Interfacial Microstructure Function in Organic Light-Emitting Diodes: Assembled Tetraaryldiamine and Copper Phthalocyanine Interlayers

Abstract

Drastically increased OLED device luminance (up to 15 000 cd/m2) and enhanced quantum efficiency (6×) are achieved by spin‐coating a siloxane‐derivatized hole injector (TPD‐Si2) onto an ITO surface. The silyl groups promote ITO–TPD interfacial cohesion, thus enabling more efficient hole injection. The Figure shows a polarized optical image of the TPD film morphology after annealing the bilayer structure (see also inside front cover).

Molecules and Electronic Materials

“…Here the device structure resembles several structures that we mentioned earlier, [41± 45,47,50,51] suggesting a direction for further work on such devices. A recent report on OLEDs [88] indeed points in that direction, as it shows significant improvement in LED performance upon chemical coupling between molecules and the ITO electrode.…”

Self‐assembled Electroactive Phosphonic Acids on ITO: Maximizing Hole‐Injection in Polymer Light‐Emitting Diodes

“…Several studies have shown that modifying the ITO with a hydrophobic layer increases charge injection to a hydrophobic HTL due to improved surface energy matching. [6,7] However, the improvement in cohesion should be the same for all three of the TPD-based SAMs, since their water contact angles are very similar. The dramatic increase in current density seen for the device made with the TPD-3 SAM on ITO compared to the TPD-1 and the TPD-2 devices is attributed to the strong hole-transporting ability of this molecule.…”

“…In addition, molecular self-assembly can occur spontaneously from a solution or gas [1] enabling the formation of highly ordered functional monolayers with few processing steps. Selfassembled monolayers (SAMs) have already shown their ability to promote crystallization of small molecules, [2] form cohesive dielectric films by layer-by-layer self-assembly, [3] change the work function of inorganic conductors, [4,5] change the morphology and orientation of organic layers, [6,7] and passivate electronic traps. [8] The need for precise control of the electrode/organic semiconductor interface is particularly apparent in the field of polymer light-emitting diodes (PLEDs), where large differences between the work function of the commonly used indium tin oxide (ITO) anode and the highest occupied molecular orbital (HOMO) level of the organic semiconductor limit hole-injection and can lead to high turn-on voltage, low brightness, and low efficiency.…”

“…[8] The need for precise control of the electrode/organic semiconductor interface is particularly apparent in the field of polymer light-emitting diodes (PLEDs), where large differences between the work function of the commonly used indium tin oxide (ITO) anode and the highest occupied molecular orbital (HOMO) level of the organic semiconductor limit hole-injection and can lead to high turn-on voltage, low brightness, and low efficiency. Several approaches have been employed in decreasing the hole-injection energy barrier, such as oxygen plasma-treatment, [9,10] ultraviolet-ozone treatment, [11,12] inserting a dipolar layer, [13,14] tuning the surface energy of ITO with SAMs, [6,7,15] depositing a layer of conductive polymer (e.g., PEDOT:PSS), [16] and using an intermediate hole-transporting layer (HTL) having a HOMO level in between that of the ITO work function and the desired organic layer. [17,18] However, these treatments have some drawbacks.…”

Electrical Characterizations of Schottky Diodes οn ITO Modified by Aromatic SAMs

“…Siloxane-derivatized hole injector (TPD-Si 2 ) was synthesized and used as SAM to improve the charge injection. But the results show that triethoxysilyl with alkyl chain as spacer of the charge injection from ITO surface is decreased because of non-conducting alkyl spacers [6].…”