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Deciphering electrical conductivity in doped organic semiconductors

Organic semiconductors enable the fabrication of large-scale printed and mechanically flexible electronic applications and have already successfully established themselves on the market for displays in the form of organic light-emitting diodes (OLEDs). In order to break into further market segments however, improvements in performance are still needed.

NE spoke to Dr. Frank Ortmann, senior researcher at Dresden University of Technology (TU) to find out more about TU’s work in deciphering electrical conductivity in doped organic semiconductors.

“Large-scale electronic applications can be achieved by printing technologies such as roll-to-roll printing because many organic semiconductors are solution processable, i.e. they are dissolvable in printing inks and so on,” he explains.

“Other organic semiconductors can be evaporated in ultrahigh vacuum, which is also possible on a large scale. The films can be made flexible because the organic semiconductor films are not as rigid as for example they are in crystalline silicon. This is due to the weaker bonding between the organic molecules in the film, making the film flexible. The layers can be coated on flexible foils made for example of PET. Another advantage is the lower weight of large area organic electronics because the organic films can be made very thin (around 100 nm or even lower in some cases).”

However, he adds, due to the weaker chemical bonding and often more disordered structure, organic semiconductors usually have a lower electrical conductivity and charge carrier mobility compared to crystalline silicon.

This means that right now the organic electronics are often slower than conventional electronics.

Dr. Ortmann gives the examples of organic transistors having a lower switching frequency, and organic solar cells, which have a lower power conversion efficiency. “This is however continuously increasing,” he adds. “If these disadvantages in performance can be overcome, organic electronics can reach much more application for example in solar cells or medical applications (many organic semiconductors are biocompatible).”

According to TU researchers, doping is the answer. In semiconductor technology, doping refers to the targeted introduction of impurities (also called dopants) into the semiconductor material of an integrated circuit. These dopants function as intentional "disturbances" in the semiconductor that can be used to specifically control the behaviour of the charge carriers and thus the electrical conductivity of the original material.

Dr Ortmann explains how: “In an undoped organic semiconductor, the number of charge carriers (e.g. electrons) that contribute to a current is very low. The dopant molecules can introduce additional charges to the organic semiconductor film, which increases the conductivity.

“Furthermore, doping can control the type of majority carriers (negatively charged electrons or positively charged holes) and the properties of charge injection or extraction between different thin films. In technical terms, it can control the electrochemical potential of the films.”

Even the smallest amounts of these "disturbances" can have a very strong influence on electrical conductivity. Molecular doping is an integral part of the majority of commercial organic electronics applications. So far, an insufficient fundamental physical understanding of the transport mechanisms of charges in doped organic semiconductors has prevented a further increase in conductivity to match the best inorganic semiconductors such as silicon.

“The conductivity in doped organic semiconductors varies over many orders of magnitude between different materials,” Dr Ortmanm says. “Up to now, it was not fully understood, why these differences are so large. In particular, there were not many design rules available which helped to synthesize new doping or host molecules.”

Researchers from the Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and the Center for Advancing Electronics Dresden (cfaed) at TU Dresden, in cooperation with Stanford University and the Institute for Molecular Science in Okazaki, have now identified key parameters that influence electrical conductivity in doped organic conductors.

“In our study,” he continues, “we now provide a comprehensive picture of the electrical conductivity by comparing different molecular and thin film parameters of a large variety of material combinations. This will help to perform a more targeted synthesis of new molecules.”

By introducing dopant molecules into organic semiconductors it creates complexes of two oppositely charged molecules. The properties of these complexes like the Coulomb attraction. This is the electrostatic attraction between a negatively charged molecule and a positively charged molecule. One of the molecules is the dopant molecule.

The density of the complexes significantly determine the energy barriers for the transport of charge carriers and thus the level of electrical conductivity.

“To have charge transport, the negative and positive charges in these complexes have to be separated and energy is needed for this,” says Dr. Ortmann. “The electrostatic attraction (binding energy) between the positively and negatively charged molecules in these complexes tends to prevent this splitting. A higher density of these complexes might improve or impede this splitting and thus affects the conductivity.”

The identification of important molecular parameters constitutes an important foundation for the development of new materials with even higher conductivity.

He continues: “The outcome of this study is a result of several years of fundamental research on doped organic semiconductors. Only the comparison of many different materials and the observation of systematic trends and correlations between different parameters led us finally to the complete picture.”

As for the next step, Dr. Ortmann points to developing new dopant molecules on the basis of the finings to achieve larger conductivities.

“This might help to considerably increase the performance of many organic electronic devices. A part of the findings is also relevant to other materials which are for example used in the active/absorbing layers of organic solar cells.

The ultimate goal is that the results will aid development of new absorbing molecules for organic solar cells with improved performance. In addition, the team hopes the findings on these small molecules should be also relevant for other material classes such as polymer semiconductors.

He concludes: “We hope that our study will motivate new scientific studies in these other material classes.”

Bethan Grylls

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