Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells

Round 1

Reviewer 1 Report

The manuscript can be improved if the authors can revise the following points:

1. Please raise the readability of this manuscript by a substantial check the manuscript thoroughly.
2. The research work represents scientific-interesting, but the research content lacks sufficient explanation.
3. The list of references should be expanding.

Author Response

Author’s response: We acknowledge reviewer 1 comments, however, we are rather confused by him/her suggestions. Point 1 raises some readability issues however it is not backed by any examples. Moreover, it is in stark contrast with reviewers 2 and 4 viewpoints, who rated the manuscript with a very high degree of readability. Then point 2 raises some concerns about the “lack of explanation”, however again, no example or reference to the text is made.

Finally, point 3 seems to imply that references should be added to the list of references. We have added over 10 references to the manuscript to address this issue.

Overall, the reviewer's comments do not point to specific guidelines for improving the quality of the manuscript.

Reviewer 2 Report

In this paper, Viterisi et al. prepared an interesting review article describing the crystallinity and molecular packing of small molecules and their development in OSCs This article is well written and separated into three main chapters: the first one is the introduction of crystallinity and the differences among polymers, liquid crystals and small molecules. In the second part, the authors demonstrate the methods in characterization of crystallinity. At last, several examples of different molecular packing of crystalline films were reviewed. The authors have chosen the key articles and have made a clear analysis. Therefore, this work is recommended for publication in Applied Sciences after addressing the following comments:

 

1. Page 12, line 487, “Indeed, the first benzodithiophene terthiophene rhodanine (BTR) type donor was reported by Chen and coworkers consisted of an oligothiophene structure with rhodamine end groups…”, rhodamine should be replaced by rhodanine. Most importantly, no reference was added here to support this (J. Am. Chem. Soc. 2012, 134, 16345; J. Am. Chem. Soc. 2013, 135, 8484).
2. Figure 9 line 499, “Adapter with permission from the Nature publishing group, copyright 2015.[75]”, adapter should be adapted.
3. There are several important papers on crystalized organic solar cell is missing (DOI: 10.1002/aenm.201600961; 10.1002/aenm.201300046).
4. The authors claimed that “The D-A or D-A-pi-A-D approach, … has undoubtedly had a major impact on devices’ characteristics”, in fact, A-D-A type molecules is still the best donor (like BTR) or acceptor (like Y6) for organic solar cells. The authors should comment these two different approach and their influence to device performance.

Author Response

Author’s response: We acknowledge that Reviewer 2 has given a very positive review on our manuscript. We are grateful for the constructive comments and suggestions. Most issues were addressed (highlighted in yellow in the manuscript), see below:

 

1. Page 12, line 487, “Indeed, the first benzodithiophene terthiophene rhodanine (BTR) type donor was reported by Chen and coworkers consisted of an oligothiophene structure with rhodamine end groups…”, rhodamine should be replaced by rhodanine. Most importantly, no reference was added here to support this (J. Am. Chem. Soc. 2012, 134, 16345; J. Am. Chem. Soc. 2013, 135, 8484).

Author’s response: The typo was corrected, and the references were added (now ref. 88 to 90). Note that the following sentence was modified to better fit with the previous sentence.

 

1. Figure 9 line 499, “Adapter with permission from the Nature publishing group, copyright 2015.[75]”, adapter should be adapted.

 

Author’s response: The typo was corrected

 

2. There are several important papers on crystalized organic solar cell is missing (DOI: 10.1002/aenm.201600961; 10.1002/aenm.201300046).

 

Author’s response: These papers, although dealing with oligothiophene molecules, were not deemed to focus sufficiently on the crystallinity of the SMs so that a reference to it in the manuscript would be somehow meaningful. We would be grateful if reviewer 2 could point out in what context these references should be added

3. The authors claimed that “The D-A or D-A-pi-A-D approach, … has undoubtedly had a major impact on devices’ characteristics”, in fact, A-D-A type molecules is still the best donor (like BTR) or acceptor (like Y6) for organic solar cells. The authors should comment these two different approach and their influence to device performance.

