White Light-emitting Diodes Based On Organic Heterojunction

Since the first report of organic light-emitting diode (OLED), it has become the research focus of new generation of flat panel displays and lighting equipments for its characteristics of thinness, lightness, active emission, wide viewing angle, high brightness, quick response, low power consumption, and good performance in low temperature and anti-earthquake, potential low cost, as well as flexible and environmentally-sound design, and many other advantages, comparing with the traditional cathode ray tubes (CRT), plasma display panel (PDP) and liquid crystal display (LCD).Currently, white organic light-emitting diode (WOLED) is the research focus of the organic light-emitting devices, because WOLED can not only be used for lighting, but also to adjust the emission color by insetting different light filters and to achieve full color display. Normally, there are currently two methods to obtain white light: by using the incomplete energy transfer of doping system to control the luminous intensity of host material and doped material to achieve white light emission, and by using the materials of different emission colors to fabricate emissive layers, then by regulating the emission intensity of each layer to achieve white light.Based on the synthesis principle of white light, firstly, we studied the influence of the thickness of BCP layer on the electroluminescence (EL) spectra of structure A [ITO/NPB/BCP/Alq₃:DCJTB/Alq₃/Al] at different applied voltages. Our experimental data show that a thin layer of BCP (³nm) can partially block holes and adjust the energy transfer between different emissive layers, but the EL spectra of these devices will change with the applied voltages. At the same time, the reasons why the EL spectra of multi-layer devices change with the applied voltage are analyzed, and it provides a theoretical basis for stable WOLED.Secondly, keeping the total thickness of Alq₃ constant, we fabricated devices with structure B [ITO/NPB/BCP/Alq₃/Alq₃:DCJTB/Alq₃/Al] by inserting a thin Alq₃ layer between BCP layer and dopant layer (Alq₃:DCJTB), and found that the relative intensity of green light has a dramatic increase in the EL spectra. Our further experimental data show that there is almost no impact on the relative intensity of green light by decreasing the thickness of the Alq₃ layer sandwiched between Al electrode and dopant layer (Alq₃:DCJTB). In addition, we studied the impact of the thickness of thin BCP layer on the electroluminescent characteristic of devices with structure B. Finally, a stable WOLED with CIE chromaticity coordinates close to the equal energy point of white light was obtained.