Charge Relaxation and Dynamics in Organic Semiconductors

摘要

Charge relaxation in dispersive materials is often described in terms of the stretched exponential function (Kohlrausch law). The process can be explained using a "hopping" model which, in principle also applies to charge transport such as current conduction. This work analyzed reported transient photoconductivity data on functionalized pentacene single crystals using a geometric hopping model developed by B. Sturman et al and extracted values (or a range of values) on the material parameters relevant to charge relaxation as well as charge transport. Using the correlated disorder model (CDM), we estimated values of the carrier mobility for the pentacene samples. From these results, we observed the following: ⅰ) the transport site density appeared to be of the same order of magnitude as the carrier density; ⅱ) it was possible to extract lower bound values on the material parameters linked to the transport process; and ⅲ) by matching the simulated charge decay to the transient photoconductivity data, we were able to refine estimates of the material parameters. The data also allowed us to simulate the stretched exponential decay. Our observations suggested that the stretching index and the carrier mobility were related. Physically, such interdependence should allow one to differentiate between localized molecular interactions and distant coulomb interactions.