Organic semiconductors that operate through the conduction of positive charges are the first choice for use in printable electronic circuitry. A device that uses electrons instead has just joined the rankings.
Transistors, the semiconductor electronic switches at the heart of any integrated circuit, come in two main flavours: p-type transistors, which switch on when a negative voltage is applied to the device's control electrode (gate); and n-type, which switch on with a positive gate voltage. Most silicon integrated circuits make use of a combination of the two types to produce complementary circuits. Such circuits can achieve higher levels of performance and yield, and lower power consumption, than can circuits made from only one type of transistor. The field of organic electronics aims to use as the semiconductor material — instead of inorganic silicon — organic molecules and polymers that can be processed from solution. That would allow large-area electronic circuits to be manufactured on flexible, plastic substrates using low-cost printing techniques1,2. Although some p-type organic transistors have the required chemical and physical properties to produce such printable electronic circuitry, researchers have so far failed to find materials for equivalent n-type devices that would offer a comparable level of performance. On page 679 of this issue, Yan et al.3 describe a polymer material that achieves this feat, allowing printed complementary circuits to be produced that have unprecedented performance.
In a semiconductor, whether it is organic or inorganic, two types of charge carrier convey the electrical current: extra, negatively charged electrons that are injected into the empty electronic energy levels of the neutral ground state of the semiconductor, and 'missing' electrons that are removed from one of the normally occupied states. The latter are called holes, and behave as if they were carrying a single positive electron charge. n-Type (n for negative) organic transistors rely on the injection of extra electrons into the lowest unoccupied molecular orbital (LUMO) of the molecules of a thin film of organic semiconductor. Electron injection is achieved by applying a positive voltage to the gate electrode, which is separated from the semiconductor film by a thin gate dielectric (a nonconducting material) (Fig. 1a). p-Type (p for positive) transistors rely on inducing holes in the highest occupied molecular orbital (HOMO) through the application of a negative gate voltage (Fig. 1b).
Because the electronic structure of the HOMO and LUMO states is similar, it should, at least in principle, be possible to have both n-type and p-type transistors in a single organic semiconductor. And indeed, a few years ago it was found that many organic semiconductors are inherently capable of both types of device operation when constructed with a suitable gate dielectric4.
The main challenge to producing technologically useful n-type organic transistors is to ensure adequate operational stability when the device is exposed to the atmosphere. This requires specially designed materials in which the energy of the molecules' LUMO states are stable enough to ensure that the extra electrons do not react with or chemically reduce atmospheric oxygen, water or other electronegative impurities5 (those that have the ability to attract electrons towards themselves); otherwise, these extra electrons would be lost as current-carrying mobile charges in the device. Organic semiconductors6 capable of stable n-type device operation are anything but scarce, but most are small organic molecules that are not sufficiently soluble in common organic solvents to be suitable for print-based manufacturing. Polymer semiconductors are useful in this respect, but to date no polymer has shown a stable n-type performance and processability comparable to those of the best p-type polymer transistors.
The new material reported by Yan et al.3 is a donor–acceptor co-polymer. It consists of alternate units of an electron-rich bithiophene donor linked to an electron-deficient naphthalenecarboxydiimide (NDI) acceptor (Fig. 1a). The latter provides the necessary stabilization of the LUMO level. Some of the donor–acceptor polymers reported previously are poorly soluble7, presumably because strong intermolecular interactions occur between donor and acceptor units on adjacent polymer chains. But the authors demonstrate that the new polymer is highly soluble in common organic solvents, and is compatible with inkjet, flexographic and gravure printing. Yan and colleagues' transistor has a 'figure of merit' — indicating the mobility of the charge carriers, which determines how much current flows in the semiconductor in response to the applied gate voltage — of 0.4–0.8 cm2 V−1s−1. This value is comparable to that of the best p-type polymer transistors8.
The NDI-based co-polymer is similar to a previously reported co-polymer that was based on perylenecarboxydiimide and bithiophene, but that was much less efficient9. Yan and colleagues' work thus illustrates how subtle variations in chemical structure can have a major influence on the electronic properties of a material. What's more, because the polymer is compatible with the gate dielectrics commonly used for p-type transistors, complementary circuits can be created. The authors put that into practice and demonstrate the feasibility of complementary logic inverters — devices that turn a high input voltage into a low input voltage, and vice versa.
Yan and colleagues' work is a major advance in the quest for printed complementary logic circuits. These can find applications in automatic object-identification techniques, such as electronic barcodes and radio-frequency identification tagging10,11. It remains to be established whether the long-term environmental and operational stability of the NDI polymer matches that of the best p-type devices. If it does, the material might also find use in applications such as flexible displays, for which p-type organic transistors are currently the first choice because of their high operational stability. The new material will undoubtedly inspire further work to synthesize other donor–acceptor polymers with even stronger stabilization of the LUMO level.
But the authors' work is not only of interest from a practical perspective. It will also help to refine our understanding of how the physics of electron transport and injection in high-mobility polymer semiconductors relates to that of holes. Electrons in polymer semiconductors are catching up with holes in the fast lane.
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Advanced Functional Materials (2012)