PhD ceremony Mr. G.A.H. Wetzelaer: Charge transport and recombination in organic-semiconductor diodes

PhD ceremony: Mr. G.A.H. Wetzelaer

Dissertation: Charge transport and recombination in organic-semiconductor diodes

Promotor(s): prof. P.W.M. Blom

Faculty: Mathematics and Natural Sciences

Organic semiconductors form an interesting class of materials, because they combine the versatility and processability of organic materials and the electrical properties of inorganic semiconductors. Organic light-emitting diodes (OLEDs) and solar cells are examples of organic-semiconductor diodes. In this research, properties of charge transport and recombination in such organic diodes are investigated. It is demonstrated that defect sites that hinder charge transport, the so-called charge traps, also play a significant role in charge recombination. This trap-assisted recombination process typically does not result in the emission of a photon and is therefore a loss mechanism in OLEDs. However, charge trapping effects can be avoided when using specifically-designed organic semiconductors with deep energy levels. Due to the use of such materials in organic solar cells, charge transport is frequently trap free in those devices and trap-assisted recombination is shown to be absent in most cases, which is beneficial for the solar-cell performance. Furthermore, the recombination rate of free carriers in organic solar cells is demonstrated to be reduced up to several orders of magnitude as compared to the expected recombination rate, which also positively affects the electrical output power. Contrary to expectations, diffusive transport of charge carriers in organic semiconductors follows the classical behavior according to the Einstein relation, despite the large energetic disorder typically present in organic semiconductors. As a result, this finding can simplify the description of the electrical behavior of organic-semiconductor devices.