A comparative study of hole transport in vapor-deposited molecular glasses of N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine Original Research Article

Hole mobilities have been measured in vapor-deposited layers of N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TTB) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD). The results are described within the framework of a formalism based on disorder, due to Bässler and co-workers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameters of the formalism are σ, the energy width of the distribution of hopping site energies, and Σ, the degree of positional disorder. For TTB, σ = 0.078 eV and Σ = 1.7. The values for TPD are 0.077 eV and 1.6. The width of the hopping site energies can be described by a model based on dipolar disorder. The degree of positional disorder is in agreement with results observed for a wide range of vapor-deposited molecular glasses and is attributed to packing constraints. The results show that transport in both compounds can be described by simple disorder-controlled hopping without invoking polaronic contributions.