Dear Isaac,

welcome,

The active layer must have high radiation efficiency where ideally every electron hole pairs in the active region produces a photon and this photon must be transmitted outside the device in the air. Practically the there is nonradiative and radiative recombination. This will reduce the emission efficiency by the the ratio= Tnonrad/(Trad +T nontad. I derived this relationship in a a previous comment to you:https://www.researchgate.net/post/How_to_estimate_Internal_photon_conversion_efficiency_from_Photo-luminescence_Spectra_or_What_is_the_appropriate_way_to_estimate_it.

So, it is required that Tnonrad >> T rad. so that the rate by radiative recombination dominate.

The ETL must have high electron concentration to be good injector of electrons in the active layer. It must also transparent to the radiation from the active layer which requires that their bandgap be equal or higher than the active layer. The active ETL must also reflect the holes in the active region.

These are the properties required for the active and the ETL layer.

Best wishes