Improved drive voltages of organic electroluminescent devices with an efficient p-type aromatic diamine hole-injection layer

An ${\mathrm{SbCl}}_{\mathrm{5}}$ -doped N,N^′-diphenyl-N,N^′-bis(3-methylphenyl)-1,1^′-biphenyl-4,4^′-diamine (TPD) thin film was studied as a hole-injection layer in low-molecular-weight organic electroluminescent (EL) devices. EL characteristics of devices with a TPD hole-injection layer doped with other oxidizing reagents, such as iodine, ${\mathrm{FeCl}}_{\mathrm{3}},$ and tris(4-bromophenyl)aminium hexachloroantimonate were compared with that of ${\mathrm{SbCl}}_{\mathrm{5}}$ -doped TPD. The device with ${\mathrm{SbCl}}_{\mathrm{5}}$ -doped TPD on a cleaned indium–tin–oxide (ITO) substrate exhibited the best performance of all the devices studied. The improvement in device performance was attributed to an increase in work function of ITO due to acid formation as a result of hydrolysis of ${\mathrm{SbCl}}_{\mathrm{5}}$ and by thinning the tunneling barrier for hole injection due to formation of the space charge region in highly doped TPD with ${\mathrm{SbCl}}_{\mathrm{5}}.$