Dear Faraz,

The colleagues do not spoke about the cleaning of the glass substrate. You have to completely clean the surface of the glass before spinning process.

Water spreads easily on glass surface if it is clean. There are standard cleaning procedures for glass surfaces.

You must clean the glass surfaces such that when you put a drop of water on the glass it spreads completely.

Please for the wetting principles please follow the link: http://web.mit.edu/nnf/education/wettability/wetting.html

Best wishes

Dear Dr. Faraz Uddin,

I suggest you to have a look at the following papers:

-SnO2-based electron transporting laye rmaterials fo rperovskite solar cells: A review of recent progress

Yichuan Chen,,Qi Meng, Linrui Zhang, Changbao Han, Hongli Gao,Yongzhe Zhang, HuiYan

JournalofEnergyChemistry, 144–167 (2019)

Available at: http://engine.scichina.com/cfs/files/files/upfile/SnO2-based-electron-transporting-layer-materials-for-per_2019_Journal-of-Ene.pdf

-Facile Solution Spin-Coating SnO2 Thin Film Covering Cracks of TiO2 Hole Blocking Layer for Perovskite Solar Cells

Haiyan Ren, Xiaoping Zou , Jin Cheng, Tao Ling, Xiao Bai and Dan Chen

Coatings, 8, 314; doi:10.3390/coatings8090314 (2018)

Available at: https://www.semanticscholar.org/paper/Facile-Solution-Spin-Coating-SnO2-Thin-Film-Cracks-Ren-Zou/0b82232e2c25e8b2438af047e6c67449f48db3ab

-MgO Nanoparticle Modified Anode for Highly Efficient SnO2-Based Planar Perovskite Solar Cells

Junjie Ma, Guang Yang, Minchao Qin, Xiaolu Zheng, Hongwei Lei, Cong Chen, Zhiliang Chen, Yaxiong Guo, Hongwei Han, Xingzhong Zhao, and Guojia Fang

Advanced Science, 4, 1700031 (2017)

Available at: https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700031

-Scaling Up Perovskite Photovoltaics: Progress, Challenges, and Outlook of a Transformational Technology

Zhen Li, Talysa R. Klein, Dong Hoe Kim, Mengjin Yang, Joseph J. Berry, Kai Zhu

This document represents the authors' peer-reviewed, accepted manuscript.

Available at: https://www.osti.gov/servlets/purl/1430821

Best regards, Pierluigi Traverso.

Faraz Uddin

What is your substrate exactly? Water is probably one of the worst liquids to use for spin coating.

Cleanliness and wetting are the 2 most important parameters in spin coating:

• Work in Class 100 conditions
• Filter all liquids to 0.22 microns or better
• Optimize the substrate cleaning to remove all particles > 0.22 microns and ensure a receptive surface
• Ensure that the liquid adequately wets the substrate - function of substrate chemistry, cleanliness, and surface tension. Lower surface tension liquids or surfactants can sometimes help
• Film cracking is often a result of mismatch in thermal expansions between coating and substrate

You may also wish to utilize a sol-gel route. I have seen very limited success with trying to spin particles onto substrates. How do you expect a film to form from particles?

Faraz Uddin

Your comment that you’re using SnO2 nano particles implies that they’re less than 100 nm and thus you should have a transparent (maybe colored) suspension. If this is not the case then you have micron sized particles or larger.

Dear Faraz Uddin

As professor Alan F Rawle said , water as a solvent is not good for the dispersion of SnO2 nanoaprticles. So you can try another solvent for the better dispersion of SnO2 nanoaprticles. Or you can use a stabilizing agent for increasing the binding between the SnO2 coating and the surface of the substrate.

Please see the following links:

Article Spin Coating Process for 10cm × 10cm Perovskite Solar Module...

file:///C:/Users/Bandna%20bharti/Downloads/Tavakoli_et_al-2018-Advanced_Energy_Materials.pdf

Dear Prof. Alan,

Thank you for your answer. I am using FTO substrate. I have tried different concentration of SnO2 nanoparticles but still it is the same. As you suggested about the solvent, I will change the solvent and then observe. Can you please suggest which solvent would be better to get crack free SnO2 layer? Alan F Rawle

Dear Dr. Bandna,

Thank you for your suggestions. Can you recommend any (best) stabilizing agent that can increase the binding between substrate and SnO2 coating? Bandna Bharti

Faraz Uddin

In what form do you get your SnO2 'nanoparticles'? If they're as a powder, then what is the SSA (specific surface area)?

BTW, a stabilizing agent is different from a coupling agent...

I agree Alan F Rawle , Your comment that you’re using SnO2 nano particles implies that they’re less than 100 nm and thus you should have a transparent (maybe colored) suspension. If this is not the case then you have micron sized particles or larger.

Hi

As mentioned above, changing the solvent and adding passivating/binder agent is a good option. Apart from that, cracking is observed when the thickness of film is high. if you will reduce the thickness with high uniformity, this problem will not happen. You can deposit one thin film and sinter, and deposit the second film and sinter again. This is a little multistep process, but you can avoid cracking effectively.

Best regards

Alan F Rawle

I have purchased SnO2 nanoparticles solution in water (15wt%) from Alfa aesar. Gunel T. Imanova .. Yes the nanoparticle solution is transparent and seems that the particle size is less than 100 nm.

Mumtaz Ali Yes, I have also tried Bi-layer and the same problem still exist. Do you have any other recommendations? I have also filtered the solution, tried Bi-layer, reduced the concentration, sonicated the solution for several hours but the cracks still exist. But I have to try other solvent according to the recommendation of Prof Alan F Rawle

Faraz Uddin we use ethanol as a solvent.

I hope by using that there will be no issue.

If you will use ethanol and water mixture then during the casting process, ethanol will evaporate first (as it has lower boiling point), thus leaving water dispersed SnO2 again.

The cracking occurs due to poor interaction between glass and SnO2. To increase mutual compatibility, they use UV-ozone-plasma treatment to make the ITO glass hydrophilic. If you don't have that machine, you can place the glass in intense UV light for a longer time or do oxygen/nitrogen plasma treatment of Washed ITO glass. It will make the glass surface hydrophilic and mutual interactions between glass and particles are improved.

Another main problem which is responsible for the cracking is fast heating and cooling rate. When we do fast heating or cooling, the difference in thermal expansion of film and glass is more pronounced, which generates micro-macro cracks. To avoid the issue, we use a heating rate of less than 2-3 degrees/min, and allow the cooling on its own (under closed chamber conditions).

Best Regards...

Dear Dr. Faraz Uddin

Please see the following link:

Article Structural, optical and magnetic properties of Cr doped SnO2...

Article Highly stable low cost anode interfacial material preparatio...

Article Influence of stabilising agents and pH on the size of SnO 2 ...