Highly crystalline thin films of organic semiconductors have great potential as a basis for low-cost flexible electronics with a good performance. But one of the obstacles engineers have run into is their limited knowledge of how to make such thin films with large monocrystalline domains, domains that do not slow down or prevent the transport of charges. In a breakthrough study supported by a prestigious ERC grant, professor Paul Heremans, imec fellow and director of imec’s large area electronics department, and his colleagues have studied how exactly crystals are formed in organic semiconductor materials. With that knowledge, they have developed new, scalable techniques to form highly-ordered crystalline organic films. And they have devised methods to integrate these films into devices such as organic thin film transistors, organic solar cells and organic light-emitting transistors.
Untapped potential – a call for fundamental research
Organic semiconductors differ radically from other classes of semiconducting materials. Their molecular nature makes them uniquely tunable and processeable, and gives them intriguing optical properties. Thanks to the latter – their excellent properties for light emission – such organic semiconductors are already used in, e.g., state-of-the-art smartphones and OLED displays.
However, the organic transistors and circuits that are made today are still lacking compared to what is theoretically possible. Their performance hinges on the speed with which charges can be transported through the thin films of material that underlie the electronics. But because of the way these films are currently deposited, they are typically highly disordered: amorphous or crystallized into areas no larger than typically one micron. So they are polycrystalline films with a random, dense pattern of grain boundaries between the crystalline domains. And these boundaries hinder the transport of charges and may induce a large variability in the transistors and circuits that are processed on top of those layers.
To be clear: the current underperformance is mainly due to how the films are deposited; it is not a property of the molecules that are used. That has been proven abundantly with single-crystal films created under lab circumstances that show much better characteristics and performances than those created with today’s state-of-the-art thin-film technology. So any improvement effort should address the way thin-films are made.