Plasma polymerized thiophene: molecular structure and electrical properties

Polythiophene (PTh) is of interest for transistors, light emitting diodes a nd sensors. Plasma polymerization is a solvent-free, room temperature proce ss that can be used to rapidly deposit thin polymer film onto a wide variet y of substrates. This paper describes the synthesis of plasma polymerized t hiophene (PPTh) and the dependence of molecular structure and properties on the polymerization conditions. Transparent plasma polymerized thiophene fi lms, deposited at about 50 nm min(-1), had a density of about 1.75 g cc(-1) , depending on the carrier gas used (if any) and on the plasma power. The m olecular structure consisted of opened thiophene rings and included signifi cant amounts of unsaturation, oxygen and nitrogen. When nitrogen was used a s a carrier gas at a low power or when a high power was used (with no carri er gas), there was significantly more oxygen, nitrogen, and sulfur-oxygen b onds. When a high power was used, the films exhibited a higher polar compon ent of surface tension and a higher internal stress. The undoped films exhi bited non-linear current-voltage (IV) behavior typical of Schottky metal-se miconductor barriers with breakdown at reverse bias. Iodine doping yielded ohmic IV behavior, perhaps reflecting the formation of a conducting iodine percolation network. (C) 2001, Elsevier Science Ltd. All rights reserved.