Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the current work, S. Spasova et al. describe the synthesis and characterization of Zinc–Tin Oxide (ZTO) thin films that are considered as an electron transport layer for perovskite solar cells. Using spin-coating and subsequent annealing, ZTO samples of different compositions (and thicknesses) were obtained, which were characterized using X-ray diffraction, SEM, and XPS. Some optical properties of the samples were studied, as well as electrical characteristics of the corresponding thin-film structures. Despite the topical area of research, in its current form the work raises major questions regarding the scientific novelty and quality of the material presentation.

1) The most serious drawback of the work is its complete detachment from the literature data in this area. In fact, studies of the functional characteristics of ZTO films have been actively conducted since the mid-2000s, including for films obtained by spin-coating from chlorine sources (similar to the current work). However, in the current work, 

• there are no references to the literature results in this area in the Introduction;

• there is no comparison of the obtained functional characteristics with the others ZTO films. 

This is unacceptable for a scientific manuscript. This information should be provided and it should be clearly emphasized in the introduction what is new in this work to the existing results.

2) It is unclear what starting materials were used in the work (composition, purity and manufacturers), as well as any conditions for spin-coating. In fact, there are many factors that affect the result, including the concentration of solutions. Moreover, the Experimental Section should clearly indicate how each sample was prepared, however, this information is missing, which nullifies the reproducibility of the results.

3) The idea of using DSC as a base to optimizing the annealing conditions seems interesting, but it raises questions. 

• First of all, it is unclear what samples were used for the study. How were they prepared: by coprecipitation, grinding, or - ??? How was their homogeneity proven? 

• Secondly, it is unclear why the DSC studies were carried out in the closed samples (lines 59-60), while drying and annealing of the films were apparently carried out in the open mode (lines 157-158). The ease of removal of decomposition products (such as water and hydrogen chloride) will certainly affect the nature and temperature of the processes.

4) When interpreting the thermal data (lines 92-153), the authors do not provide any literary references. This is surprising, since both starting materials have been known for a long time and their thermal properties have certainly been investigated. Thus, at present the interpretation of the peaks on the DSC curves may be appears speculative without any evidence.

Please use literature to correct the proposed thermal schemes. Here, the most unclear point is the "mystery" of the change in the oxidation state of tin (from +4 to +2, Sn(OH)Cl): now it is ignored, but it is necessary to indicate the reducing agent and possible products. A TGA study with analysis of the gases evolved would be useful for interpretation (if similar results are not found in the literature). Realizing that it is the New Year holidays, I will ask the Editor to give the authors more time to obtain new useful data.

5) Regarding the methodology of the article, it is unclear why the authors chose the studied set of compositions (Table 2)? What was the purpose of the study? If it was important to consider the effect of composition, then why is the study limited to x=0.4, and there are no samples with x=0.3, 0.2 or 0.1? Please add an explanation. It also seems logical to relate these compositions to the compositions of the samples studied by DSC methods. 

6) Moreover, Table 3 describes 6 samples, while 7 samples are discussed below!

7) The XRD data in the presented form (Figure 2) are not informative. 

• In fact, the halo effect does not allow us to interpret the composition. Please correct this effect, combine the graphs into one figure and add literature diffraction patterns. 

• To explain the observations, compare the evolution of the structure from the composition with the literature data. 

• Specify how the lattice parameters change relative to the reference diffraction patterns.

• Specify a possible reason for the texturing of the samples, and what significance does this have for the functional properties under consideration?

8) Please explain what the advantage of crystalline ZTO samples is over amorphous ones. It is not a fact that the appearance of clear boundaries will improve electrical properties. It would be informative to add at least one amorphous comparison sample to points 3.7 and 3.8.

9) It is methodologically unclear why it is necessary to study samples of different thicknesses if the authors wanted to reveal the effect of composition. In fact, optical properties are of course thickness-dependent. Moreover, as a rule, a larger film thickness leads to a larger crystallite/grain size and roughness. This factor should also influence both functional characteristics. Thus, the authors should either obtain a series of samples of the same thickness (as stated in the text, this is easily achievable, lines 181-182), or give a relevant explanation.

10) The SEM data are presented strangely. In fact, now the microphotographs of the surface are given only for the amorous sample, when they are not informative (Fig. 3). It is necessary to provide data for all annealed samples and compare the results of grain evaluation with the XRD data, and then use them when discussing the functional properties.

11) The discussion of the XPS data needs significant improvement. 

• There are no quantitative data. This is unacceptable. A composition table should be provided. Of particular importance are the zinc/tin ratio (relative to that specified in the solutions), stoichiometry and impurity content.

