Abstract
Most of the commonly used electron transporting materials in organic light-emitting diodes exhibit interfacial polarization resulting from partially aligned permanent dipole moments of the molecules. This property modifies the internal electric field distribution of the device and therefore enables an earlier flat band condition for the hole transporting side, leading to improved charge carrier injection. Recently, this phenomenon was studied with regard to different materials and degradation effects, however, so far the influence of dilution has not been investigated. In this paper we focus on dipolar doping of the hole transporting material 4,4-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) with the polar electron transporting material tris-(8-hydroxyquinolate) aluminum (Alq3). Impedance spectroscopy reveals that changes of the hole injection voltage do not scale in a simple linear fashion with the effective thickness of the doped layer. In fact, the measured interfacial polarization reaches a maximum value for a 1:1 blend. Taking the permanent dipole moment of Alq3 into account, an increasing degree of dipole alignment is found for decreasing Alq3 concentration. This observation can be explained by the competition between dipole-dipole interactions leading to dimerization and the driving force for vertical orientation of Alq3 dipoles at the surface of the NPB layer.
Highlights
Conductivity doping is an established approach to improve charge carrier transport in organic semiconductors.[1,2,3,4,5,6] Thereby, the carrier density in a doped layer is enhanced by charge transfer between the semiconducting matrix material and the dopant, which is either a strong electron acceptor for p-type or a strong electron donor for n-type doping, respectively
An alternative way to modify charge injection behavior in organic heterojunction devices makes use of interfacial polarization caused by partial alignment of the permanent dipole moments of polar molecules
A comparison to neat Alq[3] films in bi-layered organic light-emitting diodes (OLEDs) reveals deviations from the expected behavior, which would be a linear correlation between the amount of Alq[3] and the magnitude of interfacial polarization
Summary
Conductivity doping is an established approach to improve charge carrier transport in organic semiconductors.[1,2,3,4,5,6] Thereby, the carrier density in a doped layer is enhanced by charge transfer between the semiconducting matrix material and the dopant, which is either a strong electron acceptor for p-type or a strong electron donor for n-type doping, respectively. Carrier transport takes place, with increasing Alq[3] thickness. They explained this behavior with a fixed amount of negative charges at the NPB/Alq[3] interface. A comparison to neat Alq[3] films in bi-layered OLEDs reveals deviations from the expected behavior, which would be a linear correlation between the amount of Alq[3] and the magnitude of interfacial polarization. By simple calculations considering the magnitude of the permanent dipole moment and the packing density of Alq[3] molecules, a steadily increasing (normalized) fraction of vertically aligned Alq[3] molecules can be found for a decreasing doping concentration of the polar material
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