Electron correlation effects in electron-hole recombination in organic light-emitting diodes

Abstract

We develop a general theory of electron-hole recombination in organic light emitting diodes that leads to formation of emissive singlet excitons and nonemissive triplet excitons. We briefly review other existing theories and show how our approach is substantively different from these theories. Using an exact time-dependent approach to the interchain/intermolecular charge-transfer within a long-range interacting model we find that, (i) the relative yield of the singlet exciton in polymers is considerably larger than the 25% predicted from statistical considerations, (ii) the singlet exciton yield increases with chain length in oligomers, and, (iii) in small molecules containing nitrogen heteroatoms, the relative yield of the singlet exciton is considerably smaller and may be even close to 25%. The above results are independent of whether or not the bond-charge repulsion, X ⊥ , is included in the interchain part of the Hamiltonian for the two-chain system. The larger (smaller) yield of the singlet (triplet) exciton in carbon-based long-chain polymers is a consequence of both its ionic (covalent) nature and smaller (larger) binding energy. In nitrogen containing monomers, wavefunctions are closer to the noninteracting limit, and this decreases (increases) the relative yield of the singlet (triplet) exciton. Our results are in qualitative agreement with electroluminescence experiments involving both molecular and polymeric light emitters. The time-dependent approach developed here for describing intermolecular charge-transfer processes is completely general and may be applied to many other such processes.

Magnetic-Field Dependence of the Electroluminescence of Organic Light-Emitting Diodes: A Competition between Exciton Formation and Spin Mixing

“…This is in agreement with the claim of Segal et al that in PPV the S fraction is lower than 25% [14]. The result appears to be at odds with theoretical considerations about a faster formation rate of S than T excitons [24][25][26], based on different ionic characters and/or energies of S and T excitons. It is our assumption, however, that the step decisive for the final S fraction occurs in the very early stage of exciton formation, when these considerations may not yet apply.…”

Theory of singlet exciton yield in light-emitting polymers

“…There is also a contribution to the rates from the overlap of the relative-particle wavefunctions, which again are smaller in the triplet manifold, because the triplet exciton has a smaller particle-hole separation and has more covalent character than its singlet counterpart 18 . As a consequence, the inter-conversion rate in the triplet manifold is significantly smaller than that of the singlet manifold, and indeed it is comparable to the ISC rates.…”

Spin-dependent polaron recombination in conjugated polymers

“…In the case of the SE, however, the larger exchange energy (energy difference between SE and TE) results in a lower energy difference between the incident polarons and the SE, which facilitates the direct recombination into SE state. 8,9 Simulations were also carried out for a number of different electric field strengths. It was found that the electric field also affected the polaron recombination processes, but in different ways for singlet and triplet states.…”

“…[1][2][3][4][5][6] A considerable amount of theoretical work focused on understanding the mechanisms underlying exciton formation in conjugated polymers has also been reported. [7][8][9][10][11] So far, however, a commonly agreed theoretical picture is not available (for a review, see Ref. 12).…”