  Investigation of the interfacial electronic properties of
N,N'-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bisdicarboximide
(PDI8-CN2) organic semiconductor films grown on silicon dioxide is performed by
polarization-resolved second harmonic generation optical spectroscopy, pointing
out a spatial region where charge carriers distribution in the semiconductor
lacks inversion symmetry. By developing a model for nonlinear susceptibility in
the framework of Debye-Huckel screening theory, we show that the experimental
findings can be interpreted as resulting from the presence of a net charge
localized at the silicon dioxide, accompanied by a non-uniform charge
distribution in the organic semiconductor. Photoluminescence analysis further
reinforces this scenario. Reduction-oxidation reactions involving PDI8-CN2 and
water molecules are invoked as physical origin of the localized charge. The
work outlines a sensitive tool to probe the total charge localized at buried
semiconductor/dielectric interfaces in organic thin-film transistors without
resorting to invasive contact-based analyses.
