Electrochemical adjustment of the work function of conducting polymers

by Andreas Petr, Fapei Zhang, Heiko Peisert, Martin Knupfer, Lothar Dunsch

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].

Figure 1
Scheme of an organic light emitting diode (OLED)

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.

Figure 2
Formula of PEDOT:PSS

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 oxidation or reduction agents and a fine adjustment of the work function is usually not possible. Furthermore the presence of these redox agents in the layer after the doping/dedoping is disadvantageous. The disadvantages can be avoided by the electrochemical oxidation or reduction of the polymer layers. The correspondence of the electrochemical potential and the work function allows a precise adjustment of desired values. Usually, a difference of 4.7 eV between electrochemical potential and work function is assumed if the electrochemical potentials are referred to the Ag/AgCl-reference electrode [8]. Up to now there exist only a few reports on adjusting the work function of conducting polymers [9,10] and the electrochemical adjustment of the work function of spin cast films from chemically prepared PEDOT has not been published yet. The combination of photoemission spectroscopy with electrochemical methods allows the study of both the chemical properties such as the elemental composition and equilibrium potential and of electronic properties (e.g. work function of the sample). We changed the electronic state of PEDOT:PSS by varying the electrochemical equilibrium potential. The influence of the electrochemical treatment on both the work function and the chemical properties of the organic films have been the subject of our work.

Spin cast films of PEDOT:PSS on ITO electrodes can be reduced resp. oxidised in 0.1M toluene-4-sulfonic acid/acetonitrile [11]. The films are redox active in a wide potential range and remain at their potentials which were adjusted by electrolysis. Films with significant different doping levels could be produced by electrochemical treatment. The corresponding open circuit potentials were 0.05 V; 0.35V and 0.59V. The potential range where electrochemically treated layers are stable for a long time depends on the electrolyte and on the method of film preparation used.

Differences between the applied potentials during electrolysis and the equilibrium potentials of the films after rinsing are given in Table 1. Higher doping levels than applied in this work should be applicable because PEDOT:PSS films are very stable in a potential range up to 1.4 V. The stability of the films were checked by holding the films at different potentials and subsequent measurement of the cyclic voltammograms. Even after 1 hour at 1V no change in the cyclic voltammograms was detectable. In fact, for a short time higher doping levels could be achieved. However, an important precondition for the preparation of films with a reproducible doping level is the time where the films can maintain a constant potential. At least 5 minutes are required for rinsing the film and drying. Therefore the films were checked by applying a constant potential for 5 minutes and then measuring the time dependence of the open circuit potential. We found that at lower potentials than 0V and higher potentials than 0.7V strong changes of the equilibrium potential occur within 5 minutes. The reason is the increased current at higher potentials as shown in Fig. 3. The origin of the current in the anodic range is probably the oxidation of water traces in solution. In fact the anodic current is increasing dramatically if we add a small amount of water. The increase of the current in the cathodic range (not shown) is due to the reduction of hydrogen.

Table 1: Applied potentials, adjusted potentials and work function of PEDOT:PSS layers

Applied Potential	Adjusted Potential	Work function
0 V	0.05 V	4.7 eV
none	0.37 V	5.1 eV
0.77 V	0.59 V	5.3 eV

Figure 3
Current-time curves of an 100 nm thick PEDOT:PSS electrode. From the left to the right potential jumps from 0.1 V to 0.3 V to 0.4 V to 0.6 V to 0.8 V have been applied.

In order to obtain a direct and absolute measure of the work function of our films we investigated the films by UPS and XPS spectroscopy. In addition, these measurements also provide an elemental characterisation of the films.

Apart from the presence of O, N, C and S core level related features, the XPS spectra of the as prepared layers show a strong Na 1s peak (Fig. 4). The quantitative analysis of this peak indicates that there is roughly one sodium atom per four sulphur atoms in the film. After electrochemical treatment the Na 1s emission disappears completely. More importantly, the elimination of sodium ions is also found if the as prepared layer is immersed in pure acetonitrile which was purged with nitrogen for 10 min. This demonstrates that sodium can be completely exchanged by the solvent before or during electrolysis. Therefore sodium ions are not involved in the doping/dedoping process.

Figure 4
XPS spectra of PEDOT:PSS layers for an as prepared one , a layer after electrochemical oxidation and that one stored in acetonitrile for 10 min.

 

Absolute values of the work function of the untreated and electrochemically modified layers were determined by measuring the shift in the secondary electron cut off as shown in Fig. 5. The values for the work function given in Table 1 correlate linearly with the adjusted electrochemical potential. The linear dependence of the work function on the electrochemical potential is shown in Fig. 6. The value found for the offset between work function and electrochemical potential is in very good agreement with the value given in the literature [8] for small molecules. Moreover we found that the correlation follows the expected slope of 1. This is the first experimental evidence for the correlation of equilibrium potential and work function for conducting polymers [12]. In the case of polymers it is not trivial that the work function measured at the surface coincides with the electrochemical potential which is mainly determined by the bulk properties. It seems that there is no surface contribution to the work function. This very good agreement of both values demonstrates that the electrochemical adjustment of the chemical potential can indeed tune directly the interfacial properties in devices. The reduction of the PEDOT:PSS removes the charge carriers (holes) from these films. This is shown by UPS data which are a measure of the density of states.

Figure 5
The secondary electron cut off of the He I UPS spectra of PEDOT:PSS layers before and after electrochemical treatment.

Figure 6
Linear correlation of the absolute values of the work function measured by UPS and equilibrium potential.

In Fig. 7 the UPS signal of the untreated and a reduced film in the vicinity of the Fermi level (electrochemical potential) is given. The reduction of the density of states (Fig. 7) is in accordance with the lower conductivity of the PEDOT:PSS layers in the undoped state [13,14].

Figure 7
Reduced density of states near the Fermi level after reduction of the film. Upper curve untreated film, lower curve electrochemically reduced film.

The results presented show that the work function of spin cast PEDOT:PSS films can be adjusted in a wide range by electrolysis at constant potential. The doping level of the films is stable in the dry state. The work function measured by UPS corresponds to the adjusted electrochemical potential, the work function depends linearly (slope of 1) on the electrochemical potential. This is a further experimental proof for this well known relationship, here given for conducting polymers for the first time.

Contact

Prof. Lothar Dunsch

Address:	IFW Dresden
	Helmholtzstraße 20 01069 Dresden
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Phone:	+49 351 4659 660
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Email:	L.Dunsch@ifw-dresden.de