The theory of thermally activated delayed fluorescence for organic light emitting diodes

Abstract

The interest in organic molecules exhibiting thermally activated delayed fluorescence (TADF) has been reinvigorated in recent years owing to their potential to be exploited as emitters in highly efficient purely organic light emitting diodes (OLEDs). However, designing new molecules that exhibit efficient TADF is a non-trivial task because they would appear to require the optimisation of a number of contrasting properties. For example these molecules must exhibit rapid conversion between the singlet and triplet manifolds without the use of heavy elements to enhance spin-orbit coupling. They should also display a large fluorescence rate, but simultaneously a small energy gap between low lying singlet and triplet states. Consequently to achieve systematic material design, a detailed understanding of the fundamental factors influencing the photophysical behaviour of TADF emitters is essential. Towards achieving this goal, theory and computation is playing a crucial role. In this feature article the recent progress in the theory of organic TADF molecules in the context of OLEDs is presented, with a view of achieving a deeper understanding of these molecules and driving systematic material design.

The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence

“…7 using the kinetic equations developed in ref. 29. There is a clear correlation showing that molecules with high TADF efficiency measured by the DF/PF ratio in Fig.…”

“…This ability to reorganise derives from an elongation of the D-A bond distance in the excited state, which lowers the barrier between the two conformers. 29 For 2 and 5 the scenario is similar. However, the emissive CT state shifts to higher energies due to the methyl substituent groups.…”

“…26,27 Additionally, a small DE ST is not the only important parameter for RISC. Recent literature highlights the importance of spin-vibronic coupling, 28,29 and mixing of charge transfer and localized electronic states to promote RISC. 15,26,30,31 The motivation for the present study comes from literature reports of a series of phenothiazine (D) and dibenzothiophene-S,S-dioxide (A) TADF molecules where the donor is substituted with side bulky groups, aiming to force near D-A orthogonal geometry.…”

Environmental Control of Triplet Emission in Donor–Bridge–Acceptor Organometallics

“…The result is that the photoluminescence peak of CMA1 can be increased in energy by 210 meV, into the blue color range, without altering its chemical structure. We determine that static electrostatic interactions are one of the most important parameters for these composites, whereas for organic TADF‐type emitters, the influence of the host is typically to redshift the emission through dynamic polarizability . Despite the significant change in emission energy achieved, we find that the low activation energy for delayed emission and short room‐temperature emission lifetimes are preserved.…”

“…We determine that static electrostatic interactions are one of the most important parameters for these composites, whereas for organic TADF-type emitters, the influence of the host is typically to redshift the emission through dynamic polarizability. [16][17][18][19] Despite the significant change in emission energy achieved, we find that the low activation energy for delayed emission and short room-temperature emission lifetimes are preserved. We use these new experimental findings to test current quantum-chemical descriptions of CMA emission, and provide a better understanding of its mechanism.…”