A paper co-authored by Dr. Aram Amassian, Assistant Professor, Materials Science and Engineering, has been featured on the cover of a special issue of the Journal of Materials Chemistry. The issue focuses on the interface engineering of organic and molecular electronics.

Organic electronics is a very active area of research owing to the contribution it could make to the low-cost manufacture of lightweight, large-area electronic devices and solar cells. The ability to leverage inexpensive solution-based processing and post-processing strategies in their fabrication is key to the future adoption of these technologies. Traditionally the post-processing of materials has consisted of the use of heat to modify the properties of materials – also known as thermal annealing, which can be taxing on materials – especially if they are organic. Solvent vapor annealing has emerged as a low-cost and low thermal budget alternative to thermal annealing.

If selected and dosed with care, solvents can modify material properties and even boost the performance of devices. Solvents may be chemically selected to treat certain components of a device selectively, a highly versatile but also quite complex process.

Dr. Amassian’s paper – Solvent Vapor Annealing of an Insoluble Molecular Semiconductor – illustrates how solvent molecules such as acetone can interact with the surface of molecular thin films used in electronic and photovoltaic applications. This interaction can be used to modify the structure and properties of the films and Dr. Amassian’s research focuses on shedding light on these processes.

"We really felt that the solvent vapor annealing process, however promising, is currently very immature and is not very well understood," he said. "We therefore set out to investigate how solvent vapors interact with and modify organic semiconductors, of which pentacene is perhaps the ‘fruit fly’ system."

Dr. Amassian and his team, working with the Cornell High Energy Synchrotron Source, used a high intensity X-ray beam to probe structural evolution in pentacene thin films as it occurred.

Synchrotron radiation sources generate very intense X-ray beams, which can provide insight into the evolving structure of a molecular semiconductor as it is being annealed.

Information about changes in molecular-scale packing can provide insight into the electronic and transport properties of the semiconductor, while insight into melting and crystallization during solvent vapor annealing are important to understand and control the process.

The intense X-ray source made it possible to collect full diffraction spectra in a matter of seconds – in fact the data collected in a single second measurement was equivalent to several hours’ worth of counting using a traditional lab-based diffractometer.

"By using this method we gathered a substantial amount of information and even came upon some surprises," added Dr. Amassian. "This is because molecular materials are complex and can behave in unpredictable ways. Fortunately the bright X-ray source enabled us to capture the material behavior during the process and as a function of solvent vapor dose.

"Unlike thermal annealing, solvent vapor annealing relies on the uptake of solvent by the molecular material, so we also performed simultaneous optical reflectometry measurements to monitor the structural changes in relation to the solvent uptake."

Dr. Amassian’s research shows that solvent annealing of typically insoluble organic semiconductors such as pentacene involves a substantial uptake of solvent molecules by the film. There is evidence of important capillary forces driving the phase transformation and crystallographic alignment of the material.

The methodology used in the research is a way forward, Dr Amassian believes.

"I think we have developed a methodology through which a lot of helpful insight can be gained into solvent vapor annealing of organic semiconductors, including small-molecules and polymers, leading to improved strategies for organic electronic and solar energy device fabrication. This insight was only made possible by performing measurements during solvent vapor annealing.

"So we expect this to become the way forward for tackling some of the complex scientific and engineering problems of organic material processing and we hope that it can lead to important breakthroughs in this rapidly growing area."

Dr. Amassian and his colleagues will in the future concentrate on designing and developing new molecular materials that, in addition to yielding better devices, lend themselves well to low-cost solution processing and post-processing. He believes this will be the future for organic electronics.

"We want to work with chemists to develop soluble materials that are optimal for solution processes and which are designed to yield desirable material characteristics and device performance.

"For instance we have started investigating soluble derivatives of pentacene designed by the Anthony group and already our experiments have revealed spectacular and intriguing differences in the way they interact with solvent vapors."

Energy is another research area that interests Dr. Amassian and his colleagues.

"Our focus now is on the development of highly stable, soluble materials that can be processed into nanostructured photoactive layers for optimal harvesting of solar energy in the harsh environmental conditions of Saudi Arabia."