Concentration dependence of the efficiency and lifetime of blue organic light-emitting diodes with a dispersed fluorescent emitter

The performance dependence of blue organic light-emitting diodes (OLEDs) on the concentration of a fluorescent emitter doped in a wide bandgap host was studied. As the concentration of the 4,40-bis(9-ethyl-3-carbazovinylene)-1,10-biphenyl (BCzVBi) dopant was increased from 5 wt%, the external quantum efficiency (EQE) increased from 1.5% and reached the peak value of 2.1% at 15 wt%. Above 15 wt%, the OLEDs suffered from concentration quenching and exhibited two regimes of fast then slow efficiency falloff. The fast falloff resulted from reduced energy transfer as more holes were injected into the dopant, as indicated by the spectral change and lower voltage, whereas the slow falloff occurring above 50 wt% was due to enhanced exciton interaction in aggregated dopant, as evidenced by the dark spot formation. Our study also revealed an extended lifetime of blue fluorescent OLEDs with a host-dopant system emitter, an important benefit over pure fluorescent emitters.