THERMAL CONDUCTIVITIES OF ORGANIC SEMICONDUCTORS

Organic semiconductors have gained a lot of interest due to their ease of processing, low-cost and inherent mechanical flexibility. Although most of the research has been on their electronic and optical properties, knowledge of the thermal properties is important in the design of electronic devices as well. Our group has used ac-calorimetric techniques to measure both in-plane and transverse thermal conductivities of a variety of organic semiconductors including small-molecule crystals and polymer blends. For layered crystals composed of molecules with planar backbones and silylethynyl (or germylethynyl) sidegroups projecting between the layers, very high interplanar thermal conductivities have been observed, presumably implying that heat flows between layers mostly via interactions between librations on these sidegoups. Since most organic semiconducting devices require materials in thin film rather than bulk crystal form, I have focused on using the “3ω- technique” to measure the thermal resistances of thin films of this class of organic semiconductors, including bis(triisopropylsilylethynyl) pentacene (TIPS-pn), bis(triethylsilylethynyl) anthradithiophene (TES-ADT), and difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT). For each material, several films of different thicknesses have been measured to separate the effects of intrinsic thermal conductivity from interface thermal resistance. For sublimed films of TIPS-pn and diF-TES-ADT, with thicknesses ranging from less than 100 nm to greater than 4 μm, the thermal conductivities are similar to those of polymers and over an order of magnitude smaller than those of single crystals, presumably reflecting the large reduction in phonon mean-free path due to disorder in the films. On the other hand, the thermal resistances of thin (≤ 205 nm) crystalline films of TES-ADT, prepared by vapor-annealing of spin-cast films, are dominated by their interface resistances, possibly due to dewetting of the film from the substrate during the annealing process