Carrier concentration dependence of the mobility in organic semiconductors

Charge transport in organic materials as a function of carrier concentration is investigated. An analytical model of the concentration dependent mobility based on the variable hopping range theory is formulated. This model is applied to analyze the discrepancy between the experimental mobilities extracted from FETs and LEDs. The result shows that an exponential density of states (DOS) is a good approximation of the tail states for describing the charge transport in FETs. When applied to the low carrier concentration regime, for example to the LEDs regime, a Gaussian DOS should be assumed.