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Wednesday, 26 Jul 2017

Umeå University develops method that enables ultra fast charge transport in polymers

Their results, which enhance charge transport in polymers by more than 1,000 times, have implications for organic opto-electronic devices

28 Jan 2016 | Editor

A research team at Umeå University, led by Professor David R. Barbero, have recently published research that the researchers say shows, for the first time, that a very efficient vertical charge transport in semiconducting polymers is possible by controlled chain and crystallite orientation. Their results, which enhance charge transport in polymers by more than 1,000 times, have implications for organic opto-electronic devices and were recently published in the journal Advanced Materials.

The team of chemists and materials scientists have found a new method to align chains vertically and to produce efficient transport of electric charges through the chain backbone. In this new study, moreover, high charge transport and high mobility were obtained without any chemical doping, which is often used to artificially enhance charge transport in polymers.

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

These results may open up the route to produce more efficient organic electronic devices with vertical charge transport (e.g. OPV, OLED, lasers etc.), by a simple and inexpensive method, and without requiring chemical modification of the polymer.

The researchers pointed out that :

  • Conjugated semiconducting polymers (plastic) possess exceptional optical and electronic properties, which make them highly attractive in the production of organic opto-electronic devices, such as for instance photovoltaic solar cells (OPV), light emitting diodes (OLED) 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 a thin film from solution
  • In both OPVs and OLEDs, charges must be transported in the out of plane (vertical) direction inside the polymer film
  • However, until now the vertical charge carrier mobility of organic semiconductors, i.e. the ability of charges to move inside the material, has been too low to produce fast charge transport in electronic devices
  • Faster charge transport can occur along the polymer chain backbone - but a method to produce controlled chain orientation and high mobility in the vertical direction has remained elusive until now
Umeå University - Vertically aligned chains in the organic semiconducting polymer inside microscopic patterns

Figure: Umeå University - Vertically aligned chains in the organic semiconducting polymer inside microscopic patterns

Professor David R. Barbero at Umeå University, said, "The transport of electric charge is greatly enhanced solely by controlled chain and crystallite orientation inside the film. The mobility measured was approximately one thousand times higher than previously reported in the same organic semiconductor."
David, added, "We believe these results will impact the fields of polymer solar cells and organic photodiodes, where the charges are transported vertically in the device. 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."

Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment

Vasyl Skrypnychuk | Gert-Jan A. H. Wetzelaer | Pavlo I. Gordiichuck | Stefan C. B. Mannsfeld | Andreas Herrmann | Michael F. Toney | David R Barbero

Advanced Materials | DOI: 10.1002/adma.201503422

Abstract

A method to produce highly efficient and long range vertical charge transport is demonstrated in an undoped polythiophene thin film, with average mobilities above 3.1 cm2 V−1 s−1. These record high mobilities are achieved by controlled orientation of the polymer crystallites enabling the most efficient and fastest charge transport along the chain backbones and across multiple chains. The significant increase in mobility shown here may present a new route to producing faster and more efficient optoelectronic devices based on organic materials.

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