An Electronically Active Molecularly Doped Polyimide Hole Injection Layer for an Efficient Hybrid Organic Light-Emitting Device

An efficient hybrid organic light-emitting device (HOLED) has been fabricated with an electronically active molecularly doped polyimide (AMDPI) thin film as a hole injection layer. The AMDPI thin film was prepared by the mixed vapor deposition polymerization process using N,Nʹ-diphenyl-N,Nʹ-bis(1-naphthyl)1,1ʹ-biphenyl-4,4ʹ-diamine, pyromellitic dianhydride, and N,Nʹ-diphenyl-N,Nʹ-bis(4-aminobiphenyl)-1,1ʹbiphenyl-4,4ʹ-diamine that was chemically reduced from its nitro compound synthesized by the coupling reaction in the presence of a polar palladium organometallic catalyst. The ionization potential of the AMDPI thin film was almost similar to that of the undoped polyimide (5.1 eV). The actual contact barrier height between the ITO electrode and the polyimide hole injection layer was approximately 0.2 eV, which is slightly lower than the value (0.4 eV) calculated on the basis of the flat energy band diagram. The HOLED with the AMDPI thin film as a hole injection layer exhibited a maximum performance of ~80 000 cd/m2 and ~7 cd/A. In particular, this HOLED showed quite lower leakage current compared to the OLED with small molecules at the same thickness of hole injection layer, while a much improved device lifetime was measured for the HOLED