Thermoelectric properties of self-assembled thiophene derivatives: effect of molecular structure on doping efficiency and Fermi level alignment

Abstract

Organic materials have emerged as promising candidates for thermoelectric (TE) applications, offering distinct advantages over their inorganic counterparts. While polymers have received considerable attention in this context, small organic semiconductors have not been widely explored due to the challenge of obtaining high conducting films. Advancing this field necessitates the development of novel materials and a deeper understanding of their structure-dependent TE properties. The present study reports how slight modifications to the molecular structure affect the TE characteristics of two thiophene-based small molecules (OT1 and OT2), which differ only in their end substitution with varying acceptor strengths. This difference led to distinct morphologies when self-assembled. OT2 doped with FeCl3 displayed high electrical conductivity and power factor (52.0 μW/mK2), while OT1 exhibited significantly lower conductivity and power factor (1.6 μW/mK2) under identical conditions. The weaker acceptor strength of the end group in OT2 resulted in higher doping efficiency. Improved charge carrier transport in OT2 was due to a low-lying highest occupied molecular orbital (HOMO) energy level and enhanced density of states around the HOMO after doping. Maintaining its self-assembly after doping was also crucial for better electrical properties. These findings emphasize the importance of molecular engineering in developing high-performance TE materials.