Transparent Conducting Polymers

Abstract

A conjugated polymer in neutral state is semiconductive. It becomes conductive after it is doped by oxidation or reduction. The optical properties of neutral conjugated polymers are predominantly determined by the electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), whereas the optical properties of conducting polymers are related to new energy levels, polaron and bipolaron levels, which are generated during oxidation or reduction. The appearance of the new energy levels significantly changes the optical properties. Some conjugated polymers with low energy band gap can have high transparency in the visible range after they are oxidized and become conductive. Poly(3,4-ethylenedioxythiophene) (PEDOT), its derivatives and analogues are the most popular transparent conducting polymers. This chapter reviews the preparation, structure, properties, and application of transparent conducting polymers, particularly the PEDOTs. PEDOTs can be prepared by solution chemical polymerization, vapor-phase polymerization, or electrochemical polymerization of its monomer. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) prepared by chemical polymerization in solution is particularly interesting because it can be dispersed in water and some polar organic solvents. High-quality PEDOT:PSS films can be readily prepared through solution processing techniques. Less conductive PEDOT:PSS has been used as the buffer layer in optoelectronic devices, such as organic and polymer light-emitting diodes (OLEDs and PLEDs) and organic and polymer solar cells (OSCs and PSCs). However, as-prepared PEDOT:PSS films from aqueous solution cannot be directly used as the transparent electrode of optoelectronic devices because its conductivity is below 1 S cm−1. This conductivity is lower than the conductivity of indium tin oxide (ITO), the conventional transparent electrode material, by three to four orders of magnitude. A couple of methods have been developed to improve significantly the conductivity of PEDOT:PSS. Conductivity of more than 3000 S cm−1 was recently observed with PEDOT:PSS films after treatment with acids. This conductivity is higher than that of ITO on plastic and comparable to ITO on glass. It is anticipated that the conductivity of PEDOT:PSS can be further increased if the molecular weight of PEDOT can be increased. Thus, transparent conducting polymers are very promising candidates to replace ITO as the next-generation of transparent electrode materials.