A schematic diagram of triplet-triplet annihilation (TTA) and the mechanism by which it could increase EQEEL. The highlighted processes are: 1) TTA in which two triplet excitons (T1) on neighbouring molecules interact, resulting in one molecule returning to the ground state (S0) and the other being excited to the singlet state (S1). 2) The S1state decays radiatively to the ground state, emitting a photon. 3) The S1state forms a1CT state at an interface between donor and acceptor regions. 4) The1CT state decays radiatively to the ground state, emitting a photon. TTA is hypothesised to increase EQEELas it increases the fraction of excited states which can decay radiatively (processes 2 and 4) by forming a bright S1state from two, non-radiative T1states (process 1).
