Dyes Amount and Light Scattering Influence on the Photocurrent Enhancement of Titanium Dioxide Hierarchically Structured Photoanodes for Dye-Sensitized Solar Cells

Round 1

Reviewer 1 Report

Huang and co-workers have demonstrated hierarchically grown TiO2 photoanodes for dye-sensitized solar cells (DSSCs) and the findings are significant. This report can also be significant to the research community and therefore the manuscript can be accepted. However, there are a few minor comments which need to be addressed before accepting the manuscript and are as follows:

The device efficiency has been shown to improve, however, it is important to show the reproducibility of the device performance. Hence, it is recommended to include histogram plots of device parameters or device efficiency to show the reproducibility. Subscript and superscript need to be taken care of in the figure caption (Fig.2 and Fig. 3) and in the overall manuscript. In Fig. 9 caption, in “incident photon to conversion efficiency”, the letter “I” must be in the capital. All the figure representation should be significantly improved (especially in Figure 6).

 

Author Response

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Reviewer 2 Report

The manuscript by Huang and Hsieh deals with the investigation of the effect of hierarchic organization in TiO2 films for Dye Sensitized Solar Cells.

The matter tackles has already been thoroughly discussed in literature even if the approach developed by the authors is relatively new. 

The manuscript seriously lacks for clear discussion of the experimental results; additionally, some explanation given throughout the manuscript are just tentative and superficial. 

In the introductive section, the authors claimed (based on literature) that the obtainment of macrostructure enhance the photoelectrochemical performances of the corresponding device. Yet, on the other hand, the presence of macrostructure could prevent the effective dye loading onto the electrode. Please clarify this point, also by citing reference tackling this issue.

The use of Sodium hydroxide as desorbing agent is doubtful: the extreme basicity of NaOH could partially degrade the dye. Have the authors tried to use a softer desorbing solution as an acetic anhydride-based one?

Discussion at lines 127ff is not very clear: the authors claimed that the use of a higher amount of PVA (with fixed molecular weight) did not change the magnitudo of the interaction between the polymer and the TiO2 nanoparticle. They state that: "When a higher amount of PVA molecules was added to the paste, however, the number of hydroxyl groups in each PVA molecule did not change: the addition of PVA did not cause an enlargement of the aggregations, but just increased their number." Yet, this is not true. Even if the number of hydroxyl group in each PVA chain does not change the overall amount of -OH moieties is different when 0.2 or 0.3g of polymer are used.

Reflectance spectra of PVA-modified electrode seem to be equal each other, except for 99.000. Please discuss this. 

The mechanism (if it could be named so) reported in figure 7 is not supported by any experimental evidence and it is just conjectural. Please remove the figure or support it with some scientific evidence. 

In table 1 the authors use an arrow to indicate the magnitudo of the scattering effect. In my opinion, they should quantify this effect instead of describe it just qualitatively. Additionally, the photoelectrochemical parameters seems to not present a clear trend. Please discuss this.

Moreover, at least five device for each type of device should be characterized and the statistical errors should be added in table 1. 

The QE results reported in figure 9 are not consistent with the Jsc values summarized in table 1. In the former, device STD seems to outperform device D whereas the opposite is reported within the table. 

My suggestion is to reconsider this manuscript after MAJOR REVISIONS

Author Response

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Reviewer 3 Report

This works reports the fabrication of mesoporous TiO2 (P-25 Degussa) film with different degree of aggregation using PVA having different molecular weight as binder for the aggregation. Further, the authors studied the influence of aggregation on dye loading, light scattering, and the resulting influences on the power conversion efficiency of the device. The work is interesting and lies within the scope of the journal. However, to consider further, the manuscript should be revised on the following points.

