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

M. Wohlgenannt, Kunj Tandon, S. Mazumdar, S. Ramasesha, Z. V. Vardeny

Research output: Contribution to journalArticlepeer-review

466 Scopus citations

Abstract

Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, σS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, σT; that is, σST ≈ 1. However, recent experimental and theoretical work suggests that σST may be greater than 1. Here we report direct measurements of σST for a large number of π-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of σST on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated π-electrons, and quantify this process using exact valence bond calculations. The calculated σST reproduces the experimentally observed trend. The calculations also show that the strong dependence of σST on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.

Original languageEnglish (US)
Pages (from-to)494-497
Number of pages4
JournalNature
Volume409
Issue number6819
DOIs
StatePublished - Jan 25 2001

ASJC Scopus subject areas

  • General

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