Princeton University

School of Engineering & Applied Science

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

James Belasco
J401, E-Quad
Thursday, August 28, 2014 - 3:00pm to 4:30pm

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 work investigates the improvement of small-molecule/polymer blend OFETs. 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 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 the small molecule with another more suited to the polymer. The 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 polymer transistors via the addition of very small amounts (less than 0.3% by weight) of a soluble p-dopant. Controlled threshold voltage shifting is attained by varying doping concentrations. Further investigation of dopant effects using variable temperature current-voltage (VTIV) measurements, find that extremely small doping concentrations induce a large increase in conductivity of the organic semiconducting film.