Charge Transport Processes in Molecularly Doped Polymers: Interaction Effect Between Charge Transporting Molecules and Polymers

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Abstract
Hole transport has been investigated in films of solid solutions of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and a structural variant of TPD in bisphenol A polycarbonate, polystyrene and phenoxy. Rather than being ‘inert’ as previously assumed, the binder plays a major role in influencing the rate of charge exchange between molecules. It is found that the absolute values of the drift mobilities, their electric field dependence and the activation energies are strong functions of the binder polymer employed to cast the film. At equivalent molecular concentrations but with two different binder polymers, the mobilities can vary by as much as two orders of magnitude. In some instances mobility decreases as the electric field is increased. The role that binder plays is related to the dispersibility of the diamine molecules in the polymeric binders. It is concluded that the the best molecular dispersion is observed when polystyrene is employed.