Synthesis and Study of Janus-Dione-Based Compounds for Ternary Organic Solar Cells

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors In the present manuscript authors have tackled a problem of improving the efficiently of organic solar cells. The ternary organic cells are investigated derived from synthesis of two chromophores based on the derivatives of the Janus-dione (s-indacene-16 1,3,5,7(2H,6H)-tetraone) central acceptor fragment, namely TIIC-1 and TIIC-2. The topic of the paper is relevant for the scientific community and economy, considering constant demand for the green energy. Even in the small niche of the solar cell improvement, the organic solar cells play an important role and their improvement is very relevant. Knoevenagel condensation reaction was used for synthesis of the new material and the procedure is well explained. The authors have used proper physicochemical methods for this investigation. The figures and tables are well presented and the results are clearly shown and understandable.  The manuscript presents quite promising results where the power conversion efficiency of ternary solar cells was improved compared to binary cells from 11.9% to 12.5%. This result is not the best in the research area but can still be outlined as relevant in the scientific community. Conclusions are in line with presented results. The manuscript is exceptionally well written and should be published in present form with small grammatical corrections (mainly regarding use of subscript and superscript in the paper).

Author Response

Comments: 

In the present manuscript authors have tackled a problem of improving the efficiently of organic solar cells. The ternary organic cells are investigated derived from synthesis of two chromophores based on the derivatives of the Janus-dione (s-indacene-16 1,3,5,7(2H,6H)-tetraone) central acceptor fragment, namely TIIC-1 and TIIC-2. The topic of the paper is relevant for the scientific community and economy, considering constant demand for the green energy. Even in the small niche of the solar cell improvement, the organic solar cells play an important role and their improvement is very relevant. Knoevenagel condensation reaction was used for synthesis of the new material and the procedure is well explained. The authors have used proper physicochemical methods for this investigation. The figures and tables are well presented and the results are clearly shown and understandable. The manuscript presents quite promising results where the power conversion efficiency of ternary solar cells was improved compared to binary cells from 11.9% to 12.5%. This result is not the best in the research area but can still be outlined as relevant in the scientific community. Conclusions are in line with presented results. The manuscript is exceptionally well written and should be published in present form with small grammatical corrections (mainly regarding use of subscript and superscript in the paper).

Response: 

The authors thank the reviewer for their comments! The manuscript has been checked with the Grammarly tool to correct grammatical errors. To the best of our abilities, grammatical errors as well as the sub- and superscript errors have been corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript by Armands et al, the authors focus on the synthesis on two Janus-dione-based compounds for use in ternary solar cells. With further revision, the manuscript can be considered for publication in Materials.

1. In the ¹H NMR of TIIC-1, total 10 aromatic protons were seen, which is different with expected of 12 protons. Could the author elaborate on this?
2. Could the author explain why PM6:Y7 exhibits a PCE of 11.9%, which is much lower than reported values?
3. In the binary PM6:TIIC-1 and PM6:TIIC-2 systems, TIIC-1 and TIIC-1 did not work as good acceptors. Have authors tried TIIC-1 with other donors with better energy level alignment?
4. Have authors ever tried to test the TIIC-1 as polymers donor, i.e., TIIC-1/Y7 systems, though it may not work?
5. Please check typo: i.e., V_oc

Comments for author File: Comments.pdf

Author Response

Comment1: In the ¹H NMR of TIIC-1, total 10 aromatic protons were seen, which is different with expected of 12 protons. Could the author elaborate on this?

Response1:

A more elaborate analysis of the ¹H NMR spectra in CDCl₃ revealed that the missing aromatic signal could be related to a slight baseline bump observed at approximately 8.4 to 7.4 ppm (see Fig. S1).                             

To further elaborate, we decided to perform NMR experiments at elevated temperatures. Deuterated 1,1,2,2-tetrachloroethane-d2 (C₂Cl₄D₂) was used as the solvent due to its higher boiling point. A practically identical spectrum for aromatic signals to that of CDCl₃ was observed in C₂Cl₄D₂ at room temperature (see Fig. S2a). However, at 70 °C (see Fig. S2b), an increase in spectral resolution and a slight upfield shift of the signals were observed. Firstly, the broad signal at 7.98 ppm has been split into two sharp signals at 8.10 and 8.09 ppm, respectively. Secondly, the very broad signal stretching from approx. 8.4 to 7.4 ppm has been rectified to a more pronounced signal at approx. 8.32 ppm. Thus, the two "missing" aromatic proton signals are revealed. The observed changes in the NMR spectrum with varying temperature may be due to the presence of rotamers. As the temperature increases, the rotation rate increases and the coalesced signals narrow. The structure of possible rotational isomers is given in Fig. S3. However, it must be noted that the presence of E,E and Z,Z isomers (see Manuscript Fig. 2a) further complicates the interpretation of NMR spectra.

