Excitons mobility

A more discriminating picture of the role of traps in triplet exciton mobility can be drawn from the work by Webber (64, 65) who studied the kinetics of triplet quenching using a quencher molecule with a short lived triplet state (biacetyl) as a probe of the triplet exciton lifetime. In a study on P2VN (64), PACN (64) and PVCA (65) it has been shown that trapping is more efficient in PVCA compared to P2VN where triplet excitons exist long after the decay of delayed fluorescence (next section). 

The quenching of triplet excitons on poly(A -vinylcarbazole) by biacetyl has also been examined and delayed fluorescence (20—50 ns) observed indicating some exciton mobility occurs even after triplet-triplet annihilation has become insignificant. " An exciton-trapping model is used to describe the results. Further... 

The fundamental observable of exciton mobility and energy conduction in molecular crystals is sensitised fluorescence (compare Fig. 6.20). One irradiates a host crystal H, for example anthracene, which has a very low doping or impurity concentration of a guest G, for example tetracene, with light that can be absorbed by the host... 

There is no theory or rationalization to relate pol3mier molecular structure and triplet exciton mobility and/or self-trapping. So far for three different cases (P2VN, PAcN and PVCz) three different types of behavior have been observed. 

URL http //link.aps.org/doi/10.1103/PhysRevB.47.16631 Hillmer, H., Forchel, A., Sauer, R. Tu, C. W. (1990). Interface-roughness-controUed exciton mobilities in GaAs/Alo.syGao.eaAs quantum wells, Phys. Rev. B 42 3220-3223. 

Setz, P Knochenmuss, R. Exciton mobility and trapping in a UV-MALDI matrix. J. Phys. Chem. A 2005, 109, 4030-4037. 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

As the number of eigenstates available for coherent coupling increases, the dynamical behavior of the system becomes considerably more complex, and issues such as Coulomb interactions become more important. For example, over how many wells can the wave packet survive, if the holes remain locked in place If the holes become mobile, how will that affect the wave packet and, correspondingly, its controllability The contribution of excitons to the experimental signal must also be included [34], as well as the effects of the superposition of hole states created during the excitation process. These questions are currently under active investigation. 

Reasonably good electron transport mobility (/re > 10-5 cm1 2 3/(V s)). This aids transporting electrons to the emitter layer and efficiently confines the exciton in the EML. 

Richter, C. Schmuttenmaer, C. A., Exciton-like trap states limit electron mobility in Ti02 nanotubes. Nat. Nanotechnol. 2010,5 769-772. 

---