Surface-Passivated CsPbBr₃ for Developing Efficient and Stable Perovskite Photovoltaics

Round 1

Reviewer 1 Report

Hyeon Ju Tak, et al. Are reporting on surface passivation of all inorganic, CsPbBr3 perovskite using octylammonium bromide. Despite moderate efficiencies reported in the article (which are expected for such perovskite and structure), a clear enhancement in several aspects is observed including, the open circuit voltage, the dark current, stability, and carriers’ lifetime. Overall, the article is interesting and provides more insights on the use of long chain salts for perovskite solar cells. However, there are a few ~major and minor points that I would recommend before this article is published, please see below:

~Major:

1. Line 106, “No significant variation of morphologies of CsPbBr3 films…”. The provided figure is not clearly supporting this claim. In the figure, it seems smaller grains. Please provide some statistics or possibly some AFM images with some parameters extracted.
2. Fig 2 a, Table 1, it is not clear how many devices or batches were tested, similarly no statistics are provided, are these values average values or champion devices. What are the variations/error. Please provide details which would confirm the results and conclusions.
3. Line 119-122, “Since the OABr surface treatment did not lead to the shift of the 119 absorbance edge of the CsPbBr3 films, as shown in Figure S2, the suppression of Eloss in 120 OABr-treated PPV would imply that OABr surface treatment contributes to the reduced 121 trap density and…” or it could be related to better carriers extraction/injection, better energy level alignment. Please discuss and consider energy level alignment with selective charge electrode.
4. Was normalization of PL performed or considered. The text is comparing PL intensities, if PL intensities are compared, it sounds strange to have arbitrary units.
5. Could you please provide more reasoning behind the blue shift observed? It is mentioned, line 173, “passivation of surface traps by interaction between CsPbBr3 and OABr” Is PL also a surface measurement or bulk how would such a surface treatment affect the bulk strongly.
6. How much is the expected amount of material on the top surface? does it interact with bulk? does the used molecule stay intact, especially that Br can be volatile with annealing.
7. It is strange that only one article about octylammonium (and it is iodide) is cited and only at the very end. It should be in the introduction, and some references about perovskite with OABr highly recommended to be introduced and cited. Also several articles discussed OABr for CsPbBr3 perovskites are available in literature, such as [Nature communications. 2017 May 12;8(1):1-9.][Physical Chemistry Chemical Physics. 2019;21(39):21996-2001.][ 2021 Aug;11(8):957.]
8. In figure 3c, VTFL values are provided, but not discussed in main text, how would the change affect the devices’ performance? Please discuss and correlate in the article.

~Minor:

1. In the abstract, I would recommend adding some information about experimental part that used to investigate the reasons behind the improvement and some related findings.
2. Line 17, “Here, we developed a facile method …” the sentence is a little misleading as the material used, i.e. OABr is used before for same perovskite for relatively similar reasons.
3. In the introduction, I would recommend avoiding listing several references for citations at once, but to be specific and provide each sentence with its own citation in case different the references are discussing different aspects eg. 1-3, 4-9, 11-15, etc.
4. In several figures the text is small, eg. Figure 2, please consider a better size for the font, i.e. 8pt.
5. Figure s3 seems good to be in main text rather than in SI
6. I would recommend to provide CsPbBr3 achieved PCE in literature, maybe in general, or a comparable structure.

Author Response

Answers to the comments from Reviewer 1

Comments

Hyeon Ju Tak, et al. Are reporting on surface passivation of all inorganic, CsPbBr3 perovskite using octylammonium bromide. Despite moderate efficiencies reported in the article (which are expected for such perovskite and structure), a clear enhancement in several aspects is observed including, the open circuit voltage, the dark current, stability, and carriers’ lifetime. Overall, the article is interesting and provides more insights on the use of long chain salts for perovskite solar cells. However, there are a few ~major and minor points that I would recommend before this article is published, please see below:

Response

Thank you for comprehensive review and time spent for the insightful comments. We have tried to resolve the questions and comments thoroughly and the responses could be found below.

