Effects of carrier trapping and scattering on hole transport properties of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine thin films

The effects of trapping and scattering by dopant on the hole transport properties of N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) have been investigated by temperature dependent current–voltage characteristics, admittance spectroscopy, and luminance–voltage characteristics. Three dopants, including 4,4′,4″-tris (N-3-methylphenyl-N-phenylamine) triphenylamine (m-MTDATA) behaving as hole traps, tris-(8-hydroxyquinolinato) aluminum (Alq3) and 4,4′,4″-tri(N-carbazolyl)-triphenylamine (TCTA) acting as hole scatterers, are used in this research. It has been found that both trapping and scattering lower hole mobility in NPB, but the magnitude differs strongly. The traps are revealed to induce significantly increase in total density of traps whereas there is a slight variation for the scattering. These differences are attributed to two different charge transport mechanisms. The effect of doping on the performances of electroluminescent devices has also been studied.

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► We study the effects of trapping and scattering on the hole transport properties.
► Trapping significantly lowers the hole mobility, in constant to scattering.
► Two different transport mechanisms are revealed.
► The charge balance in the electroluminescent device is improved by trapping.