Characterizing Thermal Diffusivity of Semiconducting Polymers by Scanning Photothermal Deflection (SPD) Technique

Owing to their relatively low thermal conductivity, heat is trapped in most organic semiconducting devices resulting in operational instability and shortened device lifetime. To overcome these issues, it is important to understand and enhance device heat-dissipation. However, related studies on organic semiconductors are rare. Here, we present a self-developed scanning photothermal deflection (SPD) technique to study the thermal transport of common organic photovoltaic (OPV) polymers and bulk-heterojunctions (BHJs). In SPD, heat diffusion in an organic film is probed by photothermal deflection method utilizing the mirage effect. In general, the thermal diffusivities for conjugated polymers increase in the order of amorphous < 1D < 2D polymers in the range of 0.1 to 2 mm² s^-1 (Nano Select, 2(4), 768-778). High-performance 2D polymers such as PM6 has high thermal diffusivity values comparable to stainless steel. In contrast, fullerene- and ITIC-based BHJs have significantly lower thermal diffusivities, as the heterogeneous blend may disrupt the polycrystalline structure of the polymer and slow down the intramolecular phonon transport. On the other hand, reduction due to heterogeneous blend is less noticeable in all-polymer BHJs, indicating that a polymer-rich topology benefits dominated intramolecular phonon transport. Also, SPD technique can be applied to study Y acceptors and their BHJs (Advanced Functional Materials, 31(32), 2101627).