 

Author’s response: Indeed, as pointed out by reviewer 2, molecules of the A-D-A family are the best-performing donors to date. This was what we referred to in this sentence, the “D-A” mention was, therefore a typo. It has been changed to “A-D-A (or A-p-D-p-A)”

Reviewer 3 Report

With respect to the fact that selection of topics and aspects included in the review is a choice of authors, I think that it brings a useful summary of the published knowledge in the field. The weak point consists in the formal aspects of the article.

The abbreviations should be explained not only in abstract, but also at their first usage in the article itself. Using abbreviations for organic solar cells, for small molecules, or heterojunctions seems to be disserviceable and makes the text less reader-friendly. Many other abbreviations are used without proper explanation.

The style when always a range of references is given followed by paragraphs without specific reference to individual statements is not acceptable for a review article. I have found no reference in lines ranges 59-71, 79-150, 306-327, 390-408

The Conclusions section should be built on the information in the article body; no statement should appear first time in this section not being dealt in the text above.

Specific comments (possible mistakes are to be corrected through the whole text, not only their specified instances):
Line 25: Check for possible misprint.
Line 25: PV abbreviation used without explanation (photovoltaics?)
Line 75: P3HT not explained
Line 76: J-V used without explanation
Lines 159, 213, 266, 267: d is a physical quantity, it should be in italics d (see https://iupac.org/wp-content/uploads/2016/01/ICTNS-On-the-use-of-italic-and-roman-fonts-for-symbols-in-scientific-text.pdf )
Line 196: Physical quantities in the figures used upright, they should be in italics.
Line 195: Who is meant with "by the authors"? If authors of Ref. 29, Lee et al should be here.
Line 234: PC₆₀BM or PC₇₀BM used without explanations
Line 257: Not explained what D60 is, DCV4T used also without an explanation
Line 290, 295: SQ abbreviation used without an explanation
Line 363, 364, 368, 373, 374, 376, 378, 380, 392 ...: G and r should be in italics (G,  r), as used correctly in the equation on line 361; however, the V and E subscripts should be upright there
Line 408: The paragraph ends unexpectedly
Line 455: The DIO abbreviation used without previous explanation
Lines 523-525: Where in the above text is the text about this previous study including the reference?
Line 559: Ref. 11 is insufficient. Year and book publisher are missing.

I have not found a necessity of a change in factual article content, but I believe that extensive changes including elimination of excessive abbreviations, explanation of all abbreviations and symbols, making the text compliant with IUPAC and IUPAP standards of symbols writing, and generally making the text more reader-friendly are necessary.

Author Response

Author’s response: We acknowledge the painstaking work of reviewer 3 and are pleased that most of the suggestions made are linked with the layout of the manuscript and not the scientific content itself. However, we do not fully agree with him/her regarding the words to be abbreviated. Although we do agree with the implicit abbreviation rule suggested by reviewer 3, i.e. that abbreviations should only be used if they enhance the manuscript’s readability, we believe that abbreviating repetitive words such as organic solar cells, Small molecule, bulk heterojunctions which are used over 80 times (combined) throughout the manuscript, have a positive impact on readability. Moreover, and most importantly, these abbreviations are used very extensively in the field of organic PV (in an identical manner) and would not induce any confusion for the reader. The code names (abbreviations) of the small molecules were reproduced from the original papers. These molecules are widely known under these abbreviations in the organic photovoltaic community, and it would be confusing to refer to them using a different name.

We did not entirely comprehend Reviewer 3 comment on the referencing style. However, we believe that he suggested adding more references within the paragraphs. Therefore, as per his/her suggestion, we’ve added about 10 references embedded in the paragraphs themselves. The changes are highlighted in the manuscript.

