Switching in organic devices caused by nanoscale Schottky barrier patches.

We have identified a possible electronic origin of metal filaments, invoked to explain the switching behavior of organic devices. Interfaces of two representative organics polyparaphenylene (PPP) and poly(2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylenevinylene) with Ag are investigated using ballistic emission microscopy. Nanometer scale spatial nonuniformity of carrier injection is observed in ballistic electron emission microscopy images of both interfaces. The measured Schottky barrier (SB) appears to be consistent with metal states tailing into the gap of the PPP. We find that the SB values exhibit a distribution, even for the diodes with low ideality factors. The implications of this distribution on the measured physical properties of the diode are discussed, in light of work on devices of similar geometry, published in the literature. We also demonstrate that patches of low SB are likely to nucleate current filaments which can cause local ionization and are reported to be responsible for the switching behavior observed in metal-organic, metal-CuS and Ag-AgSe structures.