The role of LiF in improving the charge injection in Al/Alq3 based OLEDs is investigated by making and testing the current-voltage characteristics of a series of single carrier devices. It is found that thinner layers of LiF (∼2A) have a greater impact than thicker layers and that LiF has a much greater impact at the top organic/Al interface than at the bottom interface, due to differences caused by the deposition sequence. Charge transport is found to be injection limited, but doesn't quite fit the thermionic emission theory. When used with Ag electrodes, LiF has a blocking effect. Electrically poling Al/LiF/Alq3/LiF/Al devices at -15V results in negative differential resistance peaks in the IN characteristics. These peaks can be erased and reformed and are used as the basis for a new injection model. LiF also improves electron injection in devices having C60 in place of Alq3, through the formation of ohmic contacts.