Temperature-independent field-induced charge separation at doped organic/organic interfaces:: Experimental modeling of electrical properties

We examine field-induced charge-carrier separation at doped organic/organic heterointerfaces consisting of tetrafluorotetracyanoquinodimethane doped 4,4('),4(')-tris(N-1-naphtyl-N-phenylamino)-triphenylamine as hole-transporting layer and Li-doped 1,3,5-tri(phenyl-2-benzimidazole)-benzene as electron-transporting layer. Low-temperature I-V characteristics, thickness-dependent I-V characteristics, and Kelvin probe measurements are used to model the energy-level alignment at the interface. No explicit temperature dependence is observed. Thickness-dependent I-V characteristics and Kelvin probe measurements give evidence for a 5-nm-thin depletion layer adjacent to the interface. Consistent with our experimental results, we propose a model of electrons tunneling through the depletion zone from the highest occupied molecular orbit of the hole-transporting material to the lowest unoccupied molecular orbit of the electron-transporting material. This generates an electron-hole pair, which dissociates under the intense electric field close to the interface.