This work represents the first unambiguous confirmation that very large charge densities can be achieved on the surface of organic crystals by using electrolyte gates. The key result in this regard is clear detection of the Hall effect with the proper scaling with respect to magnetic field strength and current. From the Hall voltage, one determines the carrier (hole) density. Another “first” is that it has been possible to measure the organic crystal device down to 8 K. Typical single crystal OFETs cannot be cooled below 100 K without shattering the crystal. This is because the PDMS substrate contracts more on cooling than the organic crystal. We believe that insertion of ionic liquid underneath the rubrene crystal minimizes the stress on the crystal and allows it to survive to lower temperatures. The combination of very large carrier densities, good charge mobilities (up to 4 cm2/Vs) and low temperatures achieved here is exciting for ongoing work aimed at achieving a true gate induced insulator-to-metal transition in organic semiconductors. In fact, the IMT is very nearly achieved in this work.
Typical crystals are a few hundred microns thick and a few mm long. The crystals are laminated onto PDMS (rubber) stamps precoated with a thin layer of Ti/Au. The crystal sticks to the raised gold areas which then form electrical contacts. Ionic liquid is wicked into the gap between the rubrene crystal and the gate electrode. The capacitance of the ionic liquid is enormous, so that application of small voltages to the gate results in very large charge accumulation on the surface of rubrene in contact with the ionic liquid.