Improving the stability of polymer FETs by introducing fixed acceptor units into the main chain: Application to poly(alkylthiophenes)

A concept to improve the air stability of poly(3-hexylthiophene)s (P3HT)s is proposed which is based on the introduction of electron-accepting units in the main chain in order to increase the polymer ionization energy (IE). Following this approach a series of soluble acceptor-containing poly(alkylthiophene)s with varied chemical composition has been synthesized. The structural and optical properties of the new polymers were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and UV-vis optical absorption methods and the results are discussed in the present paper. The systematic change of the ionization energy relative to the conventional P3HT is measured by ultraviolet photoemission spectroscopy, and supported by quantum-mechanical calculations. Combining the results of these techniques, reveals two main effects that lead to the IE increase: (i) incorporation of acceptor units and (ii) a decreased intermolecular order in acceptor-modified polymers. Finally, the performance of OFETs comprising these new materials was evaluated, corroborating the advantages of the modified polymers with respect to operational and ambience stability.