Self-Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells

Round 1

Reviewer 1 Report

Ruthenium dyes notoriously do not work well in combination with Co complexes in DSSCs. Hence, this paper could be of merit to those in the DSSC community who pursue a way to solve this problem, as this work proposed by introducing more steric multi-nuclear Ru dyes to block charge recombination. In this aspect, I recommend publication of this paper. However, there are some points I'd like to highlight:

1. The motivation, as stated in the abstract, is that those multi-nuclear dyes effectively blocks recombination to lead to higher PCEs. I think additional experimental proof should be provided in the form of TiO2 electron lifetime measurements for instance to quantify the recombination rates.
2. Are the solar cells in Fig 8b measured at 1 sun? In general, the solar cell characterization part of this paper is very brief and should be extended, given the fact that the whole point of this study seems to be to improve DSSC performance. For instance, Table S1 should be put in the main manuscript. It is also advisable to provide more discussion on the device performance and relate it to the dye design. For example, one could discuss that the increase in Jsc and Voc could also be due to more suppressed recombination, and maybe it is advisable to give some reasoning for the lower FF for the multi-nuclear dyes as compared to Z907.
3. lambda is missing in the x-axis legend of Fig 4c.

Author Response

See attached file 

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript entitled: “Self Assembled Multinuclear Complexes for Cobalt(II/III) Mediated Sensitized Solar Cells” submitted to Applied Science- MDPI as Article presents an tetranuclear complex used as dye in dye-sensitized solar cell. Authors propose a synthesis of new complex exhibiting antenna effect, which results in extending the absorption spectra of dye. The manuscript describes synthesis and characterization of tetranuclear complexes based on three cobalt dyes connected via coordination bonding with silver cation. New complex and all intermediates were characterized by absorption spectroscopy, NMR, cyclic voltammetry, IR and finally a fully functional devices of DSSC were constructed.

Research topic regarding organic photovoltaics is an important aera, that requires cinstant investigation. The DSSC technology is still no fully mature technology and requires in dept investigation. Due to worlds’ focus on green energy this topic is of interest both academia and industry.

However, due to some relevant issues I do not recommend this work to be published in Applied Science-MDPI in its current form.

 

Comments to the manuscript:

1. General: the manuscript requires thorough English correction. Correction of grammatical error would improve readability of this work.
2. General: Authors should avoid using abbreviations without their explanation, when used for first time, for example: “SCE” (page 3, line87).
3. Introduction: I missed in the introduction information about other strategies concerning modification of dyes for DSSC. Also authors should explain a little bit more about antenna effect in other dyes.
4. Introduction: Authors should highlight better the scope of their work in separate paragraph as the last part of introduction to drive reader attention on what is the scope of this study.
5. Paragraph 2.4, DSSC fabrication: Authors should provide information about the casting knife hight in case of doctor blade technique.
6. Results and discussion: Authors should be more consistent in selecting images/figures for supplementary materials. Fig F S12 and S13 should be placed in main article instead of SI.
7. Results and discussion: the description of figures regarding changes in spectra should be unified in terms of shift presentation in “eV” or “nm” depending on units used in referred figure.
8. Results and discussion, paragraph 3.4: Authors should provide the photovoltaic characteristics in main text. A picture of constructed DSSC device would improve the attractiveness of the work.

Author Response

See attached file 

Author Response File: Author Response.docx

Reviewer 3 Report

The authors presented a study on Dye Sensitized Solar Cells comprising cobalt redox couple based on cobalt coordination complexes in combination with benchmark Z907 dye and its novel complexes 2(Z907)-Ag⁺, and 2(Z907)-Ag⁺-Ru(TMAM) and photo-physics of intermediate species.

Ruthenium compounds, despite of low abundance of Ru element, are still one of the best performing and cost-efficient group of dyes for DSSC. Due to their low performance with novel redox mediators alternative to I^-/I₃^-, (particularly based on transition metals coordination compounds) there is much room for improvement in the systems joining these classes of compounds. The main issue of their combination is intensified back electron transfer, occurring from titania conduction band to oxidized form of redox mediator. This topic gained significant attention of scientists in DSSC field and many papers on the recombination phenomena and strategies to overcome this issue in the systems comprising ruthenium sensitizers and cobalt redox mediators has been recently published. Despite the strategy presented by authors is original, they seem to neglect current state of the art in this field what disconnects their work from the broader context. This issue could be overcame if the authors extended the introduction and the discussion of their results. However my confusion about the conclusions and methodology of this work is more serious. Also there are numerous gaps in presented data set.

