One of my favorite scenes from Harry Potter is when Harry receives a gift from Hagrid, a wonderful photo album filled with moving images of his parents smiling and waving at him, and the first thought that came to my mind was ”Is someone working on that?” I know that the boundaries of science have already been tried and tested enough by the fascinating world of science fiction, and we don’t need fantasy doing it now, but for us scientists, is enough really ever enough? Just like the fact that we have perfectly working electronic devices that go from our wrists to our walls, but we’re still craving for better, for more, or sometimes, just different?
All major electronic devices and circuits work based on materials called inorganic semiconductors, the most widely used one being silicon. The way these materials are built is a lattice of atoms all put together in an arrangement that is continuous, periodic and ordered. (With minor faults here and there of course, nobody is perfect). Since the lattice is continuous, it means that the energy levels in individual atoms now line up to form continuous states, or bands through which electrons can move very easily and conduct charge. These electrons are only stopped or slowed down when they encounter aforementioned lattice imperfections. Basically, the lattice of Silicon would be like a running track with a few hurdles, but electrons are moving pretty fast in their particular track.
On the other hand, organic electronic materials, broadly classified into polymers and small molecules, don’t arrange themselves into periodic lattices. They form films where individual molecules are arranged in a more or less ordered fashion, where the more or less is decided by the way you process them. Along with this, the bonding in these molecules is a weaker Van der Waals bonding, which isn’t as strong as covalently bonded silicon. Although there isn’t any long range order, the molecular orbitals (or paths of the electrons moving in individual atoms) overlap intermittently, forming sites for the electrons to hop to and fro. Generally speaking, the behavior of electrons in these organic materials would be like rock climbing, where the electrons keep looking for points to travel forward.
The biggest advantage with organic electronics is their relatively lower processing and deposition temperatures, as these materials are often used close to room temperature. This opens up the opportunity to develop electronic devices, like transistors, solar cells, or LEDs on all kinds of plastic, flexible substrates, and even paper! (Believe me; people are really working on that). The materials are either solution-processed or thermally deposited in vacuum, and devices are patterned by a wide variety of techniques like lithography, masks, and microprinting. In the particular case of thin film transistors, low power complementary circuits have been fabricated and tested successfully. The major challenge faced by researchers today is to improve their performance to match already existing requirements. As an example, the mobility of electrons through thin film transistors, an indication of how fast they move is currently 2-3 orders of magnitude lower than silicon devices. The possible degradation of these materials overtime on exposure to air also remains a major concern. Nevertheless, instead of trying to replace silicon in traditional electronic appliances, these organic materials are really carving out their own niche in applications like intelligent labelling with RFID tags, large display sensor arrays, or portable devices.
My own research emphasizes on thin film transistors, particularly the gate dielectric component in the transistor. The gate dielectric layer separates the gate electrode from the semiconductor channel, so that when a bias is applied on the gate electrode, it induces an accumulation of charge carriers near the semiconductor-gate dielectric interface. These charge carriers then form the semiconducting channel and carry current through the transistor when a separate drain-source voltage is applied. The focus of my research is a hybrid gate dielectric layer, consisting of an inorganic metal oxide (e.g. Aluminum) and an organic self-assembled monolayer. What I intend to investigate is the effect of the gate dielectric layer, and its various aspects like the thickness, surface roughness on the properties of the transistor. Once I understand these effects, I also aim to achieve an optimum gate dielectric layer, which yields the most favorable transistor operating parameters, like threshold voltage and charge carrier mobility.
A few references for further reading:
- Embracing the Organics World, Nature Materials (Page 591, Vol 12, July 2013)
- A bright future for organic field effect transistors, Nature Materials (Vol. 5, Aug 2006)
- Low Power, High Impact Nature Materials, (Vol 6. March 2007)
- U. Zschiechang, Organic Electronics 25 (2015) 340–344