Charge source and the charging mechanism of the contact electrification of polymer powder

•Charge sources were naked-activated-mechano-anion, -cation and -radical.•An f-electron of charge source acted as the charge carrier.•Net amount of transferred-f-electron was given by average-energy-level of donors being higher than that of acceptors.•Alignment of PE-PVC-PTFE on the average-energy-level was identical to that of triboelectric series.