Marie B. O'Regan

The role of dipole moments on hole transport in triphenylamine‐doped polymers

Hole mobilities were measured in a series of triphenylamine (TPA) molecules with different dipole moments doped into apolar and highly polar poly(styrene)s. The results are described by a formalism based on disorder, due to Bassler and coworkers. The formalism is premised on the assumption that charge propagation occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. A key parameter of the formalism is the energy width of the hopping site manifold, or DOS. For the apolar poly(styrene), the width of the DOS increases with increasing dipole moment of the TPA molecule, whereas for the highly polar poly(styrene), the width is independent of the dipole moment. The results are explained by an argument based on dipolar disorder. The argument is premised on the assumption that the total width is determined by dipolar components due to the dopant molecule and the polymer repeat unit, and a van der Waals component. For the apolar poly(styrene), the width is determined by a TPA dipolar component that increases with increasing dipole moment of the TPA molecule and a van der Waals component of 0.077 eV. For the highly polar poly(styrene), the total dipolar component is 0.090 eV, independent of TPA dipole moment, and the van der Waals component 0.090 eV.

Hole photogeneration in dual layer aggregate photoreceptors

Photogeneration efficiencies have been measured in dual layer aggregate photoreceptors over a wide range of fields and wavelengths. The results are described by a surface-enhanced exciton dissociation model. The model is based on a theory of geminate recombination, originally due to Onsager. The Onsager theory is based on the assumption that the absorption of a photon creates a bound electron-hole pair which then either dissociates into a free electron and a free hole, or undergoes recombination. The key parameters of the theory are the fraction of absorbed photons that create bound pairs, and the electron-hole separation distance of the bound pair. For aggragate materials, the yield of creating the bound pairs is 0.60. The separation distances are between 20 and 60 angstrom. The high photogeneration efficiencies in these materials are attributed to both the high yield of creating the bound pairs and the low electron-hole recombination probability because of the large pair separation distances.