Spectral Characteristics of White Organic Light-emitting Diodes Based on Novel Phosphorescent Sensitizer

Abstract

White organic light-emitting diodes were fabricated by using a novel phosphorescence bis(1,2-dipheny1-1H-benzoimidazole)iridium(acetylacetonate)[(pbi)2Ir(acac)] as sensitizer and a fluorescent dye of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) codoped into a car-bazole polymer of poly(N-vinylcarbazole) (PVK). Through characterizing the UV-Vis absorption spectra, the photoluminescence spectra of (pbi)2Ir(acac) and DCJTB, and the electroluminescence spectral properties of the WOLEDs, the energy transfer mechanisms of the codoped polymer system were deduced. The results demonstrate that the luminescent spectra with different intensity of (pbi)2Ir(acac) and DCJTB were co-existent in the EL spectra of the blended system, which is ascribed to an incomplete energy transfer process in the EL process. The efficient Forster and Dexter energy transfer between the host and the guests enabled a strong yellow emission from (pbi)2Ir(acac) and DCJTB, where (pbi)2Ir(acac) plays an important role as a phosphorescent sensitizer for DCJTB. With the blue emitting-layer of N,N-diphenyl-N,N-bis(1-naphthyl)(1,1-biphenyl)-4,4-diamine, the codoped system device achieved white emission. The codoped system showed that its Commissions Internationale de 1Eclairage coordinates were more independent of the variation of bias voltage than those of phosphorescent doped PVK systems.