Tiny stamps make patterns with different energy levels for LEDs

Title: Spatially Modulating Interfacial Properties of Transparent Conductive Oxides: Patterning Work Function with Phosphonic Acid Self-Assembled Monolayers

Authors: Kristina M. Knesting , Peter J. Hotchkiss , Bradley A. MacLeod , Seth R. Marder , and David S. Ginger

Journal: Advanced Materials

Affiliation: Department of Chemistry, University of Washington, and School of Chemistry and Biochemistry, and Center for Organic Photonics and Electronics, Georgia Institute of Technology

In our world of screens, having a way to make inexpensive electronics that don’t require heavy and rigid substrates is all the more exciting  (a “substrate” in the materials world is the surface on which something resides, unlike the world of enzymes and catalysis). From the chemistry side, polymers and organic small molecules are an appealing material type because they can be chemically altered to have different properties and their electronic or optical behavior doesn’t need to rely on the substrate on which they sit. Two of the challenges in making LEDs and other optoelectronic devices from polymers or organic small molecules are:

1)      whatever substrate they’re sitting on has to keep them there (as opposed to behaving like water droplets on wax paper)

2)      electrons are going to need to move between the molecules and whatever substrate they’re sitting on

Astoundingly, a single layer (monolayer) of some mediating molecule that goes between the surface and the polymer can do wonders for both of these challenges! Do you need a surface that likes being next to long carbon chains? Make a monolayer that binds to the surface and then has a long carbon chain “tail” so your polymer’s long carbon chains interact favorably. Do you need electrons to move from the substrate into your polymer? Make a monolayer with an energy level right next to the energy level of your polymer that binds to the substrate. If you’re clever, the monolayer molecule can solve both of these problems by having good wetting properties and appropriate energy level alignment.

In this paper Knesting et al. use “microcontact printing” to stamp patterns of their monolayer molecule of choice (pentafluorobenzyl phosphonic acid) onto the surface of indium tin oxide, which is a transparent electrode. The phosphonic acid end of the monolayer molecule binds to the indium tin oxide, and the pentafluorobenzyl part sticks up from the surface to interact with the polymer. This monolayer molecule has an ionization energy/workfunction that lines up better with the polymer than the indium tin oxide itself in terms of electron injection, so the authors expect that the areas stamped with this monolayer to have higher current density and emit more brightly. They do observe this increased device performance, and they did the relevant experiments to characterize the stamped surface.

To image the surface both physically and electronically, Knesting and coworkers use scanning Kelvin probe microscopy. This technique consists of moving a very sharp tip connected to a cantilever across a sample and observing the motion of the cantilever as a function of either physical repulsion (the simpler technique called atomic force microscopy) or electronic forces due to voltages applied between the sample and the tip. The atomic force microscopy image looks flat, which isn’t surprising because the monolayer is only one molecule high. With Kelvin probe, the authors find that the stamped areas are well defined, which implies that the phosphonic acid ends that are bound to the surface stick very well and do not migrate, like thiols are known to do on gold surfaces.

This paper shows that microcontact printing can be used to effectively modulate the workfunction of a surface, thereby enabling the creation of brighter patterned organic LEDs. The authors note that this work might also enable controlled morphology and local energy alignment for organic solar cells as well.

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