Correlation between Seebeck coefficient and transport energy level in poly(3-hexylthiophene)

Understanding the behavior of dependence of Seebeck coefficient on the energy level structure for organic thermoelectric (OTE) materials is of great importance for optimizing the thermoelectric properties of OTE materials. In this work, we study the relationship between Seebeck coefficient and highest occupied molecular orbital (HOMO) energy level of π-conjugated poly(3-hexylthiophene) (P3HT) through doping with 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) to precisely tune its HOMO position with respect to the Fermi level, in an attempt to realize the correlation. By varying the F4-TCNQ doping concentration, we observed simultaneous decrease in both the Seebeck coefficient and the energetic difference between hole transport energy level and Fermi level (ETr-EF) with gradually increasing the F4-TCNQ doping content. The obtained linear trend decline in Seebeck coefficient versus hole transport energy band edge (HOMO edge) is well fitted by the mobility edge model, a general relationship that has implication on the charge transport mechanism for conventional semiconductors. The present result in this research experimentally demonstrates a direct correlation between Seebeck coefficient and charge transport energy level in OTE materials, and will make better understanding for future OTE materials exploration.