Carbon nanotubes and their composite thermoelectric (TE) materials have significant advantages in supplying power to flexible electronics due to their high electrical conductivity, excellent flexibility, and facile preparation technology. In this work, stable n-type silver ammonia-polyethyleneimine/single-walled carbon nanotube ([Ag(NH₃)₂]⁺-PEI/SWCNT) composite films were facilely prepared by solution blending and vacuum-filtration methods. The results demonstrate that light silver ammonia doping optimizes the carrier concentration and carrier mobility of the composite film, and a maximum power factor (PF) of [Ag(NH₃)₂]⁺-PEI/SWCNT of 91.9 μW m⁻¹ K⁻² was obtained, which is higher than that of PEI/SWCNT (70.0 μW m⁻¹ K⁻²). Furthermore, when the composite films were reduced by the NaBH₄ solution, the Seebeck coefficient and the PF value were further increased to −45.5 μV K⁻¹ and 115.8 μW m⁻¹ K⁻², respectively. For demonstration, a maximum output voltage of 13.8 mV and output power of 492 nW were achieved using a three p–n junction-based TE device constructed by [Ag(NH₃)₂]⁺-PEI/SWCNT at a temperature difference of 50 K. Thus, this study provides a metal complex ion doping strategy to improve thermoelectrical properties and air stability of the PEI/SWCNT composite films, which have potential applications in flexible electronics.