Berlin 2005 – scientific programme
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CPP: Chemische Physik und Polymerphysik
CPP 34: Novel materials II
CPP 34.5: Talk
Wednesday, March 9, 2005, 12:15–12:30, TU C230
Spin-conserving carrier recombination in conjugated polymers — •M. J. Walter1, M. Reufer1, P. G. Lagoudakis1, U. Scherf2, J. M. Lupton1, and J. Feldmann1 — 1Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität, Munich, Germany — 2FB Chemie, Universität Wuppertal, Gauss-Str. 20, Wuppertal, Germany
The question about the maximum internal quantum efficiency η achievable in polymeric LEDs is strongly debated but still unresolved. As generally only singlets contribute to the electroluminescence, η should be limited to 25 % according to simple spin statistics. While several publications suggest that this upper bound could be surpassed, little is known about the crucial exchange energy of exciton precursor polaron pairs (PPs) that plays a major role in the respective argumentation of increasing η.
By applying an external electric field we can store PPs over relatively long time scales and spectroscopically probe their spin-state by the use of a novel class of phosphorescent polymeric material [1]. During the electrostatic PP storage effective mixing of singlet- and triplet-PPs cannot be observed even at room temperature, suggesting an astonishingly strong exchange interaction within the PP. In the absence of fast spin-lattice-relaxation within the coulombically bound state, η>25 % appears unlikely.
[1] J. M. Lupton et al., Phys. Rev. Lett. 89, 167401 (2002).