Recent Progress of Printing Technologies for High-Efficient Organic Solar Cells

Round 1

Reviewer 1 Report

Liu and coauthors submitted the article, “Recent Progress of Printing Technologies for High-Efficient Organic Solar Cells” in the catalysts journal. In this review, the recent progress, and applications of several popular printing technologies to fabricate flexible high-efficient OSCs are summarized, including blade-coating, slot-die coating, gravure printing, screen printing, inkjet printing, etc. The strengths and weaknesses of each printing technology are also outlined in detail. Ultimately, the challenges and opportunities of printing technology to fabricate OSC devices in industrial manufacturing are also prospects. I suggest the following modifications need to be addressed before proceeding.

·       There are several research articles based on large-area organic solar cells. I failed to find the novelty of this proposed review article. Can the authors highlight the importance of their work in comparison to previously published review articles?

·       Please modify the introduction section and add some relevant studies, providing an overview of the proposed review article.

·       In the abstract section, the authors mentioned the superiority of OSCs over silicon solar cells while there are other technologies as well which are also missing in the introduction section. Please check into it.

·       Please include the section on flexible large-area OSCs.

·       Please improve the resolution of the figures. It is not uniform. In addition, I recommend the authors to thoroughly rectify the linguistic errors.

·       Please also check the equation numbers missing in some of the mathematical expressions. Also, check the superscript and subscript errors throughout the manuscript. For example, lines#265, 379, 387,439, Table 1, etc.

Besides the conclusion section, it is suggested to provide a critical analysis along with further directions for large-area solar cells. 

Author Response

Responses to the comments

Reviewer #1

1. There are several research articles based on large-area organic solar cells. I failed to find the novelty of this proposed review article. Can the authors highlight the importance of their work in comparison to previously published review articles?

Thanks for your comments. We have revised the introduction part of our manuscript by reviewing more works on the large-area OSCs based on printing technologies and highlighted the importance of their reports, as indicated below and in the revised manuscript.

"Zhao et al. [82] also used the blade coating method to fabricate PBTA-TF:IT-M-based and PBDB-TF:IT-4F-based solar cells via vacuum assisted annealing (VAA) strategy, as depicted in Figure 6a. Similar phase separation of the OSCs were observed in both blade coating and spin-casting methods, with the maximum PCEs of 10.72% and 13.55%, respectively (Figure 6b). Due to effect of the VAA process, the unfavorable morphology caused by prolong drying process was suppressed, the large-area the large-area OSCs module with 12.6 cm² large area based on PBDB-TF: IT-4F attained a PCE of 10.21% with a V_oc of 2.56 V, J_sc of 6.23 mA cm^-2 and FF of 64.02%, which proved that the VAA method is a feasible way for blade coating large-area modules."

"Min et al. [79] fabricated LBL devices based on PM6:Y6 with sequential blade coating method, achieving a high efficiency of 16.35% for small area (0.04cm²) OSCs as shown in Figure 7b-c, which were better than the BHJ-bladed devices. They found that when introduced the LBL processing, the blend films could exhibit higher absorption and enhanced charge transport ability obviously. In order to further explored the universality of this printing technique, the other non-fullerene systems PM6:Y6-2Cl, PTQ10:Y6, and PM6:Y6-C2 were also selected. Benefiting from the physical dynamics to form proper surface uniformity for the sequential blade coating films, they applied this strategy to fabricate large-scale solar modules (Figure 7d). The 11.52 cm² module delivered an impressive PCE of 11.86% with V_oc of 3.20 V, J_sc of 6.41 mA cm^-2, and an FF of 57.85%, which is the highest efficiency of large-area OSCs. These results demonstrated that LBL printing technique showed great potential for high-performance of OSCs with mass production and decrease the PCE roll-off effect"

"Recently, Wei et al [29]. studied the film-dry kinetic during the slot-die coating process systematically by selected two different NFAs Qx-1 and Qx-2. Through an in-situ UV-vis absorption measurements, they found that Qx-1 maintained an appropriate aggregation with suitable crystallization during the drying process, while Qx-2 exhibited excessive aggregation, resulting in poor device performance as shown in Figure 9d-e. Benefit from the desirable domain size of QX-1 and high thickness tolerance of blend films, the 1 cm² flexible OSC based on PM6: Qx-1 via a slot-die coating method delivered a PCE of 13.70% and FF of 71%. Furthermore, they also fabricated a 30 cm² large-area OSC modules along with outstanding PCE of 12.20% and superior storage stability over 6000h."

