Modification of TiO2 Nanowire Arrays with Sn Doping as Photoanode for Highly Efficient Dye-Sensitized Solar Cells
, Shujie Jiao, Jinzhong Wang, Yong Zhang, Bao Wang, Pu Feng and Liancheng ZhaoRound 1
Reviewer 1 Report
The authors have grown rutile TiO2 nanowires and doped them with tin (Sn). They have used an exhaustive set of tools to thoroughly characterize the materials’ composition, crystal structure, morphology, optical properties, and DSC device performance. The device results follow a pleasing trend, readily identifying an optimal Sn doping level. The authors effectively explained this based on competing trends as the Sn doping level increases, especially improved charge separation and transport vs. decreased dye uptake. Their careful work is complete and convincing that their Sn doping is real and has significant beneficial effects on the device if the correct amount is used. The language is awkward at times and would benefit from editing. But apart from that, this is a quality manuscript reporting good and novel work, and it deserves publication. I make only a couple small points about language below.
l. 75-76: The authors could omit the word “element” after V, Cr, etc.
l. 232-3: “The larger band gap is beneficial to increasing the light absorption range.” This can be confusing, since of course it decreases the light absorption range of the semiconductor itself. But it increases the range for the dye
Author Response
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript “Modification of TiO2 nanowire arrays with Sn doping as photoanode for highly efficient dye-sensitized solar cells” presents the procedure of TiO2 nanowire arrays modification by Sn element. The obtained structures were characterized by several experimental methods, including inductively coupled plasma optical emission spectroscopy, X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, filed-emission scanning electron microscopy, transmission electron microscopy, high-resolution TEM and element-mapping characterization. The used experimental techniques clearly described the structures of modified TiO2 nanowire arrays by Sn element. Further, the obtained TiO2 nanowire arrays were used as photoanodes to prepare dye-sensitized solar cells. Popular ruthenium dye (N719), iodide/triiodide electrolyte and platinized counter electrode were employed as components to DSSC. Moreover, the obtained TiO2 nanowire arrays after Sn modification can be used in the future for other DSSC systems to improve their performance. In my opinion, the obtained results are very interesting, the manuscript is well-written and could be published in Crystals.
Below are some comments that could possibly improve the manuscript, however they do not change my good opinion about article:
1.No information about the dye used to prepare DSSC in manuscript. I found the type of dye (N719) only in Supporting Information part. In my opinion, this information should be included in abstract part, keywords, page 2 line 88, page 5 line 186 (N719 dye, iodide/triiodide electrolyte and platinized counter electrode) and finally in conclusion part of the manuscript.
2. Abbreviation “DSSC” should be used consequently in all manuscript. Actually, we can found both “DSSC” and “DSC”. It should be defined in abstract part (page 1, line 12).
3. Abstract line 13-14: “low electron mobility of TiO2” Is it correct? Maybe, it should be “low electron mobility within TiO2”
4. Introduction, page 1 line 40: Some details about the ways to produce a much higher power conversion efficiency of DSSC could be added, including for example, design of the novel dyes or modification of structures of well-known dyes, the novel types of electrolyte or modification of the photoanodes and/or counter electrodes surface.
5. Introduction, page 1 line 42: “on semiconductor surface” should be added to “monolayer of dye molecule”
6. Introduction, page 2 line 60: “dye absorption” or “dye adsorption”?
7. Introduction, page 2 line 90: “…on the Sn-doped SnO2 based exhibits an remarkable…”, probably it should be corrected: “…on the Sn-doped TiO2 exhibits an remarkable…”
8. Results and discussion, page 3 line 94-95: “the ratio of Sn/Ti is about 5/95, 11/89, 15/85 and 21/89”; probably it should be corrected: “the ratio of Sn/Ti is about 5/95, 11/89, 15/85 and 21/79”.
9. According to the results in Table 1: the values of dye loading were systematically reduced from TS0 to TS4. However, the values of photocurrent density and solar cell efficiency increase from TS0 to TS2, but for TS3 and TS4 samples were lower. For TS3 and TS4 Authors concluded that the smaller thickness (from about 30 µm for the pure TiO2 to 26 µm TS3 and 20 µm TS4, respectively) is responsible for this phenomenon. On the other hand, for TS1 and TS2 the thickness of active layer is almost the same as for the pure TiO2. Better performance of DSSC for TS1 and TS2 with respect to the pure TiO2, in my opinion, is one of the most important and very interesting results from this manuscript. Maybe, it can be studied in more details. The application of electrochemical impedance spectroscopy (EIS) and comparison of the values of charge transfer resistance close to the VOC values for each sample should give the answer about the recombination between electrons in titania and electrolyte.
