Exploring Structure-Property Relationships in a Family of Ferrocene-Containing, Triphenylamine-Based Hybrid Organic Dyes

Round 1

Reviewer 1 Report

In the present study entitled “Exploring Structure-property Relationships in a Family of Ferrocene-containing, Triphenylamine-based Hybrid Organic Dyes”, the authors aim at investigating a new family of ferrocene-containing DSSC dyes based on their previously reported D-π-D-A dye architecture. For this, a family of triphenylamine-based dyes has been equipped with ferrocene-containing donors has been synthesized. Authors have studied optical, physicochemical properties, and device performance along with the DFT calculations. At first look, the works seem to be interesting and promising. However, in my opinion, the presentation of experiments, results, and discussions need some clarification and overhauling before the manuscript can be considered for publication. I would recommend an acceptance of the manuscript for publication in “Applied Sciences” only after the authors are willing to undertake the major revision.

1. In line 39, the authors have not mention the full form of NHC..?
2. Authors are requested to provide the J-V curves both in dark and under illumination. Also, it is requested to calculate the values of series and shunt resistance and provide the data in Table 2 of the manuscript. As of now, J-V data for samples 2(b-e) shows that PCE is of the order of 10^(-2) or 10^(-3) % and makes no sense. These are errors in this kind of experiment.
3. Also, it is not clear as to what is the underlying mechanism for the poor performance. The authors mention in the discussion and conclusion that there may be another adverse phenomenon present. It is not clear which phenomena authors are talking about. This seems like the authors themselves are not clear as to what is the reason for this poor performance. Additional measurements such as time-resolved PL spectroscopy or transient absorption spectroscopy should be carried out on these specimens to provide a complete picture of recombination kinetics.
4. The energy level diagram comprising the energy level of each constituent and device schematic should be provided for quick and clear understanding.
5. Abstract looks vague and doesn’t carry any meaningful information. It sounds like a brief overview in the magazine article. Authors should include some technical data in the abstract to make it clear and precise to the present study.
6. The authors should check to ask how and why ferrocenium is degrading in the presence of our electrolyte.
7. Authors should thoroughly check the manuscript for typo errors.

Author Response

Dear Reviewers,

 

Thank you for your fantastic suggestions to improve our manuscript. I apologize for the delay in our manuscript edits, but we took the added precaution of going back and remaking many of our dyes and devices and triple checking our measurements, based on your previous feedback. We feel that we have addressed the previous concerns in this version and we have a much better version of the manuscript to share with you. Because we made so many edits to the manuscript, it did not make sense to highlight everything in the manuscript, so we have addressed all the referee comments in this letter (and in the manuscript).

 

 

 

Referee 1 (and Referee 3 owing to some feedback overlap)

 

1. In line 39, the authors have not mention the full form of NHC
• N-Heterocyclic carbene has replaced NHC. (refer to line 53)

 

1. Authors are requested to provide the J-V curves both in dark and under illumination. Also, it is requested to calculate the values of series and shunt resistance and provide the data in Table 2 of the manuscript. As of now, J-V data for samples 2(b-e) shows that PCE is of the order of 10^(-2) or 10^(-3) % and makes no sense. These are errors in this kind of experiment.

 

• J-V curves and EIS in light and dark have been added to the supporting information. Not surprisingly when you have poor performing cells, the EIS and J-V curves (both in the dark and light) are also of poor quality and difficult to model/evaluate. After re-synthesizing and re-purifying (which involves chromatography) all of our dyes (and devices) we are confident that this is not short circuiting from metals or impurities. However, a new batch of cells has delivered equivalent performances, so this is a dye-based phenomenon where the more ferrocene you have on your dye, the worse the performance is. SEE NEXT POINT BELOW FOR FURTHER ELABORATION.

 

1. Also, it is not clear as to what is the underlying mechanism for the poor performance. The authors mention in the discussion and conclusion that there may be another adverse phenomenon present. It is not clear which phenomena authors are talking about. This seems like the authors themselves are not clear as to what is the reason for this poor performance.  Additional measurements such as time-resolved PL spectroscopy or transient absorption spectroscopy should be carried out on these specimens to provide a complete picture of recombination kinetics.

