Organic electroluminescent devices: enhanced carrier injection using an organosilane self assembled monolayer (SAM) derivatized ITO electrode

A protocol for the reproducible silylation of indium–tin oxide coated glass (ITO) using small molecule chlorosilanes is reported, and shown to be a convenient means of dramatically improving the performance of the ITO anode invariable used in organic electroluminescent devices. Using the model system: ITO|TPD|Alq3|Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′diphenyl-1,1′biphenyl-4,4′-diamine and Alq3 is tris(quinolin-8-olato)aluminium) luminance-voltage, current–voltage, quantum efficiency and luminance efficiency data illustrate the superior performance of the silane modified device over a reference. Static contact angle measurements confirming the successful silylation of the ITO electrode surface, are supported by direct measurement of the effect the dipolar monolayer has on the work function of the underlying ITO, using a scanning Kelvin probe. This research builds on our earlier work with dipolar phosphonic acids. However, unlike phosphonic acids, chlorosilanes are known to adhere to oxide surfaces via a covalent bond, and so the silylated ITO electrodes are expected to exhibit improved durability. Such electrode modification provides a method of tuning the work function of the ITO electrode to the HOMO of the hole-transporting layer and thus, improving device performance.