# What does the turn on voltage for LEDs depends on except for the bandgap of the material?

I am trying to find out why my LEDs show a different turn on voltage. The material emitting is the same (therefore same bandgap). The stack I have is ITO/Polyvynilcarbazole (hole injection layer)/active material/ TPBi (electron injecting material)/LiF/Al

Could that be something related to the thickness of these layers? Or energy alignment?

The only differences are an $$\eta$$ (often 1, 1.5, or 2) in the exponent $$\frac{qV}{\eta kT}$$ and a prefactor that is proportional to area and dependent on lots of stuff. But there is no real threshold.