We studied the performance enhancement of organic light-emitting diodes (OLEDs) using 2,3,5,6-fluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as the hole-transport layer. To investigate how F4-TCNQ affects the device performance, we fabricated a reference device in an ITO (170 nm)/TPD(40 nm)/Alq3(60 nm)/LiF(0.5 nm)/Al(100 nm) structure. Several types of test devices were manufactured by either doping the F4-TCNQ in the TPD layer or forming a separate F4-TCNQ layer between the ITO anode and TPD layer. N,N'-diphenyl-N,N'-di(m-tolyl)-benzidine (TPD), tri(8-hydroxyquinoline) aluminum (Alq3), and F4-TCNQ layers were formed by thermal evaporation at a pressure of 10-6 torr. The deposition rate was 1.0-1.5 Å/s for TPD and Alq3. The LiF was subsequently thermally evaporated at a deposition rate of 0.2 Å/s. The performance of the OLEDs was considered with respect to the turn-on voltage, luminance, and current efficiency. It was found that the use of F4-TCNQ in OLEDs enhances the performance of the device. In particular, the use of a separate layer of F4-TCNQ realizes better device performance than other types of OLEDs.
