Surface-Enhanced Exciton Dissociation

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. 

From Fick s Second Law, the rate of change of the exciton density, dn(x)/dt, in any interval dx will be equal to the number of excitons created minus those lost by diffusion and by spontaneous decay. Hence, 

The photogeneration efficiency 77 is assumed to be proportional to the surface exciton density, hence 

For thick samples, where the exciton diffusion length and absorption depth, 1/a, are much less than the thickness, I = 10,L i, and L 1/a. Under these conditions, exp(L/ ) exp[ - (LI i)], and Eq. (5) becomes 

a plot of lh] versus Ha should be linear with an intercept-to-slope ratio of , the exciton diffusion length. For the case where 1/a , Eq. (7) becomes 

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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. 

In addition to surface-enhanced exciton dissociation and geminate recombination, direct photoexcitation (Northrop and Simpson, 1956 Heilmeier et al., 1963 Harima et al., 1989) and exciton-exciton annihilation (Silver et al., 1963 Jortner et al., 1963 Braun, 1968 Johnston and Lyons, 1968 Foumy et al., 1968 Swenberg, 1969 Braun and Dobbs, 1970 Orlowski and Scher, 1983) arguments have been proposed. In direct photoexcitation, a free electron and free hole are created without the involvement of intermediate states. With the exception of the work of Harima et al. and Orlowski and Scher, there have been few references to direct or exciton-exciton photogeneration processes in the past one and a half decades. 

In materials where the photogeneration involves the surface-enhanced dissociation of an exciton, as is generally the case for the phthalocanines, the photogeneration efficiency defined by Kanemitsu and Imamura represents the fraction of photons that create exitons that diffuse to the interface between the generation and transport layers. The injection efficiency then represents the fraction of pairs that dissociate into free electrons and free holes. The field dependence of the photogeneration efficiency was described by the Onsager theory. A primary quantum yield of 0.50 was reported. Values of the thermalization distance and the injection efficiency were not cited. 

The existence of exciton states in polymers such as CuPA has not been considered in detail in the literature, but it seems reasonable to suppose that efficient energy transfer can occur along the polymer chains. The absorption transition has been attributed to the formation of a charge transfer state and it is therefore possible that exciton dissociation is enhanced by the local electric field. Alternatively, exciton dissociation may occur at the polymer surface, with the electron being transferred to an acceptor such as molecular oxygen. The reaction scheme... 

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