 

• Unfortunately, we do not have access to time-resolved PL spectroscopy or transient absorption. However, based on El-Zohry’s report where they already evaluated a similar family we don’t believe that this particular study would add or alter our argument. Without question this measurement would show rapid recombination. Instead, our argument is simple: It appears poor performance is a result of our electrolyte and ferrocene reacting. Since our first version of the manuscript, we have compiled a number of papers that support this hypothesis that ferrocene (photochemically or at ambient conditions) is reacting with the electrolyte. THANK YOU to the REVIEWERS for getting us to consider the energy diagram, because while considering more closely the energetics of the system and a more intensive review of the literature, we believe our ferrocene containing dyes do not possess the necessary thermodynamic driving force for regeneration, and instead are being oxidized by iodine in the electrolyte. New references have been added that showcase the ways ferrocene can react with components of our electrolyte and we have elaborated on this possibility at the end of our paper. This is consistent with our spectroelectrochemical studies as well.

 

1. The energy level diagram comprising the energy level of each constituent and device schematic should be provided for quick and clear understanding.

 

• This has been added as the final figure to our manuscript. (see Figure 5)

 

1. Abstract looks vague and doesn’t carry any meaningful information. It sounds like a brief overview in the magazine article. Authors should include some technical data in the abstract to make it clear and precise to the present study.
• We have rewritten the abstract as per the suggestion of the reviewer

 

1. The authors should check to ask how and why ferrocenium is degrading in the presence of our electrolyte.
• See response above

 

1. Authors should thoroughly check the manuscript for typo errors.
• We have checked this again.

 

Reviewer 2 Report

In the revised manuscript “Exploring Structure-property Relationships in a Family of Ferrocene-containing, Triphenylamine-based Hybrid Organic Dyes”, Tavneet K. Singh and co-workers reported a family of triphenylamine-based dyes equipped with ferrocene-containing donors, which exhibited different DSSC performance. The authors performed detailed spectroscopic and structural analysis. Nevertheless, there still exist much room to further improve the quality of this work. On the whole, I think this work can be published in Applied Sciences after minor revisions. Comments: (1) The novelty of this work should be stressed in the manuscript to provide more insight for the DSSCs and solar cells community. (2) The achieved results should be compared with the prior reports in the field. (3) The Introduction need to be improved and the current version is not enough to lead this work. Some prior reports are suggested to be included in Introduction. (4) JV and EQE curves should be provided. (5) In the introduction section “In the development of novel technologies, one of the objectives of materials science 23 is to replace expensive chemical components with low-cost building blocks” , some recent studies on low-cost building blocks should be referenced and discussed (for instance, Yang, C.; Yu, R.; Liu, C.; Li, H.; Zhang, S.; Hou, J. Achieving over 10 % Efficiency in Poly(3-hexylthiophene)-Based Organic Solar Cells via Solid Additives. ChemSusChem 2021, 14, 3607-3613; Ye, L.; Ke, H.; Liu, Y. The renaissance of polythiophene organic solar cells. Trends Chem. 2021, 3, 1074-1087; Andersen, T. R.; Weyhe, A. T.; Tao, Q.; Zhao, F.; Qin, R.; Zhang, S.; Chen, H.; Yu, D. Novel cost-effective acceptor:P3HT based organic solar cells exhibiting the highest ever reported industrial readiness factor. Mater. Adv. 2020, 1, 658-665.).

Author Response

Dear Reviewers,

 

Thank you for your fantastic suggestions to improve our manuscript. I apologize for the delay in our manuscript edits, but we took the added precaution of going back and remaking many of our dyes and devices and triple checking our measurements, based on your previous feedback. We feel that we have addressed the previous concerns in this version and we have a much better version of the manuscript to share with you. Because we made so many edits to the manuscript, it did not make sense to highlight everything in the manuscript, so we have addressed all the referee comments in this letter (and in the manuscript).

 

 

 

 

Referee 2

 

1. The achieved results should be compared with the prior reports in the field.
   • We have included the literature results in table 2 and have commented on the similar results

           

2. The novelty of this work should be stressed in the manuscript to provide more insight for the DSSCs and solar cells community. The Introduction need to be improved and the current version is not enough to lead this work. Some prior reports are suggested to be included in Introduction. In the introduction section “In the development of novel technologies, one of the objectives of materials science is to replace expensive chemical components with low-cost building blocks” , some recent studies on low-cost building blocks should be referenced and discussed (for instance, Yang, C.; Yu, R.; Liu, C.; Li, H.; Zhang, S.; Hou, J. Achieving over 10 % Efficiency in Poly(3-hexylthiophene)-Based Organic Solar Cells via Solid Additives. ChemSusChem 2021, 14, 3607-3613; Ye, L.; Ke, H.; Liu, Y. The renaissance of polythiophene organic solar cells. Trends Chem. 2021, 3, 1074-1087; Andersen, T. R.; Weyhe, A. T.; Tao, Q.; Zhao, F.; Qin, R.; Zhang, S.; Chen, H.; Yu, D. Novel cost-effective acceptor:P3HT based organic solar cells exhibiting the highest ever reported industrial readiness factor. Mater. Adv. 2020, 1, 658-665.).

