Improved Environmental Stability

A major problem in the use of OTFTs is the instability of organic semiconductors to light, atmospheric oxygen, and humidity, or a combination of these stress factors, which limits the shelf life of these components. The exposure of OTFT devices to the environment typically results in devices with high off currents, large and unstable onset voltages, and large subthreshold slopes. Encapsulation of the devices can prevent the penetration of air and humidity and thus effectively enhances the environmental stability of OTFTs. 

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Polyacetylene is considered to be the prototypical low band-gap polymer, but its potential uses in device applications have been hampered by its sensitivity to both oxygen and moisture in its pristine and doped states. Poly(thienylene vinylene) 2 has been extensively studied because it shares many of the useful attributes of polyacetylene but shows considerably improved environmental stability. The low band gap of PTV and its derivatives lends itself to potential applications in both its pristine and highly conductive doped state. Furthermore, the vinylene spacers between thiophene units allow substitution on the thiophene ring without disrupting the conjugation along the polymer backbone. 

Improved Environmental Stability oe the Biomaterial 3.5.4.1 Thermo Gravimetric Analysis... 

Pyrrole offers two positions that can be substituted to obtain soluble polymers, the N- and the 3-/or 4-positions on the aromatic ring. In general, N-substitution lowers the conductivity caused by the loss of conjugation due to steric interactions, which leads to a torsion of adjacent pyrrole units the advantage of the N-substitution is the improved environmental stability and the more straightforward structural characterization [155]. For the preparation of N-substituted and ring-substituted PPys oxidative chemical and electrochemical... 

FIGURE 5.4.7 Improved environmental stability of TFTs fabricated with self-encapsulated polymer films, (a) Transfer characteristics of a TFT exposed to air for two days, (b) Transfer characteristics of TFT as prepared and after been exposed to air for 20 days. (From Arias, A.C. et al., Adv. Mater., 18, 2900-2904, 2006. With permission.)... 

As far as applications of the doped polymer are concerned, PPV has the advantage over polyacetylene of an improved environmental stability. Much of the current research is concerned with pristine PPV because of its semiconductor properties, in particular luminescence, instead of with its conducting form. 

It is also interesting to note that, p-MS in the terpolymer not only provides a versatile reactive site for the further reactions, such as functionalization and cross-linking reaction, but also improves environmental stability and the morphology of the elastomer. 

P3HT layer at the interface was effectively encapsulated by the crystalline PE phase on the top, resulting in significantly improved environmental stability. In contrast, when a film was cast at room temperature (scheme II), PE crystallized before P3HT, which resulted in poor device performance (t[Pg.207]

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