Review on Surface Modification of SnO2 Electron Transport Layer for High-Efficiency Perovskite Solar Cells
Round 1
Reviewer 1 Report
This review paper focusses on the surface modification of SnO2 based electron transport layer (ETL) for fabricating high-performance perovskite solar cells (PSCs). Owing to its merits of suitable band energy alignment with perovskite materials, good ultraviolet-light resistance, strong charge extraction, and low photocatalytic activity, SnO2 has been widely used in the state-of-the-art PSCs, particularly in planar PSCs, for delivering high efficiency. Therefore, this timely review retrospects and updates the progress of SnO2 based ETL in PSCs. The significance and the design rules of surface modification for an efficient SnO2 based ETL is well-discussed. In addition, the effects of chemical and physical interactions on the properties of SnO2 are also described in detail. Last but not least, a future research direction is proposed for the high-performance and stable PSCs by interface engineering of SnO2. The topic of this manuscript is of great importance in the research area of perovskite solar cells. The content and structure of the manuscript are overall good, but some parts need to be reorganized and more detailed information are required. Thus, it is recommended to accept after major revision.
1). In the first paragraph of Introduction, “crystalline silicon-based solar cell accounts for 91% of the global consumption. Second-generation thin-film PVs constitute only 9% of the market”, where did the author acquire these data, please show/cite the source.
2). In page 2, the author mentioned “metal halide perovskite materials because they have excellent optoelectronic features, such as very low absorption coefficients”. Do perovskite materials have high absorption coefficients or low absorption coefficients? Please double check. “The PCE of PV perovskites has rapidly improved from the initial 3.5% to 25.5%” Please update the record efficiency of perovskite solar cells, which already reach 26.1%.
3). The author mentioned about the specific limitations of TiO2 in being a reliable and effective ETL for PSCs that “TiO2 has a slightly higher conduction band minimum (CBM) than MAPbI3”. How about other types of perovskite materials? Does TiO2 also show higher conduction band minimum?
4). In Page 6, “In general, modifying the surface of the SnO2 ETL results in a more uniform and smoother morphology than that of pristine SnO2 through AFM reduction, including synthetic and impurity incorporation, leading to a decrease in interfacial charge recombination.” How do the smooth and rough surface affect the device performance and what is the reason? Why the smoother SnO2 can reduce interfacial charge recombination? Please give more explanation.
5). In page 7, “Consequently, a 40-nm-thick film was determined to be excellent for obtaining a high fill factor.” What is the reason? Any paper already proved it? More detailed explanations are required.
6). In section 2 “IMPORTANT OF SURFACE MODIFICATION OF SnO2 ETL”, pinholes, surface roughness and thickness are discussed. Does the author think particle size and crystallinity are also important to modify SnO2 layer?
7). In page 8, “To produce a high-performance 251 PSC, the SnO2 thickness (20%) should be approximately 64 nm”. Is 20% the concentration of SnO2 precursor or anything else? Please define the value.
8). “Additionally, the film thickness affects the light transmittance.” How does it affect? The thicker the more transparent or the opposite behavior? Any data to prove? Like From xx to xx.
9). “An approximately 40-nm-thick SnO2 film with an amorphous nature was discovered to be ideal for obtaining excellent performance with minimal hysteresis.” What is the reason? More explanation
10). In page 10, “ETL layers and their interaction with the perovskite film, should be considered to increase the effectiveness of the devices used under such settings.” How to increase the effectiveness from the SnO2 based ETL?
11). In page 12, “It has been suggested that the SnO2 surface … to block electron transition.” “In addition, the interfacial trap defects can be effectively reduced by the creation of the I-Sn bonds at the surface, and Cl- diffusion in the perovskite layer positively affects film crystallinity.” Citations are required.
12). In page 14, “Compared to the bare ITO, the SnO2 ETL surface was smoother at different temperatures…” The author described the effect of different temperatures on SnO2 surface. How about the dwell time at each temperature? Whether it will affect the surface?
13). Please use the full description of UVO, WF, NGO, etc. in the main text when it first appears. Please check all abbreviations in the manuscript.
