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, poly2,7‐(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.