10. Conclusions, page 8 line 280: instead of “primitive hydrothermal methods” it should be changed to “simple hydrothermal methods”.
11. Conclusions part should be extended for a short summary about experimental techniques that were used to characterization of obtained titania samples after Sn modifications. Additionally, the lack of dye used to prepare DSSC systems.
12. Supporting Information. The abbreviations by the electrolyte components (DMII, TBP, GNCS) should be explained.
Author Response
Response to Reviewer 2 Comments
Point 1: No information about the dye used to prepare DSSC in manuscript. I found the type of dye (N719) only in Supporting Information part. In my opinion, this information should be included in abstract part, keywords, page 2 line 88, page 5 line 186 (N719 dye, iodide/triiodide electrolyte and platinized counter electrode) and finally in conclusion part of the manuscript.
Response 1: Thank you for your valuable advice. We have added the type of dye in the manuscript according to your advice. And the modification in the manuscript has been marked by blue. Change in abstract: “The power conversion efficiency (η) of DSSC based on the reasonable Sn-doped TiO2, N719 dye, platinized counter electrode and iodide/triiodide electrolyte reaches 8.75%.” Change in introduction: “In this paper, we prepared Sn-doped TiO2 NWAs by a simple hydrothermal process and then assembled DSSCs based on these photoanodes with N719 dye, platinized counter electrode and iodide/triiodide electrolyte.” Change in Results and discussion: “The pure TiO2 nanowire arrays and Sn-doped TiO2 nanowire arrays with N719 dye, platinized counter electrode and iodide/triiodide electrolyte were employed as photoanodes to prepare DSSCs. The detailed methods are shown in supporting information.” Change in conclusion: “The power conversion efficiency (η) of DSSC based on the reasonable Sn-doped TiO2, N719 dye, platinized counter electrode and iodide/triiodide electrolyte reaches 8.75%.”
Point 2: Abbreviation “DSSC” should be used consequently in all manuscript. Actually, we can found both “DSSC” and “DSC”. It should be defined in abstract part (page 1, line 12).
Response 2: Thank you for your advice. We have carefully rechecked and changed all the abbreviation to “DSSC”.
Point 3: Abstract line 13-14: “low electron mobility of TiO2” Is it correct? Maybe, it should be “low electron mobility within TiO2”
Response 3: Thank you for your careful work. We have changed “low electron mobility of TiO2” to “low electron mobility within TiO2
Point 4: Introduction, page 1 line 40: Some details about the ways to produce a much higher power conversion efficiency of DSSC could be added, including for example, design of the novel dyes or modification of structures of well-known dyes, the novel types of electrolyte or modification of the photoanodes and/or counter electrodes surface.
Response 4: We have added the ways to produce a much higher power conversion efficiency of DSSC in the manuscript. And the modification in the manuscript has been marked by blue. We add that “Much endeavor has been focused on design of or modification of the novel dyes [13], the novel types of electrolyte [14] or modification of the counter electrodes [15].
Point 5: Introduction, page 1 line 42: “on semiconductor surface” should be added to “monolayer of dye molecule”
Response 5: thank you for your advice. We have added “on semiconductor surface” in the manuscript. And the modification in the manuscript has been marked by blue.
Point 6: Introduction, page 2 line 60: “dye absorption” or “dye adsorption”?
Response 6: We are sorry for the malaprop. We have changed “dye absorption” to “dye adsorption”.
Point 7: Introduction, page 2 line 90: “…on the Sn-doped SnO2 based exhibits an remarkable…”, probably it should be corrected: “…on the Sn-doped TiO2 exhibits an remarkable…”
Response 7: Thank you for your careful work. We are sorry for the malaprop. We have changed “…on the Sn-doped TiO2 based exhibits an remarkable…” to “…on the Sn-doped TiO2 NWAs exhibits a remarkable…”.
