(Invited) A Simple and Efficient Solution-Based Technique to Electrically Dope Organic Semiconductors

Abstract

The printed electronics industry offers a paradigm change in manufacturing, cost and environmental impact when compared to the conventional semiconductor industry. Printed electronic devices are expected to be mass-produced from less-energy-demanding processes over large areas and on flexible substrates, with techniques that closely resemble the well-known mass production of printed media on paper. Still, critical to the widespread of these devices is the ability to convert lab produced “champion” figures into reliable industrial-scale products. The well-established strategies for the design and fabrication of efficient lab-scale printed electronics are not always compatible with large-area manufacturing, where several additional requirements must be met. Solution-based electrical doping of organic semiconductors has received increased attention as a technique with the potential to comply with some of these additional requirements. Stable ink-type formulations of dopant, semiconductor and solvents have been reported, some of which may be compatible with roll-to-roll coating. Yet, the ability to fabricate vertical doping gradients, critical to maintain optimal optoelectronic performance at reduced cost, has proven challenging. Vertical doping gradients would reduce the amount of coating steps without a loss in optoelectronic device functionality, simplifying manufacturing and preventing defects to propagate. In 2017 we reported a simple technique to fabricate vertical electrical p-type doping gradients in organic semiconductor films by dipping them in solutions. Still, the initial approach, which involved 12-molybdophosphoric acid (‘PMA’ dopant) and nitromethane (processing solvent), was highly solvent selective, and the use of nitromethane was found limiting. Thus, in 2018, our team replaced nitromethane with acetonitrile and showed that the PMA-acetonitrile solution became stable in air without losing its doping ability. Furthermore, organic solar cells fabricated using the updated approach showed longer air stability. In this talk we will start by discussing the basics of the solution-based technique to electrically p-type organic semiconductors using PMA, and compare it against similar, well-studied approaches. Then, we will emphasize on our latest insights on the path to unravel, from a molecular perspective, how organic semiconductors are doped down to a limited depth from solution, using PMA molecules. That is, what is the interplay between the organic film and PMA molecules in solution that leads to p-type electrical doping of the semiconducting layer? Additionally, what are the key parameters that determine the efficiency and extent of the process?