2. Please modify the introduction section and add some relevant studies, providing an overview of the proposed review article.

Thanks for your suggest. We have added some relevant research works related to the fabrication of flexible and large-area OSCs via printing technologies and provided an overview of our review article, as suggested in the introduction section of the manuscript.

"Particularly, the printing technologies are desirable and highly suitable for manufacturing large-area OSCs, such as blade-coating, slot-die coating, gravure printing, screen printing and inkjet printing et al [23-27]. Recently, Wei et al [28]. employed the slot-die method to fabricate 1 cm² flexible OSCs. With the fine tuning of active layer morphology and flexible substrate properties, the efficiency of 12.16% was achieved, which was very closed to the spin-coated rigid device (PCE of 12.37%). Furthermore, the same group studied the behaviors of film-dry kinetic during the slot-die process [29]. The PCE of 13.70% was obtained for 1 cm² large-area flexible OSCs. The 30 cm² flexible cells also delivered an impressive PCE of 12.20% due to the high tolerance of film thickness.

However, at present less attention has been paid to the printing methods relative to spin coating. The performances of the lab scale OSCs based on the printing technologies are still lower than that of spin-coating. These printing technologies are more suitable to be adopted for scalability and mass production of OSCs, which need to be further improved. Therefore, in this review, we summarized the recent progress of printing technologies including blade-coating, slot-die coating, gravure printing, screen printing and inkjet printing et al. We firstly discussed the organic active layer and interfacial layer materials used for printing OSCs. The process, characteristics, and application of the above printing methods to fabricate the OSCs are also emphasized, including the strengths and weaknesses of each printing technology are outlined. Besides, the perspective for the printing large-scale and flexible OSCs is also presented at last. We expect that this review can provide new strategies to accelerate the fabrication of OSCs with printing technology."

3. In the abstract section, the authors mentioned the superiority of OSCs over silicon solar cells while there are other technologies as well which are also missing in the introduction section. Please check into it.

Thanks for your advice. We have revised the introduction section of the manuscript according to your comment. The other solar cell technologies, including dye-sensitized solar cells, perovskite solar cells, and quantum dot solar cells have been added in the text as the third-generation solar cells, as shown in line35 in the introduction section.

4. Please include the section on flexible large-area OSCs.

Thanks for your comment. We have added more content related to the advance flexible large-area OSCs in the whole manuscript, especially with the slot-die coating method, as show in Line 48, 51, 64, 68 and 423 in the text.

5. Please improve the resolution of the figures. It is not uniform. In addition, I recommend the authors to thoroughly rectify the linguistic errors.

Thanks for your comment. According to your advice, the resolution of all the figures has been improved and the linguistic errors of our manuscript have been carefully revised, as indicated in the manuscript.

6. Please also check the equation numbers missing in some of the mathematical expressions. Also, check the superscript and subscript errors throughout the manuscript. For example, lines#265, 379, 387,439, Table 1, etc.

Thanks for your comment. We have checked and revised the mistakes in the equation expressions, superscripts and subscripts throughout the manuscript.

7. Besides the conclusion section, it is suggested to provide a critical analysis along with further directions for large-area solar cells.

Thanks for your comment. We have provided more details on the further directions for the fabrication of large-area solar cells with the printing technologies, as suggested below and in the conclusion and prospects part in the manuscript.

With the rapid development of solvent-processed OSCs, it is necessary to replace the fabrication technique from laboratory-scale spin coating method into efficient printing technologies, which are more compatible with high-throughput mass production of the devices in the future. In this review, we have summarized the recent progress of the printing technologies including blade coating, slot-die coating, inkjet printing, screen printing and gravure printing and made comparisons in their ink requirements, printing speed and pattern dimensions. In view of all the printing technologies described in this review, the blade coating and slot-die coating are the most desirable technologies for scalable fabrication of large-area OSCs with high efficiency. The blade coating process is relatively simple that can print in high speed with low material waste. Similar to the blade coating, the slot-die coating can print one-dimensional patterns which is favorable for sequential deposition process. The inkjet printing has merits in changeable digital printing patterns and contactless process. Although there are some achievements made in lab-scale OSCs, the relative slow printing speed and complex ink preparation inhibits the further application on large-area OSCs. As for screen printing, this printing method is suitable for highly viscous ink, which is inversely to the gravure printing with low viscosity of ink. Due to the high requirement of ink, there are few reports on active layer fabricated by these two technologies. Instead, both printing methods are ideal for the fabrication of electrodes for large area OSC modules

