Review Reports - Preparation of Highly Transparent (at 450–800 nm) SnO<sub>2</sub> Homojunction by Solution Method and Its Photoresponse

Round 1

Reviewer 1 Report

1. My first comment is regarding the title, and in particular the optical transmittance of the studied samples:

The authors claim “high transparent SnO2 homojunction”. However, according to the results, in particular, by analyzing figure 3 one can observe that the optical transmittance (OT) in the visible region (ca. from 380 nm to 750 nm) varies from ca. 70% to 85%, in which the OT above 80% is only achieved, for a wavelength above 560 nm, as the authors mention in table 1!

Once the visible region is considered to be in the spectral region of 380 nm to 750 nm, and the highly-transparency should be considered only above 80%, as reported, for example in Minami, Tadatsugu, et al. "Highly transparent and conductive zinc-stannate thin films prepared by RF magnetron sputtering." Japanese journal of applied physics 33.12A (1994): L1693R. Moreover, there are authors that consider the term “highly-transparency” above 90%, such as, Hagendorfer, Harald, et al. "Highly transparent and conductive ZnO: Al thin films from a low temperature aqueous solution approach." Advanced Materials 26.4 (2014): 632-636. On the other hand, some authors consider it when OT is above 70%.

I do prefer to use the term “highly-transparency” to any sample with an OT above 85%. The absence of any standard for this qualitative term is a fact, as far as I know, so I will accept that an OT>80% could be referred to any highly-transparency sample. That being said, I have two recommendations:

(a) For the sake of scientific language, I suggest that the title should use the term “highly-transparency” instead of high transparent;

(b) The spectral range should also be mentioned in the title because in the region below 450 nm all samples presented an OT below 80%. The authors could find a way to keep an appealing title without misleading the reader. This will also help the authors with their final argument regarding the potential of their samples to be used in the near ultra-violet region!

Thanks to the reviewer for explaining the "highly-transparency". We have changed "high transparent" which appeared in title (line 2, changed into "Preparation of Highly-transparency (at 450-800 nm) SnO₂ Homojunction by Solution Method and Its Photoresponse"), key words (line 30, changed into "highly-transparency") and in line 36 (changed into "The highly-transparency PN junction is the basic..." ) to "highly-transparency". And we added spectral range (450-800 nm) in the title (lines 2-4, changed into "Preparation of Highly-transparency (at 450-800 nm) SnO₂ Homojunction by Solution Method and Its Photoresponse ").

2. In the lines 129-130 the authors wrote: “Optical transmittance spectrum of SnO2: Si films of various Si concentrations within the 300- 800nm band is illustrated in Figure 3.”. This sentence should be substituted by following one “The optical transmittance spectra (300-800 nm) of five different concentrations of Si in SnO2:Si films are illustrated in Figure 3.”

We checked the sentence again and found that the use of the statement was indeed unreasonable, and it was modified according to your suggestions (lines 134-135, changed into "The optical transmittance spectra (300-800 nm) of four concentrations of Si in SnO₂: Si films are illustrated in Figure 3."), thanks.

3. By analyzing the graph shown in figure 3, it does not seem that the OT value from sample SnO2:SI(15at%) is 81.3% at 560 nm (green line, according to the authors). Although, in Table 1, the authors presented such value. With the presented graphs, I do not have the accuracy to be sure about my reading. Could the authors please check that value?

We have repeatedly checked the data in Table 1, including but not limited to the transparency of SnO₂: Si (10 at.%). We found that the transparency of SnO₂: Si (10 at.%) is indeed not 81.3%, but 80.4%. We speculate that the data was wrong due to forgetting to save some changes while writing the manuscript. And we have corrected the data in Table 1 (line 146), shown as follows, and the text (line 151, changed into " …0 at.%, 5 at.%, 10 at.% and 15 at.% Si-doped SnO₂ films are 3.60 eV, 3.61 eV, 3.56 eV, and 3.58 eV respectively."). We changed the data in Table 1 and the text to ensure the consistency of the Table and the Figure. Thanks to the reviewer for pointing out the mistakes in our paper, making the paper more rigorous. And we have checked the experimental data again in the paper to ensure the accuracy of the data.

4. By using the simple apparatus illustrated in figure 9, did the authors collect the emitted light with different angles and different average optical power excitation?

The comment mentioned by the reviewer is useful for improving the article. Unfortunately, we did not conduct experiments with different angles and different average optical power excitation due to the planning of the previous experimental plan and the limitations of the light source used in the experiment. Regarding the irradiation angle, the angle of the light source in the figure is 45 degrees (but it is not mentioned in Figure 9). We sincerely apologize for this. Due to the impact of the COVID-19, we cannot effectively supplement this part of the experiment. Hope to get your understanding!

5. In the caption of figure 9, line 240, the authors should not use the term “light is concentrated at…”. Instead, they should use “light is centered at a wavelength of 420 nm”.

We have re-examined the analysis statements in this section, and we have adopted your suggestions and modified them (lines 243-245, changed into " The red light is mainly centered at a wavelength of 630 nm;…; the purple light is centered at a wavelength of 420 nm."). Thank the reviewer for pointing out inappropriate places in the text.

