Schottky junction is the most popular model used in the junction between organic semiconductor and electrode. This model describes the charge flux due to thermal electron with the potential drop across the junction.
The junction has electrical DIODE behavior. An approximate function used in organic device is
J = J0 exp(-eV/nkT).
This function omits reversed current J0, and the contribution of the bulk resistance of the organic semiconductor.
One parameter n (n>=1) in the function is called ideal factor. It is a quantity used to describe how accurate the junction is close to ideal Schottky Junction. When its value is one, it is ideal Schottky junction. Farther away from one, more approximate the Schottky model. One side effect for large ideal factor n is that it leads to poor DIODE behavior. One indictor is Rectification ratio defined as r = IF(V)/IR(-V) Larger r, better diode. A large ideal factor results in poor diode performance.
For organic semiconductor, their ideal factor of the junction with electrode usually has a large value. It usually is greater that 3, sometimes, it reaches 10 or higher. For modeling purpose, if the ideal factor is too large (say n>4), the junction may not be well described by Schottky model. The other model(s) should be applied to consider the non-ideal effect.
In order to achieve that, we have to know what makes n value large (non ideal). There are a few reasons to cause this. The dielectric layer formed at the interface may be the most common reason. For example, the cell Al/Organic Semiconductor/ITO. Here Al is cathode electrode for OLED device. The Al can be oxidized by O2 to form Al2O3 that is a dielectric material. The source of O2 is from the leakage of air. Organic semiconductor usually has conjugated double C=C bond. It may form covalent bond with Al, which results in an insulator interface. Furthermore, the C=C bond can be oxidized by O2 or H2O impurity, which result in insulator layer.
Another popular cathode material is Ca whose work function is small to give good electron injection. Ca can be easily oxidized by O2 to form insulator CaO. This also results in large ideal factor for that junction.
Other factors such as ionic impurities may change ideal factor too. However, its mechanism is more complicated. It will not be covered here.
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