Synchronous Vapor-Phase Coating of Conducting Polymers for Flexible Optoelectronic Applications

Abstract

Since conducting polymers (CP) were first reported, poly(3,4-ethylenedioxythiophene) (PEDOT) is arguably one of the most commercially useful and most studied CPs in the last 20 years (Shirakawa et all., 1977; Chiang et al., 1977; Winther-Jensen et al., 2007; Truong et al., 2007) . PEDOT has been studied extensively on account of its many advantageous properties, such as high electrical conductivity, good transmittance and thermal stability with a low optical bandgap and thermal stability (Winther-Jensen & West, 2004; Jonas et al., 1991). These properties make PEDOT very attractive for applications, such as electrochromic windows (Welsh et al., 1999), organic electrodes for organic photovoltaic cells (OPVs) (Admassie et al., 2006; Gadisa et al., 2006) and hole injection layers (HIL) in organic light emitting devices (OLEDs) (Wakizaka et al., 2004; Hatton et al., 2009) and dye-sensitized solar cells (Saito et al., 2005). In particular, PEDOT is commonly used as a hole extraction layer in OPVs (Colladet et al., 2007; Kim et al., 2005). In most optoelectronic applications as a buffer or electrode layer, the bandgap of the layer plays an important role in determining the operating characteristics, quantum efficiency and electron/hole transport. Therefore, the main issues for electronic device applications include both the electrical conductivity and bandgap. Oxidized PEDOT can be produced in a variety of forms using different polymerization techniques. Solution processing is used most commonly in synthesizing PEDOT in the form of spin-coating, solvent-casting or ink-jet printing. However, these PEDOT systems are relatively insoluble in most solvents, making it necessary to attach soluble functional groups to the polymer or dope it with stabilizing polyelectrolytes (Terje & Skotheim, 1998). An aqueous dispersion of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOTPSS), commercially available as Baytron P, is a stable polymer system with a high transparency up to 80% (Groenendaal et al., 2000). However, the PEDOT-PSS film exhibits relatively low electrical conductivity, 10-500 S/cm (Groenendaal et al., 2000), which does not often meet the high conductivity required for most applications. In addition, scanningtunneling microscopy, neutron reflectivity measurements, and x-ray photoelectron spectroscopy have revealed a PSS rich layer on the top of the spin-coated PEDOT-PSS films due to the phase separation (Lee & Chung, 2008; Kemerink et al., 2004; Higgins et al., 2003). Since PSS is an electrical insulator, excessive PSS can limit the film conductivity (Kemerink