Charge Transport in Mixed Organic Disorder Semiconductors: Trapping, Scattering, and Effective Energetic Disorder

The effects of trapping and scattering on the transporting properties of organic disorder semiconductors have been studied by time-of-flight (TOF) method. Tris-(8-hydroxyquinoline)-aluminum (Alq(3)), 2,2',2 ''-(1,3,5-benzene-triyl)-tris-(1-phenyl-1H-benzimid-azole) (TPBi), and N,N-diphenyl-N,N-bis(1-naphthyl)-(1,1'-bipheny1)-4,4' diamine (NPB) are doped into 4,4'-N,N'-dicarbazolebiphenyl (CBP) to form traps and scatters with various energy level differences. It is found that the low scatters significantly reduce the mobility and make the TOF transients, while the deep traps and high scatters would not significantly reduce the mobility and change the nondispersive behavior of the TOF transients. The main difference between deep traps and high scatters is that the deep traps induce a great reduction of the photocurrent, while the high scatters do not obviously decrease the photocurrent. The experimental results are well explained by the Miller Abrahams hopping model and the effective energetic disorder. Furthermore, a theoretical method is established to determine the demarcation between the shallow trap (low scatter) and the deep trap (high scatter) in terms of energy level differences. These results may shed light on the understanding of charge transport in mixed organic semiconductors.