A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.
The ability to measure the structural development of organic semiconductor films and correlate it to the electrical characteristics of organic devices such as thin film transistors (OTFTs) is needed to facilitate the commercialization of this technology. Synchrotron-based Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a powerful tool that meets this need because it can non-destructively reveal both the structure and chemistry of thin organic films. The density of bonds involving Carbon, Nitrogen, Oxygen, and Fluorine can be quantified, a depth profile can be developed, and bond orientation can be determined. Here, we outline the principles of NEXAFS experimentation and data analysis with an emphasis on their application to organic semiconductor thin films. NEXAFS spectra of model organic semiconductors such as pentacene and poly(3-hexylthiophene) (P3HT) are used to demonstrate how NEXAFS can enhance understanding of the complex processing-structure relationships of semiconductor film formation and identify processing methods that may optimize device performance.
Conference Committee Involvement (1)
Organic Field-Effect Transistors VI
26 August 2007 | San Diego, California, United States