Effect of Polymer Molecular Weight on Morphology and Charge Transport of Small-Molecular Organic Semiconductors

Abstract

The utilization of polymer additives provides an intriguing pathway to tune the electrical performance of solution-grown, small-molecular organic semiconductors. In this study, we report the effect of different polymer molecular weight on the crystal growth, phase segregation and charge transport of the organic semiconductors. A semicrystalline polymer additive polyethylene oxide (PEO) with 8000 and 100 K molecular weight was blended with a well-known organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene). Distinctively different thin film morphology of the resultant TIPS pentacene/PEO blend film was identified and quantitatively characterized. In particular, PEO with a higher molecular weight of 100 K exerted a stronger effect on enhancing film formation, crystal coverage and likely mechanical properties, whereas PEO with an 8000 molecular weight yielded more superior crystal alignment and larger crystal sizes. Bottom-gate, top-contact TIPS pentacene/PEO OTFTs were fabricated to test the charge transport, which indicated loading the PEO polymer with molecular weight of 8000 demonstrated a five-fold enhancement in the extracted hole mobilities as compared to the 100 K counterpart. This work of using polymer additives with different molecular weight can be used to tune the crystallization of other solution-processed high-mobility small-molecular organic semiconductors.