Elsevier

Thin Solid Films

Volume 607, 31 May 2016, Pages 55-58
Thin Solid Films

Double-shot inkjet printing for high-conductivity polymer electrode

https://doi.org/10.1016/j.tsf.2016.03.068Get rights and content

Highlights

  • Demonstrated a double-shot inkjet printing process for high-conductivity electrodes

  • Achieved high-conductivity greater than 1000 S cm 1 only by inkjet-printing

  • Fabricated OLEDs with high-conductivity inkjet-printed anodes

Abstract

This paper presents a printing method to form a high-conductivity patterned poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film. A modified PEDOT:PSS ink containing a secondary dopant (dimethyl sulfoxide) and fluorosurfactant (Zonyl FS-300) was inkjet-printed to form a uniform conducting layer, and the dimethyl sulfoxide, conductivity enhancer, was over-printed onto it to further enhance its conductivity. We achieved high-conductivity greater than 1000 S cm 1 by only using inkjet-printing technique. The mechanism of conductivity enhancement was investigated with X-ray photoelectron spectroscopy and atomic force microscopy analyses. The printing process for high-conductivity PEDOT:PSS was applied to pattern a transparent anode for the fabrication of an organic light emitting diode.

Introduction

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been considered as one of the promising soft electrode materials for emerging flexible and printed electronic devices such as organic light emitting diodes (OLEDs) [1], [2], organic solar cells [3], [4] and organic electronic circuits [5], [6], [7], [8]. Although a pristine PEDOT:PSS film shows very low conductivity below 1 S cm 1, with the addition of a secondary dopant such as dimethyl sulfoxide (DMSO) and ethylene glycol (EG) the conductivity can be improved by 2–3 orders of magnitude, typically 600–700 S cm 1 for spin-coated films [9], [10], [11]. But, this value does not meet the requirement for the electronic applications, and several post-treatment methods have been devised in order to further increase its conductivity to over 1000 S cm 1. Chou et al. dropped a small amount of DMSO directly onto a spin-coated PEDOT:PSS film to increase its conductivity up to ~ 1000 S cm 1 [12]. More recently, Alemu et al. combined both methods (dropping & dipping of Methanol) and had the conductivity of a spin-coated PEDOT:PSS film enhanced to over 1300 S cm 1 [13]. Higher conductivity of ~ 1400 S cm 1 was achieved by dipping spin-coated samples into an EG bath for 30 min [14]. Although these post-treatments are simple and can be easily used in laboratories, they may not be applicable to roll-to-roll manufacturing of patterned flexible and transparent electrodes with high-conductivity.

This paper presents a printing method to form a high-conductivity patterned PEDOT:PSS film. We used inkjet printing technology with high positional accuracy, high speed and roll-to-roll processability. The optimized printing condition was examined to form a uniform and smooth PEDOT:PSS film, and the film was then post-processed with the same inkjet nozzle to achieve higher conductivity. We discuss the effect of printing a secondary dopant on the film with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Finally, the suggested printing method was applied to fabricate an OLEDs.

Section snippets

Experimental details

Conducting PEDOT:PSS dissolved in water was used in this work (Clevios PH 1000, Heraeus). The solid content of the PH 1000 solution was 1–1.3% and had a PEDOT to PSS ratio of 1:2.5 by weight. 5 wt% of DMSO (Sigma-Aldrich) was added into the PEDOT:PSS to enhance conductivity. The surface tension of the solution was modified by adding fluorosurfactant (Zonyl FS-300, Sigma-Aldrich). The equilibrium surface tension was measured at 23 °C with a bubble tensiometer (SITA pro line t15) and was

Results and discussion

In order to find the optimized printing conditions for uniform and smooth films, the m-PEDOT:PSS ink was printed with varying drop spacing from 40 μm to 180 μm. As seen in Fig. 1, the change in drop spacing had an effect on film morphology. At small drop spacing (40 and 60 μm) a non-uniform film was formed, and at large drop spacing resulted in a wavy film. From this result, the drop spacing of 100 μm was chosen to achieve high uniformity and smoothness of printed conducting films.

Fig. 2 shows the

Conclusion

In this study, we demonstrated an inkjet-printing process to pattern a high-conductivity polymer film. A secondary dopant, DMSO-containing PEDOT:PSS ink was first inkjet-printed to form a uniform conducting layer, and as-received DMSO was over-printed onto it to further enhance its conductivity greater than 1000 S cm 1. This is among the highest values ever attained by printing technologies for this particular conducting polymer material. We believe that our proposal of post-printing treatment is

Acknowledgments

This work was supported by the Ministry of Science, ICT and Planning of South Korea under the “IT Consilience Creative Program” (IITP-2015-R0346-15-1007) supervised by IITP (Institute for Information & Communications Technology Promotion) and by a grant (Code No. 2014M3A6A5060952) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Planning of South Korea.

References (15)

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  • Effect of surface tension and drying time on inkjet-printed PEDOT:PSS for ITO-free OLED devices

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    It is a non-contact and mask-free deposition method; therefore, it can be used for the fabrication of multilayer structures without the application of patterning processes [5,6]. During the last decade, significant results have been achieved by printing OLED layers, such as hole injection and emitting layers [7–9], even if the production of efficient fully printed devices is still challenging. Among the possible printable materials for OLED manufacturing, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is a water-soluble polymer that is widely used both as a hole injection layer and an electrode.

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    The use of a plasma treatment can also require several minutes of treatment, rendering it very expensive in R2R processes operating at speeds of several tens of m/min. Other methods to improve the wetting of PEDOT:PSS on the photoactive layer is the addition of solvents, usually isopropanol or dimethyl sulfoxide and/or surfactants, usually Zonyl FS-300 or Capstone FS-31, which improve layer formation in several coating techniques but are not effective for inkjet printing on hydrophobic surfaces [9,14,18–27]. Most techniques for the wet-chemical fabrication of organic electronics such as spin coating, wire bar coating, slot-die coating and slide coating deposit the layer as one continuous wet film [7].

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