Molecular Weight‐Induced Structural Transition of Liquid‐Crystalline Polymer Semiconductor for High‐Stability Organic Transistor

In order to fabricate polymer field‐effect transistors (PFETs) with high electrical stability under bias‐stress, it is crucial to minimize the density of charge trapping sites caused by the disordered regions. Here we report PFETs with excellent electrical stability comparable to that of single‐crystalline organic semiconductors by specifically controlling the molecular weight (MW) of the donor‐acceptor type copolymer semiconductors, poly (didodecylquaterthiophene‐alt‐didodecylbithiazole). We found that MW‐induced thermally structural transition from liquid‐crystalline to semi‐crystalline phases strongly affects the device performance (charge‐carrier mobility and electrical bias‐stability) as well as the nanostructures such as the molecular ordering and the morphological feature. In particular, for the polymer with a MW of 22 kDa, the transfer curves varied little (ΔV_th = 3～4 V) during a period of prolonged bias stress (about 50 000 s) under ambient conditions. This enhancement of the electrical bias‐stability can be attributed to highly ordered liquid‐crystalline nanostructure of copolymer semiconductors on dielectric surface via the optimization of molecular weights.