The electric field at the hole-injecting metal/organic interface controls the bias dependence of the current–voltage hole mobility

Abstract

Abstract Based upon the room temperature current–voltage data of some published organic diode structures the unique phenomenon of the decreasing hole mobility, μ, with the increasing applied electric field, E a, is interpreted. The measurable quantity, the hole drift mobility μ d is formulated in terms of E a and the electric field at the hole injecting metal/organic interface, E int, dependent algebraic function multiplied by the intrinsic hole mobility, μ max that is organic morphology dependent but E a independent scaling factor. On account that the intrinsic mobility, μ max, is uncoupled from both E a and E int it is shown that the origin of the negative field hole mobility effect occurs due to E int, that is a linear function of E a. The bias and the space distribution of the internal organic electric field, E, as well as the free hole density, p, for poly(3-hexylthiophene) is calculated in detail. Depending on the organic layer morphology the internal electric field may exhibit, at the particular value of E a, a deep well in the vicinity of the hole injecting metal/organic interface. Then the strong peak of the free hole density exists there the effect of which is spreading some 10 nm into the organic. If E int happens to be E a independent constant, then from the resulting space charge limited current density, the increasing hole drift mobility, μ d, with the increasing applied electric field, E a, is deduced. The published current–voltage data of two distinct metal-substituted phthalocyanine thin films provide an additional confirmation of the described formalism.