamine (am4) (PF2/6am4), a dopant concentration of 2 % balances the electron–hole mobility and enhances the EQE, while for polyspirobifluorene:bis(triphenyl)diamine the EQE collapses because of the large shift in electron mobility, upsetting the existing balance of the carrier injection and transport. Balancing carrier injection and transport and confining the injected carriers inside the active layer—not simply the energy-transfer mechanisms alone—are the key issues for high power and EQE.