Dear Sara Bazhair,

The answer is not so simple. The charge mobility in organics semiconductors represents only the effective value that includes effects of grain size, traps, electric field (the Poole-Frenkel effect), etc. This limitation does not allow us to make direct relation between the molecular structure and the electrical properties even though certain road-map of molecule design can be proposed.

Recently has been suggested “fused approach” in new material synthesis, in which thiophenes are connected at their C=C double bonds with other π-units in such a way that the two aromatic units share one C=C double bond. These molecules are also denoted as thienoacenes or heteroarenes and they are promising candidates for future high-mobility organic semiconductors. Organic transistors with unsubstituted DNTT and DATT exhibited mobility of 8.3 and 16.0 cm^2/Vs, respectively. These molecules are rod-like and straight. On the other hand, the N-shape molecules have been proposed also. The average mobility of decyl-substituted DNBDT (C10–DNBDT–NW) was about 12.1 cm^2/Vs, while the maximum mobility reached value of 16 cm^2/Vs.

The deposition technique cannot be omitted either. The proper growth of organic crystals can lead up to extremely high mobilities. Using off-centre spin-coating (see Yuan, Yongbo, et al. "Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method." Nature communications 5 (2014)) researcher achieved mobility up to 100 cm^2/Vs.

Because of the high-mobility materials there are new bottlenecks that is necessary to solve. One of them is the contact resistance, that limits the charge injection into the organic transistors. That is one of the reasons why we cannot fabricate small devices (with short channels). Proper design of materials (transport energy level of organic material must be in agreement with the work function of the metal electrode) as well as device structure is required for high performance devices. Regarding the device structure the vertical transistors are still very promising devices even though the fabrication technology is quite difficult and not very reproducible if it is done in low-cost way. The most important is to keep in mind that the mobility is not the only key parameter for transient behaviour, but the response time (or time-of-flight) must be suppressed. As a result, the mobility should be as high as possible, but also the channel or parasitic capacitance should be reduced.

Dear Martin Weis,

Thank you for the thoughtful answer you provided here, as I am new to the field, your answer seems to summarizes the most pressing issues in the investigations of mobility enhancement for OFET. 

as you suggested i visited the work of Yuan Yongbo, et. all, with the method of OCSC. It is a collection of various possible effects that could contribute to the achievement of high mobility in OFETs, it is also a great example of mobility enhancement through fabrication techniques as well as the use of blended materials.

would you know of any such works that were applied commercially?

Meanwhile i am also looking into some of the other methods you mentioned, I also noticed the repeated mention of trapping and injection of carriers through various papers, would you be able to shed some light as to the effects associated with these topics in organic FETs? and how they do/don't contribute to the mobility enhancement?

I'd appreciate it greatly if you could suggest some readings that could help me grasp the concepts and the research relevant.

Dear Sara Bazhair,

As far as I know at present time there is no real commercial application of organic transistors. Although there are spin-off companies such as Plastic Logic working on application of organic electronics (especially organic transistors), they did not achieve any significant success. Nevertheless, it is quite possible that some large companies will really apply it in short time since it can greatly reduce costs of display fabrication (you can omit TFT backplane made of amorphous silicon (a-Si:H) or low-temperature polycrystalline silicone (LTPS)).

Multiple charge trapping and release (MTR) mechanism is common approach in the discussion of the charge transport in materials with such great concentration of defects/traps. You can imagine it as localized states with very long life-time. Since the organic semiconductors have almost zero intrinsic charge concentration and no doping, almost all charges comes from the injection electrodes (except the case of photogeneration). Therefore the metal/organic interface is so important and the injection play a crucial role in the high performance devices. If you will reduce the injection barrier (usually represented by the contact resistance in the electrical equivalent circuit model) you will improve the charge concentration and enhance the effective charge mobility. Please note that the real charge mobility is not changed at all; however, the effective value includes many effects such as contact resistance or charge trapping. Hence, the effective mobility includes the real mobility together with all addition effects ((that we do not want to have).

once again, thank you Martin for clarifying the basics. I may come back with more relevant questions, hope to have more of your generous contribution in the future