Expanding the organic electronics toolbox
In order to realise the full potential of organic electronics, devices must not only perform to the electrical specifications of the application of interest, but also have a suitable lifetime and be manufactured in an environmentally sustainable way.
The paper in Nature Communications is here: http://go.nature.com/2GDlKwa
An example of an organic electronic device is in complementary logic for example, which is required in applications such as RFID tags, and relies on circuitry with both p-type and n-type transistors. This in turn requires the availability of organic semiconductors which possess high charge carrier mobility in both the hole and electron transport regimes, good ambient stability, and can be manufactured by a sustainable synthesis. The current bottleneck in progress is that identification of organic semiconductors with the combination of high mobility electron transport and stability under ambient conditions has been an elusive goal and considerably less developed in comparison to p-type transport counterparts.
Semiconducting polymers are typically comprised of aromatic repeat units coupled together to preserve pi electron conjugation, thus creating bandgaps in the region of 1-3 eV. Increasing the conformational rigidity of the backbone has been shown to reduce energetic disorder, enhancing charge transport properties. We exploit this design concept by designing a coupling polymerisation reaction which creates fused isoindigo repeat units in a rigid conjugated framework, avoiding single bond formation, resulting in reduced backbone torsion and low reorganisation energy. Electron mobility was further optimised through tuning of both the size of the aromatic repeat units and the lengths of the solubilising side chains.
Due to the reducing power of typical negative polarons present in an operational device, reactivity with ambient water and oxygen is high, causing degradation of the organic polymers via hydroxylation and similar routes, thus limiting device stability and lifetime. One route to overcome this problem is by increasing the electron affinity of the semiconductor beyond the electrochemical threshold for such reactions to occur. In our case, the isoindigo repeat units, formed by a common bilateral aldol condensation reaction of aromatic bifunctional isatin and oxindole monomers, were electron deficient and thus a high electron affinity can be achieved.
With typical palladium-catalysed coupling reactions, joining two electron poor aromatic units is a challenge due to the low reactivity of the organometallic species in the coupling cycle. However, this is not an issue for the Aldol condensation which allows facile coupling of these electron withdrawing momomers, affording polymers of high molecular weight, when polymer solubility is optimised. Consequently, we also avoid the typical toxic organo-metal reagents and catalysts containing precious metals, used in the conventional synthetic routes for semiconducting polymers. This metal-free route moves the field of the organic semiconductors closer to more sustainable manufacturing, enhancing its environmentally friendly objectives.
In summary, our paper demonstrates more than 300 hours of stable n-type transistor operation in air, with a promising electron mobility of 0.03 cm2/Vs, using an environmentally benign polymerisation route, and offers a route to new opportunities in applications relying on ambient electron transport.