Formation cross-sections of singlet and triplet excitons in pi-conjugated polymers

Electroluminescence in organic light-emitting diodes arises from a charge-t ransfer reaction between the injected positive and negative charges by whic h they combine to form singlet excitons that subsequently decay radiatively . The quantum yield of this process (the number of photons generated per el ectron or hole injected) is often thought(1) to have a statistical upper li mit of 25 per cent. This is based on the assumption that the formation cros s-section of singlet excitons, sigma (s), is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, s igma (T); that is, sigma (S)/sigma (T) approximate to 1. However, recent ex perimental(2) and theoretical(3) work suggests that sigma (S)/sigma (T) may be greater than 1. Here we report direct measurements of sigma (S)/sigma ( T) for a large number of pi -conjugated polymers and oligomers. We have fou nd that there exists a strong systematic, but not monotonic, dependence of sigma (S)/sigma (T) on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated pi -electrons, and quantify this process using exact valence bond calculat ions. The calculated sigma (S)/sigma (T) reproduces the experimentally obse rved trend. The calculations also show that the strong dependence of sigma (S)/sigma (T) on the optical gap is a signature of the discrete excitonic e nergy spectrum, in which higher energy excitonic levels participate in the charge recombination process.