Week 2: How can organic molecules conduct electricity?
Rohan V -
Hi all, it’s Rohan. Last week, I discussed some of the fundamental science behind silicon-based semiconductors, including why many researchers are trying to develop alternative technologies. This week, I’ll delve into what makes organic semiconductors appealing as well as methods that scientists use to gauge their effectiveness.
The term “organic” has come to mean many things in modern society. In this case, an organic molecule is defined as any substance that contains carbon to hydrogen bonds. Consequently, organic semiconductors are organic molecules which can moderately conduct electricity.
Naturally, not all organic molecules can conduct electricity. In fact, the vast majority of organic molecules are insulators: substances that impede charge flow. Rubber, a potent insulator, is a polymer made of only carbon and hydrogen atoms. Most plastics are also composed of only carbon and hydrogen. These examples beg the question, how can organic molecules be arranged to conduct electricity?
The secret to organic conductivity is quite similar to metalloid conductivity, which was discussed in last week’s post. In both cases, electrons need a way to flow between atoms. In metalloids, doping to introduce impurities with a different number of valence electrons than the metalloid of interest allows electron flow. In organic molecules, having a series of alternating double bonds (double bonds on every other carbon atom in a chain) allows electrons to flow between atoms. In simple terms, these double bonds can be ‘shared’ and create one, extended region of electrons throughout the entire molecule. In organic chemistry, this phenomenon is known as conjugation.
Because of this requirement for conductivity, the process of making viable organic semiconductors is complex. Oftentimes, research teams will estimate properties of a molecule using chemical modeling software before attempting to synthesize it. Once they have determined that a particular molecule may be viable, the starting materials, reactants and catalysts required to synthesize the molecule may be obtained. While the synthesis process takes many steps, it starts with very simple reactants that are commercially available. This availability is what makes organic semiconductors appealing. In addition, organic semiconductors are easily biodegradable and do not pose an environmental hazard. Ultimately, successful synthesis and implementation of organic semiconductors in electronic devices will significantly lower the energy consumption associated with the production of silicon-based semiconductors.
Once a potential organic semiconductor is synthesized in the lab, the next step is to measure how well it conducts electricity. I’ll discuss the commonly used experimental setups to measure single-molecule conductance in my next post.
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