Plasma surface modification of P(VDF-TrFE): Influence of surface chemistry and structure on electronic charge injection

Abstract

Reactive ion plasma treatments have been used to alter the high field electrical properties of organic dielectrics via a grafting process of chemical species within the plasma to the surface of the dielectric. This study determines the effect of a CF4/O2 plasma based processing procedure on polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] on low and high field electrical performance. Plasma treatment in conjunction with a thermal annealing procedure is analyzed in the following ways: X-ray Photoelectron Spectroscopy to determine the changes in surface chemistry of films post plasma treatment, optical profilometry to measure evolution in surface topology, water contact angle to track surface polarity as a function of plasma treatment time, and current-voltage measurements at low and high-fields to capture the electrical behavior of the films. The results indicate that plasma treatment causes the chemical modification of P(VDF-TrFE) surface through the addition of carbonyl (C=O) groups, as well as oxygen and fluorine based moieties (CF-O, C-O) which are dependent on processing condition. Contact angle with water shows an increase as a function of plasma treatment time from ∼84° to 111° in plasma treated films, indicating decreased surface polarity after plasma treatment. Finally, plasma treatment decreases film resistivity by one order of magnitude, from 8.0 × 1011 Ω m in untreated control samples to 0.8 × 1011 Ω m, as well as resulted in enhanced Schottky emission caused by decreased Schottky barrier height. Modeling I(V) data using both a surface limited (Schottky) and bulk limited (Poole-Frenkel) approaches suggest that conduction in P(VDF-TrFE) thin films results from Schottky emission and is dependent on the chemical environment of the metal/dielectric contact. This study ultimately demonstrates the ability to alter the electrical properties by plasma surface treatment and also the importance of surface chemistry in organic dielectrics to control conduction through the material for high energy and power applications.