Abstract
While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure-property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n= 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω-Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT.
Highlights
Thermoelectric materials offer a simple solution for direct heat-to-electricity conversion from a variety of heat sources via the Seebeck effect
In view of the potential thermoelectric applications, the thermal transport in the inplane direction of thin films of small-molecule semiconductors is more relevant since it aligns with the direction of fast electrical transport in these materials
Thermal conductivity measurements were performed with the 3ω–Völklein technique following a protocol with reduced measurement uncertainty
Summary
Thermoelectric materials offer a simple solution for direct heat-to-electricity conversion from a variety of heat sources via the Seebeck effect. Herringbone-stacked alkylated thienoacene-based molecular materials have recently emerged as some of the best performing OSCs, a result of p-type charge carrier mobilities that can reach over 10 cm V–1 s–1.[8,9,10] Among this family, dinaphtho[2,3-b:2′,3′-f] thieno[3,2-b]thiophene (DNTT) derivatives have been investigated in detail both experimentally and theoretically; the results point to a favorable 2D character of charge transport (i.e., within the layers).[11,12,13,14,15,16,17,18] Importantly, the origin of their high charge mobilities has recently been attributed to their reduced dynamic disorder, coupled to an isotropic electronic coupling pattern within the plane of charge transport. Reduced sample dimensions in the case of thin films worsen the situation and the experiments require extra care at every step to keep the overall uncertainty within acceptable limits
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