A Study Of Phosphorescent Organic Light-Emitting Device Utilizing Heavy Doping Ir（ppy）₃

Phosphorescent OLED had attracted more and more attentions due to its higher luminous efficiency. Since the brithday of phosphorescent OLED, the study on device structure and internal function mechanism are still two main research aspects till now. Been twenty years’ development, the performance of phosphorescent OLED has been close to the industrial standard, however, the increasingly complex device structure also brought additional difficulties in industrial production.In this paper, the device structures of phosphorescent OLED were simplified, and the electroluminescence (EL) mechanism was further studied by varying dopant concentration and thickness of emitting-layer (EML). At first, fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) was selected as the green phosphor in EML, and it was identified that Ir(ppy)3behaves remarkable charge-trap effect through the demonstration of hole/electron only devices. Based on the distinguishing feature of Ir(ppy)3, a novel Device A with different dopant concentration of Ir(ppy)3was designed and made, in which the direct charge-recombination mechanism of Ir(ppy)3would predominate in the EL mechanism. The structures of Device A were as follows:ITO/MoO3(3nm)/CBP:x wt%Ir(ppy)3(30nm)/TAZ (50nm)/LiF (1nm)/Al (100nm), x was8,15,20,25and30, respectively. For comparison, the reference phosphorescent OLED (Device R) was also fabricated, which had traditional structure of ITO/NPB (30nm)/CBP:8wt%Ir(ppy)3(30nm)/BCP (10nm)/Bphen (40nm)/LiF (1nm)/Al (100nm). By performance measurement, it is found that the current efficiency of Device A increased with rising of the Ir(ppy)3concentration. When the x was25, the Device A expressed the maximal current efficiency of46.8cd/A, which increased by64%in contrast with Device R. Such surprising experimental phenomena identified the distinguished EL mechanism between Device A and Device R. When the doping concentration of Ir(ppy)3was higer (such as Device A), the main EL mechanism is direct charge-recombination mechanism; when the doping concentration of Ir(ppy)3was lower(such as device R), the main EL mechanism is energy-transfer mechanism. Furthermore, the phosphorescent OLED with double EMLs was fabricated, in which the direct charge-recombination mechanism and energy-transfer mechanism could play a solo role in the separative EML. And, an optimized device structure of ITO/MoO3(3nm)/CBP:20wt%Ir(ppy)3(20nm)/TPBi:8wt%Ir(ppy)3(10nm)/TAZ (50nm)/LiF (1nm)/Al (100nm) was obtained, which showed a peak current efficiency of45.2cd/A and the maximum luminance of42742cd/m2. Eventually, the phosphorescent OLED with the simplest structure of ITO/TPBi:x wt%Ir(ppy)3(50nm)/LiF (1nm)/Al (100nm) was fabricated, where x was10and20, which exhibited the maximal current efficiency of24.7cd/A when x was20.