Although the efficiency of the printed OSCs fabricated via these technologies have been improved continuously, due to the mechanism of the film formation through these printing methods is still unclear, there is a big gap on device efficiency between large and small area devices. Thus, we propose several directions that should be deserved more attention, there is a big gap on device efficiency between large and small area devices. Thus, we propose several directions that should be deserved more attention. Firstly, we should recognize that most of the high-efficient materials used in OSCs required complex synthesis and treatment processes, which are unfavorable to the industrial fabrication of the devices. The design of organic materials with simple synthesis steps and easy post-processing are always crucial for the manufacture of the printable OSCs. Secondly, the films morphology and quality determine the device performance directly. How to control the fluid flow during the printing process is still a problem. It is of great significance to investigate the hydrodynamics and crystallization kinetics of active layer ink for obtaining high-quality printed films. The additives and solvent engineering have also been demonstrated as effective strategies to improve the films quality. In addition, the application of non-toxic green solvents with high boiling point can reduce the solution aggregation during the printing process and prevent the pollution caused by halogen solvents, which are important research topics in printable OSCs. Thirdly, the low stability of the organic photovoltaic materials is a big challenge, which need to be addressed before wide application and commercialization of OSCs. In order to improve the lifetime of the devices and reduce their sensitivity to light, heat or other complex conditions, the modification of active layer materials and transport layer materials are effective approaches to improve the device stability. Additionally, for large-area ITO-free flexible OSCs, it is quite necessary to maintain their excellent mechanical stability by developing novel packaging technique. These bottlenecks are expected to be solved to promote the application of printing technologies towards the commercialization of OSCs.

Thanks for your consideration again. We hope that the revised manuscript is now suitable for publication in.

Best Regards.

Author Response File: Author Response.docx

Reviewer 2 Report

This manuscript entitled “Recent Progress of Printing Technologies for High-Efficient Organic Solar Cells” has summarized the recent progress of printing technologies, including blade-coating, slot-die coating, gravure printing, screen printing and inkjet printing, used in the fabrication of flexible and large-area OSCs. The manuscript is original and well organized. However, there are some comments below, which should be further addressed. I recommend after minor revision, the paper can be published on Catalysts.

1.     The author needs to improve the writing skills of the manuscript. Some grammatical and punctuation mistakes should be revised. For example, in line 78-81, “Then, when the concept of BHJ solar cells was introduced by Heeger et al. in 1995, OSCs have developed rapidly….in Figure 1b”.

2.     The citations of some references need to be corrected carefully. For example, in line 316 of the manuscript, the reference [82] cited in the wrong place, which should be cited after Figure 6b.

3.     The authors suggested the merits of zinc oxide as the ETL in the OSCs. However, they do not give any specific report relevant to this area in details. The related reports on zinc oxide as ETL in the printable OSCs should be added in the manuscript.

4.     In 3.2, the authors presented the slot-die coating technique to fabricate the OSCs. However, the Figure 9 only showed the UV-vis. absorption and AFM characterizations of the active layer. The illustration of the slot-die coating method should be introduced in Figure 9.

Author Response

Responses to the comments

Reviewer #2

1. The author needs to improve the writing skills of the manuscript. Some grammatical and punctuation mistakes should be revised. For example, in line 78-81, “Then, when the concept of BHJ solar cells was introduced by Heeger et al. in 1995, OSCs have developed rapidly….in Figure 1b”.

Thanks for your suggestions. We have carefully checked the whole manuscript and revised the grammatical and punctuation mistakes.

2. The citations of some references need to be corrected carefully. For example, in line 316 of the manuscript, the reference [82] cited in the wrong place, which should be cited after Figure 6b.

Thanks for your comment. We have checked and revised the wrong references in the manuscript.

3. The authors suggested the merits of zinc oxide as the ETL in the OSCs. However, they do not give any specific report relevant to this area in details. The related reports on zinc oxide as ETL in the printable OSCs should be added in the manuscript.

Thanks for your comment. The relevant works on the zinc oxide as ETL in the OSCs via printing technology have been supplemented in the manuscript, as suggested below.

   "Hou et al. developed a high quality printable ZnO layer though a sol-gel technology. They found that the OSCs with ZnO precursor synthesized by n-propylamine (PA) Lewis base exhibited the best performance, which can suppress the bumps and coffee rings during the blade coating process. Thus, the 1 cm² flexible OSC fabricated with PA-ZnO ETLs showed excellent photostability and obtained a high PCE of 16.71%."

