Determination of the charge transport mechanisms in ultrathin copper phthalocyanine vertical heterojunctions

Bulky organic semiconductors have been widely applied on a variety of devices including transistors, sensors, and organic light-emitting diodes. Recently, the capability of producing stable ultrathin organic semiconductor-based junctions has opened the possibility of a variety of novel device concepts, including high-speed organic transistors, organic spin valves, and biosensors. In such context, the investigation of the charge transport mechanisms across ultrathin organic semiconductors is the key for the engineering of emerging organic-based technologies. Here, the charge transport mechanisms across heterojunctions based on physisorbed ultrathin copper phthalocyanine on gold are precisely determined and controlled over a wide range of temperatures and electric fields. We observe that the macroscopic electrical characteristics of Au/CuPc/Au heterojunctions are similar to what has been reported for chemisorbed molecular junctions. For instance, the transition from thermally activated transport to tunneling is verified regardless of the nature of the molecule-contact bonding. The Au/CuPc/Au heterojunction transport is dominated by charge localization sites at high temperatures and, upon cooling, a continuous transition from direct tunneling, via resonant tunneling, to field emission takes place by increasing the voltage bias. Such a continuous transition has not been reported for a hybrid metal/organic heterojunction yet. We have also determined the dielectric constant of the CuPc molecular layer via transport measurements, which allowed us to infer the possible molecule arrangements between the electrodes. © 2014 American Chemical Society.