Heat transport in Thermoelectric materials: from Layered systems to organic crystals

Thermoelectricity (TE) is a promising and sustainable source of energy that manage to convert wasted heat into electricity using the Seebeck effect. A good material for TE application should have with low thermal conductivity (k) and high Seebeck coefficient and electrical conductivity, because TE generators efficiency is inversely proportional to the first property and directly proportional to the other two properties. Unfortunately, materials with such characteristics are not present in nature, so as of now TE is not yet a viable form of energy due to its low efficiency. In order to increase it, we can modify some pristine materials to decrease its k and increase its electrical properties. In this thesis is presented a computational study of the thermal transport property of three materials, two belonging to the class of TMD (TaS2 and MoS2) and organic molecular crystals (BTBT, DNTT), using both ab-initio techniques and MD simulations. TMDs have high electrical properties, making them ideal for TE applications, but showed large k that decreases their possible use as TEG. Thus, after a detailed study of their thermal properties, it is shown that their inplane k greatly varies from their 2D and bulk phases, due to some inter-layer vibrational modes that are suppressed by the distancing of the layers. Furthermore, it is shown how the correct functionalization further decreases the k, making the functionalized and distanced system a good material for thermoelectric applications. While another decrease in k is found in a system with vacancies, it is observed that under the correct conditions the system showed a super periodicity that restores the original k value. BTBT and DNTT have low k, but it is shown deviations from the expected k(T) trend because of its high anharmonic properties. While a BTE-level theory fails to predict it, both Wigner theory and MD simulations reproduces this deviation correctly, giving credit to AEMD simulations in highly anharmonic crystals.