Metal evaporation dependent charge injection in organic transistors

To illuminate a long-term remaining issue on how contact metallization (metal and speed) affects charge injection, we investigated top-contact pentacene transistors using two categories of metals deposited at various rates. Differing from previous studies such as those devoted to morphological influences by microscopy, in this work we concentrated on their electrical characteristics in particular combining the low-frequency noise which provided a direct quantity of trap density and its evolution with respect to contact metal and deposition rate. It turns out that the transistors with noble metal (Au) suffer from metal-diffusion related charge trapping in the pentacene bulk close to the Au/pentacene interface, and this diffusion-limited injection is greatly tuned from bulk to interface by speeding Au deposition which leads to a Schottky-like injection due to the severe thermal damage to the upper pentacene layer. Applying a conventional contacting metal (Cu), however, Ohmic contacts with much fewer traps are always observed regardless of metallization speed. This is attributed to an ultra-thin interlayer of Cu_xO that guarantees stable Ohmic injection by introducing gap states and protecting the pentacene film so that those transistors appear to be free from Cu metallization. Our results quantitatively show the limiting factors of charge injection for different metals and at various evaporation rates.