Electrical Signature of Ultrasound-Induced Anisotropic Self-Assembly of Poly(3-Hexylthiophene) (P3HT) during Channel Formation

Round 1

Reviewer 1 Report

The manuscript reports the temporal evolution of electrical properties in P3HT during channel formation using ultrasound. An interesting source-drain signal is detected during the evaporation of solvent. The authors tried to connect the electrical properties with ultrasound irradiation-induced structural ordering. I think that the manuscript has some issues to be addressed before publication in Applied Sciences. I have the following comments to offer.

1. Why do FET transfer characteristic curves of the pristine and sonicated P3HT FETs show hysteresis, as sweeping the gate voltage? And the sonicated one seems to have larger loop than pristine sample.

2. Actually, a less sharp current peak can also be found in the pristine P3HT, as shown in figure 3b. Do the authors have more explanations on the differences in mechanisms?

3. As putting the current peak aside, why is the time shorter for sonicated solution to achieve a flat drain current, as shown in figure 3d?

4. How does the peak of source-drain current change as changing the solvent to ones with higher boiling point?

Author Response

The manuscript reports the temporal evolution of electrical properties in P3HT during channel formation using ultrasound. An interesting source-drain signal is detected during the evaporation of solvent. The authors tried to connect the electrical properties with ultrasound irradiation-induced structural ordering. I think that the manuscript has some issues to be addressed before publication in Applied Sciences. I have the following comments to offer.

1. Why do FET transfer characteristic curves of the pristine and sonicated P3HT FETs show hysteresis, as sweeping the gate voltage? And the sonicated one seems to have larger loop than pristine sample.

The presence of the electronic traps causes gate scan direction-dependent current hysteresis. As the gate voltage is scanned electrons are trapped and in the reverse scan released, resulting in current difference. To magnify, both P3HT FETs have the similar hysteresis window characterized by the threshold voltage difference in the forward and reverse directions. 

We added sentences on p.6 to address the reviewer’s point.

p.6: “For both P3HT FETs, we observed gate scan direction-dependent drain current hysteresis which is typically observed in disordered materials such as organic semiconductors. With a high density of electronic traps, induced carriers in the electrical channel by the gate electric field are easily trapped or detrapped depending on the polarity of the gate electric field.”

2. Actually, a less sharp current peak can also be found in the pristine P3HT, as shown in figure 3b. Do the authors have more explanations on the differences in mechanisms?

We have explained in the manuscript that in the pristine P3HT, less sharp peak is found in the pristine P3HT because of entangled P3HT chains suppressing self-assembly during channel formation. To clarify the reviewer’s point, we added and modified sentences, elaborating on the presence of a less sharp current peak in the pristine P3HT.

p.8:”More importantly, for the sonicated P3HT in Figure 3(c), we reproducibly observed a significant current decrease after the current peak in 12 min. following a dramatic increase in the current, while, for the pristine P3HT in Figure 3(b), a less sharp current peak in 6 min. is observed as the solvent evaporates with time.”

p.8-9: “The disentangled P3HT chains in the sonicated P3HT solution is more sensitive to solvent evaporation than the pristine P3HT with entangled chains, leading to more orientational change than in the pristine P3HT, causing a sharp peak current in comparison with a less sharp peak current in the pristine P3HT as seen in Figures 3(b) and 3(c). As solvent drying proceeds with an increase in the P3HT concentration, the distance between P3HT chains become closer. The subsequent solvent evaporation varies the extent of anisotropic molecular self-assembly of the disentangled P3HT molecules. According to Park et al., sonicated P3HT back-bones interact in solution, leading to phase transition from the isotropic to the liquid crystalline phase [16]. From Raman spectroscopy data, C=C stretching peak frequency in the thiophene rings was varied during solvent evaporation, accounting for the fact that P3HT back-bones interact with each other. Angular dependent Raman intensities indicate that the P3HT chains feature anisotropic ordering during solvent evaporation. For the sonicated P3HT solution, as the concentration increases with further evaporation, anisotropic ordering is suppressed with more entangled P3HT chains.”

p.10: “In contrast, for the sonicated P3HT chains with anisotropic ordering in solution, the abrupt solvent depletion results in a significant reduction in the mobility of the polymer chains, suppressing the self-assembly driven by the interactions between the polymer chains in solution. In the pristine P3HT, the effect of P3HT chain mobility on ordering with further evaporation is not significant because the P3HT chains are already entangled, resulting in randomly oriented polymer chains in the pristine P3HT, which is not sensitive to the change in the concentration of P3HT solution. To articulate the mechanism, however, further controlled experiments are required.”

3. As putting the current peak aside, why is the time shorter for sonicated solution to achieve a flat drain current, as shown in figure 3d?

