Abstract
Electrical properties of organic light-emitting diodes were studied in a device with 7,7,8,8-tetracyano-quinodimethane (TCNQ) to see how the TCNQ affects on the device performance. Since the TCNQ has a high electron affinity, it is used for a charge-transport and injection layer. We have made a reference device in a structure of ITO(170 nm)/TPD(40 nm)/$Alq_3$(60 nm)/LiF(0.5 nm)/Al(100 nm). And two types of devices were manufactured. One type of device is the one made by doping 5 and 10 vol% of TCNQ to N,N'-diphenyl-N,N'-di(m-tolyl)-benzidine (TPD) layer. And the other type is the one made with TCNQ layer inserted in between the ITO anode and TPD organic layer. Organic layers were formed by thermal evaporation at a pressure of $10^{-6}$ torr. It was found that for the TCNQ doped devices, turn-on voltage of the device was reduced by about 20 % and the current efficiency was improved by about three times near 6 V. And for devices with TCNQ layer inserted in between the ITO anode and TPD layer, it was found that the current efficiency was improved by more than three times even though there was not much change in turn-on voltage.
