Importance of indium tin oxide surface acido basicity for charge injection into organic materials based light emitting diodes

The influence of the indium tin oxide (ITO) electrode surface acido basicity on organic electroluminescent device characteristics is studied. As measured by photoelectron spectroscopy, acid and base treatments produce large work function shifts of the oxide compared to standard ITO treated by an oxygen plasma or water. The current onsets for triphenyldiamine (TPD) single layer diodes sandwiched between ITO and a silver electrode are in qualitative agreement with the work function of the hole injecting oxide electrode. However, saturated photovoltage measurements on single layer diodes built on acid and water treated ITO disagree with the work function obtained from photoelectron experiments. This is explained by protonation of the diamine layer close to the electrode surface producing a charged double layer. It suggests that even acidic surface hydroxyl groups of the oxide electrode are able to protonate the diamine layer, explaining the importance of plasma cleaning to remove surface hydroxyls. The same ITO treatments are investigated in tris(8-hydroxyquinoline) (Alq) aluminum single layer diodes as well as hybrid TPD/Alq light emitting devices. Implications of metal–oxide surface acido basicity on electrode treatment and molecular material design are discussed.