This work exploits the scope of doping engineering as an enabler for better-performing and thermally stable n-type organic thermoelectrics. A fullerene derivative with polar triethylene glycol type side chain (PTEG-1) is doped either by “coprocessing doping” with n-type dopants such as n-DMBI and TBAF or by “sequential doping” through thermal deposition of Cs2CO3. Solid-state diffusion of Cs2CO3 appears to dope PTEG-1 in the strongest manner, leading to the highest electrical conductivity of ∼7.5 S/cm and power factor of 32 μW/(m K2). Moreover, the behavior of differently doped PTEG-1 films under thermal stress is examined by electric and spectroscopic means. Cs2CO3-doped films are most stable, likely due to a coordinating interaction between the polar side chain and Cs+-based species, which immobilizes the dopant. The high power factor and good thermal stability of Cs2CO3-doped PTEG-1 make it very promising for tangible thermoelectric applications.