Convergence issue

bhrigu_rishibhrigu_rishi Member Posts: 1

I was trying to replicate a paper (Bao et al., Solar Energy Materials and Solar Cells 168 (2017) 78–84) in Lumerical. Random media of TiO2 with volume fraction 0.4 and radius 0.5 micrometre. The structure span 4*4*10 micrometre. The reflection I am getting similar but the sum of R+T is not equal to 1. Since media is lossless so there is no chance of absorption. I tried different things such as changing the mesh size, changing the position of the source and monitor but there is no change in the result at all. I also tried to replicate some experimental work on TiO2 random media. But again R+T is not equal to 1 and R is giving lesser than whatever reported in the paper. I have attached the results obtained from the simulation and reported in the paper (Bao et al. 2017).

One thing which I want to mention

that when I did the simulation of similar random media for smaller thicknesses such as 1 or 2 micrometres then R+T is giving 1. But as thickness increases, T approaches zero.

Thanks.

Answers

  • shkimshkim Posts: 27Ansys Employee

    The wavelength range you are currently using is quite broad. This will generally require very fine mesh over a very large simulation region. You might need to check the following settings:

    • PML proximity: To avoid the evanescent field interfering with the PML, you need to set the distance from the PML to the structure to be larger than a quarter of the max wavelength. You need a convergence test to make sure the distance is large enough.
    • Broadband source: When injecting broadband, it is possible to inject a non-negligible field outside the wavelength range, making the simulation unstable and contributing to spurious results
    • Simulation time: Check if the simulation has terminated by reaching the auto-shutoff level.

    You might find the following posts useful:

    https://forum.ansys.com/discussion/24202/general-fdtd-troubleshooting-tips-and-tricks

    https://forum.ansys.com/discussion/24201/key-fdtd-simulation-settings

    Apart from this, simulating random structures requires further attention to the source and boundary settings. If you can share the screen capture of your simulation, I can make further comments.

  • bhrigu_rishibhrigu_rishi Posts: 2Member

    Thanks for your reply. I have kept sufficient distance from the structure to PML to avoid PML proximity. For the source, I used a plane wave source. I have attached the screenshots of the source and boundary settings. Others are the default conditions I have used. Please let me know if you need anything else.


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