Characterization of triplet-triplet annihilation in organic light-emitting diodes based on anthracene derivatives

We report that the steady-state electroluminescence in organic light-emitting diodes (OLEDs) based on anthracene derivatives has a substantial contribution from annihilation of triplet states generated by recombining charge carriers. For the OLED devices of the following general structure: indium tin oxide∕N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine∕9,10-bis(2-naphthyl)-2-t-butylanthracene∕Alq3 (tris(8-hydroxyquinolate)aluminum)∕LiF∕Al, triplet-triplet annihilation contributes as much as 3%–6% of the overall electroluminescence. The intensity of triplet-triplet annihilation-related emission strongly varies with the current density and pulse width, being quadratic and linear functions of current density at low (<5mA∕cm2) and high (>10mA∕cm2) current density regimes, respectively. We find that quenching by charge carriers is the dominant decay process for the triplet states under a wide range of operating conditions, yielding triplet-state lifetimes from tens to hundreds of microseconds. The decrease in charge-carrier concentrations through improved injection and transport may be expected not only to lower operational voltage but also to enhance triplet-triplet annihilation and, consequently, overall electroluminescence efficiency.