The title should be correctly re-written. There are lots of syntax error. What is the meaning of “scattering of dye” in the title?   The abbreviations should be defined when they appear first time in the text. Which film represents the STD. It should be defined.   In the abstract, what is the meaning of “the absorbance of the attached dye”? This could be “absorbance of light by the attached dye”.   The authors assumed that the degree of light scattered by the film is directly related to the amount of light absorbed by a unit dye molecule. To justify this hypothesis, the authors should provide some relevant references.   The peak position of the absorption (Fig. 7) and the IPCE (Fig. 8) spectra is not matching. They should coincide. This issue should be resolved.   Although the aggregation was beneficial for the dye loading and the light scattering, it can hinder the charge transport. Therefore, the authors should also study the charge transport characteristics of each films using electrochemical impedance spectroscopy.

Author Response

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Reviewer 4 Report

The paper is well written and provides an interesting approach to increasing efficiency of solar cells. The paper can cite more MDPI publications, as an example there is a special issue on Nonimaging Optics in Solar Energy at MDPI Photonics journal, the authors can review and cite papers there. https://www.mdpi.com/journal/photonics/special_issues/NOSE

Figure 7 doesn’t have units for the y-axis. This should be fixed.

Author Response

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Round 2

Reviewer 2 Report

The authors did not seriously reply to reviewers' comment.

All the doubts I expressed in the first round of revision are still unchanged. The authors just slightly modified their paper without substantially improving the quality of their paper or receiving the suggestion made by the reviewers. 

At this point, I have no other option than suggest to reject the paper!

Author Response

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Reviewer 3 Report

I appreciate the authors for their revisions.  However, I would suggest the authors to improve their English on the manuscript.

I also suggest to change the term "Dyes amount and Scattering Influence.." of the title into "Dyes amount and light Scattering Influence.."

Round 3

Reviewer 2 Report

Response to First Decision-Comments

Point 1:

The addition of just a small sentence (with a reference only) is not enough to clarify the role of macrostructures in the amelioration of the photoelectrochemcial features of electrodes. I suggest to more thoroughly discussed this topic.

Point 2:

The authors did not reply to the reviewer’s question, but it is a minor point. Point 2 could be considered solved

 

Point 3:

Discussion at lines 127ff is not very clear: the authors claimed that the use of a higher amount of PVA (with fixed molecular weight) did not change the magnitudo of the interaction between the polymer and the TiO2 nanoparticle. They state that: "When a higher amount of PVA molecules was added to the paste, however, the number of hydroxyl groups in each PVA molecule did not change: the addition of PVA did not cause an enlargement of the aggregations, but just increased their number." Yet, this is not true. Even if the number of hydroxyl group in each PVA chain does not change the overall amount of -OH moieties is different when 0.2 or 0.3g of polymer are used.

Reflectance spectra of PVA-modified electrode seem to be equal each other, except for 99.000. Please discuss this.

 

Response 3: 99.000 is not mentioned in the literature. So I can't answer this question.

 

First Decision:The first part of the reviewer’s comment was not answered by the authors. This 99.000 must be a misunderstanding (?) or spelling mistake (?) which could be clarified.

 

 

STILL NO ANSWER

 

 

Point 4: 

Solved

Point 5:

 This is the table the authors presented. A clear trend is not present! If not I think I am becoming blind. Moreover, the use of arrow in a table is atypical for international publication!

 

Point 6: 

Now the IPCE seems to me MORE (but not completely) consistent with JV measurements.

 

In my opinion, the paper should be definitively REJECTED. After two round of revision, the overall merit of the manuscript is just slightly improved. Moreover, the carelessness showed by the authors is very upsetting.

Author Response

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Round 4

Reviewer 2 Report

I appreciated the effort made by the authors to reply to my comments.

Even thought the modification improved the overall merit of the manuscript, I still feel that this work is not sufficiently new and the results are not thoroughly discussed. Therefore, I think it does not deserve to be published in "Coatings"

Nevertheless, I am aware that my feelings could not be the same of other reviewers' ones. So, if they endorse the publication of the manuscript I will not oppose the publication.