Monte Carlo simulations of geminate-pair dissociation in discrete anisotropic lattices

Abstract

Abstract Field- and temperature-assisted dissociation of carrier pairs produced by the absorption of radiation has been simulated for perfectly isotropic (3D) and perfectly linear (1D) systems composed of a discrete array of hopping sites, as well as for systems with anisotropic hopping rates parallel and perpendicular to a preferred direction. For the 3D and 1D systems the exact solutions of the Onsager formalism are recovered. In neither case is Poole—Frenkel behaviour observed. The dissociation yield increases with increasing separation a of the hopping sites. The governing parameter is ΔU/kT, where ΔU, which is a function of a, is the difference in potential energy between the sites at which a carrier starts its random walk and the nearest-neighbour site in the direction of the field. Agreement with experimental data for organic solids indicates that the assumption of field-independent hopping rates implicitly contained in both simulations and analytic theory is justified. Anisotropy of the carrier transport coefficient has a marked effect on the zero-field dissociation yield. On the basis of the computer results, an analytical expression is proposed that relates φ(E = 0) and μ|/μ⊥. The effect of finite sample size on the zero-field yield is also treated quantitatively.