14). In page 16, “This may enhance the likelihood of charge recombination at the HTL/perovskite interface.” Should it be ETL/perovskite interface?
15). “It is possible to use UVO treatment to improve the perovskite layer coverage and ETL–PETL contact” What is ETL-PETL?
16). “The ETL/perovskite interface was improved by increasing the wettability of the SnO2 layer, which also decreased recombination and increased electron injection.” How and why? More explanation
17). In the section of interface modifier, please refer to this publication “Unraveling the Mechanism of Alkali Metal Fluoride Post-Treatment of SnO2 for Efficient Planar Perovskite Solar Cells”, which give the detailed explanation of the function of different alkali metal fluoride to post-treat SnO2 for high-efficiency PSCs.”
18). In page 18, “Theoretical studies and experimental results demonstrated that TBAC might be the best passivation material … internal recombination resistance.” Why is TBAC the best? The explanation in this part is hypothesis or real findings?
19). In page 20, “The addition of NaYF4:Ce and Tb@NaYF4 phosphor… The natural deterioration of perovskites is a possible explanation for this phenomenon.” Why the natural of perovskite can explain the benefits of NaYF4:Ce and Tb@NaYF4 phosphor nanocrystals? What is the relationship between them?
20). “The increased hydrophobicity of the improved film helps increase the interfacial contact, lessen the upper perovskite grain boundary defects, and prevent heterogeneous nucleation.” What is the reason? More explanation.
21). In page 22, “as a result of the smaller energy band gap of the ETL/perovskite interface.” Is it talking about the energy gap of the conduction band between ETL and perovskite?
22). In page 24, “The peak strength is not diminished”. What is the peak?
23). “As a result, the average PCE for our fully air-processed PSCs with MPTMS SAM modification was 18.75%”. Is that your work?
24). In page 28, “The grain size, when perovskite is placed, increases the film thickness threefold.” Why?
25). In the outlook and perspective section, what are the current problems or challenges of SnO2 based ETL in PSCs? What solutions you think could address these issues? The outlook is too general. The author should give an outlook about the SnO2 based PSCs since that is the topic of this review paper.
26). Please cite these publications, which are strongly related to SnO2 based PSCs.
DOI: 10.1021/acsami.9b07318; DOI: 10.1039/d0ta03951a; DOI: 10.1002/adma.201906374; DOI: 10.1002adfm.202004209
27). Some typo errors need to correct, such as Importance of surface modification of SnO2 ETL instead of “important of surface modification of SnO2 ETL” in page 4, pinholes instead of “pinhole” and surface roughness instead of “roughness surface” in the subheading, fill factor instead of “filling factor” in page 7, etc.
Language must be further polished before submission. The typo errors must be corrected.
Author Response
Thank reviewer 1 for the straightforward summary. We will address all the issues raised by reviewer 1 as much as possible
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This paper provides a comprehensive review on the influence of the main surface properties (integrity, roughness, thickness) and various surface modification procedures (both physical and chemical ones) on the SnO2-based electron transport layer performance in perovskite solar cells. In the introduction the authors state the problem of increasing the solar energy conversion efficiency in different types of photovoltaic devices and carefully formulate the requirements for the ETL material comparing the advantages of SnO2, ZnO, and TiO2. Further sections consider in detail the ETL surface properties, physical background for its performance, and the particular surface engineering procedures allowing to meet the above requirements. As a whole, the paper is well illustrated and includes 131 references to the current literature sources. Thus, it is expected to be useful for both scientists and engineers involved in the improvement of the perovskite solar cell design, so this paper can be recommended for publication in Applied Sciences in the current form.
Author Response
Thank reviewer 2 for the straightforward summary
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors undertook the Review on Surface Modification of SnO2 Electron Transport Layer for High-Efficiency Perovskite Solar Cells. This review article is interesting, however, I have few concern.
1. Authors should incorporate comparative table that focus on the efficiency and role of surface modification.
2. Authors should incorporate one new section that focus on the role of surface modification to improved conversion efficiency.
Author Response
Thank reviewer 3 for the straightforward summary. We will address all the issues raised by reviewer 3 as much as possible.
Author Response File: Author Response.pdf
