The effects of dopants on the hole-transporting properties of NPB, i.e., (N,N-diphenyl-N,N-bis(1-naphthyl)(1,1-biphenyl)-4,4 diamine), were studied by time-of-flight technique and admittance spectroscopy. Three dopants were chosen in this study. They were 4-dicyanmethylene-2-methyl-6-4H-pyran (DCM1), rubrene (RB), and tris-(8-hydroxyquinoline) aluminum (Alq3). It can be shown that DCM1 behaves as hole traps whereas Alq3 behaves as hole scatterers in NPB. Generally, both trapping and scattering lower hole mobilities in NPB. The hole mobilities decrease when DCM1 and Alq3 are introduced into NPB whereas the hole mobility remains nearly unchanged when RB is doped into NPB. The effect of doping on carrier dispersion is also studied.

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