Dr. David H. Dunlap
Department of Physics and Astronomy
University of New Mexico
This talk will review the work over several decades to quantify the role of energetic and spatial disorder on the mobility of injected charge in organic semiconductors. Theoretical interest began with the observation of universality of dispersive time-of-flight transients in the 1970's, and the reconciliation of this observation with disorder via the continous-time random walk. Emerging acceptance of the robustness of the Poole-Frenkel field-dependent mobility in these systems inspired further studies via numerical simulation of hopping conductivity, in order to more accurately account for the combined effects of energetic and spatial disorder. In the 1980's and 1990's the Gaussian Disorder Model became the theoretical tool of choice, affording further insights into the role of energetic disorder, and modeling of molecularly doped polymers allowed for insights about the nature of the two-site jump rate, as to whether it is of the Marcus form, or the Miller-Abrahams form. Experimenta
l work in the 1990's focusing on the role of permanent molecular dipoles lead to the realization that energetic disorder is spatially correlated. It became clear that the correlations arising from dipolar-disorder are what give rise to the Poole-Frenkel field dependence, yet it was also realized that correlations tend to hide the nature of the underlying jump-rate. Recent theoretical investigations are focused on the role of correlated disorder in photovoltaic devices, where the exciton diffusion length, the Coulomb radius, and the cluster size of polymer blends introduce competing length scales.
David Dunlap is a professor in the Dept. of Physics and Astronomy at the University of New Mexico. His research concerns theoretical issues of charge transport and tunneling phenomena, with emphasis on hopping in disordered materials.