Analysis of universal trap states in organic semiconductors

Organic semiconductors are promising materials for LEDs, solar cells, field-effect transistors or sensors. Although these materials have been studied thoroughly, a lack of understanding the origin of trap states prohibit the widespread use.

In recent years, trap states caused by oxygen and water are discussed to be universal for all organic semiconductors and the reason for lower electron mobility compared to holes. However, this assumption is based on simple voltage current measurements at unipolar devices and theoretical simulations. For the simplified description of the devices, a single trap level is assumed. Furthermore, a direct measurement of the energetic trap depth distribution is not done yet. Thus, it is still unclear, if these oxygen and water dependent universal trap states have a major influence on the charge carrier transport in organic semiconductors.

For the investigation of the universal trap states, a direct measurement of the unipolar energetic trap distribution is done in this thesis. For this purpose, the well-known thermally stimulated current method is modified by using a metal insulator semiconductor structure instead of a diode. The measured trap distributions are correlated to the charge carrier mobility determined with the transient space charge limited method. This method is challenging for semiconductor thin films because of the need for a low RC time constant for sample charging. A measurement circuit is developed here to improve the limits of this method by reducing this RC time constant. Additionally, a device simulation is done to analyze the physical limits of this method in terms of minimum measurable lm thickness.

The well-known materials P3HT, PCPDTBT and MDMO-PPV in pristine and controlled aged (with oxygen and water) states are analyzed. The measurement of the unipolar trap distributions and charge carrier mobilities of these materials allows for analyzing the suggested universal trap levels and its influence on the charge carrier transport.



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