In addition to Steve's suggestion
You can also try this,
btn.SetFontFile("Misc/material.ttf")
btn.SetText("person")
You don't need the bracket anymore.
For a list of material icons, you can search it online, but if you have installed the beta version of DroidScript,
you can find the list of material icon fonts at the Docs Section, MaterialUI->Overview->Run MUI Demos->Iconography
Or if you have DroidScript UI Kit app, its on Iconography section. Just scroll down in the list of controls.
Regards