A new candidate for n‑type semiconductors

Japanese scientists have produced a novel organic substance that can potentially be adapted to form an n-type semiconductor, with wide-ranging applications in the field of electronics.

Semiconductors are substances that have a conductivity between that of conductors and insulators. Due to their ability to conduct current only in specific conditions, they can be controlled or modified to suit our needs. Nowhere is the application of semiconductors more extensive or important than in electrical and electronic devices, such as diodes, transistors, solar cells and integrated circuits.

Semiconductors can be made of either organic (carbon-based) or inorganic materials, though organic semiconductors have some clear advantages over inorganic semiconductors. As noted by Professor Makoto Tadokoro of the Tokyo University of Science, “Organic semiconductor devices, unlike hard inorganic semiconductor devices, are very soft and are useful for creating adhesive portable devices that can easily fit on a person.”

However, despite the advantages of organic semiconductors, there are very few known stable molecules that bear the physical properties of n-type semiconductors, compared to inorganic n-type semiconductors. N-heteroheptacenequinone is a well-known potential candidate for n-type semiconductor materials, but it has some drawbacks: it is unstable in air and UV-visible light, and it is insoluble in organic solvents. Now, Prof Tadokoro and colleagues have identified a novel substance, derived from N-heteroheptacenequinone, that overcomes these drawbacks.

To obtain the substance, N-heteroheptacenequinone was made to undergo a four-step process of chemical reactions involving repetitive refluxing, evaporation, recrystallisation and heating. The final product achieved was C₆OAHCQ — a red solid with a crystalline near-planar structure involving two tetraazanaphthacene ‘backbones’ and one benzoquinone backbone. It has eight electron-deficient imino-N atoms and two carbonyl moieties.

To confirm its electrochemical properties, C₆OAHCQ was made to undergo a series of tests including a UV-visible absorption spectroscopy in the solution state, cyclic voltammetry and theoretical calculation of electrostatic potential. It was also compared with a tetraazapentacenequinone analog, with the results published in the journal Organic and Biomolecular Chemistry.

The tests revealed that the electron-deficient imino-N atoms and two carbonyl moieties in C₆OAHCQ provide it with an electron-accepting behaviour. In fact, the number of electrons accepted by C₆OAHCQ is more than that by fullerene C₆₀, which suggests improved conductivity. Cyclic voltammetry showed that C₆OAHCQ exhibited reversible four-step, four-electron reduction waves, which indicated that C₆OAHCQ is stable and has good electrostatic potential; the UV-visible spectroscopy also showed its stability in UV-visible light.

C₆OAHCQ also showed electrochromic properties, which enable its potential application in many specialised areas such as the development of smart windows, electrochromic mirrors and electrochromic display devices. C₆OAHCQ was also found to have excellent solubility in common organic solvents. It was overall found to be advantageous and had improved properties compared to the tetraazapentacenequinone analog.

“The identification of this organic acceptor molecular skeleton that has the property of stably receiving electrons is very important, as it can be used to develop molecular devices with new functionality,” the scientists stated. “These devices are soft, unlike hard inorganic semiconductor devices, and can help to create portable devices.”

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