Thermal Conductivities of Crystalline Organic Semiconductors

As applications for organic semiconductors grow, it is becoming increasingly important to know their thermal conductivities, k. For example, for sub-micron electronic devices, values of k\textgreater k$_{0}$ $\sim$ 5 mW/cm/K are needed, while values k\textless k$_{0}$ are required for desired thermoelectric applications. Whereas it is not surprising that semiconducting polymers typically have room temperature thermal conductivities below k$_{0}$, many molecular organic crystals also have values of k below this value. We have started measurements of both the in-plane and interplane thermal diffusivities of layered crystalline organic semiconductors using frequency\footnote{H. Zhang and J.W. Brill, J. Appl. Phys. \textbf{114}, 043508 (2013).} and position dependent\footnote{I. Hatta \textit{et al}, Jpn. Jnl Appl. Phys. \textbf{25}, L493 (1986).} ac-calorimetry; the thermal conductivities are then determined from the specific heats measured with differential scanning calorimetry. For rubrene, which has k\textless k$_{0}$, the interplane thermal conductivity is several times smaller than the in-plane value, although its temperature dependence indicates that the phonon mean-free path is at least a few layers.\footnote{H. Zhang and J.W. Brill} On the other hand, the in-plane thermal conductivity of TIPS-pentacene,\footnote{J.E. Anthony, Chem. Rev. \textbf{106}, 5028 (2006).} is several times greater than k$_{0}$, similar to that of the quasi-one dimensional organic metal TTF-TCNQ.\footnote{M.B. Salamon \textit{et al};, Phys. Rev. B \textbf{11}, 619 (1975).} Remarkably, its interlayer thermal conductivity is several times larger than its in-plane value,\footnote{H. Zhang and J.W. Brill} perhaps due to interactions between the large (triisopropylsilylethynyl) side groups on the pentacene backbone.