Optimization of High-Performance Blue Organic Light-Emitting Diodes Containing Tetraphenylsilane Molecular Glass Materials

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Abstract
Molecular glass material (4-(5-(4-(diphenylamino)phenyl)-2-oxadiazolyl)phenyl)triphenylsilane (Ph3Si(PhTPAOXD)) was used as the blue light-emitting material in the fabrication of high-performance organic light-emitting diodes (OLEDs). In the optimization of performance, five types of OLEDs were constructed from Ph3Si(PhTPAOXD): device I, ITO/NPB/Ph3Si(PhTPAOXD)/Alq3/Mg:Ag, where NPB and Alq3 are 1,4-bis(1-naphylphenylamino)biphenyl and tris(8-hydroxyquinoline)aluminum, respectively; device II, ITO/NPB/Ph3Si(PhTPAOXD)/TPBI/Mg:Ag, where TPBI is 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; device III, ITO/Ph2Si(Ph(NPA)2)2/Ph3Si(PhTPAOXD)/TPBI/Mg:Ag, where Ph2Si(Ph(NPA)2)2 is bis(3,5-bis(1-naphylphenylamino)phenyl)-diphenylsilane, a newly synthesized tetraphenylsilane-containing triarylamine as hole-transporting material; device IV, ITO/Ph2Si(Ph(NPA)2)2/NPB/Ph3Si(PhTPAOXD)/TPBI/Mg:Ag; device V, ITO/CuPc/NPB /Ph3Si(PhTPAOXD)/Alq3/LiF/Al, where CuPc is Cu(II) phthalocyanine. Device performances, including blue color purity, electroluminescence (EL) intensity, current density, and efficiency, vary drastically by changing the device thickness (100−600 Å of the light-emitting layer) and materials for hole-transporting layer (NPB and/or Ph2Si(Ph(NPA)2)2) or electron-transporting material (Alq3 or TPBI). One of the superior OLEDs is device IV, showing maximum EL near 19 000 cd/m2 with relatively low current density of 674 mA/cm2 (or near 3000 cd/m2 at 100 mA/cm2) and high external quantum efficiency of 2.4% (1.1 lm/W or 3.1 cd/A). The device possesses good blue color purity with EL emission maximum (λmaxEL) at 460 nm, corresponding to (0.16, 0.18) of blue color chromaticity on CIE coordinates. In addition, the device is reasonably stable and sustains heating over 100 °C with no loss of luminance on the basis of the annealing data for device V. Formation of the exciplex at the interface of NPB and Ph3Si(PhTPAOXD) layers is verified by EL and photoluminescence (PL) spectra studies on the devices with a combination of different charge transporting materials. The EL due to the exciplex (λmaxEL at 490−510 nm) can be properly avoided by using a 200 Å layer of Ph3Si(PhTPAOXD) in device I, which limits the charge-recombination zone away from the interface area.