This image shows the vertically aligned chains in the organic semiconducting polymer. Image: Umeå University.
This image shows the vertically aligned chains in the organic semiconducting polymer. Image: Umeå University.

A research team at Umeå University in Sweden has shown, for the first time, that very efficient vertical charge transport in semiconducting polymers can be achieved by controlling the orientation of the polymer chains. These pioneering results, which offer a way to enhance charge transport in polymers by more than 1000 times, have implications for the development of organic opto-electronic devices and were recently published in Advanced Materials.

Conjugated semiconducting polymers possess exceptional optical and electronic properties, making them highly attractive for the production of organic opto-electronic devices such as photovoltaic solar cells (OPV), light emitting diodes (OLEDs) and lasers. Polythiophene polymers such as poly(3-hexylthiophene; P3HT) have been among the most studied semiconducting polymers due to their strong optical absorbance and ease of processing into thin films from solution.

Until now, however, the vertical charge carrier mobility of semiconducting polymers, meaning the ability of electrical charges to move inside the material, has been too low to produce fast charge transport in organic opto-electronic devices. Scientists knew that faster charge transport could occur along the polymer chain backbone, but methods to produce controlled chain orientation and high mobility in the vertical direction has remained elusive until now.

In the current work, a team of chemists and materials scientists, led by David Barbero at Umeå University, has developed a new method for aligning polymer chains vertically to produce efficient transport of electric charges through the chain backbone. This high charge transport and high mobility were also obtained without any chemical doping, which is often used to enhance charge transport in polymers.

"The transport of electric charge is greatly enhanced solely by controlled chain and crystallite orientation inside the film," explains Barbero. "The mobility measured was approximately 1000 times higher than previously reported in the same organic semiconductor. "

"We believe these results will impact the fields of polymer solar cells and organic photodiodes, where the charges are transported vertically in the device," he adds. "Organic-based devices have traditionally been slower and less efficient than inorganic ones (e.g. made of silicon), in part due to the low mobility of organic (plastic) semiconductors. Typically, plastic semiconductors, which are only semi-crystalline, have hole mobilities about 10,000 times lower than doped silicon, which is used in many electronic devices. Now we show it is possible to obtain much higher mobility, and much closer to that of silicon, by controlled vertical chain alignment, and without doping."

The charge transport was measured using nanoscopic electrical measurements, and gave a mobility averaging 3.1cm2 per volt per second, which is the highest mobility ever measured in P3HT and is close to theoretical estimates of the maximum mobility in P3HT. Characterization of the crystallinity and molecular packing of the polymer was performed by synchrotron X-ray diffraction at Stanford University's National Accelerator (SLAC). This confirmed that the high mobilities were due to the re-orientation of the polymer chains and crystallites, leading to fast charge transport along the polymer backbones.

This story is adapted from material from Umeå University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.