6. In section "Conclusions", line 284, the authors mentioned “purple light”. If the authors wish to keep the term, they should put in parenthesis the spectral region, in nm, of the so-called “purple light”.

The description of purple light in the "Conclusions" is something we haven't considered. We have added the spectral region to the description of purple light and the near-ultraviolet in line 140 (changed into "…in the near-ultraviolet band (300-450 nm),…"), in line 288 (changed into " The response to purple light (420 nm) is strong…").

There are some other modifications:

(1) In line 6, we added “and” according to the manuscript template.

(2) In lines 9-10, because we used QQ mailbox before, the author's mailbox was corrected to prevent the mailbox information from being blocked.

(3) In line 113, we deleted the extra one “)” (changed into: “… (model: shimazu uv-2600)”).

(4) In line 118, we added a word “model” (changed into: “… (model: Agilent 4155C)”)

(5) In line 125, we modify “SnO₂:Si” to “SnO₂: Si” to better meet the format requirements.

(6) In line 130, we adjusted the format of Figure 2, shown as follows.

(7) In lines 141-143 and line 278, because we determined the spectral range of near-ultraviolet light, we have corrected the description of related experimental phenomena (changed into “As the spectral wavelength decreases, the transmittance of SnO₂: Si films show a gradually decreasing trend, which is attributed to the increased absorption of light in near-ultraviolet band.”, “…;the absorbs of light in the near-ultraviolet region enhances for SnO₂: Si film with increasing Si doping concentrations…”).

(8) In line 165-166, we modified the format of the title in Figure 5 (changed into “(a) Optical transmittance spectrum of SnO₂: 10at.% Ga films; (b) The (αℎυ)² versus ℎυ plots of SnO₂: 10at.% Ga films.”).

(9) In line 191, we added a word “Si” in order to not misunderstand (changed into “…thin films with different Si doping concentrations.”).

(10) In line 241, the Figure 9, we changed the light around sample to purple instead of yellow.

(11) In line 251, we corrected the title of Figure 11 (changed into “The I-V characteristic of SnO₂: 15at.% Si/SnO₂: 10at.% Ga homojunction under excitation of different color lights and in the dark.”).

(12) In section “Conference”, line 302-376, we have corrected the format of the reference.

The manuscript is, in general, well written, its length is appropriate, and I consider that the authors did a reasonable description of the experiments, as well as the discussion of the results. The thin-film preparation seems to be of excellent quality, as well as its structural characterization. The final results show that this type of homojunction has the potential to be used in the spectral range of 350-400 nm, in particular, as the authors mentioned, for near-ultraviolet detectors. Although to be entirely sure about this potential application, the optical tests showed in figure 9, must be carefully executed. The author should take into account different optical parameters, such us, a range of average optical powers, different angles of excitation and collection, as well as, the full spectral characterization of the light source.

After the comments above are addressed and solved, I will decide on the acceptance of the manuscript.

Reviewer 2 Report

The authors of the manuscript investigate the optical properties of Si- and Ga- doped SnO2 layers, obtained by a chemical method. They use various of methods to characterize the samples.

In my opinion the manuscript needs major revision.

1.I think it should be added at the end of the Introduction section that the investigation is on both Si- and Ga- doped SnO2 layers. It is not clear which case is investigated – doping with Si or doping with Si and with Ga.

Thank the reviewer for pointing out the shortcomings of the paper. At the end of the "Introduction", we have added relevant instructions (lines 82-85): "Based on this, the n-type and p-type transparent SnO₂ films were prepared by Si doping and Ga doping, and a highly-transparency SnO₂ homojunction was prepared by solution method. The effects of different Si doping concentrations on the structure and performance of homojunction were emphatically studied, and the current response of the device under different frequencies of light was further discussed.".

2.What are the structure and composition of the layers? I would like to recommend XRD analysis (or XPS, or EDS analysis (in SEM/TEM to check the atomic ratio between Sn and O)) in order to be sure that SnO2 is obtained.

Thank you very much for the targeted suggestions mentioned by the reviewer. Although we did not investigate the composition of layers in this research, we refer to our team's previous experiment records. And cited the literature which our team published before which we published before (lines 79-82, added "Otherwise, in our previous research [17], the X-ray photoelectron spectroscopy (XPS) characterization shows that the atomic ratio of SnO₂ in the SnOx film which is annealed at 100℃ by solution method is 94.49%. It can be deduced from this that the composition of the film samples annealing at 300℃ is mostly SnO₂ in this research."). And the literature is “Preparation and optimization of SnOx thin film by solution method at low temperature.”

3. Do the authors know the thickness of the layers and how do they controll it? Please, insert the data in the text.

Sorry, we did not characterize the film thickness in time due to the damage of the experimental instrument in the previous experiment. But in this research, we conducted a study on the number of spin coatings on the film. We found that the homojunction performance of single-layer Si- and Ga-doped SnO₂ films was poor. For example, there was a situation like that shown in Figure 1. At first, we speculated that it was caused by the thickness of the film. So we improved the experimental operation and spin-coated the Si- and Ga-doped SnO₂ film three times to increase the thickness of the film. The experiment found that the performance of using the above film to prepare homojunction has been effectively improved, as shown in Figure 2. In addition, due to the impact of the epidemic, we failed to supplement the relevant experiments. We sincerely hope to get your understanding.