Charge-recombination processes in oligomer- and polymer-based light-emitting diodes: A molecular picture

An overview of our recent work on the mechanisms of singlet and triplet exciton formation in electroluminescent π-conjugated materials will be presented. According to simple spin statistics, only one-fourth of the excitons are formed as singlets. However, deviations from that statistics can occur if the initially formed triplet charge-transfer (CT) excited states are amenable to intersystem crossing or dissociation. Although the electronic couplings between the CT states and the neutral exciton states are expected to be largest for the lowest singlet and triplet excitons (S¹ and T₁, respectively), the possibility for direct recombination into T₁ is always very small due to the large exchange energy. In small molecules, spin statistics is expected to be observed because both singlet and triplet exciton formations proceed via higher-lying S_n/T_n states with similar electronic couplings and fast formation rates. In extended conjugated chains, however, that the ¹CT → S₁ pathway is faster while the ³CT→ T_n channels become much slower, opening the route to intersystem crossing or dissociation among the ³CT states.