Stability of Charge Transfer States in F4TCNQ-doped P3HT

Printable electronic devices from organic semiconductors are highly desired but limited by low conductivity and stability relative to their inorganic counterparts. p-Doping of poly(3-hexyl)thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) increases conductivity through integer charge transfer (ICT) to form mobile carriers in the P3HT. An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, trap-like state for the hole on P3HT. This effort addresses 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 with time using a combination of infrared and photoelectron spectroscopies and supported by a complete loss of film conductivity with increased CPX state concentration. Both the fraction and rate of conversion to the CPX s...