In recent years, most of the work on p-type organic thermoelectrics focus on improving the thermoelectric properties of PEDOT:PSS through a sequential doping-dedoping process. However, the air-stability of thermoelectric parameters of these systems, which is essential for the realization of reliable devices remains largely unexplored. In this study, Poly (ethyleneimine)-ethoxylate (PEIE) acts as a work function modification agent and encapsulation layer to improve the thermoelectric performance and air-stability of nitric acid (HNO3) doped PEDOT:PSS films. The evaporation of HNO3 is responsible for a simultaneous decrease in electrical conductivity and an increase in the Seebeck coefficient leading to the degradation of the power factor. PEIE reduces the evaporation of HNO3 from PEDOT:PSS, and increases the power factor from 72 to 168 μW m-1K-2. After a week of exposure to air, the films show a power factor of 124 μW m-1K-2, retaining 74% of its initial thermoelectric merits. These results underscore the importance of PEIE as a material for enhancing thermoelectric performance and air-stability in the development of polymer-based thermoelectrics.