Elastic electronics: Rules for fabrication of ordered monolayers of semiconducting polymers have been established

Organic materials will change the face of electronics. Devices will become not only cheaper, thinner and lighter but will also gain unprecedented properties. It will be possible to roll a display into a tube or produce it from transparent elements and place directly on windows, for instance, in cars. However, before elastic electronics wins the mass market, the rules governing the fabrication of ordered thin layers of organic semiconductors must be learnt.

A group of scientists from the Institute of Physical Chemistry of the Polish Academy of Sciences (headed by Assoc. Prof. Robert Nowakowski) and the Warsaw University of Technology (Prof. Małgorzata Zagórska) and the Atomic Energy Commission in Grenoble (Prof. Adam Proń) has achieved considerable progress in this respect. "We have examined how the organisation of molecules changes within layers, depending on the length of molecules. Thanks to this we understand why shorter molecules form ordered two-dimensional structures, while their long analogues create chaotic aggregates. We can sometimes eliminate this last effect completely," says Prof. Robert Nowakowski from the Microscopy and Spectroscopy Research Group of the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS).

Organic molecules may conduct current as well as metals. However, in metals electron cloud can move in any direction whereas current carriers in organic molecules move along the so-called conjugated double bonds. This means that carriers are very mobile only in one direction: along the long axis of the molecule. In this situation the conductivity can be improved in layers consisting of very long molecules, that is by using high-molecular compounds -- polymers. However, this solution has a significant drawback. It is more difficult for high-molecular compounds (polymers) to create ordered layers. As a result they often arrange themselves randomly, which leads to the chaotic movement of charge carriers (the carrier, having passed through a long macromolecule of coil-like shape, may reappear nearly in the same place in which it started its journey). The chaotic structure leads to low charge carriers mobility. The problem described above can be solved through the use of molecules that are longer than typical organic molecules but in the same time short enough to show natural tendency to self-ordering, i.e. oligomers. As a result of mutual interactions such molecules arrange themselves into parallel rows.

At present it is assumed that in future, organic electronic systems will be made from ordered layers of molecules that guarantee high mobility of carriers in the direction specified for a given device. The optimisation of structure of organic semiconductor layers consists in finding a compromise between the length of an oligomer chain and its self-organisation ability. "Chemists from the Warsaw University of Technology prepared for us new polymers and oligomers, derivatives of thiophene. However, the structural and microscopic examination of thin layers of these compounds showed that they were disordered. We suspected that this disorder resulted from polydispersity, that is the coexistence of molecules of different lengths. This phenomenon occurs in almost all synthetic polymers," explains Prof. Nowakowski. In order to verify this assumption, scientists from the IPC PAS developed a unique method for the separation of a mixture after polymerisation into fractions of molecules of identical length. High performance liquid chromatography and thin layer chromatography were used for this purpose. Then monolayers were deposited on a graphite substrate from these fractions, and they were examined with the use of a scanning tunnelling microscope.

The assumption regarding polydispersity turned out to be correct. The ordering of molecules is connected with the existence of long and elastic alkyl groups introduced into a molecule to increase its solubility. The shortest molecules create two-dimensional structures in the layer as a result of mutual interaction (interdigitation) of alkyl groups of neighbouring molecules in two perpendicular directions. The elongation of a molecule increases the number of alkyl groups interacting only perpendicularly to its longitudinal axis and leads to the asymmetry of intermolecular interactions. This results in the change of the type of ordering from two-dimensional islands, observed for shorter oligomers, into one-dimensional columns created by longer oligomers. "It turns out that the chaos in the layers is caused by the fact that they are created from a mixture of macromolecules of various lengths, each of which aims at a different type of ordering," says a PhD student Tomasz Jaroch from the IPC PAS.

The ordering of molecules in a layer has its origin in their structure. Even a small change in the structure of a mer (a repeat unit from which a polimer or oligomer chain is made) may affect the self-organisation process. The group of Prof. Małgorzata Zagórska from the Warsaw University of Technology synthesised oligomers with alkyl groups attached to the carbon atoms of the thiophene ring in different positions as compared to the case of oligomers examined previously. This change results in a decrease of the distance between alkyl groups within the mer unit and consequently, a change in interactions between molecules in the layer. In compounds synthesised in this way no negative effects of self-organisation have been observed: molecules of different lengths created ordered two-dimensional islands. The ordered layers prepared in this way show good semiconducting properties because the cores interacting directly along the oblong axis guarantee an increase in the effective mobility of charge carriers. The researchers from the IPC PAS confirmed experimentally the lack of interdigitation of the alkyl groups in this direction by demonstrating on microscopic pictures that it was possible to move a single oligomer within the layer. The interdigitation of alkyl groups along the axis of a molecule would make such an operation impossible.

The results of the research have great practical significance since they allow predicting the behaviour of oligomers and polymers in layers, and consequently, they open the way to the creation of ordered layers which guarantee better mobility of charge carriers in organic electronics devices.