On the iodine doping process of plasma polymerised thiophene layers

To make a fair comparison of the conductive properties of plasma polymerised thiophene (PPT) layers deposited under different conditions, optimal doping procedures should be applied. The iodine doping process of PPT layers deposited at high (HP) and low (LP) pressure has been studied in detail. Doping time, thickness, and exposure time to air before and after doping were varied. Iodine doping generates charge carriers by the formation of charge transfer (CT) complexes, which are in equilibrium with absorbed iodine. The conductivity of the LP-PPT layers decreases with increasing thickness due to (vacuum-) UV modification. Upon exposure to air before doping, LP-PPT layers loose their capability to form CT complexes, whereas HP-PPT layers do not. Consequently, the conductivity for LP-PPT layers exposed to air before doping decreases with exposure time. After iodine doping, a lower rate of oxidation upon exposure to air is observed compared with as deposited PPT layers. Furthermore, the conductivity decreases slowly due to diffusion of absorbed iodine out of the PPT layers. The conductivity of the HP-PPT layers increases faster with doping time and shows a higher stability towards exposure to air after doping than LP-PPT layers. Consequently, the optimal doping procedure may vary for PPT layers deposited under different conditions.