Tailoring the work function of indium tin oxide electrodes in electrophosphorescent organic light-emitting diodes

We investigate the use of organic surface modifiers based on phosphonic acid anchoring groups that react with the surface of indium tin oxide (ITO) in order to modify its work function, surface energy, and the charge injection in organic multilayer electrophosphorescent devices. The phosphonic acid surface modifiers, possessing different substituting groups, are found to tune the work function of ITO in the range of 4.40-5.40 eV. These surface modifiers have been tested as an interfacial layer between the ITO anode and hole transport layers (HTL) that are either processed from the vapor phase or from solution. The use of this interfacial layer with a solution-processible HTL results in high quantum and luminous efficiencies of 20.6% and 68 cd/A at 100 cd/m(2) (17.5% and 60 cd/A at 1000 cd/m(2)). The enhanced performance of the devices incorporating phosphonic acid modifiers could be associated with an improved charge injection and a better compatibility with the hydrophobic nature of the organic layer. The performance of these devices is also compared to that of devices in which ITO is modified with other well-known techniques such as air plasma treatment or the use of a layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate).