Feb 1 2016
A team of researchers at Umeå University have demonstrated, for the first time, the possibility of an efficient charge transport in semiconducting polymers, by controlled chain and crystallite orientation. These pioneering results, while enhancing polymer charge transport by over 1,000 times, can influence organic opto-electronic devices, and were published in a recent issue of Advanced Materials.
Vertically aligned chains in the organic semiconducting polymer inside microscopic patterns (Credit: Umeå University).
Conjugated semiconducting polymers (plastic) have excellent optical and electronic properties, making them attractive for the production of organic opto-electronic devices; such as photovoltaic solar cells (OPVs), lasers, and light emitting diodes (OLEDs).
Polythiophene polymers, including poly(3-hexylthiophene) P3HT, have been among the most researched semiconducting polymers because of their strong optical absorbance, and their easy processability into a thin film from solution. In OPVs and OLEDs, charges must be moved in the vertical (out of plane) direction inside the polymer film.
However, till recently the vertical charge carrier movement of organic semiconductors, i.e. the ability to move inside the material, has been very low for effecting rapid charge transport in electronic devices. Faster charge transport occurs all along the polymer chain backbone. A method for producing high mobility in the vertical direction and controlled chain orientation has not been successful till recently.
In the work, conducted at Umeå University, a team of materials scientists and chemists, headed by Professor David R. Barbero, have found a new process to align chains vertically, and produce effective transport of electric charges in the chain backbone. In this study, high charge transport and high mobility were obtained, without chemical doping, which is usually used to improve polymer charge transport.
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. 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.
David R. Barbero, Umeå University
The charge transport was determined using nanoscopic electrical measurements, and provided a movement averaging 3.1 cm2/V.s, which is the maximum mobility to be measured in P3HT, and is close to a theoretical estimation of the highest mobility in P3HT. Molecular packing characterization of the polymer and crystallinity performed by synchrotron X-ray diffraction at Stanford University’s National Accelerator (SLAC) confirmed that the high movements measured were a result of the re-orientation of the crystallites and polymer chains, leading to quick charge transport all along the polymer backbones.
These results could enable the production of better and efficient organic electronic devices (OPVs, OLEDs, lasers) with vertical charge transport, using an easy and cost-effective method, and avoiding chemical modification of the polymer.