Electrical and Thermal Transport of Organic Semiconductors Blended with Insulators

Significant advancements have been made in the application of organic semiconductors (OSCs) in organic field-effect transistors (OFETs); however, their commercial applications have been constrained by the low reproducibility and poor stability of OSCs. Blending OSCs with commodity-insulating polymers is one possible tactic to get around these restrictions. The resultant bicomponent blends have surprising advantages over neat OSCs, including low cost, high performance, and long-time retention. However, a mechanistic comprehension of the role of insulators in these blends is still elusive.

We first looked into how the blending of insulators affected the electrical and thermal transport of three n-type polymers. Despite that commercial polystyrene (PS) is electrically and thermally insulating, the blended films universally provide improved electronic properties and heat transfer as verified by the OFET, time-resolved photoluminescence, and scanning photothermal deflection (SPD) measurements. These outcomes can be rationalized by a two-phase model in which PS blends inhomogeneously with the OSCs. As a result, for the charge transport, the insulator dilutes the trap states in the amorphous region of OSCs, contributing to the improved effective mobility and current density in the blended films. For thermal transport, the PS molecules dispersed among the disordered polymer chains in the amorphous region provide “highways” for phonon propagation.

With the knowledge of the insulator effect on the thermal behaviors of OSCs, coupled with the fact that the thermal conduction of existing OSCs is poor and rarely explored in comparison with traditional inorganic semiconductors. We thus further provide an understanding of the heat transfer mechanism in these insulator blends. Here, PC71BM, a well-known poor heat conductor but widely used electron transporter, serves as a host OSC. PS is a blended guest material. The thermal behaviors of the PC71BM/PS films were systematically investigated by the SPD technique and an infrared thermal camera. Furthermore, we found that the low-molecular-weight PS in the blended film is intimately mixed with the OSC phase, allowing for a better- interconnected phonon transport network and efficient heat conduction. The phase thermodynamics of the blended system discloses the role of insulators in heat transfer, providing new insights into this blending strategy.

Finally, the concept of advancement in the thermal transport of OSCs was extended to fluorinated insulators as guest materials, i.e., poly(4-fluorostyrene) (FPS) and poly(pentafluorostyrene) (5FPS). Starting with 20% 5FPS, the blends can sustain its mobility under high-temperature stress (250 oC for 5 hrs). While the neat device achieved only 10.2% of the mobility of the fresh samples under the same test conditions. Both photothermal deflection spectroscopy (PDS) and temperature-dependent transport reveal that the thermally induced energetic disorder can be effectively inhibited upon blending insulators. As a result, the thermal transport of blended films is more efficient than that of the neat counterpart. We also discovered that a closer mixture of the binary phases improves heat transfer capability. This strategy offers a facile method to operate organic electronics under harsh thermal conditions. Such insights from the disclosed role of insulators provide a guideline for better thermal management in organic electronic applications.

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