Query #Major 1-1

Line 106, “No significant variation of morphologies of CsPbBr3 films…”. The provided figure is not clearly supporting this claim. In the figure, it seems smaller grains. Please provide some statistics or possibly some AFM images with some parameters extracted.

Response

We now provide the grain size distribution obtained from SEM in order to clearly show the morphology variations of CsPbBr₃ films before and after surface treatment. As shown in Figure S1c, the CsPbBr₃ films before and after surface treatment showed similar grain size distributions.

Figure S1. (c) Grain size distribution obtained from SEM images.

Query # Major 1-2

Fig 2 a, Table 1, it is not clear how many devices or batches were tested, similarly no statistics are provided, are these values average values or champion devices. What are the variations/error. Please provide details which would confirm the results and conclusions.

Response

We now provide the statistical data of our PPVs in Figure S3 and Table 1 of the revised manuscript.

Table 1. Photovoltaic parameters of CsPbBr₃ PPVs with and without surface treatment using OABr in reverse can.

¹ Data in brackets are averaged values from 16 devices.

² Calculated from R-F avg. = ( PCE_REV + PCE_FWD ) / 2.

Figure S3. Detailed distribution data of (a)V_OC, (b) J_SC, (c) FF and (d) PCE for control and OABr-treated CsPbBr₃ PPVs.

Query # Major 1-3

Line 119-122, “Since the OABr surface treatment did not lead to the shift of the 119 absorbance edge of the CsPbBr3 films, as shown in Figure S2, the suppression of Eloss in 120 OABr-treated PPV would imply that OABr surface treatment contributes to the reduced 121 trap density and…” or it could be related to better carriers extraction/injection, better energy level alignment. Please discuss and consider energy level alignment with selective charge electrode.

Response

Although the optical bandgap of perovskite films before and after surface treatment were not significantly changed, the energy level of perovskite surface could be varied after OABr treatment, which would affect the charge extraction in PPVs. Therefore, as the reviewer commented, we provide additional discussion about carbon electrode in the revised manuscript with appropriate references.

“Additional possible reason for the reduced E_loss of PPVs after OABr treatment is the low dipole moment arising from the long alkyl chains, leading to the facilitated charge transfer at perovskite/carbon electrode interface in the PPV by inducing the better energy alignment [21-25].”

21. Wang, G.; Dong, W.; Gurung, A.; Chen, K.; Wu, F.; He, Q.; Pathak, R.; Qiao, Q. Improving photovoltaic performance of carbon-based CsPbBr₃ perovskite solar cells by interfacial engineering using P3HT interlayer. Power Sources 2019, 432, 48–54.
22. Wu, X.; Xie, L.; Lin, K.; Lu, J.; Wang, K.; Feng, W.; Fan, B.; Yin, P.; Wei, Z. Efficient and stable carbon-based perovskite solar cells enabled by inorganic interface of CuSCN and carbon nanotubes. Mater. Chem. A 2019, 7, 12236–12243.
23. Gholipour, S.; Correa-Baena, J.-P.; Domanski, K.; Matsui, T.; Steier, L.; Giordano, F.; Tajabadi, F.; Tress, W.; Saliba, M.; Abate, A.; et al. Highly efficient and stable perovskite solar cells based on a low-cost carbon cloth. Energy Mater. 2016, 6, 1601116.
24. Ding, J.; Duan, J.; Guo, C.; Tang, Q. Toward charge extraction in all-inorganic perovskite solar cells by interfacial engineering. Mater. Chem. A 2018, 6, 21999–22004.
25. Duan, J.; Wang, M.; Wang, Y.; Zhang, J.; Guo, Q.; Zhang, Q.; Duan, Y.; Tang, Q. Effect of side-group-regulated dipolar passivating molecules on CsPbBr₃ perovskite solar cells. ACS Energy Lett. 2021, 6, 2336–

Query # Major 1-4

Was normalization of PL performed or considered. The text is comparing PL intensities, if PL intensities are compared, it sounds strange to have arbitrary units.

Response

Normalization of the steady-state PL was not performed. Therefore, we rectify the units for steady-state PL data in the revised manuscript from arbitrary units to c.p.s. (counts per second).

Figure 3. (a) Steady-state PL measurements for CsPbBr₃ films before and after OABr surface treatment.

