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.

Spectroscopic evidence for trap-dominated magnetic field effects in organic semiconductors

“…Besides enhancing the number of traps through electrical stressing, we have investigated the possibility to chemically engineer traps with green and red dyes incorporated into the (blue) backbone [22]. The highest occupied molecular orbital (HOMO) of the dyes aligns perfectly with the HOMO of the backbone, while the lowest unoccupied molecular orbital (LUMO) of the dyes lies lower in energy.…”

“…Figure 3(b) shows how the strength of the green dye emission remains constant with voltage, while the red dye emission reduces drastically. The reason is that the EL arising from the green dyes can be attributed to energy transfer of excitons, while the red dyes clearly exhibit trap-assisted recombination [22]. This allows us to investigate the effect of these two separate processes on the MFEs.…”

“…These copolymers have radiative dyes incorporated in the backbone, which can act as trap sites. We obtained the same polymers as used by Nicolai et al [22], which were fabricated by Merck (for the chemical structure, see Fig. 1): a blue backbone copolymer, a backbone with green dyes, and a backbone with green and red dyes.…”

Tuning Emission Colors from Blue to Green in Polymeric Light-Emitting Diodes Fabricated using Polyfluorene Blends

“…Different colors can be achieved by preparing different combinations of fluorescent or phosphorescent compounds, 5−21 building multilayer systems in which each layer is composed of molecules or polymers as the active medium, 4,11,13,22 combining multiple layers in a tandem diode architecture, 17 using a single polymer with multiple functional groups, 3,5,18 using mixtures of polymers with small phosphorescent 19 or fluorescent 5,20 molecules (host/guest systems), quantum dots, 22,26 or nanorods/nanotubes, 10,23 using systems with excimer or exciplex emissions, 24,25 using systems that form exciplexes in bilayer structures, 4,6 or preparing blends with conjugated polymers. 7,11,15,16,26−32 The use of polymer blends represents a relatively inexpensive approach to the preparation of novel polymeric electroluminescent (EL) diodes with acceptable performances.…”

Monolithically integrated organic resistive switches for luminance and emission color manipulation in polymer light emitting diodes

“…33 Unlike in other blend systems, the used configuration exhibits significant bias dependent emission spectra, which usually undesired, can be exploited for exact color temperature tuning in the white light emitting diode in the vicinity of the white point. The voltage dependent emission spectrum is caused by the saturation of the dopant at higher current densities [33][34][35] due to the strong imbalance of excited state lifetimes between fluorescent ($ ns) and phosphorescent ($ls-ms) materials.…”