Electro-reflectance study of low-voltage turn-on in triplet fusion OLED materials

Coherent spin processes like singlet-fission (SF) and triplet fusion (TF) are gaining in interest to the scientific community, as SF could lead to an increase in performance of organic photovoltaics and TF in the emission efficiency of OLED devices. Recently reported results have suggested that higher order effects such as TF lead to a reduction of the luminescence turn-on voltage and is necessary for what was referred to as “sub-band gap turn-on”. However, we have shown that heterojunction band alignment can describe the low voltage turn-on. In this presentation we will present electro-reflection (ER) studies that elucidate the built-in potentials in 2 prototypical OLED devices based on Rubrene and DiFTES-ADT as the emitting layer. The results are put into context of the low luminescence turn-on, which is observed in both cases, and potential higher order effects. Furthermore, differences in the luminescence - current density-voltage-characteristics, L-J-V and magneto-electro-luminescence, MEL, between the two systems will be presented. A kinetic model captures zero field splitting and various exciton and charge carrier recombination pathways.