Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures

Indium tin oxide (ITO), Au and Pt are materials of interest as high work function contacts for organic semiconductor devices. In this paper the relative energy level line-up of these materials is investigated both as bare surfaces or part of a polymer/conductor interface. Kelvin probe (KP) measurements show that the estimated work function for Au and Pt surfaces, evaporated under normal high vacuum (HV) fabrication conditions and measured in air, can be significantly lower (by similar to 0.2 to 0.9 eV) than those of clean surfaces evaporated and measured in the more demanding and clean ultra high vacuum (UHV), that are often used as reference values. The work function of all surfaces increases significantly (from similar to 0.4 to > 1 eV) after an oxygen plasma, but then decreases upon air exposure, with different rates for different materials. The effect of the plasma wears off most rapidly for Au whilst it is more resilient for ITO. Most interestingly, via KP and electroabsorption measurements of the built-in potential on polymer/conductor and conductor/polymer/conductor structures, we demonstrate that a plasma-induced enhancement of the work function is "frozen in" by the application of a polymer semiconductor layer over the plasma-treated surfaces and can be made to last for years by proper device encapsulation. These results have strong implications on the understanding, fabrication, design and stability of organic semiconductor devices. (C) 2011 Elsevier B. V. All rights reserved.