The additional discussion has been added to the manuscript, page 5.

Comment2: Could the author explain why PM6:Y7 exhibits a PCE of 11.9%, which is much lower than reported values?

Response2: 

We agree that the achieved PM6:Y7 cell performance falls short compared to values reported in the literature. Compared with the literature, the J_SC values of our devices are relatively high (over 24 mA/cm²). Currently, the main reason for the relatively low efficiency is the lower FF values (around 0.60 in our case, compared to >0.75 in the literature). This is despite our repeated efforts to improve efficiency by following several published cell manufacturing protocols (using the same solvents, annealing time and temperature, active layer thickness, etc.).

Comment3: In the binary PM6:TIIC-1 and PM6:TIIC-2 systems, TIIC-1 and TIIC-1 did not work as good acceptors. Have authors tried TIIC-1 with other donors with better energy level alignment?

Response3: 

For now, we have not tried TIIC-1 and TIIC-2 with other donor polymers. Based purely on the energy level values, TIIC-1 should have worked even better than Y7. As the electron affinity level for TIIC-1 is higher than the electron affinity level of Y7 (see Fig. 5c), the resulting V_OC should have been higher for the cell with TIIC-1. In our experiments, we obtained V_OC=0.83 V for the PM6:Y7 cell and only V_OC=0.44 V for the PM6:TIIC-1 cell.

Additionally, the ionization energy level for other polymers is often shallower than that of PM6, which should lead to even lower V_OC values.

Comment4: Have authors ever tried to test the TIIC-1 as polymers donor, i.e., TIIC-1/Y7 systems, though it may not work?

Response4: 

Thank you for this suggestion!

The ITO/PEDOT:PSS/TIIC-1:Y7/PDINO/Ag cell was made (a cell without polymer PM6). Unfortunately, the results showed absolutely no photovoltaic effect: the I-V curve had the characteristics of a simple resistor.

Figure S5 in the Supplementary Materials file has been changed to show this measurement.

The following paragraph has been added to the manuscript, page 11:

Additionally, an attempt was made to evaluate the feasibility of using the TIIC-1 as an electron donor. A solar cell was made with the active layer of TIIC-1:Y7 (without the polymer PM6). In this case, there was absolutely no photovoltaic effect- the measured I-V curve had characteristics of a simple resistor (see Fig. S5 a).

Comment5: Please check typo: i.e., V_oc

Response5: Throughout the manuscript, all instances of "U_OC" have been changed to "V_OC".

 

To better understand the additional measurements, all comments and responses have been collected in the attached Word document.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In this contribution, the authors present the synthesis and characterization of two novel D-A-D chromophores (TIIC-1 and TIIC-2) based on a thieno[3,2-b]indole donor and a s-indacene-1,3,5,7(2H,6H)-tetraone (Janus-dione) acceptor core. These compounds are evaluated as third components in ternary organic solar cells (TOSCs) using the PM6:Y7 binary host. The study is methodical, combining synthesis, theoretical modeling, photophysical and electrochemical characterization, and device testing. The central result, an improvement in power conversion efficiency (PCE) from 11.9% to 12.5% with the addition of TIIC-1, is well-supported by the data and provides a useful example of performance gain via cascade energy level engineering in ternary blends.

While the experimental work is thorough and the conclusions are largely justified, several aspects of the manuscript's narrative, contextualization, and data presentation require strengthening to enhance its impact and clarity.

