Origin of Sub-Bandgap Electroluminescence in Organic Light-Emitting Diodes

Sub-bandgap electroluminescence in organic light emitting diodes is a phenomenon in which the electroluminescence turn-on voltage is lower than the bandgap voltage of the emitter. Based on the results of transient electroluminescence (EL) and photoluminescence and electroabsorption spectroscopy measurements, it is concluded that in rubrene/C-60 devices, charge transfer excitons are generated at the rubrene/C-60 interface under sub-bandgap driving conditions, leading to the formation of triplet excitons, and sub-bandgap EL is the result of the subsequent triplet-triplet annihilation process.