Excitons dissociation efficiency

The improved order in films cast from toluene may also be induced by prolonged illumination at low temperatures, as recently discovered at the Technion [155]. Delocalized photoexcitations among adjacent chains may lead to several characteristic properties that are not common in isolated chains. These are [71] (i) reduced PL quantum efficiency, (ii) PL redshift, (iii) relatively large generation of PP excitations, rather than intrachain excitons, (iv) more substantial delayed PL due to PP recombination, (v) reduced formation efficiency of triplet excitons since the intersystem process is hampered by the interchain delocalization [156], and (vi) increased exciton dissociation efficiency in Ceo" doped polymer films. It is thus important to review the ultrafast excitation dynamics in MEH-PPV in comparison with those of DOO-PPV. 

Fig. 4 Exciton dissociation efficiency dotted line), charge collection efficiency dashed line), and internal quantum efficiency in blend structures of different lengthscales (quantified in terms of heterojunction interfacial area per unit volume). Modified with permission from [10],...

Fig. 6 Left Two-dimensional slices through representative structures with diffuse interfaces (Cl), pure domains with diffuse interfaces (DII) and pure domains with abrupt interfaces (SII). Right KMC predictions of short-circuit current density (top), exciton dissociation efficiency (middle), and carrier collection efficiency (bottom) for the Cl (circles), DO (triangles), and SII (squares) morphologies. Adapted from [18] by permission of The Royal Society of Chemistry...

Arkhipov VI, Heremans P, Bassler H (2003) Why is exciton dissociation so efficient at the interface between a conjugated polymer and an electron acceptor Appl Phys Lett 82 4605... 

The PF formalism treats the electron-hole pair dissociation as a one step carrier escapes from the Coulomb field of the countercharge due to external field-assisted thermal activation over the barrier. The two steps in the formation of a free carrier pair, as shown in Fig. 48, are undistinguishable. In other models of the exciton dissociation process, these two steps are separable, and the overall quantum efficiency expressed by the product... 

Surface-enhanced exciton dissociation models are based on the assumption that the absorption of a photon creates an exciton that diffuses to the surface where it dissociates into a free electron-hole pair, or a free and deeply trapped carrier of opposite sign, through an interaction with a donor or acceptor center associated with the surface. Such a process was first proposed by Lyons (1955). Evidence for this was largely based on early studies of anthracene, where the photogeneration efficiency increases with the absorption coefficient. Further evidence was that gases, particularly O2, adsorbed on the surface of anthracene crystals significantly change the photoconductivity, even for weakly absorbed radiation. 

Depending on the ratio of the diffusion length to the thickness and absorption depth, the variation of the photogeneration efficiency with the absorption coefficient can be predicted. Surface-enhanced exciton dissociation arguments have been widely invoked to describe photogeneration in organic materials, particularly where the efficiency is dependent upon the ambient atmosphere. Evidence for surface-enhanced exciton dissociation is usually inferred from the relationship of the absorption spectrum and the spectral dependence of the photogeneration efficiency. 

The radiative decay of singlet excitons is clearly an important process in the operation of polymer LEDs. This rate is denoted by kr, where for PPV, (k, rl 1200 ns.19 Radiative decay competes with various nonradiative decay processes, such as quenching of excitons by defects, exciton dissociation, and intersystem crossing to form triplet states. Assuming that both radiative and nonradiative decays are monoexponential, the photoluminescence quantum efficiency, PLeff, defined as the number of photons emitted per photon absorbed, is given by... 

The leitmotifs of these devices include bespoke dye sensitisers, space-quantised nanoscale structures that enable hot carrier or multiple exciton generation, molecular and solid-state junction architectures that lead to efficient exciton dissociation and charge separation, and charge collection by percolation through porous or mesoscale phases. Another common theme underlying the devices discussed in this book is the... 

Within the bulk heterojimction, the donor and acceptor domains are generally disordered in volume. For exciton dissociation and charge generation a fine nanoscale intermixing is required, whereas for the efficient transport of charge carriers percolation and a certain phase separation are needed to ensure imdisturbed transport. Hence the optimization of the nanomorphology of the photoactive blend is a key issue for improving the efficiency of the photovoltaic operation [62,66,67]. 

---