Query # Major 1-5

Could you please provide more reasoning behind the blue shift observed? It is mentioned, line 173, “passivation of surface traps by interaction between CsPbBr3 and OABr” Is PL also a surface measurement or bulk how would such a surface treatment affect the bulk strongly.

Response

We now provide additional explanation about the steady-state PL data in the revised manuscript.

“The enhanced PL intensity would be referred to the suppression of non-radiative recombination [30], and the blue-shift of PL peak possibly indicates the reduced trap density of CsPbBr3 film by the interaction between CsPbBr₃ and OABr after surface treatment [31].”

30. Ma, C.; Park, N.-G. Paradoxical approach with a hydrophilic passivation layer for moisture-stable, 23% efficient perovskite solar cells. ACS Energy Lett. 2020, 5, 3268–3275
31. He, Q.; Worku, M.; Xu, L.; Zhou, C.; Lteif, S.; Schlenoff, J.; Ma, B. Surface passivation of perovskite thin films by phosphonium halides for efficient and stable solar cells. Mater. Chem. A 2020, 8, 2039–2046.

Query # Major 1-6

How much is the expected amount of material on the top surface? does it interact with bulk? does the used molecule stay intact, especially that Br can be volatile with annealing.

Response

The existence of OABr on the surface of perovskite film after surface treatment would be identified by the variation of optoelectronic properties of CsPbBr₃ films before and after OABr treatment. We carefully suggested that the surface traps would be de-passivated if the OABr is removed after deposition of carbon electrode or annealing process because of the limited interaction between perovskite and carbon electrode. Our PPVs after OABr treatment showed enhanced performance compared with control device, indicating the passivation effect of surface treatment. By considering the fact that methyl ammonium bromide is thermally stable under 200 °C (Chem. Mater. 2014, 26, 6160), we also suggested that the OABr would have enough thermal stability for the deposition of carbon electrode. In this work, the optimized treatment was obtained by controlling the concentration of OABr in chloroform solution and the 15 mM solution showed the best performance of PPVs. Additionally, we used spin-coating method for the surface treatment (please see Materials and Methods Section), affording the short-time exposure of OABr to the perovskite film, and thus the main contribution of OABr is the surface of perovskite film and their penetration into the bulk perovskite would be very limited. This is possibly supported by no significant variation of optical properties of perovskite films before and after surface treatment. Due to the limitation of revision deadline, the detailed analysis on the exact amount of OABr on the surface of perovskite will be performed and reported in our next study.

Query # Major 1-7

It is strange that only one article about octylammonium (and it is iodide) is cited and only at the very end. It should be in the introduction, and some references about perovskite with OABr highly recommended to be introduced and cited. Also several articles discussed OABr for CsPbBr3 perovskites are available in literature, such as [Nature communications. 2017 May 12;8(1):1-9.][Physical Chemistry Chemical Physics. 2019;21(39):21996-2001.][ 2021 Aug;11(8):957.]

Response

As the reviewer commented, we additional references related to surface treatment of perovskite films in the revised manuscript (ref. 14-17).

Query # Major 1-8

In figure 3c, V_TFL values are provided, but not discussed in main text, how would the change affect the devices’ performance? Please discuss and correlate in the article.

Response

In this work, V_TFL values were used for calculating the trap density of devices and its related equations were stated in the manuscript (equation 4). After received the reviewer’s comment, we modified the manuscript to enhance the clarity of the relationship between V_TFL and trap density.

“The trap densities of devices were calculated from the V_TFL using the below equation.”

Query # Minor 1-1

In the abstract, I would recommend adding some information about experimental part that used to investigate the reasons behind the improvement and some related findings.

Response

As the reviewer commented, we added the additional sentence related to analysis in the Introduction part of the revised manuscript.

“Then, the enhanced performance of CsPbBr₃ PPV after surface treatment was investigated using light-intensity dependent photovoltaic performances, photoluminescence (PL) measurements, and trap density analysis.”

Query # Minor 1-2

Line 17, “Here, we developed a facile method …” the sentence is a little misleading as the material used, i.e. OABr is used before for same perovskite for relatively similar reasons.

