Carrier trapping and scattering in amorphous organic hole transporter

The effects of dopants on the hole transporting properties of N, N′ -diphenyl- N, N′ -bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine (NPB) have been studied by time of flight. Five dopants: copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(p -dimethylaminostyrle)-4 H -pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1 H,5 H -benzo[ij] quinolizin-8-yl)vinyl]-4 H -pyran (DCM2), 2-(4-biphenyl)-5-(4-tert-butylphenyl)- 1,3,4-oxadiazole (tBu-PBD), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are used in this study. The dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbital relative to that of NPB. Generally, traps are found to induce significant reduction in hole mobility while there is a slight reduction for scattering. Two different underlying charge transport mechanisms are proposed.