 In Figure 3(d) represented by log scale, the earlier current peaks in 4 and 10 mins in the sonicated p3HT result from self-assembly process of the P3HT chains. In the Log scale, the current seems to be saturated after the earlier peak (in 4 min) in the sonicated P3HT, but actually the current keeps increasing up to a real peak (in 12 min) as clearly seen in a linear scale (Figure 3c). We modified a sentence to clarify it.

p.8:”More importantly, for the sonicated P3HT in Figure 3(c), we reproducibly observed a significant current decrease after the current peak in 12 min. following a dramatic increase in the current, while, for the pristine P3HT in Figure 3(b), a less sharp current peak in 6 min. is observed as the solvent evaporates with time.”

4. How does the peak of source-drain current change as changing the solvent to ones with higher boiling point?

We believe that a sharp peak current is not seen because slow evaporation due to a high boiling point, suppressing a rapid increase in the concentration during channel formation. Therefore, in case with a higher boiling point solvent, the difference in the temporal evolution between the pristine and the sonicated P3HT would be reduced. Broader current in the both cases will be expected. We find the idea is very interesting and plan to do experiments. We appreciate to the reviewer’s valuable comments.

Reviewer 2 Report

In the present work, Y. Kim et al. reported on the ultrasound irradiation-induced anisotropic self-assembly of the P3HT chains in solution, leading to the enhanced electrical current of the FET device. The work is interesting and I would like to recommend its publication in Applied Science. Before the final acceptance, the following concern should be addressed:

• As the authors said that the color change of the P3HT solution from bright orange to dark brown is a feature of aggregation of P3HT polymer chains, leading to the enhanced crystallinity of P3HT film. Could the authors explain how the anisotropic self-assembled P3HT chains in solution leads to improved ordering of the P3HT film. As we know that the aggregation of polymer chains in solution did not ensure the molecules will assembled well on substrate. Sometimes the aggregate clusters would lead a problem for film assembly due to the steric effect.

Author Response

In the present work, Y. Kim et al. reported on the ultrasound irradiation-induced anisotropic self-assembly of the P3HT chains in solution, leading to the enhanced electrical current of the FET device. The work is interesting and I would like to recommend its publication in Applied Science. Before the final acceptance, the following concern should be addressed:

As the authors said that the color change of the P3HT solution from bright orange to dark brown is a feature of aggregation of P3HT polymer chains, leading to the enhanced crystallinity of P3HT film. Could the authors explain how the anisotropic self-assembled P3HT chains in solution leads to improved ordering of the P3HT film. As we know that the aggregation of polymer chains in solution did not ensure the molecules will assembled well on substrate. Sometimes the aggregate clusters would lead a problem for film assembly due to the steric effect.

As the reviewer pointed out, retention of the solution phase in the film phase involves many factors including molecular weight, the choice of solvent, concentration, regioregularity of polymers, and interaction with substrate. As found in this manuscript, the FET mobility is maximized at a concentration at a narrow range, evidencing many factors are entangled. As the reviewer mentioned, despite the presence of disentangled P3HT chains in solution due to sonication, aggregate clusters may lead to a negative effect on charge transport at a particular condition resulting from the complicating factors. However, according to our results in this manuscript, the effect of sonication in solution state on structural ordering in the film phase governs over the other factors, as evidenced in the insitu channel formation study here. We believe that anisotropic nature of the P3HT chains arising from sonivation facilitated the formation of the nano-crystallites during solidification. Indeed, given that sonication-induced ordering effect is more effective to regioregular P3HT chains than that of the regiorandom P3HT chains, ultrasonic irradiation is believed to act on facile formation of nano-crystallites by enhanced side chain interactions of P3HT. To address the reviewer’s concern, we added sentences on p. 11-12, addressing the relevant factors associated with retention of the P3HT solution phase into the solid film phase.   

1. 11-12: “Retention of anisotropic phase in solution into the ordered film phase is often suppressed by many factors such as molecular weight, the choice of solvent, polymer regioregularity, concentration and interaction with substrate. As found in earlier experiments, indeed, the FET mobility in the sonicated P3HT is maximized at a narrow concentration range, evidencing that retention of a solution phase into a film phase involves many complicating factors. However, far higher current observed during channel formation in the sonicated P3HT, whether in solution state or in film state, points out that aggregated P3HT clusters formed nano-crystallites both in the sonicated P3HT solution and solid film phase, dominating other factors in determining the film structure, thereby the magnitude of the electrical current. Given that sonication-induced ordering effect is more effective to regioregular P3HT chains than regiorandom P3HT chains, [7, 15] ultrasonic irradiation is believed to act on facile formation of nano-crystallites by enhanced side chain interactions between P3HT molecules.”

Round 2

Reviewer 1 Report

My concerns have been addressed in the revised manuscript. I recommend the publication.