White organic light-emitting devices based on phosphor-sensitized fluorescence

Although phosphorescent organic light-emitting devices (OLEDs) can have an internal quantum efficiency (IQE) of 100%, the IQE usually decays at high current densities due to triplet-triplet annihilation. Phosphor-sensitized fluorescence can realize the energy transfer between phosphorescent emitter and fluorescent emitter, and can be used to suppress the efficiency fluctuations and adjust the color of the device. With this in mind, white light emission including different colors of phosphorescent emitter and fluorescent emitter can be expected. Herein, phosphor-sensitized fluorescent white OLEDs are fabricated by combining ultra-thin layer insertion and doping, in which laser dyes DCM (4-(Dicyanomethylene)-2-methyl-6-(4-dimethyl-aminostyryl)-4H-pyran), iridium complexes Ir(ppy)<sub>3</sub> (tris(2-phenylpyridine)iridium), and biphenyl ethylene derivatives BCzVB (1,4-bis[2- (3-N-ethylcarbazoryl)vinyl]benzene) are used as red, green and blue emitters, respectively. By adjusting the doping concentration of Ir(ppy)<sub>3</sub> phosphorescent green emitter in CBP (4,4’-N,N’-dicarbazole-biphyenyl) host, with ultra-thin layers of BCzVB fluorescent blue emitter on both sides of CBP:Ir(ppy)<sub>3</sub> doping system and with ultra-thin layer of DCM fluorescent red emitter inserting in CBP:Ir(ppy)<sub>3</sub> layer, the three colors can be balanced. White emissions are obtained in the device, the highest external quantum efficiency is 2.5% (current efficiency of 5.1 cd/A), the maximum brightness is 12400 cd/m<sup>2</sup>, and Commission Internationale de l'Eclairage (CIE) co-ordinates can reach the ideal white light equilibrium point (0.33, 0.33) at a current density of 1 mA/cm<sup>2</sup>. The acquisition of white light is attributed to the appropriate doping ratio of Ir(ppy)<sub>3</sub> and the position of DCM, which effectively balances the emission ratio of three primary colors: red, green, and blue. The results indicate that the partially energy transfer of triplet excitons to singlet excitons by phosphor-sensitized fluorescence scheme can be used to realize high-efficiency white organic electroluminescent devices, thereby reducing energy consumption and providing more room for promoting OLED applications.