Synthesis, Surface Grafting, and Fabrication of Ultrathin Polymeric SAMFETs with High Field-Effect Mobility

Densely surface-grafted monolayer (3-4 nm) poly(3-hexylthiophene) (P3HT) brushes are prepared by click chemistry. For this, P3HT chains with alkyne end groups were synthesized and chemically coupled to a surface-immobilized self-assembled monolayer (SAM) having azide functionality in an organic field-effect transistor channel. The grafted P3HT-alkyne with a molecular weight of M_n,MALDI = 11 400 g mol^-1 ( M_n,SEC = 17 400 g mol^-1) and a narrow distribution of Đ = 1.15, has the highest reported molecular weight for surface-immobilized P3HT brushes. We show the successful grafting of P3HT on the substrate surface with atomic force microscopy, contact angle, and absorption studies. From the film thickness, we can calculate the reduced tethered densities of ∑ = 10.3-12.1, which is indicative of the monolayers being in the true brush regime with high grafting density that is enough to form a compact self-assembled monolayer. The aggregation behavior of the films is characterized by UV-vis spectroscopy and compared to linear P3HT and a bottlebrush copolymer polystyrene- g-P3HT (PS- g-P3HT) with similar P3HT lengths. For such an SAM-based organic field-effect transistor (SAMFET) nanodevice with an ultrathin P3HT layer of 3-4 nm, a very high field-effect mobility of up to 1.8 × 10^-3 cm² V^-1 s^-1 is achieved in channel lengths of 5-20 μm, which is nearly 2 orders of magnitude higher than reported values for polymer-based SAMFETs.