Improving and Controlling Organic Field Effect Transistor Performance through Dual Solvent Processing and Molecular Doping

Abstract

Solution processed organic electronic devices such as organic field effect transistors (OFETs) show the potential for low cost manufacturing due to their low temperature processing and possible fabrication via inkjet or roll-to-roll processing. However, while OFETs have made strides in performance, they currently remain an emergent technology, and fully organic devices command high consumer prices. Further technological developments that easily and effectively enhance device performance are still needed. This work focuses on processes and techniques that are demonstrated to improve OFET performance and actively control device characteristics.

This thesis investigates the improvement of small-molecule/polymer blend OFETs, realized by mixing 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and amorphous polystyrene in solution and fabricating bottom gate/bottom contact devices via a spin coating process. Precise manipulation of substrate conditions, processing parameters, and solvent combinations of these OFETs was undertaken in an effort to improve device performance, specifically transistor mobility. The use of pentafluorobenzenthiol (PFBT) as a self-assembled monolayer (SAM) to modify the source and drain electrodes was also investigated as a way to decrease contact resistance and improve semiconductor crystallinity. It was found that mobility could be increased by almost an order of magnitude via application of a dual-solvent solution that combines a solvent more suited to TIPS-pentacene with another more suited to the polystyrene. This result is examined via the use of atomic force microscopy (AFM) and polarized microscopy, which reveal considerable morphological differences between device films from different solvents.

A technique for manipulating the threshold voltage characteristics of OFETs through the use of ultra-low doping of the organic semiconductor solution is also demonstrated. This technique is applied to the dual-solvent transistors and poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl-benzidine)] (poly-TPD) polymer transistors via the addition of very small amounts (less than 0.3\% by weight) of the soluble p-dopant molybdenum tris-[1-(trifluoroethanoyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] [Mo(tfd-COCF3)3]. Controlled threshold voltage shifting is attained by varying doping concentrations. Further investigation of dopant effects on film characteristics are undertaken on doped TIPS-pentacene using variable temperature current-voltage (VTIV) measurements, where extremely small doping concentrations are found to induce a large increase in conductivity of the organic semiconducting film.