Temperature Dependence of the Exciton-Exciton Annihilation Rate Constant in Poly(DI-N-Hexylsilane)
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*Sandia National Laboratories, Albuquerque, NM 87185 "**Department of Chemistry, Princeton University, Princeton, NJ 08544
ABSTRACT
Fluorescence depolarization studies of polysilane chains in solution have shown that energy transfer along the polymer chains occurs for only a very short time relative to the excited state lifetime and only over short distances before the excited states become trapped in long, low-energy segments. However, in solid films we have shown in previous work that excitons are highly mobile throughout their 600 ps lifetime at room temperature, presumably because energy transfer among neighboring, parallel chain segments becomes possible. In this paper we report that the exciton-exciton annihilation rate constant decreases by only a factor of five between room temperature and 12 K, showing that the excitons do not become trapped even at low temperatures. INTRODUCTION
With the advent of organic light emitting diodes, interest in those phenomena which might limit the quantum efficiency for fluorescence in organic solid films has increased markedly. Exciton-exciton annihilation, in which two singlet excitons interact to create one highly excited state which nonradiatively decays to a single exciton, is one of those phenomena. In previous work we have shown that this process is very efficient in polysilanes, a-conjugated polymers with an all silicon backbone.
The absorption spectra of polysilanes consist of broad, featureless bands which, it is generally assumed, result from absorption by random length, ordered, all-trans segments on the order of 20 silicon atoms long, which are separated by undefined, conformational chain defects1' 2. The fluorescence spectra are narrower and on the low energy side of the absorption band. These observations have led to the conclusion that energy is transferred from shorter, higher energy segments to longer, lower energy segments before fluorescence occurs 2. Fluorescence depolarization experiments on molecules in solution 3 have shown that this energy transfer occurs only on a very short time scale,
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