Researchers have identified a class of polymer with a disordered structure yet excellent semiconducting properties. The discovery could lead to the development of new flexible, high-quality displays for tablets and smartphones.
Researchers have sought semiconducting polymers for years that could be solution processed and printed, yet retain good electronic properties. However, such polymers typically show a disordered spaghetti-like internal structure rather than the ordered crystal lattice structure found in most electronic devices; this disorder slows electrons down. Now, researchers have identified polymers that, while disordered at the microstructural level, paradoxically still allow electrons to move quickly and freely (Nature, doi:10.1038/nature13854).
‘This class of polymer is very soluble, highly solution processable, printable in many different solvents, but somehow immune to the energetic disorder usually seen in processed polymers,’ says senior author Henning Sirringhaus, electron device physicist at the University of Cambridge, UK, who compares such disorder to hills and valleys in an energetic landscape crossed by electrons.
In the new class of polymers, it seems electrons can follow a flat path through this landscape and electrons move as freely as in expensive crystalline inorganic semiconductors.
‘The most astonishing aspect is that the material is not crystalline yet at the energetic level it is as ordered as inorganic semiconductors,’ says Sirringhaus. ‘The energetic fluctuations are about the same as for gallium nitride,’ the semiconductor used today in high-end LEDs.
Organic LEDs have recently become available in high-end phones and TV displays, replacing liquid crystal based displays. ‘There is a lot of interest in making these displays flexible,’ says Sirringhaus, ‘but in order to make them flexible you need a transistor technology that’s compatible with flexible plastic substrates, and organic transistors are in principle able to do that.’
Sirringhaus and colleagues compared several high-mobility conjugated polymers and pointed to a recently reported indacenodithiophene (IDTBT) semiconducting polymer as a standout (J. Am. Chem. Soc, doi: 10.1021/ja1049324). Calculations at Mons University, Belgium, suggested that the key to this amorphous polymer having a small degree of energetic disorder is the relatively low torsional twist of its backbone, which preserves pi-conjugation of its electronic orbitals.
‘In these polymers, we kind of achieve the conformation of graphene, but retain the processability that traditionally has been seen as an advantage in conjugated polymers,’ explains Sirringhaus.
‘The conclusion is that while there can be several types of structural disorder, they don’t all contribute equally to energetic disorder. Some types of structural disorder are relatively OK, some are not,’ observes material scientist C. Daniel Frisbee of the University of Minnesota, US, who says he is intrigued by the findings. ‘In polymers it may be more important to worry about the backbone configurations.’
The plan now is to plug the results into molecular design guidelines for a wider class of disorder-free conjugated polymers, which would open up a new range of flexible electronic applications, such as more lightweight, flexible and robust displays for smartphones and tablets.