Efficient deep-blue organic light-emitting diodes based on 9,9-Bis(4-biphenylyl)fluorene derivatives

We fabricated highly efficient deep-blue organic light-emitting diodes (OLEDs) composed of 2,7-bis(4'-diphenylamino-1,1'-biphenyl-4-yl)-9,9-bis(1,1'- biphenyl-4-yl)-9H-fluorene (FLBD) as an emitter doped into a wider energy gap host of 2,7-bis(1,1'-biphenyl-4-yl)-9,9-bis(1,1'-biphenyl-4-yl)-9H-fluorene (FLBH). We investigated the dominant factors for external electroluminescence quantum efficiency (η_ext) in the deep-blue OLEDs by focusing on the carrier balance in the emitting layer (EML), the location and width of the carrier recombination zone, and carrier-induced exciton quenching. The η_ext value of the OLEDs strongly depended on the hole injection efficiency from a hole transport layer (HTL) to the EML. By controlling the hole injection barrier into the EML and balancing the number of electrons and holes in the EML using HTLs having various highest occupied molecular orbital levels, we obtained the highest η_ext of 5.4% with deep-blue Commission International De L'Eclairage coordinates of x = 0.15 and y = 0.09. Additionally, we found that the width of the carrier recombination zone tightly correlates to the hole injection efficiency into the EML.