Manipulation and control of the interfacial polarization in organic light-emitting diodes by dipolar doping

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.

Self-Assembled Electret for Vibration-Based Power Generator

“…Furthermore, the driving forces of molecular orientation remain uncertain in terms of solid-state physics, leading to several researches focusing on OLED materials 44,45,50-53 . Additionally, various experiments and simulations have been conducted over the previous decade with an objective of understanding the molecular orientation of SAE materials, which are also known to form amorphous films 28,29,32,42,54,55 .…”

Acridine-Triphenylamine Based Hole-Transporting and Hole-Injecting Material for Highly Efficient Phosphorescent-Based Organic Light Emitting Diodes

“…The observed values are more suitable for thermally stable device applications and which enhance the morphological properties during device fabrication. Consequently, our materials revealed a higher melting point of 351 • C. TPA-1A showed outstanding thermal properties than well-known reference hole-transporting materials NPB (98 • C) and TPD (65 • C) due to its highly conjugated structure and rigid diphenyl acridine moiety attached to the triphenylamine central core [38,39]. …”

“…Consequently, our materials revealed a higher melting point of 351 °C. TPA-1A showed outstanding thermal properties than wellknown reference hole-transporting materials NPB (98 °C) and TPD (65 °C) due to its highly conjugated structure and rigid diphenyl acridine moiety attached to the triphenylamine central core [38,39]. The morphological studies of films are supported by scanning electron microscope (SEM) imagery and depicted in Figure 2.…”

The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes

“…J€ ager et al have shown that by mixing Alq 3 into an a-NPD matrix the polarity of the HTL can be controlled. 56 The electron transport material Al(7-Prq) 3 has been demonstrated by Noguchi et al to exhibit an inverted polarization with a sheet charge density of þ3.1 mC/m 2 . 57 In a bilayer configuration as shown in Fig.…”

“…In Alq 3 , the polar sheet density has been reported by several groups to lie around À1.1 mC/m 2 -with the negative polarity at the HTL/ETL interface-confirmed by displacement current measurement, 58,59 capacitance-voltage, 34 and ). 56 Obviously, the fabrication conditions play a role in the formation of the macroscopic polarization. Other polar electron transport materials exhibiting orientation polarization are TPBi (À1.1 mC/m .…”