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

Abstract

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∕ Al q 3 (tris(8-hydroxyquinolate)aluminum)∕ Li F ∕ 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.