Comparison of the Mobility–Carrier Density Relation in Polymer and Single-Crystal Organic Transistors Employing Vacuum and Liquid Gate Dielectrics
✍ Scribed by Yu Xia; Jeong Ho Cho; Jiyoul Lee; P. Paul Ruden; C. Daniel Frisbie
- Publisher
- John Wiley and Sons
- Year
- 2009
- Tongue
- English
- Weight
- 361 KB
- Volume
- 21
- Category
- Article
- ISSN
- 0935-9648
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✦ Synopsis
Charge mobility is a key figure of merit for many applications of organic semiconductors. In organic field-effect transistors (OFETs), higher charge mobility leads to larger ON-to-OFF current ratios and higher switching speeds, which enhances the utility of these devices for electronic circuitry. [1] It is generally appreciated that the carrier mobility in organic semiconductors correlates with the degree of structural order, for example, high crystallinity often (but not always) favors higher mobilities because the intermolecular electronic coupling can be greater than in amorphous or semicrystalline materials. [2] In addition, structural disorder leads to in-gap or ''band-tail'' states that can trap charge. For amorphous or semicrystalline polymer semiconductors, structural disorder is significant and the charge mobility is consequently a strong function of total carrier density. [3] Tanase et al. and Shimotani et al. have shown that the carrier mobility in polymer semiconductors can increase by several orders of magnitude as carrier density is increased. [4,5] The explanation is that at higher carrier concentrations, disorder-induced traps are filled to a greater extent, which increases the average mobility of the remaining carriers because they sample fewer and shallower traps.
In contrast, Morpurgo and colleagues reported that for OFETs based on high-mobility rubrene single crystals, the trend is opposite; [6] OFETs with higher gate-dielectric constants, and thus higher gate-induced charge densities, have lower field-effect mobilities, which they attributed not to carrier-density effects (at least for carrier densities <10 13 cm À2 ), but to coupling of the charges in the channel with the polarizable dielectric. The fundamental mobility-charge density relation is apparently very different for polymer semiconductors compared with smallmolecule single crystals. It is expected that, to a first approximation, this is because of the vastly different degrees of static disorder in the two systems.