1. The authors state that thieno[3,2-b]indole "has been widely used as a donor fragment in organic electronics" and cite four references [28-31]. While these citations demonstrate diverse applications, they provide a narrow and somewhat arbitrary snapshot. The introduction would be significantly strengthened by a more focused and informative review of thieno[3,2-b]indole role specifically in photovoltaic materials. Citing key studies that established its merits for high-performance donor or acceptor units in binary or ternary OSCs would better justify its selection for this work. A broader, more targeted literature review on this specific material class for photovoltaics is needed to frame the authors' design strategy within the field's ongoing developments.
2. The manuscript uses a fixed mass ratio of 10:10:1 (PM6:Y7:TIIC) for the ternary devices. The effect of the third component is highly concentration-dependent, influencing morphology, energy transfer, and charge transport pathways. The reported performance improvement with TIIC-1 is promising, but the absence of a composition study leaves a critical question unanswered: Is this ratio optimal? Presenting data on PCE, FF, J_SC, and V_OC as a function of TIIC-1 concentration would transform the finding from a single data point into a more generalizable and insightful structure-property relationship, particularly relevant for the discussed "alloy model."
3. The proposed cascade charge transfer pathway (PM6 → TIIC-1 → Y7) is central to the interpretation but is inferred primarily from energy level diagrams. More direct experimental validation would solidify the claim. Techniques such as photoluminescence quenching studies (comparing quenching efficiency in binary vs. ternary blends) or transient absorption spectroscopy could provide more conclusive evidence for the role of TIIC-1 as an effective charge relay. Furthermore, the discussion would benefit from linking the modest increase in Fill Factor (FF) to potential morphological changes induced by the ternary component. Atomic force microscopy (AFM) or similar characterization of the active layer morphology is absent but would be highly informative.
4. The table is poorly formatted in the provided text, making it difficult to parse the data for TIIC-1 and TIIC-2. A clear, side-by-side comparison is essential for readability.
5. The abstract and introduction reference OSC efficiencies approaching 20%. While the absolute PCE of 12.5% is reasonable for a proof-of-concept study, briefly contextualizing this gain within the specific realm of PM6:Y7-based ternary cells would help readers assess its significance more accurately.

In summary, this manuscript reports competent synthetic work and a solid foundational study on new ternary component materials with a clear, positive result. To elevate its contribution, the authors should address the aforementioned points. Specifically, providing a more focused literature review on the chosen donor moiety, including basic device optimization data, and strengthening the evidence for the proposed operating mechanism are crucial improvements. Addressing the formatting and clarity issues is also necessary. The core science is sound and the findings are of interest, but the revisions outlined above are required to fully substantiate the authors' claims, improve the manuscript's coherence, and maximize its value to the readership.

Author Response

Comment1: The authors state that thieno[3,2-b]indole "has been widely used as a donor fragment in organic electronics" and cite four references [28-31]. While these citations demonstrate diverse applications, they provide a narrow and somewhat arbitrary snapshot. The introduction would be significantly strengthened by a more focused and informative review of thieno[3,2-b]indole role specifically in photovoltaic materials. Citing key studies that established its merits for high-performance donor or acceptor units in binary or ternary OSCs would better justify its selection for this work. A broader, more targeted literature review on this specific material class for photovoltaics is needed to frame the authors' design strategy within the field's ongoing developments.

Response1:  The introduction (Page 2) has been supplemented with:

 Due to its electron-rich, planar, π-conjugated structure, thienoindole has been widely used as a donor fragment in organic electronics [28–31]. The planarity of the thienoindole unit is a direct consequence of its fused, aromatic structure. This structural rigidity and high degree of planarity provide advantages for photovoltaic performance. The flat surface of the thienoindole allows molecules to pack tightly in a "face-on" orientation with respect to one another. This reduces the π-stacking distance, thereby significantly facilitating inter-chain charge hopping and increasing charge mobility [additional references A and B have been added]. Additionally, the straightforward synthesis of thienoindole derivatives allows structural modifications, such as the attachment of various solubilizing alkyl groups [32]. 

 

Additional references:

A) Juae Kim et al.,Conjugated polymers containing 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) and isoindigo for organic photovoltaics, Polymer, Volume 95, 2016, Pages 36-44, https://doi.org/10.1016/j.polymer.2016.04.061.

B) Xingbao Zhou et al., Thieno[3,2-b]indole (TI) bridged A-π−D-π−A small molecules: Synthesis, characterizations and organic solar cell applications, Dyes and Pigments, Volume 160, 2019, Pages 16-24, https://doi.org/10.1016/j.dyepig.2018.07.009.

Comment2: The manuscript uses a fixed mass ratio of 10:10:1 (PM6:Y7:TIIC) for the ternary devices. The effect of the third component is highly concentration-dependent, influencing morphology, energy transfer, and charge transport pathways. The reported performance improvement with TIIC-1 is promising, but the absence of a composition study leaves a critical question unanswered: Is this ratio optimal? Presenting data on PCE, FF, J_SC, and V_OC as a function of TIIC-1 concentration would transform the finding from a single data point into a more generalizable and insightful structure-property relationship, particularly relevant for the discussed "alloy model."

