A Photoelectrochemical Study of Hybrid Organic and Donor—Acceptor Dyes as Sensitizers for Dye-Sensitized Solar Cells

Round 1

Reviewer 1 Report

Manucript by Barichello et al. presents a set of photoelectrochemical studies on two types of Graetzel-type solar cells with two different types of dyes and tho different electrolytes. Generally the topic is worth studying, however the justification of selection of studied dyes is missing. Such selection does not seem to be justified, and there is no convincing justification in the manuscript.  

Both dyes are commercially available, there are no clear relation between those two structures so the observed relations are simply random - any pair of randomly selected dyes could yield analogous paper. Therefore the manuscript lacks the scientific novelty. There is not clear research hypothesis defined in the introduction.

The albedo effects can be easily related to transmission spectra of the cells - reflecting/scatteirng layer improves performance just due to increased effective light flux - again no siginificant scientific novelty.

Finally, the differences of geometrical arrangement of dye molecules at the surface should be related to the spacial arrangement of frontier molecular orbitals involved in the electron transfer. Detailed expalnation of observed facts should be supported by careful DFT modelling. Does it make any sense for well-known molecules?  No - there is novetly in the approach.

To sum up, whereas the whole study is correctly performed and the data analysis could be improved (e.g. bo theoretical models and optical transmission spectra), it does not yield any siginificant progress in the field of solar cells, does not prove or falsify any meaningful research hypothesis.

It is a report on a series of experiments, but the question is, wheather this is a good and valuable scientific contribution. In my opinion no - it is a routine and incremental study of well-known materials applied in a well-known device.

The only really novel and challenging part of the story is related to the bifaciality factor and its dependence on the dye and electrolyte. In my opinion authors should focus on facts collected in table 1 and build a solid story around it.

Therefore, taking into account the scope of the manuscript and the lacks of scientific novelty I cannot recommend publication of the title manuscript in Applied Sciences, at leats in the present form.

A detailed revision, taking into account deep remodelling of the manuscript, highlighting the novelty and presenting deep and thoughfull  analysis of bifaciality and albedo will of high value. 

Author Response

  Reviewer 1

Manucript by Barichello et al. presents a set of photoelectrochemical studies on two types of Graetzel-type solar cells with two different types of dyes and tho different electrolytes. Generally, the topic is worth studying, however the justification of selection of studied dyes is missing. Such selection does not seem to be justified, and there is no convincing justification in the manuscript.  

Both dyes are commercially available, there are no clear relation between those two structures so the observed relations are simply random - any pair of randomly selected dyes could yield analogous paper. Therefore the manuscript lacks the scientific novelty. There is not clear research hypothesis defined in the introduction.

This work, as reported in the acknowledgements section, was financed by Best4U Project. In this project, we started with a screening of several sensitized to be used for DSSC as top cell for a tandem system as reported in the following table presented to the workshop of the Project:

We decided, for this paper, to focus our investigation on the best two dyes (C106 and Y123).

Moreover, C106 has been reported in literature as one of the most efficient representative dye between based Metal-Ru-complexes dyes [21.          G. Calogero, A. Bartolotta, G. Di Marco, A. Di Carlo, F. Bonaccorso, Vegetable-based dye-sensitized solar cells Chem. Soc. Rev., 2015, 44 (10), 3244.]. On the other hand, Y123 has been investigated in literature as one of most representative of the metal-free group of dyes. Furthermore, in a recent paper (“Andrea Capasso et al.2019 2D Mater. 6 035007”), written with the contribution of one of the Corresponding Author (Dr. Giuseppe Calogero) these two dyes were investigated as the top evolution of two categories of dyes ( one metal-free    and other metal based on Ru-polypyridine complexes) but conditions were different (thickness, electrolyte, device’s fabrication).

In accordance with your comment, we inserted in the introduction the reason we choose these two dyes.

From line 67 to line 72  ‘The choice to compare in this work the C106 and Y123 dyes is due to the fact they are considered as the most representative ones of the two categories, metal-Ru-complexes and metal-free-dye respectively. Indeed, C106 has reached one of the highest power conversion efficiencies of 11.3 % [21, 27] and it demonstrated a robust stability when tested at 60 °C under light soaking. Y123, on the other hand, belongs to a group of new organic dyes (together with C218 and JF419) worth of interest due to their large extinction coefficient with one –COOH group [30].’