Response

The phrase is now corrected from “developed” to “adopted”.

Query # Minor 1-3

In the introduction, I would recommend avoiding listing several references for citations at once, but to be specific and provide each sentence with its own citation in case different the references are discussing different aspects eg. 1-3, 4-9, 11-15, etc.

Response

We notated references according to the recommended manner and rearranged the numbers accordingly.

Query # Minor 1-4

In several figures the text is small, eg. Figure 2, please consider a better size for the font, i.e. 8pt.

Response

To improve better reading for Figure 2, we have increased the font size.

Query # Minor 1-5

Figure s3 seems good to be in main text rather than in SI

Response

We now transferred reverse and forward curves in the main text.

Figure 2. (a) J–V characteristics of control and OABr-treated CsPbBr₃ PPVs under AM 1.5G illumination (100 mW cm⁻²). Solid and dashed lines are the data measured in reverse and forward scans, respectively.

Query # Minor 1-6

I would recommend to provide CsPbBr3 achieved PCE in literature, maybe in general, or a comparable structure.

Response

We have included efficiency table for reported CsPbBr₃ PPVs and corresponding references in Supporting Information.

Table S2. Photovoltaic parameters of reported CsPbBr3 PPVs.

• Y, Ren.; N, Zhang.; Z, Arain.; M, Mateen.; J, Chen.; Y, Sun.; Z, Li. Polymer-induced lattice expansion leads to all-inorganic CsPbBr3 perovskite solar cells with reduced trap density, Power Sources. 2020, 475, 228676.
• D, Huang.; P, Xie.; Z, Pan.; H, Rao.; X, Zhong. One-step solution deposition of CsPbBr3 based on precursor engineering for efficient all-inorganic perovskite solar cells, Mater. Chem. A. 2019, 7, 22420–22428.
• J, Duan.; Y, Zhao.; B, He.; Q, Tang. Simplified Perovskite Solar Cell with 4.1% Efficiency Employing Inorganic CsPbBr3 as Light Absorber. Small. 2018, 14, 1704443.
• Z, Liu.; B, Sun.; X, Liu.; J, Han.; H, Ye.; T, Shi.; Z, Tang.; G, Liao. Efficient Carbon-Based CsPbBr3 Inorganic Perovskite Solar Cells by Using Cu-Phthalocyanine as Hole Transport Material. Nano-Micro Lett. 2018, 10, 34.
• G, Wang.; W, Dong.; A, Gurung.; K, Chen.; F, Wu.; Q, He.; R, Pathak.; Q, Qiao. Improving photovoltaic performance of carbon-based CsPbBr3 perovskite solar cells by interfacial engineering using P3HT interlayer. Power Sources. 2019, 432, 48.
• J, Liang.; C, Wang.; Y, Wang.; Z, Xu.; Z, Lu.; Y, Ma.; H, Zhu.; Y, Hu.; C, Xiao.; X, Yi.; G, Zhu.; H, Lv.; L, Ma.; T, Chen.; Z, Tie.; Z, Jin.; J, Liu. All-Inorganic Perovskite Solar Cells. Am. Chem. Soc. 2016, 138, 15829–15832.
• H, Guo.; Y, Pei.; J, Zhang.; C, Cai.; K, Zhou.; Y, Zhu. Doping with SnBr2 in CsPbBr3 to enhance the efficiency of all-inorganic perovskite solar cells. J. Mater. Chem. C. 2019, 7, 11234–11243.
• J, Duan.; Y, Zhao.; B, He.; Q, Tang. High-Purity Inorganic Perovskite Films for Solar Cells with 9.72 % Efficiency. Chem. Int. Ed. 2018, 57, 3787.
• Y, Liu.; B, He.; J, Duan.; Y, Zhao.; Y, Ding.; M, Tang.; H, Chen.; Q, Tang. Poly(3-hexylthiophene)/zinc phthalocyanine composites for advanced interface engineering of 10.03%-efficiency CsPbBr3 perovskite solar cells. Mater. Chem. A. 2019, 7, 12635.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. It would be nice to include the state-of-the-art research process of CsPbBr3 in the introduction. since authors are targeting on improving the device performance and long term stability of the CsPbBr3 based PSCs, thus it is important to show what is the current status related (e.g. what is the highest PCE with CsPbBr3 based PSC, etc).
2. Is there a reason why the authors did not use a hole transporting layer for potentially better charge transfer?
3. Authors should clarify more details on the ageing condition of the devices under mppt, e.g. what is the ageing temperature, atmosphere (did it test under N2 gas or ambient), and relative humidity.