• The authors do not provide information on whether these are surface spectra or after etching. The etching conditions and time should be added.

• The phrase "Depending on the film thicknesses, additional peaks Na1s and Si2p appear" raises questions (line 211). In fact, what film thickness must be to successfully etch it to silicon? How is the film thickness related to the presence of sodium? What is the reason for the appearance of sodium?

• Regarding the interpretation of peak positions, the calibration relative to C1s is obsolete. Authors may use it only if they compare with earlier data based on the same calibration (this should be stated and added to the text). Otherwise, the spectra should be recalibrated and taking into account the position of the Fermi level.

• All key spectra should be deconvolution into components (i.e., spectra fittings), not just O1s. Supplementary materials can be used for this data.

• The results of fitting O1s are difficult to understand now. A quantitative table should be provided, probably in the Supplementary Materials.

• In fact, the position of the Zn2p and Sn3d spectra for sample 5 stands out from the rest, but this point is not discussed in the text.

12) According to XPS, the authors interpret tin as a single oxidation state of +4. This contradicts the DSC data (par. 3.1), where tin +2 was formed (Sn(OH)Cl or SnO), and then, at 400℃ there was either disproportionation (“2SnO→SnO₂ + Sn”, line 133) or its absence (“In the case of samples with a mixed composition, this process was not registered”, line 134).

13) Technical notes:

• Figure 1a is currently unreadable because the inscriptions overlap. Make separate Figures with the peaks labeled and include them in the Supplementary Materials.

• Very small captions (including numbers on the axes) are present in Figures 2, 3, 4, 5.

• The first two references should be replaced with thematic reviews. 

• The sentences from the beginning of the Introduction should be depersonalized (remove "we"), since these are not the results obtained by the authors of the current work.

 

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

We appreciate the valuable remarks and suggestions of the esteemed reviewer and present

our responses and explanations in the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors managed to study the Zinc–Tin Oxide (ZTO) thin films (ZnO)x(SnO2)1-x, deposited by spin coating on glass substrates at room temperature. The gradual phase transitions were registered by Differential Scanning Calorimetry (DSC) method. The X-ray photoelectron spectroscopy (XPS) data were used to profiling of the elemental composition across the surface. X-ray diffraction (XRD) measurements offered information about the phase composition, crystal structure and orientation. The surface  morphology of the prepared samples was studied using a high resolution scanning electron microscope. The transmittance was measured by optical investigation  and the current-voltage measurements were also done and studied.

Overall the study has a purpose and sound results but the authors must work on figure and text. There are missing information and details.

The originality part is missing, the used materials have already been studied with this method by others, and I do not know what is the significant differences between the previous research done by others and this one.

Thereby I consider that the information presented is appropriate for the journal but the manuscript should undergo a few corrections before publishing. A better description of the purpose and what brings new compared with the existing material on the subject and method.

Questions for authors:

Q1:  In Table 1 there are samples A to E, what is the difference between them?

Q2: In Figure 1 a) the legend is missing. Because Figure 1b is a stand-alone it should be renamed Figure 2.

Q3: Why is  ” 3.2. Deposition” after ” 3.1. DSC analysis”?

Q4: line 174 what does ” the same syngony” means?

Q5: Above Figure 4 it is a figure without number, in this figure what does ZS1…ZS7 means?

Q6: In figures 5 a), b), c) and d) please add legends for the 1 – 7 lines, or specify them in text.

As a side comment: if you have a figure with multiple captions, and you decide to count them as a), b)…., then you should have them one next/under each other. Not separated by writing. Please rearrange and rename Fig 5 (a….f ??!!)

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English could be improved to more clearly express the research

Author Response

We appreciate the valuable remarks and suggestions of the esteemed reviewer and present

our responses and explanations in the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript reports the deposition of ZnO/Sno oxides with different stoichiometries for application to perowskite solar cells. The deposited films were characterised using spectroscopic, microscopic and diffractometric techniques and the band energy diagram was constructed as a function of material composition.

I have a few points to raise to improve the soundness of the manuscript: 

1) Figures 1a and 1b present the same data. I feel that figure 1b represents the data in a more understandable way and that figure 1a is not useful.

2) Section 3.2 should be moved to the materials section.

3) The UV Vis and XPS data of seven samples are shown in Figures 4 and 5 respectively, while the composition of six samples is described in the tables.

4) The Sn3d and O1s signals for sample 1 are very different from the other samples. A discussion should be included in the text.

Author Response

We appreciate the valuable remarks and suggestions of the esteemed reviewer and present

our responses and explanations in the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Unfortunately, the corrections made were minimal and did not solve the key problems of the manuscript.

