The effect of 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) doping on electroluminescent properties of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD) was investigated. A series of organic light-emitting devices (OLEDs) integrated with (i) single-layer poly-TPD, (ii) blended single-layer poly-TPD:TPBi or (iii) bilayer poly-TPD/TPBi were fabricated and characterized. An excimer emission band at 500nm was found in the poly-TPD film, poly-TPD:TPBi (1:1) blend film, and poly-TPD/TPBi bilayer film. It was observed that the planar geometry of poly-TPD was related to the formation of excimers. The electromer emission, which was absent in photoluminescence, was investigated by applying an external electrical field to devices with non-doped and TPBi-doped poly-TPD. Only the electromer emission was observed in the devices with TPBi-doped poly-TPD, due to the impeded intrinsic and excimer emissions. The planar geometry of poly-TPD molecules may be destroyed due to the longer inter-ion distance with the doping of TPBi. 
Electroluminescent properties of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] doped with 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene