Rough electrode surface: Effect on charge carrier injection and transport in organic devices

The effect of electrode roughness on charge carrier injection and transport is considered. An explicit formula connecting the roughness profile of an electrode with the distribution of the electric field at its surface (and the electrostatic potential in the bulk of transport layer) is derived for the case of smooth roughness, when the typical height of roughness element is small in comparison with its size across the surface (this is a very typical situation). This formula gives us an opportunity to measure the electrode surface profile (e.g., by AFM) and then to calculate various injection properties of this particular electrode for any kind of injection. General properties of the electrode - organic layer interface in the case of significant (not smooth) roughness are considered and a suitable numeric procedure for the calculation of the surface electric field distribution is proposed. It is also shown that rough surface of electrodes generates an additional energy disorder in the bulk of transport layer. This principal result indicates that the electrode roughness affects not only carrier injection but carrier transport as well. Roughness-induced energetic disorder produces a channel-like structure in the vicinity of the electrode, thus providing separation of electrons and holes. Such separation should decrease the charge recombination rate and, hence, the emitting efficiency of light emitting devices. At the same time, the separation is favorable to solar cells.