Voltage dependent capacitance -- a measure of energy level bending in naphthalene-tetra-carboxylic- di-imide based transistors

We demonstrate transient capacitance measurements using charge extraction by a linearly increasing voltage (CELIV) on the small molecule naphthalene-tetra-carboxylic- di-imide (NTCDI) based organic transistors. The OFETs use Aluminum (Al) and Aluminum Oxide (AlO$_{x})$ as bottom gate and dielectric, with gold (Au) source and drain electrodes. The Al/AlO$_{x}$ gate is modified using two different self assembled monolayers, triethoxy(octyl)silane and perfluorooctyltriethoxysilane, in order to tune the turn-on voltage. We have clarified the voltage dependent capacitance in diode structures and found that when the transistor is in the fully on state a charge reservoir is formed at the AlO$_{x}$ interface and a saturation of the steady-state capacitance is seen, equaling the capacitance of the AlO$_{x}$ layer. When the transistor is in the fully off state the steady state capacitance saturates to the capacitance of the semiconductor bulk. We interpret this as a build-up of a charge reservoir in the semi conductor bulk when going from the off to the on state making it possible to charge the AlO$_{x}$ capacitance. By going from the on state towards the off state using a linearly increasing voltage pulse the dynamics of the depletion of the reservoir gives information about the energy level bending in the bulk.