This is a good question. LED is usually large compared to OLED. You did not mension the size. Here is just a brief introduction. For current and heat simulations, you can use CHARGE and HEAT. For the optics side, you can either use FDTD, or use stackdipole and stackpurcell functions:
stackpurcell - Script command
You will need to simulate saparately and with the results imported to FDTD. You may first try to use HEAT,CHARGE and see if the large size device can be simulated. If FDTD is challenge, the scripts should work.
However, currently you may not directly use np density as index perturbation for the stack functions.
if needed please file a feature request. https://forum.ansys.com/forums/topic/vote-new-features-and-file-your-feature-request/#latest
If you are a commercial user, please contact sales for more support: https://www.lumerical.com/contact-us/find-a-local-rep/
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