The main conclusions of this work are PV performance enhancement of compound 3 and 4 in comparison with Z907 in the solar cells mediated by [Co(bpy)₃]^3+/2+  and the occurrence of energy transfer between Ru(TMAM) and Z907 moieties in the complex 4. Even though the difference between 3 and 4 performance is marginal (errors are not given), the authors give different explanations for the enhancement in comparison with Z907. This is very disturbing, since the authors shown that the compound 4 is not photo-stable and decompose to compound 3 and Ru(TMAM) compound. The energy transfer model which is proposed for compound 4 (From Ru(TMAM) to Z907) is supported mainly by the lack of Ru(TMAM) luminescence in the emission spectra measured for 4 at ZrO₂, while this can be easily explained by the lack of Ru(TMAM) on its place caused by degradation of the labile complex on any stage of sample preparation. Also the reasoning, methodology, used terminology and general manuscript organization raises many doubts. I will try to list the majority of them.

 

The authors noticed photo-degradation of 4 in solution so they made measurements at ZrO₂ to “immobilize” the compound on a substrate which would not accept the electrons from a sensitizer. However, despite the ZrO₂ conduction band edge potential is more negative than the LUMO of the sensitizer, there are still numerous trap states within the energy gap of mesoporous semiconductor which may accept the electrons donated by sensitizer. Such electrons would not contribute to the photocurrent in complete DSSC based on ZrO₂ but the injection occurrs. Thus the measurement in such a conditions is actually very destructive for the sensitizer as it stays in oxidized state on the semiconductor's surface until it gets rereduced or decomposed. Similarly,any conclusions should not be drawn from the measurements at TiO₂. Actually time resolved spectroscopic measurements performed on sensitized photoanodes not assembled in complete DSSC devices are inconclusive for the DSSC systems, since the presence of electrolyte solution significantly changes the conditions in which studied phenomena occur. An again a severe degradation of the sensitizer takes place on illuminated semiconductor's surface.

The authors use time resolved absorption measurements of compounds solutions and describe observed bands, but did not draw any new conclusions different than obtained in the stationary emission measurements. Thus why are this results important?

The authors give the values of yields of different electron transfer processes but do not provide the explanation of methods or expressions applied to obtain them.

Figure 3a. – Why is Z907 emission in solution not shown?

Figure 3b. - Why is complex 1 emission on ZrO₂ not shown?

Complex 4 is the only one for whom TCSPC decay is shown. Why?

Complex 3 is the only one for whom transient absorption trace describing charge separated state kinetics is shown. Why?

In the lines 304/305 the authors write:

“The compound 3 (2907‐Ag+) exhibits an emission centred at 750 nm, substantially convolved with laser pulse.”

What kind of convolution is that? Spectral? Temporal?

The excitation wavelength for time resolved emission measurements should be given as well as pump pulse energy density. The later is also missing in transient absorptiom measurements.

In the description of transient absorption measurements (lines 274 and 282) the authors use terms “bleach” and “stimulated emission” as synonyms. Despite they are both causing negative transient absorption signal a stimulated emission is not the same as bleaching which means higher transmittance of the sample. On the contrary stimulated emission gives additional light to the probe, so the origin of negative signal is different.

Fig 6. – a comparison with bare TiO₂ would be helpful in assessment of dye’s contribution to the absorption spectra (especially for compound 2).

In the IPCE expression (line 387) a dye regeneration yield factor is missing.

In line 389 The authors write “The LHE is 90% for Z907 and 95% for the other two cases (...)”. How is this calculated? For what spectral range? The method must be provided.

The number of studied DSSC devices per system nor the errors of obtained photovoltaic parameters must be given.

Non-scientific issues:

All the abbreviations (TBPa etc.) must be explained.

Figure S16 needs description. Otherwise the reader needs to guess what are the red and blue curves representing.

In the figure 4 the charts are not in order. Different layouts are applied. A spline should not be used to join experimental data points – it suggests fitting what is misleading. A label providing a compound indication in every chart would help the reader in comparison of data and drawing conclusions.

