The electrical conductivity and doping stability of poly(3-hexylthiophene) (P3HT) in ambient air are significantly improved using a gold chloride dopant, where the coexistence of AuCl4- and Au nanoparticles contributes to high electrical conductivity and doping stability at the same time. The uniformly formed Au nanoparticles induce a facile charge transfer in a thin film of AuCl3-doped P3HT as examined by the crystallography and transmission electron microscopy. P3HT doped with AuCl3 exhibits a high electrical conductivity of 207 S cm-1, a Seebeck coefficient of 73.9 ?V K-1, and an optimized power factor of 110 ?W m- 1 K-2. Importantly, AuCl3-doped P3HT shows excellent air stability. After exposure to the ambient condition for 300 h, the optimized power factor of the AuCl3-doped P3HT maintains over 80% of the initial, whereas that of the FeCl3-doped P3HT decreases to only 20% of the original. Finally, a flexible and organic thermoelectric generator (OTEG) is fabricated using the AuCl3-doped P3HT by slot-die coating. The OTEG shows a relatively high open-circuit voltage (Voc) of 7.96 mV and a short-circuit current (Isc) of 0.93 ?A, resulting in a power density of 18.5 nW cm-2 at a temperature gradient of 10 ?C.