Examination of Au, Cu, and Al contacts in organic field-effect transistors via displacement current measurements

Displacement current measurements (DCM) on long-channel capacitors (LCCs) were used to examine carrier injection and extraction processes in organic field-effect transistors (OFETs). An LCC can be seen as an OFET with one channel contact removed and the conducting channel elongated to millimeter range. While carrier injection and extraction processes occur simultaneously in OFETs during traditional DC measurements, these two processes can be examined separately with LCCs and DCMs. In addition, the numbers of injected, extracted, and trapped carriers can be determined by integrating the displacement current with respect to time. Pentacene LCCs with Au, Cu, and Al contacts were fabricated. Surprisingly, it was found that more carriers were trapped in the Au devices than in the Cu devices even though the pentacene film and dielectric layers were essentially identical. We propose that carrier trapping in the long channel of the Au devices is indirectly caused by the deep trap states at the pentacene-dielectric interface in the contact region generated by Au penetration. In addition, ambipolar injection and transport were observed in an LCC with an Al contact and a PMMA buffer layer between pentacene and SiO ₂.