The albedo effects can be easily related to transmission spectra of the cells - reflecting/scatteirng layer improves performance just due to increased effective light flux - again no siginificant scientific novelty.

You are right. The albedo effect alone has no novelty but, for these dyes, there are not in literature any of these studies. Indeed, for the project that has the objective to prepare and to study a 4 terminals – solar cells, the albedo effect is very important to improve the PCE, and the study of this effect in literature has not been made at the best of our knowledge. We modified the text about this effect according with your comment therefore we emphasized the novelty.

From line 108 to 110  ‘At the best of our knowledge, in literature a PCE as high as 12.8 % with Y123 dye has not been reported yet, since no one studied the potential of Y123 under the albedo effect.’

Finally, the differences of geometrical arrangement of dye molecules at the surface should be related to the spacial arrangement of frontier molecular orbitals involved in the electron transfer. Detailed expalnation of observed facts should be supported by careful DFT modelling. Does it make any sense for well-known molecules?  No - there is novetly in the approach.

You are right indeed we did not do any of these studies.

To sum up, whereas the whole study is correctly performed and the data analysis could be improved (e.g. bo theoretical models and optical transmission spectra), it does not yield any siginificant progress in the field of solar cells, does not prove or falsify any meaningful research hypothesis. It is a report on a series of experiments, but the question is, wheather this is a good and valuable scientific contribution. In my opinion no - it is a routine and incremental study of well-known materials applied in a well-known device.

This is scientific research that answers to some questions:  why Y123 dye exhibits higher extinction coefficient than C106 but it results in a worse PCE performance? In this work, we front these issues, we discuss PCE variation related to different thicknesses, and we explained the scientific reasons.  In literature, no one stop to reason about the potential of the albedo effect (a well-known effect) for DSSC application as BIPV or in indoor environment instead the key for DSSC future is application is there.

The only really novel and challenging part of the story is related to the bifaciality factor and its dependence on the dye and electrolyte. In my opinion authors should focus on facts collected in table 1 and build a solid story around it.

Therefore, taking into account the scope of the manuscript and the lacks of scientific novelty I cannot recommend publication of the title manuscript in Applied Sciences, at leats in the present form.

A detailed revision, taking into account deep remodelling of the manuscript, highlighting the novelty and presenting deep and thoughfull  analysis of bifaciality and albedo will of high value. 

We modified the manuscript addressing most of the reviewer suggestions.

From line 290 to line 310 we implemented discussion on BF.

‘Considering results in table 1, it is possible to notice that BF is influenced by the electrolyte and by the dye. J8-electrolyte based devices have a BF higher than HSE-based ones and this depends on the different viscosity of the employed solvents. As discussed previously, J8 solvent allows a faster regeneration of the redox couple and this brings to better performances also in the rear side. Analyzing BF differences for both electrolytes, we notice the most influenced parameter is the current density, indeed, considering HSE-C106 device from rear to front the current doubles (from 6.42 to 11.99 mA/cm2), while in the case of J8-C106 (from 9.62 to 13.94 mA/cm2) . A BF of 93 % was achieved due to the exploitation of the albedo effect with C106 on a white background: more scattered photons, less differences between Jsc from rear to front (from 19.34 to 21.07 mA/cm2). It is worth to highlight that exploiting the albedo effect, a standard DSSC cell reached the bifaciality factor of 93 % and this is mandatory to understand the great potential of this technology when used as BIPV in windows or indoor environment even with different angle inclinations.

C106 has a higher BF than Y123 (therefore less differences between front and rear side measurements) and it is possible to find the explanation observing their absorbance spectra (figure 2b). A sensitized C106-dye in TiO2 surface shows two maximum peaks at 401 and 541 nm. Iodine/iodide electrolyte absorbs from 300 to 400 nm [44] therefore, in rear side, when the electrolyte for first filters the light, this cannot block the C106 excitation in the second band at 540 nm. On the other hand, Y123 has one band from 400 to 500 nm partially influenced by the filtered light from the electrolyte from the rear side, this probably brings to a higher differences from front to rear with Y123 dye than for C106 dye.’

