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/Alq(3) (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 (< 5 mA/cm(2)) and high (> 10 mA/cm(2)) 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. (c) 2007 American Institute of Physics.