Toward establishing self-dopant design principles in n-type organic thermoelectrics

Doping methods in n-type organic materials heavily rely on the addition of extrinsic compounds with relatively small ionization potentials. The introduction of dopants modifies the density of states near the Fermi-energy such that populated states now exist at some energy relative to the LUMO of the organic semiconductor. Upon charge transfer the energies of these states distort in ways that may be very difficult to predict, leading to a broader density of states. Furthermore, dopants can disrupt the packing structure of organic films and hinder charge mobility. The dopants may also aggregate during film casting, decreasing the number of charge carriers available to the system and creating additional grain boundaries. These challenges may be mitigated by intrinsically doping the n-type organic semiconductor via a process dubbed self-doping. Presently, the relationship between self-dopant structure and doping efficiency remains unknown. We have investigated the effect of dopant structure on the doping efficiency in a variety of perylene diimides. We believe our findings provide fundamental design principles for the fabrication of effective self-dopants geared toward increasing the thermoelectric properties of n-type organic semiconductors.