Electric Field Induced Electron Transfer between a Single-Walled Carbon Nanotube and a Molecularly Doped Hole Transport Layer

In this work, we report a novel, electric field induced hole injection reaction between thin films of single-walled carbon nanotubes (CNT) and a molecularly doped charge (hole) transport layer (CTL) comprising a hole transport molecule (TPD) doped in polycarbonate in the bilayer device configuration. The CNT bilayer device was shown to be able to charge and then discharge in the dark, whereas the controlled bilayer device, constructed by replacing the CNT film with a metal (TiZr) layer, remains charged up after charging. The discharge of the charged CNT bilayer device suggests that hole injection from the CNT film to the CTL (or electron transfer from the CTL to the CNT film) occurs and that the injected holes are neutralized by a series of isoenergetic electron transfer across the CTL. Results show that the rate of the initial discharge increases as the surface conductivity of the CNT film increases. Current–voltage and time-of-flight measurements suggest that the discharge process is limited by the mobility of the CTL when the surface resistivity of the CNT film is ≤250 ohm/sq. For bilayer devices formulated with more resistive CNT films, the discharge process is limited by the hole injection efficiency. Since conductive CNT films are an essentially dense network of carbon nanotubes on Mylar, the decrease in the hole injection efficiency for bilayer devices fabricated from more resistive CNT films can be attributed to the increase in porosity in the CNT network in these films. The increased porosity limits hole injection between the carbon nanotube and the CTL. Since micrometer size pixels of CNT film can be made on a flexible substrate by inkjet printing and a nano imprinting technique, we suggest to couple this electric field induced hole injection reaction with an active-matrix backplane for a novel digital printing application.