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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?

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Are you looking at the current or at the light intensity?

A real turn-on voltage does not exist. When the current is small, ohmic contributions to the voltage drop can be neglected. The current is then exponentially dependent on forward voltage according to the diode equation.

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.

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