Hole Transport in Bisphenol‑A Polycarbonate Doped with N,N′‑Diphenyl‑N,N′‑bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine
ABSTRACT: The time-of-flight (TOF) transients of solution-cast, free-standing films of N,N′-diphenyl-N,N-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′diamine (TPD) in bisphenol A polycarbonate (PC) have been studied using electron gun induced charge generation. This molecularly doped polymer (MDP) has been shown to exhibit perfectly flat plateaus on its time-of-flight curves with optical excitation. Our TOF results with continuously changing electron energies, as well as numerical calculations using a multiple trapping model with a Gaussian trap distribution (MTMg), suggest that charge carrier transport in this molecularly doped polymer is nonequilibrium and the flat plateaus can be explained by the presence of a thin surface layer depleted of transport material. The depleted surface layers on samples of this molecularly doped polymer are extremely thin (less than 0.12 μm), with those relating to the release side (contacting a substrate during coating/drying procedure) being much smaller than for the free side exposed to air. Since TPD-doped PC and a tetraphenylbenzidine polymer containing the TPD moiety in its main chain served as the prototype materials for the concept of “trap-free” carrier transport, we have also discussed this in detail.