Investigation of surface energy for organic light emitting polymers and indium tin oxide

The contact angles on the thin films of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and indium tin oxide (ITO) were measured by the sessile-drop technique. The surface energies of the films were calculated using the 2-liquid geometric mean and 3-liquid Lifshitz-van der Waals acid-base(LWAB) approaches. The overall total surface energies (gamma(S)) of MEH-PPV and the as-received ITO were 26.4 and 25.8 mJ/m(2), respectively. Both approaches yielded almost the same surface energies. The surface energies were mainly contributed to by dispersion interactions or Lifshitz-van der Waals (LW) interactions for both MEH-PPV and ITO. The changes in the contact angles and surface energies of the ITO films, due to different solvent cleaning processes and argon plasma treatments, were analyzed. Experimental results revealed that the total surface energy of the ITO films increased after various cleaning processes. In comparison with different solvents used in this study, we found that methanol is an effective solvent for ITO cleaning, as a higher surface energy was observed. ITO films treated with argon plasma showed the highest surface energy. This work demonstrated that contact angle measurement is a useful method to diagnose the cleaning effect on ITO films. (C) 2000 Elsevier Science S.A. All rights reserved.