A team of engineers has determined why some transistors made of organic crystals don't perform well, yielding ideas about how to make them work better.
Stanford researchers have shown that the way boundaries between individual crystals in a film are aligned can make a 70-fold difference in how easily current, or electrical charges, can move through transistors. The research could help engineers design better digital displays and other devices [Rivnay et al., Nature Materials (2009), 8, 952].
Organic semiconductors have a lot to offer in electronics. They are cheap and flexible, and the production process is much simpler than for traditional silicon chips. Applications such as computer display screens, digital signs or magazines made of "electronic paper" have been possibilities for more than a decade, but their full potential seems always just around the corner. A persistent problem is that performance from transistor to transistor varies much more than can be allowed in commercially viable devices.
"You can make a single device that has high 'charge mobility,' but you really need to make thousands of them," said Alberto Salleo, an assistant professor of materials science and engineering at Stanford and a senior co-author of the paper. "Most research groups report a high variation in that mobility. What we did here is try to understand what causes the variation."
Salleo's group led a multidisciplinary team of researchers in making a systematic study of a likely culprit of the inconsistent transistor performance in polycrystalline devices: the "grain" boundaries between crystals.
To examine the role that boundary alignment plays the researchers grew crystals of an organic semiconductor called PDI8-CN2 using a process that ensures consistent alignment from crystal to crystal in a particular direction.
They then made transistors in which charges could flow through molecules that were well aligned with each other, and others where the molecules were misaligned across the grain boundaries. The first kind of transistors performed far better. He went further to link the properties of these boundaries to the molecular packing in the crystals.
The papers lead author comments, "The problem of understanding defects in organic electronic materials including grain boundaries is very important for any device application, by better understanding what goes on at these boundaries, and how detrimental they are, improvements can be made at the chemistry end as well as at the design and fabrication end of the process. This way devices can be more reproducible and better performing."
The team's work could strongly influence how organic crystal electronics are made in the future.