Author Response

Answers to the comments from Reviewer 2

Query # 2-1

It would be nice to include the state-of-the-art research process of CsPbBr3 in the introduction. since authors are targeting on improving the device performance and long term stability of the CsPbBr3 based PSCs, thus it is important to show what is the current status related (e.g. what is the highest PCE with CsPbBr₃ based PSC, etc).

Response

Thank you for your kind comments. Based on the comment, we revised the introduction part in our manuscript. Additionally, we included the efficiency table for the reported CsPbBr₃ PPVs and corresponding references in Table S2.

“Particularly, CsPbBr₃, which has been regarded as a promising wide bandgap pho-to-absorber with exceptional stability, and its corresponding PPVs have shown efficiencies up to 11.08% [10,11]. Moreover, CsPbBr₃ PPVs have been typically investigated by adapting the device structure with the direct junction between perovskite layer and carbon electrode [12], and, as a result, these PPVs have still afforded the inadequate contact accompanied by insufficient interfacial interaction, leading to large open-circuit voltage (V_OC) deficit of ~0.6 eV [13].”

10. Zhou, Q.; Duan, J.; Du, J.; Guo, Q.; Zhang, Q.; Yang, X.; Duan, Y.; Tnag, Q. Tailored lattice “Tape” to confine tensile interface for 11.08%-efficiency all-inorganic CsPbBr3 perovskite solar cell with an ultrahigh voltage of 1.702 V. Adv. Sci. 2021, 8, 2101418.
11. Jeong, M.; Choi, I.W.; Go, E.M.; Cho. Y.; Kim, M.; Lee, B.; Jeong, S.; Jo, Y.; Choi, H.W.; Lee, J.; et al. Stable perovskite solar cells with efficiency exceeding 24.8% and 0.3-V voltage loss. Science 2020, 369, 1615–1620.
12. Duan, J.; Hu, T.; Zhao, Y.; He, B.; Tang, Q. Carbon-electrode-tailored all-inorganic perovskite solar cells to harvest solar and water-vapor energy. Angew. Chem. Int. Ed. 2018, 57, 5746–5749.
13. Liu, T.; Wang, Z.; Lou, L.; Xiao, S.; Zheng, S.; Yang, S. Interfacial post-treatment for enhancing the performance of printable carbon-based perovskite solar cells. Sol. RRL 2020, 4, 1900278.

Query # 2-2

Is there a reason why the authors did not use a hole transporting layer for potentially better charge transfer?

Response

Up to now, CsPbBr₃ PPVs has typically investigated using the direct contact between perovskite layer and carbon electrode, as shown in Table S2. Recently, some research groups have successfully introduced hole transporting layer into CsPbBr₃ PPVs. However, our group did not obtain any meaningful devices with the deposition of hole transporting layer, such as P3HT. This is because of the poor wettability of carbon electrode on the surface of hole transporting layer and we would ascribe this to the differences in the used chemicals and the deposition method for carbon electrode.

Query # 2-3

Authors should clarify more details on the ageing condition of the devices under mppt, e.g. what is the ageing temperature, atmosphere (did it test under N2 gas or ambient), and relative humidity.

Response

We have added the measuring condition in Figure 4 as well as the explanation in the main text.

Figure 4. (a) Storage stability test for control and OABr-treated CsPbBr₃ PPVs. Devices were kept in air condition without encapsulation. (b) Maximum power point tracking measurement. Devices were measured under 1.0 V bias and AM 1.5G illumination (100 mW cm⁻²).

“Additionally, maximum power point tracking of PPVs under 1-sun illumination were studied under the temperature of ~45 ºC and the constant bias of 1.0 V, as shown in Figure 4b.”

Author Response File: Author Response.pdf