Manifestation of Charge Recombination Mechanisms in Electroluminescence of Organic Solids

Abstract

The electroluminescence (EL) emanated from organic light-emitting-diodes (LEDs) is demonstrated to provide quantitative information concerning recombination mechanisms in organic solids. Based upon the experimentally observed non-monotonic dependence of the quantum EL yield (ϕEL) on applied electric field (F) for molecularly-doped polymer hole transporting layer and Alq3 emitter consisting LEDs, it is shown that while the low-field regime increase of ϕEL(F) can be explained in the framework of the commonly used Langevin formalism, the high-field (F>106 V/cm) decrease in ϕEL(F) requires to take into account a finite capture time of the carriers, which being comparable with the carrier motion time prior to the ultimate recombination step suggests the Thomson-like recombination to operate in organic solids. The electron-hole (CT) pair appears to be a plausible precursor of the ultimate recombination product (a localized neutral state). This is the branching ratio between its recombination decay (mutual carrier capture) and strongly field-dependent dissociation into free carriers that decides about ϕEL at high electric fields. It also accounts for the field value for which ϕ(F) reaches its maximum. This paper is dedicated to Prof. Edgar Silinsh's Memory. He had continuously challenged the organic solid state community and his insights into electronic properties of molecular crystals will he kept in mind as having important impact on the organic solid state international community.