Charge transport in copper phthalocyanine (CuPc), partially fluorinated CuPc(F4CuPc), and fluorinated CuPc(F16CuPc) based organic thin film transistors is studied using dual SiO2/ polymethylmethacrylate gate dielectrics. We demonstrate the strong influence of air/moisture induced electron traps on electron transport when the lowest unoccupied molecular orbital is close to the vacuum irrespective of hydroxyl-free dielectrics used in the devices. © 2010 American Institute of Physics.