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

Abstract

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%+0.5×75%=62.5%. The estimates of maximum EQE of the fluorescent OLEDs should be revised to at least 0.2×62.5%=12.5% and, likely, even higher to account for optical outcoupling exceeding 0.2.

Triplet–triplet annihilation in highly efficient fluorescent organic light-emitting diodes: current state and future outlook

“…Because the magnetic field only affects the singlet channel [34], it can be used to examine the competition between the two annihilation channels and determine their relative contributions to TTA. Analysis of time-resolved electroluminescence with and without the magnetic field yielded a 1.3 branching ratio, which is substantially higher than the value of 0.33 expected for a purely statistical annihilation process [29]. Interestingly, the branching ratio obtained in this study of magnetic field effects was in excellent agreement with the observed fraction of TTA-related emission in total electroluminescence.…”

“…However, the observed electroluminescence can be readily understood without invoking any new mechanisms if one notices that the recombination leading to the triplet state of rubrene is not forbidden by energy, and, furthermore, the rubrene triplets readily undergo TTA to generate the emissive singlet state. In fact, by using a similar heterojunction in doped OLEDs resembling highly efficient OLEDs [29], we have been able to construct red-emitting devices operating relatively efficiently at similarly low voltages and demonstrate that their electroluminescence is dominated by the delayed component that shows all typical tell-tale signs of TTA (T. D. Pawlik, D. Y. Kondakov 2009, unpublished data).…”

“…In fact, the high endothermicity of quintet state formation prevents annihilation of triplet pairs that would otherwise lead to molecular quintets for many organic molecules, including anthracene and tetracene derivatives [29,35]. For example, in the case of rubrene, the density functional calculations show that formation of the lowest molecular quintet state from two triplets is deficient in energy by more than 2 eV [29].…”

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

“…In this case, the yield of singlet emissive states is highly dependent on the relative energy order of the excited singlet and triplet energy levels in the molecule, and the maximum total singlet yield is limited to 62.5%. OLEDs with TTA contribution as large as 40% have been demonstrated but the relative merit of this approach is still unsatisfactory [6,7].…”

“…In this case, the yield of singlet emissive states is highly dependent on the relative energy order of the excited singlet and triplet energy levels in the molecule, and the maximum total singlet yield is limited to 62.5%. OLEDs with TTA contribution as large as 40% have been demonstrated but the relative merit of this approach is still unsatisfactory [6,7].The TADF mechanism uses the thermal energy to assist reverse intersystem crossing and promote the up-conversion of lower triplet states into more energetic but emissive singlet states [8]. The efficiency of the TADF mechanism is controlled by two main parameters, the energy splitting between the singlet and triplet states ( E ST ) and the efficiency of non-radiative pathways available for the excited singlet and triplet states to decay [9,10].…”