Frequency-Dependent Photothermal Measurement of Transverse Thermal Diffusivity of Organic Semiconductors

TL;DR

This study employs a frequency-dependent photothermal method to measure the transverse thermal diffusivity of organic semiconductors, revealing how diffusivity varies with material structure and thickness.

Contribution

It introduces a novel photothermal measurement technique for transverse thermal diffusivity and applies it to organic semiconductors, providing new insights into their heat transport properties.

Findings

NFC-PEDOT diffusivity inversely related to thickness

Interlayer diffusivity in TIPS-pn exceeds in-plane values

Low-frequency phonons likely dominate heat conduction

Abstract

We have used a photothermal technique, in which chopped light heats the front surface of a small ( ~ 1 mm2) sample and the chopping frequency dependence of thermal radiation from the back surface is measured with a liquid nitrogen cooled infrared detector. In our system, the sample is placed directly in front of the detector within its dewar. Because the detector is also sensitive to some of the incident light which leaks around or through the sample, measurements are made for the detector signal that is in quadrature with the chopped light. Results are presented for layered crystals of semiconducting 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pn) and for papers of cellulose nanofibrils coated with semiconducting poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) (NFC-PEDOT). For NFC-PEDOT, we have found that the transverse diffusivity, smaller than the in-plane value, varies inversely with thickness, suggesting that texturing of the papers varies with thickness. For TIPS-pn, we have found that the interlayer diffusivity is an order of magnitude larger than the in-plane value, consistent with previous estimates, suggesting that low-frequency optical phonons, presumably associated with librations in the TIPS side-groups, carry most of the heat.