The orientation of the TDM of the emitters, which impacts the light out-coupling efficiency of the device, is correlated with the EQE of the OLED. Due to the rigid and planar structure of TRZ linked to extended donors, a number of TRZ-based compounds have been documented to show preferential horizontal orientation of their TDM, leading to enhanced light outcoupling efficiency in the device, with EQEmax > 30%. The parameters controlling the orientation of the TDM during thermal evaporation remain unclear and this is clearly a design feature that can be exploited further.