Self-assembled electroactive phosphonic acids on ITO: Maximizing hole-injection in polymer light-emitting diodes

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 tetra-phenyl-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.