  Charge injection from electrodes into doped organic films is a widespread
technology used in the majority of state-of-the-art organic semiconductor
devices. Although such interfaces are commonly considered to form Ohmic
contacts via strong band bending, an experiment that directly measures the
contact resistance has not yet been demonstrated. In this study, we use a
simple metal/doped organic semiconductor/metal stack and study its
voltage-dependent resistance. A transport layer thickness variation proves that
the presented experiment gains direct access to the contact resistance of the
device. We can quantify that for an operating current density of 10mA/cm2 the
investigated material system exhibits a voltage drop over the metal/organic
interface of about 200mV, which can be reduced by more than one order of
magnitude when employing an additional injection layer. The presented
experiment proposes a simple strategy to measure the contact resistance between
any metal and doped organic film without applying numerical tools or elaborate
techniques. Furthermore, the simplistic device architecture allows for very
high, homogeneous, and tunable electric fields within the organic layer, which
enables a clear investigation of the Poole-Frenkel effect.