 

After consideration of above mentioned issues I recommend to reject the manuscript from being published in Applied Sciences.

Author Response

see attached file 

Author Response File: Author Response.docx

Reviewer 4 Report

The authors presented tetranuclear self‐assembled dye for the application in DSSCs. The article represents complex investigation, however, several parts require additional explanation or correction. 

The authors observed partial photodissociation of the complex 4 upon 450 nm excitation, which also resulted in the formation of a silver colloid in solution. How long the material could be stable under UV light and visible light (green for example)?

Why IPCE spectra is demonstrated only for 450 -850 nm? Please, update the plot to the UV part (at least to 350 - 400 nm). Also, It is necessary to add the value of Jsc extracted from IPCE spectra via integration.

Comments for Figure 8 with JV curves:

The JV curves for samples 3 (2Z907-Ag+)  and 4 (2Z907-Ag+-Ru(TMAM)) could not be analyzed in the present form. Please, update the plot with an inset JV plot and different scale.

Authors should provide the statistical distribution of the IV output parameters for the fabricated DSSCs.

Update the device band diagram with a description of all energy levels in the device including electrodes.

 

Author Response

see attached file 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors improved the manuscript, however they did not refer to all the remarks given.

My biggest concern was indeed the matter of time resolved photophysical measurements. First, the method using ZrO₂ as reference substrate for photophysical measurements is of course well known, and was widely used in DSSC community for years, and is still used however, the results obtained in this way should be handled with care since electron injection to trap states was reported for both ZrO₂ and Al₂O₃ semiconductors, predominately used as reference substrate for TiO₂.

Oum, K.; Lohse, P. W.; Flender, O.; Klein, J. R.; Scholz, M.; Lenzer, T.; Duc, J.; Oekermannz, T. Ultrafast Dynamics of the Indoline Dye D149 on Electrodeposited ZnO and sintered ZrO2 and TiO2 Thin Films. Phys. Chem. Chem. Phys. 2012, 14, 15429−15437.

Oum K, Flender O, Lohse PW, Scholz M, Hagfeldt A, Boschloo G, et al. Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent. Phys Chem Chem Phys. 2014;16:8019-29.

Sobuś, J.; Burdziński, G.; Karolczak, J.; Idígoras, J.; Anta, J.A.; and Ziółek M. Comparison of TiO2 and ZnO Solar Cells Sensitized with an Indoline Dye: Time-Resolved Laser Spectroscopy Studies of Partial Charge Separation Processes Langmuir 2014 30 (9), 2505-2512

Especially while decomposition of complex 4 occurs in the solution at least a comment should be provided on the reason why the complex 4 decomposes in solution and does not decompose on the electrode. In the situation when photovoltaic and stationary absorption measurements of samples with complex 3 and 4 are hardly distinguishable, the authors should not be surprised that the reader may be sceptic about stability of complex 4. Nevertheless in light of new electron lifetime measurements (where complexes 3 and 4 are distinguishable at last) I can agree that complex 4 must have been stable enough in the measurements supporting the conclusions of this work.

However I still have got some remarks and questions.

The information (given in Figure 6 caption) that the TCSPC measurements on ZrO₂ were done in the presence of ACN with 0.1M of Li⁺ solution is of great importance, . Was this also a case in transient absorption measurements of the films? It is important for main conclusions of this work, according to again:

Oum K, Flender O, Lohse PW, Scholz M, Hagfeldt A, Boschloo G, et al. Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent. Phys Chem Chem Phys. 2014;16:8019-29.

The introduction still lacks broader context, skipping reference to works on different strategies of dealing with the recombination of electrons from CB and oxidized form of cobalt based redox mediator. It needs to be added.

I understand why the authors did not provide the pump pulse energy density in TCSPC experiment, but it is still not given for transient absorption measurements – this information needs to be added.

Why the results in figure 8 are given in different spectral range?

Figure S16 still needs description. The red line is not described.

There are different line thicknesses used on different charts within one figure (Figure 5)

I’m going to reconsider acceptance of this manuscript after major revision.

Author Response

See attached file 

Author Response File: Author Response.docx

Round 3

Reviewer 3 Report

The manuscript has been notably improoved and can be published in current form.