Polytetrafluoroethylene (PTFE) is one of the most efficient polymers for green energy-harvesting devices like triboelectric nanogenerators because of its high capability of acquiring and retaining negative charge. Despite its extensive use, the relation between PTFE triboelectric behavior and its electronic properties has never been analyzed. To shed light on this important feature, we have studied the electronic properties of PTFE low-index surfaces in the high-pressure phase by means of density functional theory. We start by showing that adding either a positive or a negative charge on pristine surfaces is energetically unfavorable. We then demonstrate the role played by surface defects. When a surface fluorine vacancy is introduced, the analysis of the band structure reveals that the defect generates a trap state that enables the surface to acquire and retain negative charge.