Conductivity, work function, and environmental stability of PEDOT:PSS thin films treated with sorbitol
Introduction
The electronic properties of conducting polymers are of interest for physical concepts and technological applications. Accordingly, the development of highly conducting polymers with good stability and processability has been the focus of many recent studies [1]. Currently, a variety of conducting polymers is available for various applications, such as polyaniline, polypyrrole, and polythiophenes [2]. In this direction, poly(3,4-ethylenedioxythiophene) (PEDOT) has been found to exhibit a relatively high conductivity and appears to be the most stable conducting polymer currently available [3], [4]. PEDOT itself is an insoluble material but when it is synthesized in the presence of poly(4-styrenesulfonate) (PSS) an aqueous dispersion can be obtained that can be cast into thin films. In the films the polycationic PEDOT chains are incorporated into a polyanionic PSS matrix to compensate the charges (Fig. 1). Thin films deposited from an aqueous PEDOT:PSS dispersion have been utilized in a wide range of applications [3], [4], [5], for example in antistatic coatings [6], as electrode in light-emitting diodes (LEDs) [7], photovoltaics (PV) [8], memories [9], sensors [10], and as active material for electrochromic devices [11], field-effect transistors [12] and circuits in general [13].
Thin PEDOT:PSS films are extremely hygroscopic [14] and post-deposition treatments in air by thermal annealing are generally unstable due to the fast water uptake. For example, reported conductivities measured in air are roughly one order of magnitude lower compared to those measured under an inert environment [14], [15].
Addition of sorbitol, a polyhydroxy alcohol, to the aqueous PEDOT:PSS dispersion is known to enhance the conductivity of the thin films by several orders of magnitude, depending on its concentration [14]. The dramatic effect of sorbitol as processing additive arises from a further reorganization and stabilization of the PEDOT and PSS chains during subsequent thermal annealing of the films by a plasticizing effect [16]. While this method to enhance the conductivity has been previously studied, the reasons for the remarkable behavior are still under debate [16], [17], [18], [19], [20], [21], [22].
Apart from a change in conductivity, sorbitol treatment may also affect other properties of PEDOT:PSS relevant to device operation, e.g. the electrode work function χ. For PEDOT:PSS thin films, a range of work functions has been reported from 4.7 to 5.4 eV [23], [24], [25], [26] and it has been found that this level can be tuned to minimize the hole-injection barrier at the anode [27] in organic LEDs [28], [29] and PV cells [30]. The spread in work function values is assumed to be related to differences in the top layer, which may contain an excess of PSS [14], [31], [32], [33]. Since the PSS-rich top layer may be modified by the addition of high-boiling solvents [14], [31], [33] or (non-) intentionally by other processing conditions [34], an effect of sorbitol treatment on the work function may be expected.
Here we investigate the conductivity and environmental stability of PEDOT:PSS thin films treated with different sorbitol concentrations. We combine in situ conductivity and water-uptake measurements during thermal treatment and exposure to humidity to further investigate the various properties of pristine and sorbitol-treated PEDOT:PSS thin films [14], [35]. Using scanning Kelvin probe microscopy (SKPM) we find that sorbitol treatment also causes a reduction in work function from 5.1 to 4.8 eV. Furthermore, the increased conductivity by sorbitol-treatment is accompanied by an enhanced environmental stability towards uptake from water. This effect is attributed to a denser packing of the sorbitol-treated films, reducing the water uptake, in combination with a morphology that is less susceptible to swelling. The results presented here have immediate implications for device making.
Section snippets
Experimental
Soda lime glass substrates (3 × 3 cm2) were grooved into pieces of 1 × 1 cm2 on the back side with a diamond pen. They were cleaned with soap and then sonicated in baths of acetone and isopropanol for 20 min each and rinsed with deionized water after each process. Residual organic contaminations were removed using a 30 min UV-ozone treatment (UV-Ozone Photoreactor, PR-100, Ultraviolet Products). Four electrodes (1 × 6 mm2 with 1 mm spacing) were deposited on the cleaned glass substrates using a shadow mask
Effect of thermal annealing
The conductivity of PEDOT:PSS films was monitored in real-time in dry N2 environment, while gradually increasing the temperature at a rate of ∼10 °C/min to 200 °C, followed by 2 min at 200 °C, and subsequent natural cooling to room temperature in about 3 h (Fig. 2).
For pristine PEDOT:PSS the conductivity increases upon annealing by about one order of magnitude from ∼4 × 10−4 to 3 × 10−3 S/cm (Fig. 2a). Subsequent cooling to room temperature, results in a decrease to about 10−3 S/cm. Rapid thermal
Discussion
To clarify the complex effects of the humidity dependency presented in this paper, we recall that spin coated PEDOT:PSS films possess a phase segregated morphology, consisting of conductive PEDOT-rich grains surrounded by a shell formed by excess PSS [15], [38]. PSS is a highly hydrophilic polymer electrolyte, having anionic immobile sulfonic acid groups and a proton (or Na+ impurities, ∼300 ppm) as mobile counter ion (Fig. 1). Hence, in PSS, ionic transport may occur through protons (or Na+ [14]
Conclusion
We have studied and further clarified the role of sorbitol as a typical high-boiling processing additive on the conductivity of PEDOT:PSS thin films under thermal annealing and exposure to humidity. The well-established conductivity enhancement of thin PEDO:PSS films caused by adding sorbitol to the aqueous dispersion occurs during thermal annealing and coincides with the evaporation of sorbitol from the films. The increased conductivity after annealing is accompanied by a lowering of the work
Acknowledgments
We thank Dr. Albert van Breemen of TNO Science and Industry for generously providing materials, samples and for fruitful discussions and Dr. X. Lou and J. L. J. van Dongen for their assistance with mass spectrometry experiments. A. M. Nardes acknowledges the Alβan Program (the European Union Programme of High Level Scholarships for Latin America, ID#E03D19439BR) for the financial support.
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