Flexible thermoelectric generators (f-TEGs) are a promising technology capable of harvesting electricity from waste heat generated by the human body. For stable operation of f-TEGs, mechanical integrity, flexibility, and high thermoelectric (TE) performance are required. Carbon nanotubes (CNTs) are attractive materials for f-TEG applications due to their excellent flexibility and high electrical conductivity. However, their inherently low solution dispersibility makes uniform large-area coating challenging. Therefore, this study effectively secures TE performance by applying an optimized three-step process to CNT/PET film using oxidized CNTs with improved dispersibility in water. Furthermore, p- or n-doping optimization using various dopants provides stable p- or n-type TE performances of CNT- materials and presents new possibilities for future f-TEG development.