Response2: 

Thank you for the comment!

The performance of the solar cells is indeed highly sensitive to the concentration of the third component.

A series of samples was prepared varying the amount of TIIC-1 in the active layer. The lowest amount (sample with a material ratio of PM6:Y7:TIIC-1 of 10:10:0.5) showed very little improvement: an increase of around 0.2 mA/cm2 in J_SC and 0.1% in PCE, within the margin of error. The sample with 10:10:1 PM6:Y7:TIIC-1 mass ratio was the most efficient, reaching 12.5% efficiency. Further increase in the amount of TIIC-1 in the active layer leads to a decrease in all solar cell parameters (J_SC, V_OC, FF, and PCE). We have added Figure S4 and Table S1 to the Supplementary information file, showing the dependence of solar cell performance on the concentration of the third component.

This discussion has been added to the manuscript, page 10.

 

Comment3: The proposed cascade charge transfer pathway (PM6 → TIIC-1 → Y7) is central to the interpretation but is inferred primarily from energy level diagrams. More direct experimental validation would solidify the claim. Techniques such as photoluminescence quenching studies (comparing quenching efficiency in binary vs. ternary blends) or transient absorption spectroscopy could provide more conclusive evidence for the role of TIIC-1 as an effective charge relay. Furthermore, the discussion would benefit from linking the modest increase in Fill Factor (FF) to potential morphological changes induced by the ternary component. Atomic force microscopy (AFM) or similar characterization of the active layer morphology is absent but would be highly informative.

Response3: 

We conducted a photoluminescence quenching experiment by examining different ratio combinations of TIIC-1 and Y7 in toluene solution to offer additional proof for the cascade charge transfer pathway (Fig. S7). It is evident that there is an effective excitation transfer between the two compounds because the emission of TIIC-1 is being effectively quenched with the introduction of Y7 in the mixture.

The additional discussion has been added to the manuscript, Page 9, Figure S7 has been added to the Supplementary Materials file.

Comment4: The table is poorly formatted in the provided text, making it difficult to parse the data for TIIC-1 and TIIC-2. A clear, side-by-side comparison is essential for readability.

Response4: We thank the reviewer for this comment! Table 3 has been changed. The parameters of binary (PM6:TIIC-1 and PM6:TIIC-2) cells have been moved to Table S2 in the Supplementary Materials file. In the manuscript, Table 3 now contains only information about the reference cell (PM6:Y7) and the ternary cells.

Comment5: The abstract and introduction reference OSC efficiencies approaching 20%. While the absolute PCE of 12.5% is reasonable for a proof-of-concept study, briefly contextualizing this gain within the specific realm of PM6:Y7-based ternary cells would help readers assess its significance more accurately.

Response5: 

Thank you for this comment! 

We believe the reported results are significant, as they convincingly demonstrate that by constructing ternary devices incorporating our novel NFA molecules, the performance of the parent two-component OSCs can be enhanced.

While some aspects were less favorable for TIIC-1 (absorption maximum position, the presence of E,E and Z,Z isomers) compared to TIIC-2, TIIC-1 showed better compatibility with the reference system PM6:Y7. It shows that a thorough investigation is needed to assess the potential of each novel material. In this work, we do not report devices with record-high performance, yet the addition of a third component (TIIC-1) has shown promising improvements in the binary system.

We have collected all the comments and responses with additional graphs in a single Word document

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have successfully addressed all the concerns raised by this reviewer. The manuscript can be accepted in the present form.

Reviewer 3 Report

Comments and Suggestions for Authors

In conclusion, the authors have comprehensively addressed the key concerns raised during the review process. The introduction now provides a much stronger, property-focused rationale for the thienoindole donor choice. The most critical addition is the systematic optimization of the ternary blend ratio, which convincingly demonstrates an optimal composition window and transforms a singular observation into a robust finding. The photoluminescence quenching experiment offers valuable direct evidence supporting the proposed cascade energy transfer mechanism. With these substantive revisions, along with improved data presentation, the manuscript presents a complete, rigorous, and compelling study. The work makes a clear and valuable contribution to the field of ternary organic solar cells. Therefore, I recommend acceptance for publication in its present form.