Finally, concerning the conclusion, the reviewer’s comment is somewhat puzzling. Indeed, our conclusion section seems to fit in all aspects with the reviewers’ guidelines since most, if not all, statements relate to the analysis in previous sections of the manuscript (mainly section 4). The fact that reviewer 3 does not mention particular statements makes it difficult to find out the exact places he/she seems to be concerned about. See below:  

 

“The advent of the D-A-type donors, particularly DTS, oligothiophene, BDT SM donors, marked the start of a new era in SM-BHJ in a way that the crystallinity and, more importantly, the packing of molecules shifted to a more disordered liquid-crystalline-like type. This trend is likely the result of incorporating a large number of alkyl chains in the chemical structure of the donors with respect to early SM donors. Indeed, it is remarkable that SM donors that led to efficiencies above 7%have a ratio of aliphatic to aromatic carbons often superior to one. Therefore, it is consistent with the fact that the added disorder incurred by the alkyl chains likely directs intermolecular interactions towards exclusive p-p stacking and hydrophobic interactions. The disordered liquid crystalline-like packing type is consequently greatly favoured.

 

Author’s comment: This section sums up the trend in crystallization habits vs molecular structure of SMs, dealt with throughout the manuscript (section 4).

 

 

 Although the link between this increase in disorder, brought about by the numerous alkyl chains, and the higher PCEs, is counterintuitive, it can be readily rationalised by the fact that an ordered packing type is rather difficult to achieve in such a complex medium as that of OSCs’ active layers. As demonstrated in our previous study, the subtle thermodynamics and kinetic parameters underpinning the nucleation and growth process are extremely difficult to control. As a consequence, the crystallites formed in such active layers often display extreme features, e.g. either too small or too large crystallites with sub-optimum anisotropic features.

 

Author’s comment: This section sums up the result that arose from our own research was dealt with in section 4

 

Liquid crystalline-like packing SMs can limit the interactions to the only two types mentioned above, forming much more predictable stacks of molecules. Despite the fact that disorder is more predominant in the crystalline domains, the gain in crystallisation control outcompetes the former setback to a large extent. The system is more forgiving towards the formation of homogeneously distributed (size and orientation) crystalline domains, hence the consistent increase in PCE over the years.”

 

Author’s comment: This final statement is a closing remark that sums up our view on the evolution of crystallinity in SM-BHJ and the role it played in the increase in OSC’s efficiency over the years.

 

 

Author’s response: All other issues pointed out by reviewer 3 were addressed, see below (highlighted in yellow in the manuscript):

Line 25: Check for possible misprint.

Line 25: PV abbreviation used without explanation (photovoltaics?)

Line 75: P3HT not explained

Line 76: J-V used without explanation

Lines 159, 213, 266, 267: d is a physical quantity, it should be in italics d (see https://iupac.org/wp-content/uploads/2016/01/ICTNS-On-the-use-of-italic-and-roman-fonts-for-symbols-in-scientific-text.pdf)

 

Author’s response: The above issues were addressed

 

Line 196: Physical quantities in the figures used upright, they should be in italics.

 

Author’s response: The figure reviewer 3 refers to was adapted from a previously published study. For the sake of authenticity, we believe it is better left as is since it does not induce a loss in comprehension.

Line 195: Who is meant with “by the authors”? If authors of Ref. 29, Lee et al should be here.

 

Author’s response: The sentence was modified for the reader to understand to which authors the sentence refers.

 

Line 234: PC₆₀BM or PC₇₀BM used without explanations

Line 257: Not explained what D60 is, DCV4T used also without an explanation

Line 290, 295: SQ abbreviation used without an explanation

Line 363, 364, 368, 373, 374, 376, 378, 380, 392 ...: G and r should be in italics (G,  r), as used correctly in the equation on line 361; however, the V and E subscripts should be upright there

Line 408: The paragraph ends unexpectedly

Line 455: The DIO abbreviation used without previous explanation

 

Author’s response: The above issues were addressed

 

Lines 523-525: Where in the above text is the text about this previous study including the reference?

 

Author’s response: This sentence was rephrased as suggested by reviewer 3.

 

Line 559: Ref. 11 is insufficient. Year and book publisher are missing.