We thank reviewer 1 for useful discussion and suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper discusses about the photo-electrochemical studies of two different types of dyes as sensitizers in DSSC’s. However, the following key points must be answered before accepting the manuscript for publication.

1. Please define and abbreviate these terms: HSE, J8, bpy in the main text.
2. The purity of these dyes must be mentioned.
3. In line 174 and line 187, it should be Figure 2 (a) and (b) not figure 2.1 and 2.2.
4. There are few grammatical errors in the manuscript. It must be improved.
5. Why the 3^rd absorption peak of C106 shifts drastically from 388 to 401 nm whereas the other peak only shifts from 538 to 541 nm respectively?
6. In the introduction, these articles can be included to extend the scope of this work to a wider audience.

(A)https://doi.org/10.1016/j.electacta.2017.02.154,

(B) https://doi.org/10.1021/acsami.0c07075 and (C) 10.1039/C9RA08278F

7. The units in y axis of Figure 2 are reversed. And the starting letter should be in capital. Please check and correct these changes in all the figures.
8. The authors are requested to comment on the performance and output of this work with the work by Andrea Capasso et al.2019 2D Mater. 6 035007.
9. Please provide a table to compare your results with other published works.
10. Reference style should be made uniform.

Author Response

Reviewer 2

This paper discusses about the photo-electrochemical studies of two different types of dyes as sensitizers in DSSC’s. However, the following key points must be answered before accepting the manuscript for publication.

1. Please define and abbreviate these terms: HSE, J8, bpy in the main text.

• We defined HSE from line 148 to line 151:

‘High-stability-electrolyte (El-HSE) was purchased from Great Solar; its composition is not quantitatively indicated and it consists in a dissolved redox couple of triiodide/iodide in 3-methoxypropionitrile (3MPN) solvent with inorganic iodide salt, organic iodide salt and imidazole compound as additives as reported from the seller.’

 J8* is defined from line 144 to line 148

‘The home-made electrolytic solution J8* was prepared in acetonitrile (AN), a low viscous solvent (0.47 cp), and valeronitrile (whose viscosity is 0.78 cp) in a volumetric ratio of 85:15 v / v.  In this way, a less volatile solution has been obtained resulting in a mixture with the following composition: LiI 0.1 M, I2 0.05 M, Tert butyl pyridina (TBP) 0.5 M, Methyl-propyl imidazolium iodide (MPII) 0.6 M (J8 *) [ref 25].’

We define bpy in line 58 ‘bypiridyl (bpy)’ and in line 219 ‘2,2′-bipyridyl (bpy)’

1. The purity of these dyes must be mentioned.

2) Both dyes are commercially available and we bought C106 from Dyesol company and Y123 from Dyenamo company. We insert in the text the purity of these dyes reported by the company. From line 140 to line 141.

‘C106 (purity ≥ 85 %) and Y123 (purity 99 %) dyes were purchased from Dyesol and Dyenamo respectively.’

1. In line 174 and line 187, it should be Figure 2 (a) and (b) not figure 2.1 and 2.2.

 3) You are right and we corrected.

1. There are few grammatical errors in the manuscript. It must be improved.

4) We revised English grammatical errors as requested.

1. Why the 3^rd absorption peak of C106 shifts drastically from 388 to 401 nm whereas the other peak only shifts from 538 to 541 nm respectively?

5) Thank you for the revision, we corrected. It was a mistake of us as it is possible to see from the graph, we corrected in line 210 ‘the third absorption band undergoes a slight bathochromic shift, from 398 to 401 nm and from 538 nm to 541 nm, respectively’

1. In the introduction, these articles can be included to extend the scope of this work to a wider audience.

(A)https://doi.org/10.1016/j.electacta.2017.02.154,

(B) https://doi.org/10.1021/acsami.0c07075 and (C) 10.1039/C9RA08278F

6) We thank the reviewer for the suggestion, we agree and we included these references in the introduction numbers (A) 28, (B) 11 and (C) 12.

1. The units in y axis of Figure 2 are reversed. And the starting letter should be in capital. Please check and correct these changes in all the figures.

7) We did as requested.

