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Energy relaxation dynamics and universal scaling laws in organic light‐emitting diodes
Author(s) -
Bittner Eric R.,
Karabunarliev Stoyan
Publication year - 2003
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.10754
Subject(s) - exciton , physics , dipole , singlet state , molecular physics , population , relaxation (psychology) , phonon , scaling , condensed matter physics , chemistry , atomic physics , excited state , quantum mechanics , psychology , social psychology , demography , sociology , geometry , mathematics
Electron–hole (e–h) capture in luminescent conjugated polymers (LCPs) is modeled by the dissipative dynamics of a multilevel electronic system coupled to a phonon bath. Electroinjected e–h pairs are simulated by a mixed quantum state, which relaxes via phonon‐driven internal conversions to low‐lying charge‐transfer (CT) and excitonic (XT) states. The underlying two‐band polymer model reflects PPV and spans monoexcited configuration interaction singlets (S) and triplets (T), coupled to Franck–Condon active CC stretches and ring torsions. Focusing entirely upon long PPV chains, we consider the recombination kinetics of an initially separated CT pair. Our model calculations indicate that S and T recombination proceeds according to a branched, two‐step mechanism dictated by near e–h symmetry. The initial relaxation occurs rapidly with nearly half of the population going into excitons ( S XT or T XT ), while the remaining portion remains locked in metastable CT states. While formation rates of S CT and T CT are nearly equal, S XT is formed about twice as fast T XT in concurrence with experimental observations of these systems. Further, breaking e–h symmetry suppresses the XT to CT branching ratio for triplets and opens a slow CT → XT conversion channel exclusively for singlets due to dipole–dipole interactions between geminate and nongeminate configurations. Finally, our calculations yield a remarkable linear relation between chain length and singlet/triplet branching ratio that can be explained in terms of the binding energies of the respective final excitonic states and the scaling of singlet–triplet energy gap with chain length. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003