 

• We have re-written the abstract and the introduction to better frame the importance of this work and we have include the two most relevant citations provided.

 

3. JV and EQE curves should be provided.
   • JV curves have been included in the SI. Unfortunately, we do not have access to EQE measurements, but with such poor device performance in our ferrocene dyes, we are not sure this measurement would be relevant.

Reviewer 3 Report

 

                  This manuscript reports the synthesis and DSSC device performance of a series of ferrocene-functionalized triphenylamine dyes. Authors reported the synthesis method and the results applied to a dye-sensitized solar cell.

             First of all, authors reported the application to dye-sensitized solar cells, which are not used much recently. DSSCs are no longer “next-generation photovoltaic devices” because there is no significant change in their efficiency for a long time. Authors claims that “ferrocene, owing to its redox robustness, will lead to more stable and efficient dyes” [page 1, line 16]; however, this is not true. As well shown in Fig 2(a), the absorption range of these ferrocene modified dyes are about 400 to 500 nm, where the solar radiation is not strong. For application to solar cells, it is necessary to absorb light in the region where sunlight is strongest. That is, it must absorb light in the region of about 800 nm. Therefore, efficient dyes for DSSC (i.e., dyes such as N719 dye) show low band gap absorption characteristic and wide absorption spectrum range. Therefore the dyes synthesized in this experiment can’t be an efficient dyes. If the authors really want efficient dyes, they need to synthesize materials with low light absorption moiety. The authors targeted to make efficient dyes for dye-sensitized solar cells, but there was no effect when these dyes were used in DSSC application. Where can they use these dyes? The goal of the experiment was set incorrectly (i.e. targeted for efficient dyes, but the light absorption range is not fit for strong solar radiation range) and the purpose of the experiment was not achieved (i.e. very poor DSSC application), what is the conclusion that will help the world other than the conclusion that these synthesized dyes should not be used?

             For these reasons, I recommend a publication after major revision with additional data and a better structuring of the findings. Other issues with the manuscript as written that must be addressed before it is suitable for publication in Applied Sciences are follows :

 

1. The author wrote as follows; [in page 7, line 272 ~ 275] (“the Knoevenagel condensation does little to perturb the highest occupied molecular orbital (HOMO) energies of the dye electrochemically (vide infra), it is safe to conclude that this bathochromic shift is a result of a decrease in the lowest unoccupied molecular orbital (LUMO) energy”), This is a very dangerous assumption. Since there is no problem with the authors doing CVs, the authors should take CV to confirm a first oxidation penitential of dyes and aldehyde precursor.
2. Basically, in a dye-sensitized solar cell, the open circuit voltage is determined by the difference between the Fermi energy level of the transparent electrode and the oxidation/reduction level of the electrolyte. I think that the reason that the open-circuit voltage of the device does not appear at all is because the diode is not formed. This usually occurs due to the poor purity of the synthesized material. If the dyes were well made without impurities, there is no reason to leave a large difference in Voc as shown in Table 2 because the HOMO values of the dyes are similar. The metal catalyst (palladium in this experiment) used in the synthesis is famous for dissipating all surrounding charges even if there is only a small amount of impurities. It is necessary to confirm the presence of impurities through analysis such as RBS (Rutherford Backscattering Spectrometry), which can detect even a small amount of metallic impurities.
3. If these dyes to be used in solar cell applications, it is necessary to modify the dyes to change the light-absorbing range to a range that can absorb solar radiation better. Or the authors should find other applications field for the synthesized dyes.