 

Author’s response: The issue was addressed

 

 

I have not found a necessity of a change in factual article content, but I believe that extensive changes including elimination of excessive abbreviations, explanation of all abbreviations and symbols, making the text compliant with IUPAC and IUPAP standards of symbols writing, and generally making the text more reader-friendly are necessary.

 

 

Reviewer 4 Report

The manuscript seems promising with the nice representation of the field. However some problems must be solved:

1) Introduction section lacks newest information in this field. it must be update with the newest research in this field.

2) Lack of proper citation. For example Chapter 2 has a lot of text., but almost no citation for it. It must be corrected by adding citations.

3) The Chapter 3 is about characterization techniques, however it is almost all about X-ray diffraction use. Ir must be update with mote techniques or the name of chapter must be corrected.

Author Response

Author’s response: We acknowledge that Reviewer 4 has given a very positive review of our manuscript. We are grateful for the constructive comments and suggestions. Most issues were addressed. See below:

 

• Introduction section lacks newest information in this field. it must be update with the newest research in this field.

 

Author’s response: We emphasize on the fact that the paper is intended as an overview of crystallinity on Small-molecule BHJ solar cell. We have added a small paragraph in the introduction on the state-of-the-art record PCEs of SM-BHJ OSCs.

 

• Lack of proper citation. For example Chapter 2 has a lot of text., but almost no citation for it. It must be corrected by adding citations.

 

Author’s response: Paragraph 2 deals with somewhat general concepts of crystallinity. This chapter is intended as a very broad overview of the crystallization behaviour of different types of molecules. The content of the paragraph does not refer to any specific study, in particular, however, we have added a significant amount of references as per Reviewer 4 suggestions.

 

• The Chapter 3 is about characterization techniques, however it is almost all about X-ray diffraction use. It must be update with mote techniques or the name of chapter must be corrected.

 

 

Author’s response: Indeed, the focus of the review is on the crystallinity of OSC active layers. Therefore this section’s content is purposely focused on X-Ray diffraction. The title was somehow misleading, and was changed to “X-Ray diffraction characterisation techniques” as suggested by reviewer 4.

 

Round 2

Reviewer 3 Report

The authors reflected the comments very carefully and mostly either have modified the text, or have justified their choice.
I still mean that abbreviations are overused, but I will respect authors' choice and reasons.

To the "We did not entirely comprehend Reviewer 3 comment on the referencing style. However, we believe that he suggested adding more references within the paragraphs." answer:
For example, a group of references [23-28] is mentioned in line 82 (in the first version, it was [20-25]). In the first version, a long text followed, where it was not indicated which statement comes from which paper of the group of [20-25].
I have not required adding new references, but I see in this version, that statements are now with references having been newly added, not [23-28].
For each statement in the article, it should be clear whether it is an own finding or conclusion published here for the first time, or from which paper(s) it is cited.

Specific comments: 
line 262: I still have not noticed full name at the first usage of DCV4T.
line 306: Figure X should be probably Figure 4.
lines 311-331: In line 313, the authors state: "we developed an experimental turnaround consisting of recording a diffractogram of a so-coined "powdered" active layers."
   If this review paper is the first one publishing this turnaround, it deserves a more detailed description. If it was published in their previous paper, reference should be added after "we developed", even if it is still ref. [63] (line 291).
line 395: Similarly, was the relative quantification of the interfacial energy γ of the amorphous fraction of the active layers in the early stage of crystalline growth (r=0) performed for this review paper and it is not referred from any previous paper? If referred from a previous paper, reference to it should be added.
line 399: Are the authors sure that "fus" should not be used as a subscript?
line 471: Brabec and coworkers are mentioned, but this name is not found in ref. 81. When so many authors are listed in the reference, it is confusing when only the last two are replaced by "et al.", in particular when the name of the author hidden under this "et al." is used to refer the paper.
   In my opinion, it is more common to use the name of the first author followed by "and coworkers" or "et al." even if the head of the team is later in the list of authors. My personal preference is not to use "et al." and always to list all authors in the reference list; if the authors have a different choice, they should keep the rule of the first author when mentioning the paper and in no case use names of authors left out from the reference list to refer the paper.
   The similar concerns also line 420 and Chen: In a group of papers, using a name of a leader coauthoring all papers is more acceptable even if he is not the first author, but not combining it with leaving out this author from the authors list summarizing under "et al."

