Device Physics of White Polymer Light‐Emitting Diodes

Abstract

The charge transport and recombination in white‐emitting polymer light‐ emitting diodes (PLEDs) are studied. The PLED investigated has a single emissive layer consisting of a copolymer in which a green and red dye are incorporated in a blue backbone. From single‐carrier devices the effect of the green‐ and red‐emitting dyes on the hole and electron transport is determined. The red dye acts as a deep electron trap thereby strongly reducing the electron transport. By incorporating trap‐assisted recombination for the red emission and bimolecular Langevin recombination for the blue emission, the current and light output of the white PLED can be consistently described. The color shift of single‐layer white‐emitting PLEDs can be explained by the different voltage dependencies of trap‐assisted and bimolecular recombination.

Spin-Dependent Exciton Funneling to a Dendritic Fluorophore Mediated by a Thermally Activated Delayed Fluorescence Material as an Exciton-Harvesting Host

“…In fact, such direct carrier trapping on the guest fluorophore is evident in the current density–voltage characteristics (Figure c) that show lower current densities for devices incorporating the guest fluorophores, compared to the control device based on the neat XAc-XT emission layer. Similar phenomena have also been reported for several host–guest systems in OLEDs. − Furthermore, the decrease in current density is more significant for G0 than G1, indicating that the encapsulated structure of G1 partially inhibits direct carrier trapping on the fluorescent core. Therefore, we conclude that the improved EL efficiency of the device incorporating G1 is attributable to the suppression of both triplet energy transfer and direct carrier trapping, which minimizes the nonradiative decay of the triplet excitons.…”

25th Anniversary Article: Charge Transport and Recombination in Polymer Light‐Emitting Diodes

“…We observe that the blue Langevin recombination exhibits a stronger dependence on voltage than the red SRH process, as shown by the relative decrease of red emission compared to the blue peak with increasing voltage . The stronger voltage dependence of the bimolecular Langevin recombination arises from the fact that it is quadratically dependent on the carrier density, whereas SRH recombination only exhibits a linear dependence.…”

Solution-processed single-emissive-layer WOLEDs with high efficiency and ultra-high color rendering index beyond 90

“…2d). 10 Since Au-1 and Au-7 have lower LUMO and higher HOMO energy levels than the PYD2 host (Scheme 1), the EL process allows direct trapping of holes and electrons and subsequent exciton formation on these Au( iii ) emitters. In such a charge-trapping-controlled device, charges are prone to being trapped by Au-7 at low voltage due to the lower LUMO level of Au-7 (−3.3 eV) compared with Au-1 (−2.7 eV).…”