1. The authors are requested to comment on the performance and output of this work with the work by Andrea Capasso et al.2019 2D Mater. 6 035007.

Please provide a table to compare your results with other published works.

8) In Andrea Capasso et al. al.2019 2D Mater. 6 035007 work, a CE based on CVD-graphene is studied and it shows an excellent optical transparency (Tr > 97% for visible wavelengths ranging between 400 and 800 nm). Unlike CE based on graphene flakes deposition, graphene-A and graphene-B retain Tr ~ 25% and ~14%, respectively, at 700 nm, while graphene- C exhibits low Tr (<5% for all the visible wavelength) due to the high mass loading of the graphene flakes (i.e. 0.48 mg cm−2). Concerning the Pt based CE, it exhibits a transmittance around 44% between 700-800 nm while this value decrease to 10% at shorter wavelengths.

Capasso et al.  employed C106 as dye and an iodine-iodide commercial electrolyte High Performance Electrolyte (HPE), similar to HSE, was utilized; the main difference between HPE and HSE is the solvent, HPE employs acetonitrile, less viscous and more performant for DSSCs than HSE that is composed by 3-methoxyproprionitrile. Despite the use of a more performant electrolyte, Pt-based/C106 device in Capasso et al. shows similar PCE (4.9 %) to our work (4.8 %) in similar condition (black background and thickness around 7-8 µm). This is due to in Capasso et al. the DSSC device was fabricated without the use of blocking layer as we made in this work. Considering our Pt-based C106 device, with J8* electrolyte similar to HPE, we obtained 5.6 % of PCE, an higher result due to the use of blocking layer in our device as discussed before. The PCE of CVD graphene based cell is less than 0.01%, indicating the need of a catalytic layer for obtaining an efficient electrolyte regeneration process. Concerning Y123 dye, in Capasso et al. a similar procedure to us was utilized to made the cells. Indeed Y123 based device results in better performances: PCE in Capasso et al. is 9 % while in this work is 4.92 % in similar conditions (black background and thickness). In this case, the difference is because of the use of a cobalt-based electrolyte more performant.

 In order to made a comparison between Y123 and C106 in the same conditions, we decided to develop this work, although there are others works about C106 and Y123, this is the only work with a comparison made in the same condition.

We prefer to discuss other work results directly on the text due to the difficulties to create a table without the same conditions.

It is possible to find the comparison with other works in paragraph 3.2.1 The Electrolyte from line 252 to 268

‘In Capasso et al. [41], a C106 based device with an iodine-iodide commercial electrolyte, High Performance Electrolyte (HPE), similar to HSE, was developed. The main difference between HPE and HSE is the solvent: HPE employs acetonitrile, while HSE contains MPN. Despite the use of a more performant electrolyte for reasons discussed above, Pt-based/C106 device in Capasso et al. shows similar PCE (4.9 %) to our work (4.8 %) in similar condition (black background and thickness around 7-8 µm). This is due to the fact that Capasso et al. fabricated the DSSC device without the use of a blocking layer as we made in this work. Considering our Pt-based C106 device, with J8* electrolyte with a similar solvent to HPE, we obtained 5.6 % of PCE, an higher result due to the use of blocking layer in our device as discussed before. Concerning Y123 dye, in Capasso et al. a similar procedure to ours was used to fabricate the cells. Indeed, Y123 based device results in better performances: PCE in Capasso et al. is 9 % while in this work is 4.92 % in similar conditions (black background and thickness). In this case, the difference lays on the use of a cobalt-based electrolyte more performant than a iodide/triiodide one. In a recent work that employs Y123 as a dye [30], with similar material and conditions to the ones of this work, a PCE of 5.3 % was obtained. The slight difference between 4.92 % (this work) and 5.3 % [30] on a black background concerns on the use of a scattering layer on the last work. Indeed, when our cell is measured in a white background (albedo effect) that works as a scattering layer our result (5.6 %) accords to that one in literature’

1. Reference style should be made uniform.

9) We agree and we changed as requested.

We thank reviewer 2 for useful discussion and suggestions.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Most of my concerns have been addressed appropriately.

I recommend acceptance of the manuscript in the present form.

Reviewer 2 Report

Accept