Author Response

Dear Reviewers,

 

Thank you for your fantastic suggestions to improve our manuscript. I apologize for the delay in our manuscript edits, but we took the added precaution of going back and remaking many of our dyes and devices and triple checking our measurements, based on your previous feedback. We feel that we have addressed the previous concerns in this version and we have a much better version of the manuscript to share with you. Because we made so many edits to the manuscript, it did not make sense to highlight everything in the manuscript, so we have addressed all the referee comments in this letter (and in the manuscript).

 

 

 

Referee 3

 

1. First of all, authors reported the application to dye-sensitized solar cells, which are not used much recently. DSSCs are no longer “next-generation photovoltaic devices” because there is no significant change in their efficiency for a long time.

• We have changed the introduction to tone down the “next-gen” concept for DSSC.

 

2. Authors claims that “ferrocene, owing to its redox robustness, will lead to more stable and efficient dyes” [page 1, line 16]; however, this is not true.

• We replaced the word “will” for “could”

 

3. As well shown in Fig 2(a), the absorption range of these ferrocene modified dyes are about 400 to 500 nm, where the solar radiation is not strong.

• While not part of the paper, we model all of our dyes using TD-DFT, before we spend months synthesizing them. Compared to the parent, the optical properties of the dyes were superior to the parent dye. However, this was not observed experimentally in the structure property relationship. We appreciate the need to design molecules with longer wavelength absorptions.

 

4. Where can they use these dyes? The goal of the experiment was set incorrectly (i.e. targeted for efficient dyes, but the light absorption range is not fit for strong solar radiation range) and the purpose of the experiment was not achieved (i.e. very poor DSSC application), what is the conclusion that will help the world other than the conclusion that these synthesized dyes should not be used?

• We believe that making new molecules and studying them using hypotheses-driven approaches is something that is being forgotten in chemistry. Just because our hypothesis was disproven, does not make this work less scientific. If this reviewer is not a materials chemist, then let me assure you that making these molecules was a herculean effort, and that trail has now been blazed for anyone to follow and apply these materials elsewhere. Frankly, we are not sure where these dyes would have value, perhaps integrated devices, but we expect that they will inspire others who appreciate molecular materials.

 

1. The author wrote as follows; [in page 7, line 272 ~ 275] (“the Knoevenagel condensation does little to perturb the highest occupied molecular orbital (HOMO) energies of the dye electrochemically (vide infra), it is safe to conclude that this bathochromic shift is a result of a decrease in the lowest unoccupied molecular orbital (LUMO) energy”), This is a very dangerous assumption. Since there is no problem with the authors doing CVs, the authors should take CV to confirm a first oxidation penitential of dyes and aldehyde precursor.

• This requested CV figure was in our original submission in the SI (Figure S1…) but having made many cyanoacetic acid dyes previously we would push back a little to say that our statement is a very safe assumption for this molecular family.

 

1. Basically, in a dye-sensitized solar cell, the open circuit voltage is determined by the difference between the Fermi energy level of the transparent electrode and the oxidation/reduction level of the electrolyte. I think that the reason that the open-circuit voltage of the device does not appear at all is because the diode is not formed. This usually occurs due to the poor purity of the synthesized material. If the dyes were well made without impurities, there is no reason to leave a large difference in Voc as shown in Table 2 because the HOMO values of the dyes are similar. The metal catalyst (palladium in this experiment) used in the synthesis is famous for dissipating all surrounding charges even if there is only a small amount of impurities. It is necessary to confirm the presence of impurities through analysis such as RBS (Rutherford Backscattering Spectrometry), which can detect even a small amount of metallic impurities.

 

• So unfortunately, we do not have access to RBS to determine if there is any metal impurity. However, we feel that this might be a red herring, because it is possible that the ferrocene itself is decomposing in our device. I don’t expect it is decomposing to produce metallic iron, but the way we purify our dye precursors using column chromatography, precludes the possibility that palladium is a contaminant in our dyes or devices. Instead after looking at the literature, and sketching the energy diagram, we actually hypothesize that the Voc so poor because the dye is being oxidized by iodine, setting up a new electrochemical equilibrium – please also refer to response #4 to reviewer 1

 

1. If these dyes to be used in solar cell applications, it is necessary to modify the dyes to change the light-absorbing range to a range that can absorb solar radiation better. Or the authors should find other applications field for the synthesized dyes.

 

• Agreed, See point 4 above.

Round 2

Reviewer 1 Report

The authors have thoroughly answered all my queries concerning this paper.

So it is a pleasure for me to say very simply O.K. for publication.

Reviewer 3 Report

Accept.