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Summary form only given. Because of its centrosymmetric structure, the poly(p-phenylene vinylene) (PPV) layer does not contribute to the SH signal, whereas the ITO-PPV and the PPV-aluminum interfaces do. A very interesting phenomenon can be observed if a reverse bias is applied to the diode. With increasing reverse bias, the SH intensity first decreases to a minimum value, and then it increases quadratically. For a maximum voltage of 20 V, the SH intensity is 25 times larger than when no bias is applied. We explain this behavior as follows: the total nonlinear susceptibility X/sub tot/ consists of two contributions: a constant part X from the ITO-PPV interface and a second part X(u) that results from the depletion layer at the PPV-aluminum Schottky contact and is similar to the effective voltage at the contact.