Charge exci tons

Besides charge transfer interactions, dipolar coupling between ttk transitions of bases may lead to delocalization of the excited states. In order to obtain some guidelines for our experimental studies, we have undertaken the calculation of excited Frank-Condon states within the framework of the exci-ton theory [26]. These studies were enriched by combining data from quantum chemistry and molecular dynamics calculations in collaboration with Krystyna Zakrzewska and Richard Lavery [26,27,27-29]. The general formalism is described in the Chapter by E. Bittner and A. Czader in the present volume. 

The commonly observed larger scale of phase separation of the toluene cast MDMO-PPV PCBM blends has generally been interpreted as the main reason for the reduced photocurrents in comparison to those of the chlorobenzene cast blends. It can then be expected that a lower charge carrier generation efficiency may result when exciton diffusion lengths of 10-20 nm are well exceeded by the PCBM cluster size (200-500 nm), since many exci-tons are generated within these clusters. Experimentally, it has been identified that indeed some unquenched photoexcitations give rise to residual PCBM photoluminescence in toluene cast blends, whereas in chlorobenzene cast... 

FlG. 11.3. The steady state number of CTEs (ni) occupying the donor-acceptor interface and the number of dissociated pairs (52) as a function of the pumping intensity. The pumping intensity S is equal to the number of charge-transfer exci-tons produced at the interface during a CTE lifetime in the absence of dissociation processes. The results are from the numerical simulations of the CTE system described in the text. Reprinted with permission from Kiselev et al. (20). Copyright Elsevier (1998). 

In this equation, y is the charge equilibrium factor, is the probability of exci-ton formation from one electron-hole pair, and jjpl is the quantum yield for the photoluminescence. 

The effect of fusion on the electronic transitions of the nitrate ion in the sodium to cesium salts is in contrast to that with the halides in that the aforementioned n tr transition shifts to the blue (by 0.06 to 0.32 ev) (Rhodes and Ubbelohde, 1959 Cleaver et al, 1961). However, the transition involved would appear to be fundamentally different from the exci-ton charge transfer with the halides in that here charge density is moved away from, rather than toward, the cations. Thus the blue shift on fusion seems qualitatively very reasonable in view of the general decrease in distances at this point. The effect is qualitatively quite analogous to the result of a decrease in cation size on the same nitrate transition which was just considered. A more detailed understanding of the effect of fusion on transitions of the nonspherical nitrate ion is lacking further experimental and theoretical investigations of this and related problems would seem very desirable. 

---