1) The three lines added to Introduction (line 48-50) do not reflect the scientific landscape in the field of ZTO film production. Thus, in its current form, Introduction still cannot be accepted.

Please add methods for ZTO film production (general overview). Then note the advantages of spin-coating in light of the considered practical applications of films. Then focus on the works describing spin-coating of ZTO films from various precursors. This can also be presented in the form of a table reflecting "precursors and solvent" - "annealing conditions" - "film characteristics (including thickness, purity, etc.)" - "what functional characteristics were measured". In fact, at present, the novelty of the work is not proven, since similar chloride precursors have already been used for spin-coating of ZTO films.

2) The obtained functional properties must be compared with those of the literature data. Add the relevant information to Results and discussion (possibly in the form of a table). Otherwise, the scientific significance of the work is still not confirmed.

3) The added references to thermal behavior do not support the proposed process schemes. According to [20], the compound of stoichiometry Zn(OH)Cl is formed at temperatures above 150 degrees, which contradicts what is written on lines 138-140 (see zinc to chlorine ratio). Similarly, reference [22] does not describe the formation of Sn(OH)2Cl2. Thus, the discussion needs further development.

4) Regarding the methodology, i.e. the limits of variation of x in the samples, the authors did not provide an explanation for their choice. Of course, the authors have the right to choose them in accordance with their wishes. However, it is inappropriate to deceive the readers' expectations, and the current version of the article leads to such a sad situation. Therefore, the interval of x from 0.4 to 0.9 should be indicated in the Abstract and in the Introduction.

5) Contrary to the Responses to Comments, the authors did not remove the "extra/duplicate" sample 7 from Figures 5, 6, 7 and 8.

6) The Response to the previous Comment 7 is unacceptable. Film thicknesses from 30 to 300 nm (line 194) are sufficient for X-ray diffraction analysis without any limitations. So far, the XRD data remain completely uninformative (Figure 2). This is unacceptable for the specifics of the Coatings journal.

• In fact, the halo effect does not allow us to interpret the composition. Please correct this effect, combine the graphs into one figure and add literature diffraction patterns. 

• To explain the observations, compare the evolution of the structure from the composition with the literature data. 

• Specify how the lattice parameters change relative to the reference diffraction patterns.

• Specify a possible reason for the texturing of the samples, and what significance does this have for the functional properties under consideration?

7) If the authors consider it necessary to use annealed samples, they should show the composition and functional properties of the unannealed sample (as-deposited, as in Fig. 3a). One example will suffice.

8) Comparison of optical and electrical properties of films of different thickness is incorrect. In fact, then it will not be possible to identify the influence of x (which was the purpose of the work), since the thickness differs up to 10 times!! Thus, in tables 3 and 5, in addition to x, it is necessary to indicate the film thickness.

9) Since grain/crystallite size has a significant influence on the functional characteristics studied, these data are necessary to be added. 

10) Most of the comments on the RFES remained unanswered. Thus, the discussion of the XPS data still needs significant improvement. 

• The added quantitative data is currently too precise and needs to be rounded off according to the method's error.

• Information on the surface composition does not provide information on the purity of the films, since the surface always absorbs external contaminants. It is necessary to prove the purity of the films, since this is important for the functional characteristics being studied.

• The XPS beam cannot "break through" a film 30 nm thick or more (line 194) to the substrate. Thus, the phrase "Depending on the film thicknesses, additional peaks Na1s and Si2p appear" raises questions (line 224). In fact, what film thickness must be to successfully etch it to silicon? How is the film thickness related to the presence of sodium? What is the reason for the appearance of sodium?

• Regarding the interpretation of peak positions, the calibration relative to C1s is obsolete. Authors may use it only if they compare with earlier data based on the same calibration (this should be stated and added to the text). Otherwise, the spectra should be recalibrated and taking into account the position of the Fermi level.

• All key spectra should be deconvolution into components (i.e., spectra fittings), not just O1s. Supplementary materials can be used for this data.

• The results of fitting O1s are difficult to understand now. A quantitative table should be provided, probably in the Supplementary Materials.

• In fact, the position of the Zn2p and Sn3d spectra for sample 5 stands out from the rest, but this point is not discussed in the text.

11) Technical notes:

• Very small captions (including numbers on the axes) are still present in Figures 2, 4, 9.

• The first two references must be replaced with thematic reviews. 

• Some added lines have formatting issues (eg lines 62-64)

 

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Dear reviewer,

Thank you very much for the extensive and constructive review. We hope to have addressed properly your remarks.

Best Regards,

Eng. Spasova

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors revised their manuscript according to my requests

Author Response

Thank you for approving the second revision of the manuscript without any comments! Much appreciated.