4. In 3.2, the authors presented the slot-die coating technique to fabricate the OSCs. However, the Figure 9 only showed the UV-vis. absorption and AFM characterizations of the active layer. The illustration of the slot-die coating method should be introduced in Figure 9.

Thanks for your comment. We have revised the figure 9 and added the related illustration of slot-die coating in this figure, as depicted in the manuscript.

Thanks for your consideration again. We hope that the revised manuscript is now suitable for publication in.

Best Regards.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors did an impressive study in this review manuscript. However, I would like to suggest authors consider rewriting the manuscript using short paragraphs and providing information.

This version makes the writing a little complicated for readers to follow. Moreover, I would like to suggest the author make a separate part to discuss the drawback of mentioned methods. The comparison can also be addressed.

The conclusion also needs to be reconsidered.

Author Response

Responses to the comments

Reviewer #3

1. The authors did an impressive study in this review manuscript. However, I would like to suggest authors consider rewriting the manuscript using short paragraphs and providing information.

Thanks for your comment. We have revised the manuscript and modify the layout of the text with short paragraphs and provided more information on the relevant works in details, as shown in the manuscript.

2. This version makes the writing a little complicated for readers to follow. Moreover, I would like to suggest the author make a separate part to discuss the drawback of mentioned methods. The comparison can also be addressed.

Thanks for your comment. We have discussed and compared the features, advantages and disadvantages of each printing method in the manuscript and added a summary of the merits and drawbacks of the mentioned printing technologies in the conclusion and prospects part, as shown below.

"In view of all the printing technologies described in this review, the blade coating and slot-die coating are the most desirable technologies for scalable fabrication of large-area OSCs with high efficiency. The blade coating process is relatively simple that can print in high speed with low material waste. Similar to the blade coating, the slot-die coating can print one-dimensional patterns which is favorable for sequential deposition process. The inkjet printing has merits in changeable digital printing patterns and contactless process. Although there are some achievements made in lab-scale OSCs, the relative slow printing speed and complex ink preparation inhibits the further application on large-area OSCs. As for screen printing, this printing method is suitable for highly viscous ink, which is inversely to the gravure printing with low viscosity of ink. Due to the high requirement of ink, there are few reports on active layer fabricated by these two technologies. Instead, both printing methods are ideal for the fabrication of electrodes for large area OSC modules."

3. The conclusion also needs to be reconsidered.

Thanks for your suggestion. We have revised the conclusion and prospects part in the end of the manuscript according to your advice, as present in the manuscript.

"Although the efficiency of the printed OSCs fabricated via these technologies have been improved continuously, due to the mechanism of the film formation through these printing methods is still unclear, there is a big gap on device efficiency between large and small area devices. Thus, we propose several directions that should be deserved more attention, there is a big gap on device efficiency between large and small area devices. Thus, we propose several directions that should be deserved more attention. Firstly, we should recognize that most of the high-efficient materials used in OSCs required complex synthesis and treatment processes, which are unfavorable to the industrial fabrication of the devices. The design of organic materials with simple synthesis steps and easy post-processing are always crucial for the manufacture of the printable OSCs. Secondly, the films morphology and quality determine the device performance directly. How to control the fluid flow during the printing process is still a problem. It is of great significance to investigate the hydrodynamics and crystallization kinetics of active layer ink for obtaining high-quality printed films. The additives and solvent engineering have also been demonstrated as effective strategies to improve the films quality. In addition, the application of non-toxic green solvents with high boiling point can reduce the solution aggregation during the printing process and prevent the pollution caused by halogen solvents, which are important research topics in printable OSCs. Thirdly, the low stability of the organic photovoltaic materials is a big challenge, which need to be addressed before wide application and commercialization of OSCs. In order to improve the lifetime of the devices and reduce their sensitivity to light, heat or other complex conditions, the modification of active layer materials and transport layer materials are effective approaches to improve the device stability. Additionally, for large-area ITO-free flexible OSCs, it is quite necessary to maintain their excellent mechanical stability by developing novel packaging technique. These bottlenecks are expected to be solved to promote the application of printing technologies towards the commercialization of OSCs."

Thanks for your consideration again. We hope that the revised manuscript is now suitable for publication in.

Best Regards.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

It is improved now. Accept it