6.1 Introduction

The carrier mobilities in organic semiconductors are typically low. If the injection barrier between the metal and the semiconductor is small and carriers can be efficiently injected into the device, charge transport in such devices can be described using the theory of space charge limited current (SCLC), which plays an important role in investigating the efficiency of charge injection in OLEDs and estimating parameters such as mobility and trap concentration in organic semiconductors [120,121]. The theory of SCLC in a trap-free dielectric was formulated by Mott and Gurney [122]. Later it was extended to account for SCLC controlled by shallow traps with exponential energy distribution [123]. However, it is generally accepted that the DOS is a Gaussian distribution, and the states in the tail act as trapped states [124].

In the following, we extend Campbell's work [124] to derive a single-carrier SCLC model for organic semiconductors with a Gaussian DOS distribution. A single carrier diode can be easily fabricated by choosing metals with appropriate work functions for the contact.