 

Author Response

Reviewer 3 comments:

The authors reflected the comments very carefully and mostly either have modified the text, or have justified their choice.
I still mean that abbreviations are overused, but I will respect authors' choice and reasons.

To the "We did not entirely comprehend Reviewer 3 comment on the referencing style. However, we believe that he suggested adding more references within the paragraphs." answer:
For example, a group of references [23-28] is mentioned in line 82 (in the first version, it was [20-25]). In the first version, a long text followed, where it was not indicated which statement comes from which paper of the group of [20-25].
I have not required adding new references, but I see in this version, that statements are now with references having been newly added, not [23-28].
For each statement in the article, it should be clear whether it is an own finding or conclusion published here for the first time, or from which paper(s) it is cited.

Author’s response: We now understand what Reviewer 3 was referring to; however, we wonder if it can be considered of significant importance. Indeed, in our understanding, a group of references placed at the end of a sentence relates to the sentence itself, not to the following paragraph. Considering the example brought up by Reviewer 3:

“Indeed, the need for an active layer annealing to achieve improved current density-voltage (J-V) characteristics, which was initially mostly linked with phase segregation, was soon associated with polymer crystallisation.[23-28] However, the concept of crystallinity for polymers is somewhat different from the crystallinity of small molecules since, in the former case, crystallinity is understood as the presence of ordered stacks of molecular fragments in an otherwise disordered matrix of polymer chains.”

The group of references are examples of key papers on “polymer crystallisation”. The following sentence is a general statement made by the authors which does not relate to those examples. Reviewer 3 may have expected that we expand on these particular examples, however, we believe that it is not the purpose of this review. Its primary aim is to give a broad overview of trends in crystallisation habits of small molecules in SM-BHJ organic solar cells. It is by no means intended to be comprehensive.

Specific comments: 

line 262: I still have not noticed full name at the first usage of DCV4T.

Author’s response: The full name of the abbreviation was reported

line 306: Figure X should be probably Figure 4.

Author’s response: The typo was corrected

lines 311-331: In line 313, the authors state: "we developed an experimental turnaround consisting of recording a diffractogram of a so-coined "powdered" active layers."
   If this review paper is the first one publishing this turnaround, it deserves a more detailed description. If it was published in their previous paper, reference should be added after "we developed", even if it is still ref. [63] (line 291).
line 395: Similarly, was the relative quantification of the interfacial energy γ of the amorphous fraction of the active layers in the early stage of crystalline growth (r=0) performed for this review paper and it is not referred from any previous paper? If referred from a previous paper, reference to it should be added.

Author’s response:  The references to our own articles were added again after the statements, as suggested by Reviewer 3.

line 399: Are the authors sure that "fus" should not be used as a subscript?

Author’s response: The typo was corrected

line 471: Brabec and coworkers are mentioned, but this name is not found in ref. 81. When so many authors are listed in the reference, it is confusing when only the last two are replaced by "et al.", in particular when the name of the author hidden under this "et al." is used to refer the paper. In my opinion, it is more common to use the name of the first author followed by "and coworkers" or "et al." even if the head of the team is later in the list of authors. My personal preference is not to use "et al." and always to list all authors in the reference list; if the authors have a different choice, they should keep the rule of the first author when mentioning the paper and in no case use names of authors left out from the reference list to refer the paper.
   The similar concerns also line 420 and Chen: In a group of papers, using a name of a leader coauthoring all papers is more acceptable even if he is not the first author, but not combining it with leaving out this author from the authors list summarizing under "et al."

 Author’s response: The name of Brabec was not find in reference 81 since the MDPI reference style limits the number of authors names in the bibliography section. We changed the sentence to “ first author et. Al.” as suggested by Reviewer 3.