Excitons diffusion

Using a variety of transient and CW spectroscopies spanning the time domains from ps to ms, we have identified the dominant intrachain photoexcitations in C )-doped PPV films. These are spin-correlated polaron pairs, which are formed within picoseconds following exciton diffusion and subsequent dissociation at photoinduced PPV+/Cw> defect centers. We found that the higher-energy PA band of polaron pairs is blue-shifted by about 0.4 eV compared to that of isolated polarons in PPV. 

B.W. D Andrade, M.E. Thompson, and S.R. Forrest, Controlling exciton diffusion in multilayer white phosphorescent organic light emitting devices, Adv. Mater., 14 147-151 (2002). 

The second type of solar cell is based on a /m-heterojunction in analogy to semiconductor devices [274]. Excitons generated by light, diffuse and dissociate at the interface between a hole and an electron-conducting material. The optimum layer thickness was calculated to be 1.5 times the exciton diffusion length [275]. 

An important question concerning energy trapping is whether its kinetics are limited substantially by (a) exciton diffusion from the antenna to RCs or (b) electron transfer reactions which occur within the RC itself. The former is known as the diffusion limited model while the latter is trap limited. For many years PSII was considered to be diffusion limited, due mainly to the extensive kinetic modelling studies of Butler and coworkers [232,233] in which this hypothesis was assumed. More recently this point of view has been strongly contested by Holzwarth and coworkers [230,234,235] who have convincingly analyzed the main open RC PSII fluorescence decay components (200-300 ps, 500-600 ps for PSII with outer plus inner antenna) in terms of exciton dynamics within a system of first order rate processes. A similar analysis has also been presented to explain the two PSII photovoltage rise components (300 ps, 500 ps)... 

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

This exciton diffuses to the donor/acceptor interface via an energy-transfer mechanism (i.e., no net transport of mass or charge occurs). (3) Charge-transfer quenching of the exciton at the D/A interface produces a charge- transfer (CT) state, in the form of a coulombically interacting donor/acceptor complex (D A ). The nomenclature used to describe this species has been relatively imprecise, and has... 

Stubinger T, Bruiting W (2001) Exciton diffusion and optical interference in organic donor-acceptor photovoltaic cells. J Appl Phys 90 3632... 

Lunt RR, Giebink NC, Belak AA, Benziger JB, Forrest SR (2009) Exciton diffusion lengths... 

Luhman WA, Holmes RJ (2009) Enhanced exciton diffusion in an organic photovoltaic cell by energy transfer using a phosphorescent sensitizer. Appl Phys Lett 94 153304... 

The 7 constant (in cm3s-1) of mobile excitons is directly related to the exciton diffusion constant D 7 = S-nDR, (in cm2s-1). 

The kinetics captured in disordered systems like polymers, glasses and poly-cristalline structures has been often described in terms of continuous relaxation times and exciton diffusion at recombination centers [10]. Assuming a <5— pulse function, the temporal data are best fitted by a monomolecular kinetic equation,... 

Fig. 5.5. Luminescence quenching (bullets, right hand, axis) and short-circuit current Ac (black squares, left hand axis) vs. molar fullerene concentration in a bulk hetero junction composite. The different onsets for percolation for the two phenomena (exciton diffusion versus ambipolar carrier transport) can be clearly seen...

We can now couple our calculated monchromatic standing wave to the exciton diffusion process, as a source term in the diffusion equation for excited states. In this way we can obtain the photocurrent from the device under the given assumptions. 

Such bilayers were studied by photoluminescence, with a view to extracting the degree of photoluminescence quenching induced by a thin acceptor layer on the polymer [18]. Studies of photoluminescence quenching in bilayers have corroborated the picture derived from studies of photoelectrical performance. The short exciton diffusion length in PEOPT is 5 nm, consistent with both PL quenching and photodiode performance. 

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