ICT2014 Abstracts

Layered, small molecule, crystalline organic semiconductor have been suggested as room temperature thermoelectric power devices if they can be doped to retain their relatively high electronic mobilities (> 1 cm^2/Vs).  It is therefore important to know their thermal conductivities, k, with the hope that k < k0 ~ 10 mW/cm K.  We have started measurements of both the in-plane and interplane thermal diffusivities of several of these materials using frequency and position dependent ac-calorimetry; the thermal conductivities are then determined from the specific heats measured with differential scanning calorimetry.  For rubrene, the interplane thermal conductivity is several times smaller than the in-plane value, which is smaller than k0.  On the other hand, for materials based on alkyne-functionalized acenes, such as TIPS-pentacene, the in-plane thermal conductivities are slightly larger than k0 but their interlayer thermal conductivities are an order of magnitude larger than this.  This inverted anisotropy suggests significant interactions between the organometallic side groups, leading to optical phonons with significant interplane dispersion.  If these materials are to be used in thermoelectric devices, it will be important to design the apparatus such that the interlayer thermal conductivity does not thermally short out the sample.  Research supported by US-NSF, Grants DMR-0800367, EPS-0814194, and DMR-1262261.