Transverse Thermal Diffusivity of Polymer and Small Molecule Organic Semiconductors Measured with an ac-Photothermal Technique

We have developed a simplified ac-photothermal apparatus [1] for measurement of the transverse (i.e. through-plane for partly aligned polymers and interlayer for layered crystals) thermal diffusivity of small samples. Our technique is essentially the Fourier transform of the laser flash method. The sample, with a typical area of 5 mm^2 and heated with chopped light, is placed in the dewar of an infrared detector close to the detector, which measures the thermal radiation from the back of the sample. For optically opaque samples, an analysis of the complex frequency dependence of the detector signal gives the transverse diffusivity; results will be presented for free-standing PEDOT:PSS films and samples of cellulose nanofibrils coated with PEDOT:PSS (samples provided by X. Crispin, Linkoping U.) For samples which are not opaque, the same analysis, overlooking the finite optical absorption length, can lead to a very large overestimate of the diffusivity. Here we show how the technique can be adapted and present a more complete analysis for less absorbing samples, and present results for TIPS-pentacene (provided by J. Anthony, U. Kentucky), correcting the value we previously presented [1].
[1] J. W. Brill, et al, J. Appl. Phys. 118, 235501 (2015)