Application of ultrathin layer in white organic electroluminescent devices

Based on 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) acting as dopant and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) acting as hole-blocking layer, the EL spectra of two kinds of organic light-emitting diodes (OLEDs) ITO/NPB/BCP/Alq₃:DCJTB/Alq₃/Al (structure A) and ITO/NPB/BCP/Alq₃/Alq₃:DCJTB/Alq3/Al (structure B) were investigated. Our experimental data showed that the relative emission intensity of green light was low in the EL spectra of devices with structure A, and it was hard to enhance the relative emission intensity of green light by only increasing the thickness of Alq₃ layer. On the other hand, the thin layer of Alq₃ inserted between BCP layer and dopant layer (Alq₃:DCJTB) had a tremendous impact on the relative emission intensity of green light for devices with structure B, and a strong green light emission could be obtained by inserting an ultrathin layer of Alq₃. Further, by optimizing the device structure, a stable white OLED ITO/NPB(50 nm)/ BCP(3 nm)/Alq₃(3 nm)/Alq₃:DCJTB(1%(w))(5 nm)/Alq₃(7 nm)/Al was obtained with ultrathin organic layers of BCP, Alq₃ and Alq₃:DCJTB. The CIE of the device almost kept at (0.33, 0.37) with increasing the applied voltages (14-18 V), and its luminance reached 11521 cd· m^-2 at the current density of 432 mA· cm^-2.