Electrical conduction of LiF interlayers in organic diodes

An interlayer of LiF in between a metal and an organic semiconductor is commonly used to improve the electron injection. Here, we investigate the effect of moderate bias voltages on the electrical properties of Al/LiF/poly(spirofluorene)/Ba/Al diodes by systematically varying the thickness of the LiF layer (2-50 nm). Application of forward bias V below the bandgap of LiF (V < Eg ∼ 14 V) results in reversible formation of an electrical double layer at the LiF/poly(spirofluorene) hetero-junction. Electrons are trapped on the poly(spirofluorene) side of the junction, while positively charged defects accumulate in the LiF with number densities as high as 1025/m3. Optoelectronic measurements confirm the built-up of aggregated, ionized F centres in the LiF as the positive trapped charges. The charged defects result in efficient transport of electrons from the polymer across the LiF, with current densities that are practically independent of the thickness of the LiF layer.