Point 8: Results and discussion, page 3 line 94-95: “the ratio of Sn/Ti is about 5/95, 11/89, 15/85 and 21/89”; probably it should be corrected: “the ratio of Sn/Ti is about 5/95, 11/89, 15/85 and 21/79”.
Response 8: Thank you for your careful reading of our manuscript. We are sorry for the miscalculation. We have changed “the ratio of Sn/Ti is about 5/95, 11/89, 15/85 and 21/89” to “the ratio of Sn/Ti is about 5/95, 11/89, 15/85 and 21/79”. And the modification in the manuscript has been marked by blue.
Point 9: According to the results in Table 1: the values of dye loading were systematically reduced from TS0 to TS4. However, the values of photocurrent density and solar cell efficiency increase from TS0 to TS2, but for TS3 and TS4 samples were lower. For TS3 and TS4 Authors concluded that the smaller thickness (from about 30 µm for the pure TiO2 to 26 µm TS3 and 20 µm TS4, respectively) is responsible for this phenomenon. On the other hand, for TS1 and TS2 the thickness of active layer is almost the same as for the pure TiO2. Better performance of DSSC for TS1 and TS2 with respect to the pure TiO2, in my opinion, is one of the most important and very interesting results from this manuscript. Maybe, it can be studied in more details. The application of electrochemical impedance spectroscopy (EIS) and comparison of the values of charge transfer resistance close to the VOC values for each sample should give the answer about the recombination between electrons in titania and electrolyte.
Response 9: Thank you for your precious advice. We have added the EIS analysis in the manuscript. And the modification in the manuscript has been marked by blue. We add that: “To further analyze charge transfer kinetics in these DSSCs, electrochemical impedance spectroscopy (EIS) is carried out under 750 mV bias in the dark condition, as shown in inset of Figure S8. All obtained curves show two arcs in low frequency and middle frequency regions. According to previous researches, the enlarged low-frequency arc implies the charge transfer resistance of the photoanode, as shown in Figure S8 [80]. Through the fitting with Z-view, the resistance of the electrode could be calculated, as shown in Table S1. With the doping amount of Sn increasing, the charge transfer resistance gradually decrease, which certifies that the Sn doping can effectively improve the charge transfer performance.
Figure S8 The enlarged low-frequency arc and full scale (inset graph) Nyquist plots of the DSSCs based on the pure TiO2 NWAs (TS0) and Sn-doped TiO2 NWAs (TS1, TS2, TS3, TS4) measured under 750 mV bias in the dark condition
Point 10: Conclusions, page 8 line 280: instead of “primitive hydrothermal methods” it should be changed to “simple hydrothermal methods”.
Response 10: Thank you for your advice. We have changed “primitive hydrothermal methods” to “simple hydrothermal methods”. And the modification in the manuscript has been marked by blue.
Point 11: Conclusions part should be extended for a short summary about experimental techniques that were used to characterization of obtained titania samples after Sn modifications. Additionally, the lack of dye used to prepare DSSC systems.
Response 11: Thank you very much to point out deficiency of our manuscript. We have changed in manuscript. The new conclusion is “We have successfully produced Sn-doped TiO2 NWAs through simple hydrothermal methods and analysed their performance as photoanode in DSSCs. The prepared Sn-doped is 30μm length with Square section of 15 nm average square edge length. XRD, Raman and XPS results confirmed the success of Sn doping. It is found that Sn doping well augments the electron mobility and raises the flat band potential to improve the performance of DSSC. The power conversion efficiency (η) of DSSC based on the reasonable Sn-doped TiO2, N719 dye, platinized counter electrode and iodide/triiodide electrolyte reaches 8.75%. Furthermore, with a anatase TiO2 light scattering layer, DSSC based on the Sn-doped TiO2 NWAs exhibits a remarkable power conversion efficiency of 9.43%, especially useful in the condition of weak lights.” And the modification in the manuscript has been marked by blue.
Point 12: Supporting Information. The abbreviations by the electrolyte components (DMII, TBP, GNCS) should be explained.
Response 12: Thank you for your advice. DMII is the abbreviation of 1,3-dimethylimidazolium, TBP is the abbreviation of 4-tert-butylpyridine, and GNCS is the abbreviation of guanidinium thiocyanate. We have added these in supporting information. And the modification in the supporting information has been marked by blue.
Author Response File: Author Response.pdf
