Stability of Charge Transfer States in F4TCNQ-Doped P3HT

Printable electronic devices from organic semiconductors are strongly desired but limited by their low conductivity and stability relative to those of their inorganic counterparts. p-Doping of poly(3-hexyl)thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane increases conductivity through integer charge transfer (ICT) to form mobile carriers in P3HT. An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, traplike state for the hole on P3HT. This effort addresses the stability of the free carrier states, once formed. Herein, we demonstrate that, while the ICT state may be kinetically preferred, the CPX state is thermodynamically more stable. Conversion of the ICT state to the CPX state is monitored here over time using a combination of infrared and photoelectron spectroscopies and supported by a complete loss of film conductivity with an increased CPX state concentration. Both the fraction and the rate of conversion to the CPX state are influenced by polymer molecular weight, dopant concentration, and storage conditions, with ambient storage conditions accelerating the conversion. This work suggests that a renewed focus on dopant–matrix reaction chemistry should be considered in the context of both kinetic and thermodynamic considerations.