Abstract
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.
Highlights
Among the inorganic semiconductors, lithium fluoride has the largest bandgap ($14 eV (Ref. 1)) and the largest known negative electron affinity (À3 eV (Ref. 2))
For diodes with LiF layer exceeding 10 nm in thickness and bias voltages !þ4 V, we notice that the initial decay of the current density due to charging and polarization is followed by an increase in current density over time at times t > 1 s
Our experimental data indicate that under prolonged bias voltage stress of Al/LiF/poly(spirofluorene)/ Ba/Al diodes first electrons are accumulated at the polymer side of LiF/poly(spirofluorene) interface
Summary
Lithium fluoride has the largest bandgap ($14 eV (Ref. 1)) and the largest known negative electron affinity (À3 eV (Ref. 2)). Upon biasing the top Ba/Al electrode negative with respect to the bottom Al electrode, with a voltage exceeding the built-in voltage of about 1.5 V, viz., the difference between the work function of the bottom Al and the top Ba/Al electrode, we expect injection of electrons into the polymeric semiconductor (Fig. 1(c)). Specific interface related electronic states may be populated by applying a medium bias voltage below the bandgap of the alkali halide. We employ electro-optical methods to find further evidence for accumulation of charges near the LiF/ poly(spirofluorene) interface and estimate the doping density in the LiF layers
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