Enhancement of external quantum efficiency and reduction of roll-off in blue phosphorescent organic light emitt diodes using TCTA inter-layer

•BCP and TPBi were used as an ETL material with a QTW structure in OLEDs.•The inter-layer of TCTA and mCP was used to reduce the roll-off effect.•The thickness of inter-layer affects efficiency of devices and roll-off properties.•It had a 23% of roll-off improvement.•The difference of the highest and the lowest efficiency is 64% and 52%. The improved external quantum efficiency (EQE) and reduced roll-off properties of blue phosphorescent organic light-emitting diodes (PHOLEDs), were fabricated with structure, ITO/NPB (400Å)/TCTA (200Å)/mCP:FIrpic (7%)(300Å)/TPBi (300Å)/Liq (20Å)/Al (800Å) by incorporating an 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA) interlayer. We compared the properties of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) as the electron transport layer (ETL) with a typical structure of hole transport layer (HTL)/emissive layer (EML)/ETL in OLEDs and utilized inter-layer in the optimized structure to enhance EQE to 52% at 5.5V, also stabilize the roll-off of 23%. The use of inter-layer in blue PHOLEDs exhibits a current efficiency of 10.04cd/A, an EQE of 6.20% at 5.5V and the highest luminance of 10310cd/m2 at 9.5V. We have identified the properties of electroluminescence through the inter-layer in blue PHOLEDs which can be divided into singlet excitons and triplet excitons which emit fluorescence of N,N′-bis(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) at 420nm and phosphorescence of Iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate(FIrpic) at 470nm, 494nm, respectively.