Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes

We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The nonstatistical branching ratio of the two annihilation channels is attributed to the favorable relationship between the energies of the excited singlet and triplet states of rubrene-emissive layer host. We surmise that, with the appropriate emissive layer materials, the fluorescent OLED devices are capable of using a considerably larger fraction of triplet states than was previously believed. In principle, the upper limit for the singlet excited state yield in the TTA process is 0.5, which makes the maximum internal quantum efficiency of fluorescent OLEDs to be 25%+0.5x75%=62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2x62.5%=12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.