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

Abstract

In order to fulfill the promise of organic electronic devices, performance‐limiting factors, such as the energetic discontinuity of the material interfaces, must be overcome. Here, improved performance of polymer light‐emitting diodes (PLEDs) is demonstrated using self‐assembled monolayers (SAMs) of triarylamine‐based hole‐transporting molecules with phosphonic acid‐binding groups to modify the surface of the indium tin oxide (ITO) anode. The modified ITO surfaces are used in multilayer PLEDs, in which a green‐emitting polymer, poly[2,7‐(9,9‐dihexylfluorene)‐co‐4,7‐(2,1,3‐benzothiadiazole)] (PFBT5), is sandwiched between a thermally crosslinked hole‐transporting layer (HTL) and an electron‐transporting layer (ETL). All tetraphenyl‐diamine (TPD)‐based SAMs show significantly improved hole‐injection between ITO and the HTL compared to oxygen plasma‐treated ITO and simple aromatic SAMs on ITO. The device performance is consistent with the hole‐transporting properties of triarylamine groups (measured by electrochemical measurements) and improved surface energy matching with the HTL. The turn‐on voltage of the devices using SAM‐modified anodes can be lowered by up to 3 V compared to bare ITO, yielding up to 18‐fold increases in current density and up to 17‐fold increases in brightness at 10 V. Variations in hole‐injection and turn‐on voltage between the different TPD‐based molecules are attributed to the position of alkyl‐spacers within the molecules.

Modification of ITO surface using aromatic small molecules with carboxylic acid groups for OLED applications

“…This behavior can be attributed to the direct attachment of the carboxylic acid group into triarylamine moiety. Its attachment allows the electron density of triarylamine core to be redistributed to the electronwithdrawing carboxylic acid group, so that only a single reversible oxidation peak is observed in the given potential range [35].…”

Spatially Modulating Interfacial Properties of Transparent Conductive Oxides: Patterning Work Function with Phosphonic Acid Self‐Assembled Monolayers

“…[13] A number of groups have incorporated SAMs into OLEDs, [1][2][3][4]6,[10][11][12]18,19] with many focusing on the use of phosphonic acid SAMs to modulate interfacial properties and improve device performance. [20][21][22][23][24][25][26] Given the strong influence SAM modifiers can have on the performance of organic electronic devices, the ability to microcontact print SAMs with large work function contrast is both scientifically interesting from the standpoint of creating model systems to explore the role of barriers and energy level offsets on charge injection in OLEDs, and technologically useful in the context of applications including low-cost illuminated signs and displays. [3,6] Although a limited amount of work has been performed in this area, notably by microcontact printing thiols on gold, [3,6] silanes on hydroxyl-terminated surfaces, [2,11,12] or phosphoryl chlorides on indium tin oxide (ITO), [27] these functional group/substrate combinations are not necessarily ideal for integration into OPV and OLED applications.…”

Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces

“…Common surface treatments include halogenation, [16][17][18][19][20] plasma reactions, [21][22][23][24][25][26][27] UV-ozone [28][29][30] and controlled air exposure. [31][32][33][34][35][36] Buffer layers include vapor-deposited small molecules, [37][38][39] spincast polymers, [40][41][42][43][44][45][46] inorganic salts, 12,47-52 covalently bound selfassembled monolayers [53][54][55][56][57][58] and thin metal oxide layers. This review concentrates on the use of transition metal oxides as buffer layers in organic electronics.…”