Electrochemical adjustment of the work function of conducting polymers

During the last two decades conducting polymers were used for conductive coatings, for corrosion protection and as materials for sensors and separation membranes [1]. More recently it has been demonstrated that conducting polymers are applicable as an active layer in solar cells and organic light emitting diodes (OLED), see Fig. 1, [2-4]. In all applications the conducting polymers are used in connection with other materials. Usually they are sandwiched between layers of metals or organic materials. Therefore the charge transport in any device strongly depends on the properties of the interfaces. The work function of the materials is one of the fundamental parameters in the determination of the interface properties and is a key for improving the performance. The most promising candidate for an application in organic electronics among conducting polymers is poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). Its chemical structure is shown in Fig. 2. This polymer was used at first for the antistatic equipment of photographic materials [5]. Nowadays PEDOT:PSS serves as a buffer layer in OLED to improve hole injection. These buffer layers have a high morphological and redox stability and very good film forming properties [5,6], by which the rough surface of optical transparent electrodes is smoothed out. For many applications the commercially available aqueous suspension of PEDOT : PSS Baytron P (Bayer AG) is used. This solution has a doping level of about 30% and a PEDOT to PSS monomer ratio of 1.6 [7]. This doping level corresponds to a work function which is still too low to remove the barrier for hole injection into common hole transport layers of OLED. Our aim was to adjust the work function of such layers. Generally this can be done by chemical doping or dedoping or by an electrochemical treatment. The chemical way includes the addition of oxida-