uted; consequently, the formation of a conductive region is hindered. Also, in this figure, (b) shows the film surface obtained by performing the dipping treatment in which the unevenly generated aggregates of PEDOT:PSS particles were fractionated and distributed, and (c) shows the film surface obtained by applying the polar solvent to each layer. As shown in (c), the finer aggregates of PEDOT:PSS particles were distributed.

Figure 4 shows the changes in the resistance value according to the number of printing times when the treatment method was changed. The resistance value of the film was lower when applying the polar solvent to each layer than when using the dipping treatment. Therefore, it was revealed that the change in the aggregation state of PEDOT:PSS particles affected the conductivity.

Figure 2. Changes in the resistance value and visible light transmittance according to the number of printing times.

(a) (b) (c)

Figure 3. Microscope image of the surface of a PEDOT:PSS (annealing temperature: 100˚C, 3 times printing, ×100). (a): No treatment. (b): Dipping treatment. (c): Spraying treatment in each layer.

Figure 5 shows the changes in the visible light transmittance according to the number of printing times when the treatment method using the polar solvent was changed. When the number of printing times was two, since the amount of PEDOT:PSS in the thin film was small, the visible light transmittance varied. However, when the number of printing times was three or four, the visible light transmittance of the film was lower when using the dipping treatment than when not using the polar solvent. The reason for this was probably that the aggregates of PEDOT:PSS particles were fractionated and a homogeneous thin film with a small number of gaps was formed; consequently, the transmission of light was hindered. The visible light transmittance was higher in a thin film obtained by applying the polar solvent to each layer than in a thin film obtained by performing the dipping treat- ment. The reason for this was probably that the aggregates of PEDOT:PSS particles were fractionated, the unevenness on the film surface was reduced, and the scattering of light was inhibited; consequently, the visible light transmittance increased.

Both the conductivity and visible light transmittance were higher in a thin film obtained by applying the polar solvent to each layer than in a thin film obtained by performing the dipping treatment. The surface homogeneity and electrical and optical properties of the thin film were improved by the application method. However, the visible light transmittance was more affected by the number of printing times than by the fractionation of the aggregates of PEDOT:PSS particles. Therefore, by adopting three printing times instead of four, the characteristics of a PEDOT:PSS thin film would improve, and the visible light transmittance would be near 80%.

3.3. Examination of Annealing Time between Printing Operations

Annealing is required each time after printing. Although annealing decreases the visible light transmittance, it can also remove impurities that decrease conductivity from a film. Therefore, when the resistance value is taken into consideration, annealing is indispensable. In the present study, the annealing temperature was set at 100˚C, by which the best characteristics were observed in our previous experiments.

Figure 6 shows images of the surfaces of PEDOT:PSS thin films obtained by changing the annealing time. No change was observed in the surface state of a film due to the annealing time. The surface state of the thin film became more heterogeneous as the annealing temperature increased [28] . In the annealing process, PEDOT:PSS particles can move and aggregate, resulting in an uneven surface. Even though the annealing time was changed, the aggregate state did not change.

Figure 4. Dependence of the resistance value due to each treatment and the number of printing times.

Figure 5. Dependence of the visible light transmittance due to each treatment and the number of printing time.

Figure 7 shows the changes in the resistance value and visible light transmittance due to the annealing time between printing operations when three printing times were adopted. As shown in this figure, the resistance value decreased with time, and it was lowest when the annealing time was 30 min. After 30 min, the resistance value did not change. Therefore, to remove impurities by annealing, a 30 min annealing time was sufficient. Moreover, no change was observed in the visible light transmittance due to the annealing time. Therefore, when the surface state of a film did not change, the visible light transmittance also did not change.

Next, a thin film was formed by applying the polar solvent to each layer and changing the annealing time between printing operations from 15 to 30 min. Figure 8 shows the changes in the resistance value due to the annealing time and number of printing times. The resistance value decreased when the number of printing times was three or four, and the characteristics of a thin film became stable after formation. In particular, when the number of printing times was three, the resistance value decreased the most. Figure 9 shows the changes in visible light

(a) (b)

Figure 6. Microscope image of the surface of a PEDOT:PSS (annealing temperature: 100˚C, 3 times printing, ×100). (a): 15 minutes. (b): 30 minutes.

Figure 7. Dependence of the resistance value and visible light transmittance due to the annealing time between printing operations (3 times printing).

Figure 8. Dependence of the resistance value due to the annealing time and the number of printing times.

Figure 9. Dependence of the visible light transmittance due to the annealing time and the number of printing times.

transmittance due to the annealing time and number of printing times. The transmittance did not change, although the annealing time was changed. In printing three times, resistance value was 390.4 Ω and the visible light transmittance was 86.6%. As for these values, the resistivity of PEDOT:PSS thin film was nearly twice as high as that of ITO thin film used commonly and visible light transmittance of the former was nearly 3% lower than that of the latter. The lowest resistance value was measured when the number of printing times was four but the visible light transmittance was nearly 80%. The transmittance was more affected by the number of printing times than by the annealing time. When the transmittance is taken into consideration, to reduce the resistance value of a PEDOT:PSS thin film, treatment methods should be examined when the film is formed by three printing times.

4. Conclusions

To improve the characteristics of a PEDOT:PSS transparent conductive film produced using an ink-jet printer, the present study examined not only a method in which a polar solvent was applied to each layer but also the effects of the annealing time between printing operations on the resistance value and visible light transmittance of the produced thin film. Because of the effect of the number of printing times on the resistance value and visible light transmittance, we focused on PEDOT:PSS thin films, which were formed by two to four printing times.

The resistance value was lowered from 459.8 Ω to 434.0 Ω in a thin film obtained by applying the polar solvent to each layer than in a thin film obtained by performing the dipping treatment. The change in the aggregation state of PEDOT:PSS particles was revealed to affect the conductivity. The visible light transmittance was raised 3% or so in a thin film obtained by applying the polar solvent to each layer than in a thin film obtained by performing the dipping treatment. The reason for this was probably that the aggregates of PEDOT:PSS particles were fractionated, the unevenness on the film surface was reduced, and the scattering of light was inhibited; con- sequently, the visible light transmittance increased.

The annealing time between printing operations was examined, and the resistance value was revealed to decrease by performing annealing for 30 min between printing operations. The reason for this is probably that, since impurities in each layer were removed, a conductive region was formed. No change was observed in the surface state of a thin film due to the annealing time, and the annealing time did not affect the transmittance. When drying a thin film for 15 min, which was formed by three printing times and by applying a polar solvent between printing operations, was changed to annealing the thin film for 30 min between printing operations, the resistance value decreased from 434.0 to 390.4 Ω. The number of printing times affected the transmittance most. When the transmittance is taken into consideration, the characteristics of a PEDOT:PSS thin film should be examined when the film is formed by three printing times.

Cite this paper

Atsushi Nitta,Kazuya Kawahara,Kohei Miyata, (2016) Characteristics Improvement of PEDOT:PSS Transparent Conductive Film Prepared by Ink-Jet Printing. Advances in Materials Physics and Chemistry,06,239-247. doi: 10.4236/ampc.2016.68024

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NOTES

*Corresponding author.

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