Processing optimisation, conductivity enhancement & adhesion improvement of PEDOT:PSS films

Carter, Joseph Lewis ORCID: 0000-0001-5647-5214 (2022). Processing optimisation, conductivity enhancement & adhesion improvement of PEDOT:PSS films. University of Birmingham. Ph.D.

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Abstract

The use of aqueous poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as a replacement for indium tin oxide (ITO) in optoelectronic devices has gathered a large degree of interest in the last 40 years. This is due to reduced costs, improved mechanical flexibility and solution processibility (e.g., roll-2-roll, ink jet printing) of PEDOT:PSS. However, pristine PEDOT:PSS conductivity is approximately 1 Scm\(^{-1}\), which is considerably lower than that of ITO (4500 Scm\(^{-1}\)). Current conductivity enhancing methods mainly employ volatile and harmful substances, such as concentrated acids (e.g., H\(_2\)SO\(_4\)). Additionally, the adhesion of PEDOT:PSS to polymeric substrates commonly used in flexible optoelectronic devices or during bulk manufacturing processes is weak and, therefore, needs to be addressed.

The aim of this study is to increase the conductivity of PEDOT:PSS with the addition of the environmentally safe, non-ionic surfactant Tween 80. Processing and surfactant concentration were optimised to provide the best conductivity, while solution properties were analysed to indicate the suitability of this system in common bulk processing methods. Other conductivity enhancement methods were also explored, such as multiple layer application, methyl ethyl ketone (MEK) addition, and solvent washing. Mechanisms of conductivity enhancement were assessed using atomic force microscopy (AFM), x-ray diffraction (XRD) and Raman spectroscopy. Finally, investigation was carried out into the use of polydopamine (PDA) as a primer layer for PEDOT:PSS adhesion improvement on polymeric substrates.

The lowest sheet resistivity obtained in this study was 16.75 Ω□\(^{-1}\) for a 5 layered pristine PEDOT:PSS sample, whereas the greatest conductivity was achieved by washing pristine PEDOT:PSS with methanol (74.4 Scm\(^{-1}\)). However, both methods were deemed impractical approaches to improving conductivity on a bulk manufacturing scale. Therefore, Tween 80 was found to be promising alternative to enhance the electrical properties of PEDOT:PSS. At a concentration of 1 wt% Tween 80, sheet resistivity improved from approximately 1000 to 130 Ω□\(^{-1}\), corresponding to a conductivity increase from 3 to 20 Scm\(^{-1}\). The greatest conductivity achieved due to surfactant addition was 26.8 Scm\(^{-1}\) at 1.40 wt%, however, film quality began to deteriorate at this concentration. It was established, via AFM, that phase separation of the PEDOT and PSS regions occurred when Tween 80 was added, and alignment was detected with XRD. Conductivity enhancement was attributed to both of these mechanisms, and it was suspected that a benzoid to quinoid structural change within PEDOT was occurring, although this was not detected by the Raman analysis in this study.

Solution properties of PEDOT:PSS were also improved when using Tween 80 as an additive. Wettability on polypropene (PP) and poly(ethylene terephthalate) (PET) was shown to increase with greater surfactant concentrations. This led to a corresponding improvement in adhesion and film quality on both substrates. Finally, the use of a PDA primer was also shown to have a positive effect on wettability and adhesion. This was shown to be the case across all substrates for PEDOT:PSS/Tween 80 films. Adhesion improvement was greatest for pristine PEDOT:PSS films, however, there is scope to further expand on the work presented in this study.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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