In amorphous organic semiconductors in which electron or hole transport is due to hopping in an exponential density of states (DOS), the mobility is proportional to $n^b$, where $n$ is the carrier density and where $b$ increases with increasing width of the DOS. Exact analytical expressions are given for the steady-state and frequency-dependent current density in single-carrier metal/semiconductor/metal devices, based on such materials. For $b⪢$ 1, the cross over frequency between a conductive and capacitive ac response is shown to be much larger than the inverse steady-state carrier transit time. The relevance to the analysis of the ac small-signal response in small-molecule and polymer layers, such as are used in organic light-emitting devices, is discussed.

• Received 1 March 2006

DOI:https://doi.org/10.1103/PhysRevB.73.233306